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Phylogeographic relationships and the evolutionary history of the Carassius auratus complex with a newly born homodiploid raw fish (2nNCRC)

Background An important aspect of studying evolution is to understand how new species are formed and their uniqueness is maintained. Hybridization can lead to the formation of new species through reorganization of the adaptive system and significant changes in phenotype. Interestingly, eight stable strains of 2nNCRC derived from interspecies hybridization have been established in our laboratory. To examine the phylogeographical pattern of the widely distributed genus Carassius across Eurasia and investigate the possible homoploid hybrid origin of the Carassius auratus complex lineage in light of past climatic events, the mitochondrial genome (mtDNA) and one nuclear DNA were used to reconstruct the phylogenetic relationship between the C. auratus complex and 2nNCRC and to assess how demographic history, dispersal and barriers to gene flow have led to the current distribution of the C. auratus complex. Results As expected, 2nNCRC had a very close relationship with the C. auratus complex and similar morphological characteristics to those of the C. auratus complex, which is genetically distinct from the other three species of Carassius. The estimation of divergence time and ancestral state demonstrated that the C. auratus complex possibly originated from the Yangtze River basin in China. There were seven sublineages of the C. auratus complex across Eurasia and at least four mtDNA lineages endemic to particular geographical regions in China. The primary colonization route from China to Mongolia and the Far East (Russia) occurred during the Late Pliocene, and the diversification of other sublineages of the C. auratus complex specifically coincided with the interglacial stage during the Early and Mid-Pleistocene in China. Conclusion Our results support the origin of the C. auratus complex in China, and its wide distribution across Eurasia was mainly due to natural Pleistocene dispersal and recent anthropogenic translocation. The sympatric distribution of the ancestral area for both parents of 2nNCRC and the C. auratus complex, as well as the significant changes in the structure of pharyngeal teeth and morphological characteristics between 2nNCRC and its parents, imply that homoploid hybrid speciation (HHS) for C. auratus could likely have occurred in nature. The diversification pattern indicated an independent evolutionary history of the C. auratus complex, which was not separated from the most recent common ancestor of C. carassius or C. cuvieri. Considering that the paleoclimate oscillation and the development of an eastward-flowing drainage system during the Pliocene and Pleistocene in China provided an opportunity for hybridization between divergent lineages, the formation of 2nNCRC in our laboratory could be a good candidate for explaining the HHS of C. auratus in nature. Supplementary Information The online version contains supplementary material available at 10.1186/s12864-022-08468-x.


Background
A fundamental question in evolutionary biology is whether speciation is gradual or punctuated [1]. Hybridization is regarded as an 'evolutionary catalyst' that may be an important source for the origination of a new species or an increase the amount of genetic variability. Hybridization can result in the reorganization of adaptive systems and lead to the formation of new species [2]. Hybridization between species can trigger vast genetic and genomic imbalances, including a high rate of DNA mutations and combinations [3], often resulting in significant changes in phenotypes and genotypes of hybrid offspring, which may facilitate speciation and adaptive radiation.
Drastic climatic and geological oscillations, such as glacial expansion and megadroughts, forced freshwater species to contract their distribution ranges and reside in small refuges in many cases. Potential gene exchange and hybrid speciation likely occurred owing to the close contact between distant species. Increasingly, studies suggest that interspecies hybridization and gene introgression are not uncommon in fish, such as in the families Poeciliidae, Atherinidae, Cyprinidae, Cobitidae, and even Carcharhinidae [4][5][6], especially East African cichlids, which have attracted much attention in recent years [7][8][9]. As the most common species of vertebrates, more than 34,000 fish species have been recorded (https:// www. fishb ase. de/). The formation of many fishes has been determined to be associated with hybridization [10,11]. Ancestral hybridization has been suggested to have played a key role in facilitating species diversification of cichlid fish in Lake Malawi, Lake Victoria, and Lake Tanganyika [7][8][9].
However, for a long time, there has been a lack of enough direct evidence to prove that fish hybridization will produce new species. Distant hybridization can generate allotetraploid and autotetraploid fish, supplying important and direct evidence for hybridization speciation in fish [12]. It was reported that a newly born homodiploid raw fish (2nNCRC, 2n =100), which is very similar to the crucian carp with no beard, was derived from interspecies hybridization between Cyprinus carpio (female, 2n = 100) and Megalobrama amblycephala (male, 2n = 48) by artificial propagation [13]. Fortunately, we repeated this experiment three times with similar results in our laboratory. In addition, eight stable generations of 2nNCRC with a very clear genetic background have been founded in our laboratory since 2014, supplying a valuable model system for studying hybrid speciation in fish.
It is widely believed that five species exist in the genus Carassius, including Carassius carassius, Carassius cuvieri, Carassius langsdorfii, Carassius gibelio and Carassius auratus, which are generally accepted as valid species and they can be found in FishBase (https:// www. fishb ase. de/). Our previous study showed that the hybrid speciation would be a possible route of formation of C. auratus [13]. It is particularly interesting to find out whether the origin of C. auratus is identical to that of 2nNCRC and whether C. auratus evolved from 2nNCRC. First and foremost, it is necessary to clarify the historical evolutionary processes of C. auratus. Phylogeographic studies might address this issue because they are usually used to determine the historical processes that affect vicariance, dispersal, extinction and radiation that then lead to the geographic distributions of genetic lineages [14,15].
The recent climatic oscillation and geological events during the Pliocene-Pleistocene have played a fundamental role in shaping contemporary patterns of biodiversity and species diversification and distribution [16,17]. Pleistocene glaciations are known to have had a far-reaching influence on the evolution of organisms in the Northern Hemisphere [18,19]. For example, paleontological and genogeographical studies indicate that European and North American species experienced repeated episodes of contraction and expansion of their ranges due to major climatic oscillations [20]. The Carassius species complex has a wide distribution across the Eurasian continent and neighboring islands [21,22]. According to the fossil record of Carassius species in the Pliocene epoch (5.3-2.6 million years ago, Mya) discovered in northern China [23], Quaternary paleoenvironmental changes in East Asia and Europe had a great influence on the speciation and diversification of Carassius species. In freshwater fishes, the dynamics of recolonization are tightly linked to the history of river drainage systems [24]. During glacial melt periods, ephemeral rivers and periglacial lakes could arise, and the reconfiguration of the landscape caused by drastic climatic change and geological events may have allowed species to disperse into new habitats, providing opportunities for colonization and new species interactions.
These processes have resulted in complicated recolonization scenarios for the Carassius species complex in East Asia, where the haplotypes of the mitochondrial control region and tf alleles are clustered into four and three major lineages, respectively, and based on this information, it has been speculated that the Yangtze River basin was the potential origination center for the Carassius species complex then radiated across East Asia [22,25]. Gao et al. [21] deduced that the close relationship between the C. auratus complex from eastern mainland China and south-central Ryukyus was the result of natural Pleistocene dispersal. However, the existence of two distinct lineages of C. carassius in Europe [24] was mainly due to the Danubian catchment being separated from other river systems by the Alps, the Sudetes Mountains and the Carpathian Mountains.
According to the formation of the newly born 2nNCRC, combined with the close phylogenetic relationship between the C. carpio and Carassius species complex [26,27], we speculated that the existence of a possible route through distant hybridization under natural conditions may have generated C. auratus, and this hybridization likely occurred during the Pliocene-Pleistocene based on the following hypotheses: (1) 2nNCRC has a very close relationship with the C. auratus complex in terms of phenotype and genotype; (2) several geographic lineages of Carassius are endemic to specific regions of Eurasia, especially the C. auratus complex in China; and (3) the most recent common ancestor (TMRCA) of both Cyprinus and Megalobrama had a much earlier divergence time than that of the C. auratus complex, and all had a sympatric distribution in history. According to the distribution of current fish species and fossil data [23,28], the speciation of C. auratus in China might have been punctuated and not derived from the processing of lineage differentiation with other species of Carassius. Thus, establishing a robust time-calibrated phylogeny is the first requirement for tracing the possible origin and diversification patterns of the Carassius species complex.
In the present study, the mitochondrial genome and nuclear DNA were used to reconstruct the phylogenetic relationship between 2nNCRC and the C. auratus complex. The cytb was the largest data set including the Genbank data, including the population information for the species of Carassius, were obtained and used to investigate the evolutionary history of the C. auratus complex.

