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Cryptic Haploid Stages in the Life Cycle of Leathesia marina (Chordariaceae, Phaeophyceae) Under In Vitro Culture

We evaluated the life cycle of Leathesia marina through molecular analyses, culture studies, morphological observations, and ploidy measurements. Macroscopic sporophytes were collected from two localities in Atlantic Patagonia and were cultured under long‐day (LD) and short‐day (SD) conditions. Molecular identification of the microscopic and macroscopic phases was performed through the cox3 and rbcL genes and the phylogeny was assessed on the basis of single gene and concatenated datasets. Nuclear ploidy of each phase was estimated from the DNA contents of individual nuclei through epifluorescence microscopy and flow cytometry. Molecular results confirmed the identity of the Argentinian specimens as L. marina and revealed their conspecificity with L. marina from New Zealand, Germany, and Japan. The sporophytic macrothalli (2n) released mitospores from plurilocular sporangia, which developed into globular microthalli (2n), morphologically similar to the sporophytes but not in size, constituting a generation of small diploid thalli, with a mean fluorescent nuclei cross‐sectional area of 3.21 ± 0.7 μm2. The unilocular sporangia released meiospores that developed two morphologically different types of microthalli: erect branched microthalli (n) with a nuclear area of 1.48 ± 0.07 µm2 that reproduces asexually, and prostrate branched microthalli (n) with a nuclear area of 1.24 ± 0.10 µm2 that reproduces sexually. The prostrate microthalli released gametes in LD conditions, which merged and produced macroscopic thalli with a nuclear cross‐sectional area of 3.45 ± 0.09 µm2. Flow cytometry confirmed that the erect and prostrate microthalli were haploid and that the globular microthalli and macrothalli were diploid.

We evaluated the life cycle of Leathesia marina through molecular analyses, culture studies, morphological observations, and ploidy measurements. Macroscopic sporophytes were collected from two localities in Atlantic Patagonia and were cultured under long-day (LD) and short-day (SD) conditions. Molecular identification of the microscopic and macroscopic phases was performed through the cox3 and rbcL genes and the phylogeny was assessed on the basis of single gene and concatenated datasets. Nuclear ploidy of each phase was estimated from the DNA contents of individual nuclei through epifluorescence microscopy and flow cytometry. Molecular results confirmed the identity of the Argentinian specimens as L. marina and revealed their conspecificity with L. marina from New Zealand, Germany, and Japan. The sporophytic macrothalli (2n) released mitospores from plurilocular sporangia, which developed into globular microthalli (2n), morphologically similar to the sporophytes but not in size, constituting a generation of small diploid thalli, with a mean fluorescent nuclei cross-sectional area of 3.21 AE 0.7 lm 2 . The unilocular sporangia released meiospores that developed two morphologically different types of microthalli: erect branched microthalli (n) with a nuclear area of 1.48 AE 0.07 µm 2 that reproduces asexually, and prostrate branched microthalli (n) with a nuclear area of 1.24 AE 0.10 µm 2 that reproduces sexually. The prostrate microthalli released gametes in LD conditions, which merged and produced macroscopic thalli with a nuclear cross-sectional area of 3.45 AE 0.09 µm 2 . Flow cytometry confirmed that the erect and prostrate microthalli were haploid and that the globular microthalli and macrothalli were diploid.
Key index words: flow cytometry; fluorescent nuclei; microthalli; molecular identification; ploidy Abbreviations: BI, Bayesian Inference; DAPI, fluorescent dye 4 0 , 6-diamidino-2-phenylindole; GTR, general time reversible model; I, proportion of invariable sites; LD, long day; ML, maximum likelihood; PI, propidium iodide; RAxML-NG, next-generation randomized maximum likelihood; SD, short day; Γ, distribution The brown algae (Phaeophyceae) constitute a taxonomically diverse and ecologically important group in marine environments (Mann 1982, Andersen 1992. These algae are of particular interest in evolution studies as they have evolved complex characteristics (complex multicellular and metabolic organization, cell walls containing various polysaccharides and high resistance to osmotic stress) independent of other lineages, such as Plantae, Fungi, and Metazoa . Furthermore, brown algae exhibit a diverse range of life cycles with wide variety of sexual traits and reproductive strategies, where the transitions between different types of life cycle could reveal key adaptive events in the evolution of this group (Cock et al. 2014).
The ancestral brown alga appears to have had a diplohaplontic life cycle with similar dominance of the diploid phase (sporophyte) and haploid phase (gametophyte), in size and complexity (Phillips et al. 2008, Heesch et al. 2019. However, over evolutionary time, in some clades, the diploid phase became dominant, whereas in others, they evolved toward greater haploid dominance, exhibiting in both phases a wide variety of morphologies and complexity (Heesch et al. 2019). This variability ranges from diploid life cycles, to the isomorphic haploid-diploid life cycles with morphologically similar generations, and heteromorphic haploiddiploid life cycles with more or less marked differences between the two generations, in terms of size and morphology, where either the sporophyte or the gametophyte generation being dominant in terms of size (Clayton 1988, Cock et al. 2014. Also, the correspondence between the sporophyte and gametophyte generations and the ploidy levels of each generation is not absolute and mutations or natural variations that uncouple ploidy and life cycle generation intervene in life cycle progression in brown algae (Cock et al. 2014).
These life cycle variations occur at the ordinal level in brown algae. For example, in the order Ectocarpales, we find families with isomorphic generations (i.e., Acinetosporaceae, Ectocarpaceae) and families with heteromorphic generations, in which either the sporophyte (Scytosiphonaceae, except the genera Myelophycus and Melanosiphon) or the gametophyte (Chordariaceae, Adenocystaceae) is microscopic (Peters and Ram ırez 2001). Also, evidence of mutations was detected in Ectocarpus that produce gametophytes, when the non-mutated algae would have produced sporophytes (Arun et al. 2018).
It has been proposed that life-history variations are a result of complex adaptation as well as a successful solution to a particular ecological problem (Russell 1986). On the other hand, advances in molecular studies suggest that specific developmental programs are deployed at precise points in the life cycle to generate either a sporophyte or a gametophyte, where the intervention of the TALE (three amino acids loop extension) homeodomain transcription factors (very ancient protein family) and chromatin modification seem to participate in the regulation of the life cycle (Cock et al. 2014, Arun et al. 2019, Bourdareau et al. 2020. For this reason, brown algae represent a particularly interesting group to explore life cycle functions, since they exhibit a broad range of phenomena involving developmental processes, reproductive skills, dispersal modes, and adaptation to the local environment (Cock et al. 2014).
The family Chordariaceae with heteromorphic life cycles is the largest and most morphologically diverse lineage within the order Ectocarpales (Van den Hoek et al. 1995). In heteromorphic life cycles, the phases exhibit a high degree of independence and differentiation; usually, each phase has unique ecological and evolutionary constraints (Schiel and Foster 2006). In many species of marine macroalgae with heteromorphic life cycles, the microscopic generation is usually the most tolerant to unfavorable light, temperature, and nutrient availability (Carney and Edwards 2006).
Leathesia marina (Chordariaceae) is widely distributed in cold subtropical to temperate waters of both hemispheres, occurring as epiphytes in rocky intertidal communities (Oates 1989). However, it is not found in the tropics nor in Antarctica (Quartino and Boraso de Zaixso 1996). Life-history studies on Leathesia marina have shown that the alga exhibits a typical heteromorphic life cycle, where a prostrate microscopic gametophyte alternates with a macroscopic sporophyte (Dangeard 1965, 1969, Peters 1987. Under culture conditions, L. marina microthalli have been reported, but neither sexual reproduction nor young macrothalli have been observed (Damman 1930, Kylin 1933. Therefore, the life cycle of L. marina is yet to be elucidated. The most common approach to elucidate algal life cycles is by observing the development of their life stages under variable culture conditions. Additionally, several methods have been developed to determine the ploidy of the different stages of algae. Chromosome counting is a classical method to determine the nuclear phase; however, this technique is difficult to perform on algae due to the rapid cell division, and the results are often not precise due to the small size of the nuclei and chromosomes (Cole 1990). Another method is based on the correlation between the DNA content and nuclear size (Sparrow and Miksche 1961, Price 1976, Whittick 1986, wherein relative ploidy is determined in stained nuclei using a fluorometer and an image analysis system (Choi et al. 1994). In recent years, flow cytometry has been used as a convenient alternative to quantify the nuclear DNA content in marine algae (Le Gall et al. 1993), allowing the analysis of thousands of nuclei in a few minutes. Additionally, molecular analyses based on mitochondrial and plastidial markers proved to be excellent tools to assist in the identification of macroalgae (Le Gall and Saunders 2010) and are very useful for assisting with the identification of the microscopic generations.
In this study, we aim to evaluate the life history and the characteristics of the different phases of Leathesia marina under culture conditions. For this 1350 purpose, we integrate ploidy quantification by epifluorescence microscopy and flow cytometry, and molecular studies to complement the morphological observations.