Morphological traits
Twelve to twenty samples of each generation of 2nNCRC were randomly selected to summarize the morphological characteristics of qualitative and quantitative traits (Table S1 in Supplementary Information). Both 2nNCRC and C. auratus have no barbells and almost the same quantitative traits. In particular, they have the same morphotype and pattern of pharyngeal teeth and four tabular teeth on each side of the pharyngeal bone, which are very different from those of C. carpio and M. amblycephala (Fig. 1). 2nNCRC (n = 30), C. auratus (n = 27), C. carpio (n = 38) and M. amblycephala (n = 28) were used to examine shape variation through principal component analysis (PCA) and canonical variance analysis (CVA). The results showed that the first three PCs explained 94.13% of the variation in shape morphology (Table S2 in Supplementary Information), while the first two CVs explained 97.94% of the variation (Table S3 in Supplementary Information). The scatter plot of CV1 and CV2 showed clear differentiation between 2nNCRC and its parents ( Fig. 1) but a close relationship and partial overlap between 2nNCRC and C. auratus. The Mahalanobis distances based on geometrical morphology data showed a significant difference among 2nNCRC and the three species (P < 0.0001) ( Table S4 in Supplementary Information), while the distance values between 2nNCRC and C. auratus were much lower than those between 2nNCRC and its parents.

Phylogenetic tree and haplotype network
The aligned and concatenated sequences (14 mitochondrial genes) were 13,634 bp with 5841 variable sites. Of these, 816 were parsimony-informative. The maximum likelihood (ML) and Bayesian tree showed a clear identification of the species of Carassius and most species of Cyprinidae (Fig. 2a). All species from Carassius containing 2nNCRC formed a monophyletic clade with high support values, and three valid species of Carassius were observed (Fig. 2a, in bold). However, 2nNCRC, together with C. gibelio, was unable to be separated from C. auratus but formed a single genetic cluster, and the genetic distances among 2nNCRC, C. auratus and C. gibelio were much lower than those among 2nNCRC and other species of Carassius (Table S5 in Supplementary Information). Consequently, C. auratus and C. gibelio are named the C. auratus complex in the present study.
Another phylogenetic tree using HoxA2b (nuclear DNA) also clearly identified most species of Cyprinidae and two distinct clusters in the C. auratus complex with 2nNCRC, which were monophyletic and sister to Procypris merus and C. carpio. However, it was also impossible to distinguish between 2nNCRC and C. auratus (Fig. 2b).
The median joining network of cytb showed that there was no shared haplotype among the C. carassius, C. cuvieri, C. langsdorfii and C. auratus complexes (Fig. 3a). The haplotype of C. carassius was distributed in Europe with two distinct sublineages. The haplotype of C. cuvieri had a narrow distribution in Japan, and most of the haplotypes of C. langsdorfii were distributed in Japan with very few shared haplotypes (Fig. 3a). In addition, there were two shared haplotypes (H1 and H3) between 2nNCRC and C. auratus. However, the C. auratus complex had an admixture distribution across Eurasia with many shared haplotypes, such as H2, H4, H6 and H7, which could be found in Europe, China, Western Asia and Siberia (Fig. 3a). Surprisingly, a significant phylogeographical pattern with four specific regions was found for the C. auratus complex in China, two in Southeast China (Sublineage B1 in Fujian Province, Sublineage B4 in Taiwan Island), one in North China (Sublineage B2 in Amur River Basin), and one in the middle of the Yangtze River basin (Sublineage B5) (Fig. 3b).
Within the widely distributed C. auratus complex, several sublineages were found in Eurasia, and most were consistent with the six sublineages (sublineages B1-B6) distributed in China, except for sublineage A from the Far East (Russia) and Mongolia, which first diverged from the C. auratus complex at approximately 3.

Ancestral range reconstruction
The results of the ancestral range reconstruction are depicted in Fig. 5 and Fig. S2 (in Supplementary Information). Among the six models of geographic-range evolution compared in the likelihood framework in Bio-GeoBEARS, the DIVA + j model was chosen for both 24 cytb sequences and 26 concatenated dataset (twelve proteins and two rRNAs in mitogenomes) according to the best likelihood and AICc associated scores (Table S6 in Supplementary Information). According to the ancestral area reconstruction using the 24 cytb gene across Eurasia, 41 dispersal and 21 vicariance events occurred within the evolution of the studied Carassius. TMRCA of Carassius probably diverged into two distinct clades ( Fig.   S2) in the late Miocene. One clade included C. carassius, which was mainly distributed in Europe, while the other clade further split into two separate lineages through two dispersal events and one vicariance event; the dispersal events corresponding to TMRCA of C. cuvieri, which was likely to disperse from Eurasia during early Pliocene according to the molecular dating (Fig. 4b). And then it diverged in two lineages through a vicariance event, the first one corresponding to lineages distributed in Japan (C. cuvieri and C. langsdorfii, node 43, G: 65.86%), and the second one corresponding to the lineage of C. auratus complex, whose ancestor was most likely to live in China (node 40). Three vicariance events occurred within the first lineage: a first one about 4.03 Ma separating C. cuvieri from C. langsdorfii, and other events during Pleistocene leading to different sublineages of C. langsdorfii across Eurasia. Within the second lineage, fifteen diversification events occurred: a vicariance event between Far East (Sulineage A) and China (Sublineage B) about 3.12 Mya, and fourteen events between 1.6-0.34 Mya leading to sublineages B1-B6. The dispersal events from the China to the Southeast Asia (Vietnam) explain the actual distribution of Sublineage B1 in both areas. The same condition can be found for the Sublineage B2 co-occur in the North China (Amur River Basin) and Far East, as well as the Sublineage B3 co-occur in China and Ryukyus and Japanese Islands. According to the ancestral area reconstruction using the 26 concatenated dataset of the C. auratus complex, Cyprinus and Megalobrama were distributed in China, and both TMRCAs of Cyprinus (node 42) and Megalobrama (node 50) were mainly distributed in the Pearl River basin, while TMRCA of the C. auratus complex was mainly distributed in the Yangtze River basin (node 31, A: 80.28%) (Fig. 5).

Discussion
This study represents the first attempt to better understand the relationship of artificial hybrid fish species and naturally evolving fish. 2nNCRC is very similar to C. auratus in terms of morphological characteristics based on qualitative and quantitative traits, as well as the geometric shape of fish. Our phylogenetic analysis showed that C. auratus and C. gibelio were unable to be separated from each other based on mtDNA and HoxA2b but were distinct from the other three species of Carassius. The intraspecies relationship between 2nNCRC and C. auratus complex was uncovered by both mtDNA and HoxA2b. The C. auratus complex was likely to have originated in China, and the admixture of mtDNA haplotype lineages across Eurasia was mainly due to natural Pleistocene dispersal and/or anthropogenic translocation.

Prerequisite conditions and ecological selection for the HHS of 2nNCRC
2nNCRC has two sets of chromosomes, one each from the maternal C. carpio, and the great morphologic and genetic variation in the parents can be seen as HHS [13]. Compared with allopolyploid hybrid speciation, HHS has at least two issues. First, reproductive isolation can easily be generated between the allopolyploid hybrid species and their parents due to the inaccuracy of chromosomal pairing and the inability to formulate germ cells [34], while reproductive isolation cannot be immediately generated between HHS and parents, such as 2nNCRC and C. carpio. Second, two complete genomes from their parents can be found in allopolyploid hybrid species; they possess a protective mechanism for the genome, while HHS lacks this mechanism. However, the F 1 2nNCRC females and males are fertile, and eight stable strains of the new homoploid species have been established in our laboratory since 2014, suggesting that HHS not only is common in plants but also occurs in fish.
The prerequisite condition for HHS is that the distribution of parents should overlap in history, making hybrid speciation possible [35]. Both TMRCAs of Cyprinus and Megalobrama had a sympatric distribution in the Pearl River basin and Yangtze River basin (Fig. 5), and currently, they have a wide distribution with the C. auratus complex in these river systems. In particular, the connectivity between the Yangtze River and Pearl River system before the early Pleistocene would have facilitated the dispersion of freshwater species between the two river systems [36,37]. Consequently, the connectivity of river systems and flooding mediated dispersion and encounters among freshwater species.
In addition, occupying a different and suitable ecological niche from parents is very important for HHS [7,37], and this scenario which may be lost during continuous backcrossing with parents. Fortunately, the reproductive trait of 2nNCRC was the same as that of the C. auratus complex, both reaching sexual maturity at the age of 1 year [13]. C. carpio and M. amblycephala reach sexual maturity at least 2 years old. Furthermore, the same shape and dental pattern of pharyngeal teeth were found for 2nNCRC and the C. auratus complex (Fig. 1). The same compressed teeth in C. auratus and 2nNCRC suggested that they had the same omnivorous feeding habit but mainly fed on plant food (the stem and leaf of aquatic plants). However, rounded and ridged molariform teeth are found in C. carpio, which feeds on omnivores but mainly on animal food (freshwater snails and aquatic insects). M. amblycephala possesses recurved and uncinate teeth and mainly feeds on aquatic plants. In terms of freshwater aquatic resources, in comparison to C. carpio and M. amblycephala, the homoploid 2nNCRC and C. auratus have stronger adaptability, as they have a wider range of food in the aquatic ecosystem. Ecological selection is a major factor promoting HHS, and the foundation of a novel ecological niche supplies an opportunity for the evolution of new species while maintaining its own unique phenotype [37]. The family Cyprinidae exhibits species-specific numbers and arrangements of pharyngeal teeth, which are an important feeding organ and taxonomic characteristic, and extensive variation in tooth shape with high hereditary stability [38]. The same pharyngeal teeth were found among the C. auratus complex and different generations of 2nNCRC. Although the homoploid 2nNCRC and C. auratus showed very similar phenotypes, there were also significant differences between them, including the qualitative and quantitative traits found by Wang et al. [13] and the geometrical morphological shapes, which exhibited a significant humped back in 2nNCRC, found in the present study (Fig. 1). The morphological variation between 2nNCRC and C. auratus might be attributed to long-term morphological evolution.

Evolutionary relationships of the C. auratus complex with the homoploid 2nNCRC
In the phylogenetic tree, ML and Bayesian analyses based on the concatenated sequence produced essentially the same tree topology with high ML bootstrap values and Bayesian posterior probabilities. We determined that the robust tree topology adequately reflected the phylogeny of the species in Carassius and Cyprinus, as well as other species in Cyprinidae. Carassius clustered with Cyprinus first and then clustered with other species in Cyprinidae, Barbinae, Schizothoracinae and Labeoninae, all of which Node ages are shown together with 95% highest posterior density bars indicating a range of age estimates belong to Barbini, a monophyletic group in Cyprinidae, and this result was consistent with those of previous phylogenetic studies and the morphological characteristics of Cyprinidae [39,40]. Carassius, as a monophyletic clade, included four definite interspecies clusters, and there was no shared haplotype among C. carassius, C. cuvieri, C. langsdorfii and other species, including C. gibelio and C. auratus (Fig. 3a). The haplotypes of C. carassius observed in Europe were separated into two clades without shared haplotypes, in accordance with the existence of two C. carassius lineages in Europe [24]. The haplotypes of C. cuvieri were restricted to mainland Japan, and the major haplotypes of C. langsdorfii observed across Eurasia were distributed on Japanese islands. Considering that the distribution of haplotypes was restricted to specific areas, we considered that the three species evolved independently for a considerable period of time on the main Japanese islands and Europe. However, the widely distributed species C. gibelio and C. auratus were paraphyletic, and they had many shared haplotypes. It has been documented that the triploid C. gibelio has a more shared haplotype with the sympatric diploid C. auratus than with allopatric triploids [41]. Previous studies have proposed that an autopolyploidy event might have resulted in the formation of C. gibelio (3n = 150) [42,43]. Liu et al. [22] considered that the polyploid C. gibelio might have originated from the sympatric C. auratus (2n = 100) via autopolyploidy. However, it was reported that the autopolyploidy has not contributed to the separation between diploid C. auratus and triploid C. gibelio as they were both intermixed in the same lineages and share the same alleles [22,25]. Although C. gibelio is naturally gynogenic, many phenomena similar to those in normal sexual diploid species, such as normal meiosis, multiple modes of unisexual gynogenesis and sexual reproduction, have been identified in some strains of C. gibelio [44,45]. Hence, there exist the possibility of the breeding between diploid C. auratus and triploid C. gibelio in theory. Furthermore, the shared haplotype between the sympatric C. gibelio and C. auratus found at present and in previous studies confirmed the existence of gene flow between them. Consequently, it is likely that extensive gene flow and no substantial genetic separation exist between C. gibelio and C. auratus. Though the lack of genetic separation between them could be also the result of recent divergence, TMRCA of C. auratus complex and divergence between sublineage A and B, as well as among B1-B6 could be traced back to as early as the later Pliocene (Fig. 4b) indicating a complex evolutionary history for C. auratus complex.
The preliminary analysis of the nuclear genome of the homoploid 2nNCRC suggested that no characteristic exists that distinguishes the homoploid 2nNCRC from the diploid C. auratus [13]. The homoploid 2nNCRC was clustered with C. auratus based on mtDNA and nuclear DNA (HoxA2b) data. However, the genus Carassius was not a monophyletic group in the HoxA2b tree and showed a closer relationship between the C. auratus complex and C. carpio (Fig. 2b). The nuclear-mitochondrial discordance in the phylogenetic placement of C. auratus and C. cuvieri suggested a possible complex evolutionary history of the genus Carassius. Certainly, more nuclear markers and specimens are needed to further investigate the phylogenetic relationship in Carassius, and further studies including genomic data are required. We found six sublineages, and four had high regional specificity that were composed of endemic populations indigenous to each region in China (Fig. 3b), including Southeast China (Fujian), North China (Aumer River basin), Taiwan Island, and the middle and lower Yangtze River basin. However, a perplexing but concurrent event occurred in which the C. auratus complex found in Japan was not related to the Japanese crucian carp (C. langsdorfii or C. cuvieri) and the C. auratus complex widely distributed in Europe was not related to European crucian carp (C. carassius). The C. cuvieri was the monophyletic group (Fig. 2a) and was native to Japan [21]. However, the C. auratus complex in Japan was clustered with Chinese lineages. Similarily, most of the C. auratus complex across Europe examined in this study was nested in native Chinese lineages, and there were many shared haplotypes between Europe and China (Figs. 3a, 4b). The C. auratus complex distributed in Europe possibly originated via artificial introduction from Asia [41]. In contrast, genealogical analyses clustered the haplotype of C. gibelio distributed in Far East Russia (H29) with strong support, as well as two haplotypes with typical regions (H12 and H16) found in Ryukyus (Fig. 3a) and three specific haplotypes (H81, H82 and H83) found in Vietnam, suggesting that the C. auratus complex rapidly radiated from mainland China rather being distributed through recent anthropogenic translocation [41].

Biogeographic history of the C. auratus complex
Patterns of divergence suggest historical biogeographic events. Sublineage A from Far East Russia and Mongolia first split from the C. auratus complex at approximately 3.12 Mya (Fig. 3b), far preceding the Pleistocene glacial epoch in China, suggesting that dispersal from China rather than vicariance created the current distribution pattern of the C. auratus complex in Asia (Fig. 4a). With the humid and warm climate during the early Pliocene in North China [46], which supplied a suitable habitat for freshwater species, the well-dated Carassius fossil in North China has been recorded in the Pliocene epoch. Furthermore, recurrent glaciations during the Pleistocene were another contributor to species diversification in East Asia. The early separation of sublineage B1 at approximately 1.6 Mya exactly coincided with the Poyang Glaciation (1.8 ~ 1.5 Mya) in China [47,48] and Gonzi Glaciation in Europe. Sublineage B1 only existed in warm and moist tropical or subtropical areas (Vietnam and Fujian, China), which provided important refugia for freshwater species during glaciations [21,49]. Many haplotypes found in sublineage B2 and sublineage A were C. gibelio, while sublineage B2 has a closer genealogical relationship with the C. auratus complex in China; the separation of sublineage B2 in the Amur River basin of China occurred at approximately 1.49 Mya, far postdating the divergence of sublineage A, suggesting that sublineage B2 was the result of another dispersal event during the Poyang-Dagu interglacial stage (1.5-1.1 Mya) [47,48]. Furthermore, during interglacial times, the warm, wet East Asian summer monsoons intensified, which also promoted the dispersion of the C. auratus complex from China to Ryukyus [21]. In sublineage B3, the divergence between the C. auratus complex in Ryukyus and China dated to approximately 0.84 Mya, exactly coinciding with the Poyang-Dagu interglacial stage (0.9 ~ 0.4 Mya) in the Mid-Pleistocene [47,48], and this result was consistent with our previous speculation that natural dispersed from mainland China to Ryukyus occurred in the Pleistocene. Sublineages B3 and B6 distributed across mainland China may be the result of repeated episodes of range contraction and expansion during glacial cycling. Most of the eastward river systems in mainland China are known to have repeatedly anastomosed with one another during the postglacial and Holocene due to erosion and stream capture [50], especially with recent anthropogenic translocations. In turn, these events likely had a dramatic effect on the evolution of many freshwater species, and might have promoted genetic exchange between previously isolated C. auratus complex lineages and even removed any geographic signatures. A similar situation can be found in other aquatic organisms, such as freshwater snails and frogs [49,51]. However, a specific sublineage, B5, was found in the middle and lower Yangtze River basin, as well as three other sublineages (B1, B2 and B4) in China with specific regions, suggesting that the genetic structure in the C. auratus complex has not been completely changed by recent human-mediated translocations [21].

C. auratus complex not a lineage differentiated from the common ancestor of C. cuvieri or C. carassius
Combined with the reconstruction of ancestral state and haplotype networks in the present study and a previous study on the population genetics of the C. auratus complex in China [21,25], we concluded that the C. auratus complex originated in the Yangtze River basin in China (Fig. 5). TMRCA of C. carassius was mostly distributed in Europe, and both C. langsdorfii and C. cuvieri were on the main islands of Japan, essentially correspond to the results of a previous report on the distribution of some lineages of Carassius endemic to particular geographical regions [52][53][54][55][56].
It was believed that the freshwater ichthyofaunas from the islands of Japan and China resembled each other during the Miocene and Pliocene, as the main islands of Japan and mainland China are known to have formed a contiguous land mass in the late Pliocene [57]. Fossils indicate that in the late Pliocene, C. auratus was distributed in North China and Japan [23,58], coinciding with the dating of TMRCA of the C. auratus complex in this study. Thus, we hypothesized that the C. auratus complex lineage should be separated from the C. cuvieri lineage based on molecular data and that a recent common ancestor existed for those distributed in Asia. The divergent and current distribution patterns of Carassius were mainly caused by dispersal that was aided by the formation of land bridges between Japanese archipelago islands and the continent [59,60]. The common ancestor of the C. cuvieri and C. auratus complex likely dispersed from the Eurasian continent to the islands of Japan rather than in the reverse direction. Consequently, TMRCAs of C. cuvieri and C. auratus should be distributed on mainland China, which is incompatible with the present study (Fig. S2, node 44). Hence, there are reasons to believe that C. auratus did not diverge from TMRCA of C. cuvieri.
Similarly, TMRCA of C. carassius was distributed in Europe. However, the C. auratus complex currently distributed in Europe has been verified as dispersing from Asia by anthropogenic translocation. For instance, recent genetic evidence supports the anthropogenic introduction of the crucian carp to the UK during the fifteenth century [61]. C. carassius is native to parts of central, eastern and northern Europe and almost exclusively restricted to lentic ecosystems [56,62]. A strong geographic structure was found because of the susceptibility of C. carassius to genetic isolation and bottlenecks due to their small population sizes and especially their low dispersal rates [24]. Two genetically distinct sublineages of C. carassius distributed in Europe were found in the present study and in a previous study [24]; therefore, the Carpathian Mountains and the Central European Highlands are inferred to have acted as barriers to the colonization of C. carassius in northern European drainages during the Pleistocene. According to the reconstructed ancestral areas, C. carassius was mainly distributed in Europe and restricted to its native areas (Fig. S2). Consequently, very few records of C. carassius distributed in Asia were found, except in West Siberia, Kazakhstan, Uzbekistan and Turkey, all of which are very close to Europe with no significant geographical barrier with Europe. However, the Central Siberian Plateau, Mongolian Plateau, Altay Mountains, Tianshan Mountains, Pamir Mountains, Himalayas, Iran Plateau and Great Caucasus, with elevations greater than 1000 m, formed natural dispersal barriers in the watersheds impeding freshwater species exchange. Furthermore, the separation of C. carassius from other species of Carassius far postdated the formation of high-elevation orogens such as the Tibetan Plateau, Mongolian Plateau and Tian-Shan, which occurred during the Miocene and even earlier [63]; in addition, well-dated Carassius fossils have been found in North China [23] that have far postdated the formation of these mountains, suggesting that C. auratus is not a lineage that diverged from TMRCA of C. carassius.

Paleoclimate and geological events facilitated distant hybridization
Based on the above analysis and the homoploid 2nNCRC derived from distant hybridization in our laboratory [13], our results also reinforce a possible naturally occurring hybrid origin of C. auratus. Rapid hybrid speciation was recently shown to occur by Lamichhaney et al. [64], and many species of cichlid fishes in East Africa have been verified as experiencing ancestral distant hybridization that has been suggested to play a key role in facilitating species diversification [7][8][9]. Our findings further supported that this mechanism was very likely to have occurred in the Cyprinidae family.
The robust, time-calibrated molecular phylogeny suggested that the separation of C. carpio and Carassius occurred at approximately 11.86 Mya (Fig. 4b), close to the divergence between goldfish and common carp (10.0 Mya) [65], and the origin of the common ancestor of common carp and crucian carp (11.4-8.1 Mya) [66], all of which occurred close to the time of the hybrid speciation of allotetraploid C. carpio (12.4 Mya) as estimated by Xu et al. [27]. Our results showed that extreme climate oscillations and geological events that occurred during the Pliocene and Pleistocene might have had a great influence on the speciation and radiation of the C. auratus complex in China.
Landscape features and geological history are hypothesized to strongly affect biogeographical processes, and this can be especially true in freshwater systems [67,68]. One major consequence of extreme climatic changes and geological activities is ecosystem and landscape reconfigurations. A specifically large event was the drastic uplift of the Tibetan Plateau that which ultimately led to the formation of eastward-flowing drainage systems in China, such as the Yangtze River, which formed between the Oligocene and Miocene, but the unification of the upper and middle Yangtze River in the Three Gorges Mountain region occurred between the Late Pliocene and Early Pleistocene [69,70]. The Yangtze River and Han River flowed into the middle of the Jianghan basin. The connection of the upper and middle Yangtze River facilitated species dispersal from west to east, similar to what was observed for the frog Quasipaa boulengeri [49] and the freshwater snail Bellamya aeruginosa [51,71]. Therefore, the establishment of the eastward-flowing Yangtze River and the changes in river systems could have provided opportunities for dispersal, gene flow and even hybridization between closely related freshwater species, especially fish. Furthermore, the intensified tectonic movement and glacial cycling in the Pleistocene, as well as the prevailing monsoon, had major impacts on East Asian biota starting in the late Pliocene [72,73]. As a result, the weather was cold and dry, and rivers and lakes experienced frequent range expansions and regressions in the Jianghan and Dongting Lake basins [74,75]. The regression of lakes and rivers could dramatically reduce the habitats available to fish, and this precarious situation was further exacerbated in the glacial period. Consequently, restricted habitat regions caused hybridization between close and even divergent lineages. An example of a specifically large event is in the LGM, where the sea level was 130-150 m lower than that today, and this event resulted in strengthening downcutting processes of rivers. Most lakes in the plains region of eastern China have opened and dried, and this scenario has been verified in Taihu Lake, Boyang Lake and Dongting Lake [76]. The cooler and drier climates intensified during this period, and the fish habitats in these lakes were fragmented or even lost. Hybridization between divergent lineages would have been common when habitats were repeatedly lost, as the propagation of vast majority of freshwater fishes is oviparity, creating opportunities for new speciation and diversification, especially for sympatric species possessing similar ecological and breeding habits, such as the parents of 2nNCRC in the Yangtze River basin. Both C. carpio and M. amblycephala with sympatric distribution in history (Fig. 5) mostly occupy the middle and lower layers in freshwater, and they usually breed during May-June in every year [77]. A typical example of this scenario is the repeated range expansions and regressions of lakes that likely contributed to the high diversity of African cichlids [78,79]. Since East Africa underwent dramatic climatic and geological changes in the Pleistocene over the past few million years, the constant expansion and regression of the great East African lakes have led to the repeated loss of habitats or the formation of new habitats [51,79]. For example, hybridization might have facilitated speciation bursts for the cichlids in Lake Tanganyika, and time-calibrated trees support the concept that the radiation of Tanganyika cichlids coincided with lake formation [8]. Specially, the ancient hybridization of Lake Victoria cichlids is considered to be related to the capture of Malagarasi (Congo) tributaries and East African mega-droughts, providing an opportunity for the Congolese lineage to colonize the Lake Victoria region and the Congo-Nilotic admixture event to occur [7]. It has been speculated that hybridization between distant relatives, when coincident with ecological opportunity, may facilitate rapid adaptive radiation through recombination and sorting of admixture-derived polymorphisms by natural and sexual selection [7]. The C. auratus complex exhibits remarkable morphological and genetic diversity, and the different ploidy forms, unisexual gynogenesis and bisexual reproduction [21,80,81], might also benefit from hybridization-derived genetic variants. Future investigation will test for evidence of ancient admixture among the C. auratus complex, C. carpio, and M. amblycephala based on whole-genome sequencing data and the effect of ancestral hybridization on adaptive radiation, ecology and life history.

Conclusions
The newly born homodiploid 2nNCRC derived from the hybridization of C. carpio and M. amblycephala showed very similar phenotypic and genetic characteristics with the diploid C. auratus complex in nature. The very similar morphological characteristics and identical structures of the pharyngeal teeth between 2nNCRC and the C. auratus complex were totally different from those of C. carpio and M. amblycephala. The new structure of pharyngeal teeth suggested that 2nNCRC occupied a novel and suitable ecological niche from its parents, suggesting typical HHS in fish. Molecular phylogenetic analyses revealed an intraspecies relationship between 2nNCRC and the C. auratus complex. According to the reconstruction of ancestral areas and molecular data, the C. auratus complex lineage most likely originated from the Yangtze River basin in China during the Pliocene. The divergence pattern of the C. auratus complex in China was attributed to Pleistocene radiation, and the molecular data exactly coincided with the interglacial periods that occurred during the early and mid-Pleistocene in China. The diversification in the Carassius indicated that the C. auratus complex was impossible to derive from the recent common ancestor of C. carassius or C. cuvieri. HHS for 2nNCRC provides a good candidate speciation route for C. auratus, as the prerequisite condition of HHS and the hybridization between divergence lineages caused by the oscillation of the geologic climate during the Pliocene and Pleistocene could be satisfied in China.

Samples and sequence data preparation
All of the 2nNCRC samples, including F 1 to F 5 generations used in this study, were cultured in ponds at the Protection Station of Polyploidy Fish, Hunan Normal University, and fed artificial feed. The F 1 generation of 2nNCRC was denoted as 2nNCRC-F 1 , the F 2 generation of 2nNCRC was the F 1 self-crossed offspring and denoted as 2nNCRC-F 2 , and the naming procedure was followed for 2nNCRC-F 3 , 2nNCRC-F 4 and 2nNCRC-F 5 . One specimen of each generation (F 1 to F 3 ) was collected, and a total of three mitochondrial genomes of 2nNCRC were sequenced to construct a phylogenetic tree based on two rRNAs and twelve protein-coding genes (ND6 was excluded) (the methods for DNA extraction, amplification, and sequencing are given in Additional file 1 in the Supplementary Information). To perform a comprehensive phylogenetic analysis, another 85 specimens of representative Cyprinidae fish (including 29 specimens of Carassius, 19 of Cyprinus, 14 of Megalobrama and 23 of other cyprinid fish) and 10 Catostomidae fish with complete mitochondrial genomes (both rRNAs and twelve protein-coding genes) were retrieved from GenBank (see Additional file 2 in Supplementary Information). In addition, a total of 831 cytb genes of the specimens (including three specimens in each generation of 2nNCRC-F 1 to F 5 ) in the genus Carassius widely distributed across Eurasia were also acquired from GenBank (see Additional file 3 in Supplementary Information) to obtain the phylogeographic structure of Carassius across Eurasia.
As very few nuclear DNA data are shared for Carassius and other Cyprinidae in GenBank, only HoxA2b was used to reconstruct the phylogenetic tree in present study. Two specimens of each generation (2nNCRC-F 1 to F 3 ) were sampled with two clones for each individual, and a total of 12 partial cds genes for HoxA2b were amplified (see Additional file 1 in Supplementary Information). In addition, the partial cds of HoxA2b for both parents (one for M. amblycephala and two for C. carpio) of 2nNCRC were also amplified, and 40 other specimens of Cyprinidae representing 21 species (see Additional file 4 in Supplementary Information) were obtained from GenBank in this study.
A previous study showed that the morphometrics of crucian carp-like fish (2nNCRC) were very similar to those of C. auratus in terms of barbel loss and a few lateral scales and even some measurable traits [13]. In this study, the morphological characteristics of other qualitative and quantitative traits were compared among the species of Carassius and 2nNCRC, as well as their parents (see Table S1 in Supplementary Information). To further test the morphological differences among 2nNCRC, C. auratus, C. carpio and M. amblycephala, geometric morphometrics was used to examine shape variation through principal component analysis (PCA) and canonical variance analysis (CVA) in MorphoJ v2.0 [82]. The left side of a specimen was placed on a moist foam mat at the base to maintain its natural configuration and photographed with a Nikon CoolPix 4500. Then, we digitized sixteen landmarks (see Fig. S1 in Supplementary Information) in each fish to capture the variation across the head and body shape, and the X and Y coordinates were recorded according to Zamani-Faradonbe et al. [83] by the software TpsDig2 [84]. For each specimen, the X and Y landmark coordinates were translated to the origin, rotated, scaled and superimposed by GPA (generalized Procrustes analysis) [85] in TpsRelwarp [86] to remove nonshape variations (such as location, orientation and scale). The specimen number of each species was more than 25 in this study. The CVA differentiates mutually exclusive groups by analyzing their relative positions in the morphospace and mapping shape differences onto the two first variates that explain the majority of the morphospace. This process is performed through the construction of a coordinate system called the canonical variates by determining the scores on those axes for all individuals, and they are referred to as eigenvalues [87].
Furthermore, pairwise genetic distances of inter-and intraspecific variation in the Carassius genus (see Table  S5 in Supplementary Information) were calculated under the K2P model for the cytb dataset using MEGA 7 [88], as the cytb dataset is the most even of the datasets in GenBank.

Phylogenetic analysis
All sequences were aligned with MAFFT [89] as implemented in Geneious 4.8.3. For the 98 mitochondrial genomes, the species of Catostomidae (see Additional file 2 in Supplementary Information) that shared a more recent common ancestry with Cyprinidae than with any other extant group of Cypriniform based on the pharyngeal feeding structures [90] were used as outgroups, and all alignments (twelve proteins and two rRNAs) were combined. We tested the 14 genes' data for saturation using DAMBE v. 6.4.41 [91]. The test revealed an ISS value that was significantly lower than the ISS.c in all cases (P < 0.0000), indicating the suitability of the fourteen genes for phylogenetic analysis. The homogeneity of the fourteen genes was tested in *PAUP [92], and the P value was 0.69 (> 0.05). The 57 partial cds of HoxA2b (nuclear DNA) included 12 clones of 2nNCRC and 43 specimens of other Cyprinidae species, as well as the outgroup Colossoma macropomum and Esox Lucius (see Additional file 4 in Supplementary Information). The alignments (the conservative sequence after removing GAP) were also tested for saturation using DAMBE v. 6.4.41, and the ISS-value was significantly lower than the ISS.c value in all cases (P < 0.0000). To test our first hypothesis that 2nNCRC is C. auratus, ML and Bayesian trees using both the HoxA2b and mitochondrial genomes (incorporating the 14-gene data) were generated using RAxML v. 8.0 [93] with 1000 bootstrap replicates and MrBayes v. 3.2 [94], respectively. In the phylogenetic tree analysis, we determined the best-fitting substitution models for each gene fragment using Moldetest 3.7 [95], which are shown in Table S6 in the Supplementary Information. Mixing of the MCMC chains of the two independent runs was monitored with TRACER v. 1.7.1 [96], and the analysis was terminated after the average standard deviation of the split frequencies fell to less than 0.01. The first 25% of the sampled approximately 20 million generations were discarded as burn-in. The final trees were visualized in FIGTREE v. 1.4.4 [97].
To test our second hypothesis that several geographic lineages of Carassius endemic to specific regions of Eurasia exist, especially for C. auratus in China, we generated two haplotype networks using 831 cytb genes for all in Eurasia and 128 cytb for samples only in China and colored each haplotype by the geographic region from which it was collected (see Additional file 3 in Supplementary Information). Only the cytb gene was used in the haplotype network, as it had the most abundant data with population information for the five species of Carassius and is often used for population genetics in fish. The haplotype networks were constructed using network v. 10 (www. fluxus-engin eering. com/ share net. htm) and applying the median-joining and maximum parsimony options.

Divergence time estimation
To test our third hypothesis that C. auratus originated from the hybridization between C. carpio and M. amblycephala in China, the precondition would need to be that TMRCA of both Cyprinus and Megalobrama had a much earlier divergence than that of the C. auratus complex, and all would have had to have a sympatric distribution in history. Furthermore, these lineages of the C. auratus complex distributed in Eurasia should belong to native lineages distributed in China. The divergence time was estimated using a molecular clock approach as implemented in BEAST. We used the combined data (98 specimens) including twelve proteins and two rRNAs in the phylogenetic tree and employed a (uncorrelated lognormal) relaxed clock, as a likelihood-ratio test (LRT) rejected the strict molecular clock hypothesis for the data (P < 0.01). As the lack of credible and exact fossil data can be found for the species in this study, we used a conservative approach by choosing three calibration points with normal distribution priors: the oldest fossil of Plesiomyxocyprinus arratiae, which is similar to Myxocyprinus asiaticus, was constrained from the middle Eocene or earlier, approximately 40-38 Mya [32], which is often used as the calibration point of M. asiaticus [30,90]; the minimum age of Catostomidae is 60 Ma based on a catostomid fossil from the Paleocene [31]; and the timing of the drastic uplift of the Tibet Plateau occurred between 25 and 17 Mya [33] and was utilized as one calibration point for the Schizothoracinae as the species endemic to Qinghai-Tibet Plateau. The clade corresponding to each calibration point was constrained to be monophyletic. The GTR + I + G model was the best fit for the combined dataset (Table S6 in Supplementary Information). The 'speciation: Yule process' tree prior was used to construct the tree. We ran four independent runs for 20 million generations, logging trees every 2000 generations. Convergence was checked with TRACER v. 1.7.1. Posterior trees from the four runs were combined after removing the first 10% as burn-in in LogCombiner v. 2.5.2 (http:// beast. bio. ed. ac. uk/ logco mbiner). The maximum credibility tree was created in TreeAnnotator v. 2.5.2 available in the BEAST package.
Furthermore, another divergence time was estimated using the cytb data. We used a reduced dataset of cytb: specimens for which two or more sequences from the same region were included. This approach resulted in 247 terminals -237 representatives of the five species of Carassius and 13 outgroup sequences (C. carpio) from GenBank (see Additional file 5 in Supplementary  Information). To minimize error, we used a conservative approach by employing calibration points from previous studies [24]. TMRCA of C. carassius in the northern and central-eastern European drainages and the Danubian catchment was constrained to 2.18-2.15 Mya. Another calibration point was the fossil of Carassius in the Pliocene epoch (5.3-2.6 Mya) in northern China (Yushe, Shanxi Province). The GTR + I + G model was also the best fit for the cytb dataset. The 'speciation: Yule process' tree prior was used to construct the tree. We ran four independent runs for 50 million generations, logging trees every 5000 generations. The other settings were same as those noted above.

Reconstruction of ancestral areas
We further performed a biogeographic reconstruction of ancestral areas for species of Carassius using BioGeoBE-ARS [98] in RASP 4.02 [99] to determine whether overlapping distributions existed between C. auratus and other species of Carassius. The analyses were conducted on a fully resolved topology from the BEAST analysis containing 24 cytb sequences and five species of Carassius (see Additional file 6 in Supplementary Information). Seven major geographical areas were defined based on the worldwide distribution of Carassius according to its current distribution and Jeffries et al. All six models of geographic range evolution were compared in a likelihood framework (DEC, DEC + j, DIVALIKE, DIVALIKE + j, BAYAREALIKE and BAYAR-EALIKE + j). The best-fit model was assessed by comparing Akaike's information criterion and likelihood-ratio tests, and DIVALIKE + j was chosen (see Table S7 in Supplementary Information). To account for phylogenetic uncertainty, 4000 post burn-in trees resulting from the BEAST analysis were integrated for inference. The maximum number of ancestral areas was set to four, as C. auratus and C. carpio can be widespread.