MATERIALS AND METHODS
Macroscopic sporophytes of Leathesia marina (henceforth referred to as globose macrothalli) were collected from the lower intertidal region of two coastal localities on Atlantic Patagonia. Las Grutas (40°50 0 S, 65°6 0 W), the northern location, is~260 km away from the southern location at Puerto Madryn (42°46 0 S, 62°59 0 W). Vouchers of all collected specimens were deposited in the Herbarium of Universidad Nacional del Sur (BBB), Bah ıa Blanca, Buenos Aires, Argentina.
Mitochondrial cox3 and plastidial rbcL markers were used to infer the phylogeny of the Argentinian specimens of Leathesia marina collected in the two localities and to confirm the identity of the microscopic generations obtained in culture. Sections of macrothalli and microscopic thalli were dried in silica gel, and the total genomic DNA was extracted using QuickExtract TM FFPE DNA Extraction Kit according to the manufacturer's instructions. Amplification and sequencing of the cox3 and rbcL genes were carried out as described in Santiañez et al. (2018). All newly generated sequences were submitted to GenBank (Table S1 in the Supporting Information).
Global alignment for the analyses of single gene (cox3: 650 bp) and concatenated data set (rbcL: 1,399 bp + cox3: 509 bp = 1,908 bp) were constructed together with available sequence data of the Chordariaceae in GenBank. Phylogenetic analyses based on single gene and concatenated datasets (partitioned by gene and codon position) were performed using maximum likelihood (ML) and Bayesian Inference (BI) under the GTR + I + Γ model. ML analyses were conducted in RAxML v.8 (Stamatakis 2014) with 1,000 bootstrap pseudoreplicates through the Cipres Phylogenetic Portal (Miller et al. 2010). BI was performed in MrBayes v.3.2.1 (Huelsenbeck and Ronquist 2001) wherein 25% burn-in was set prior to calculating the trees. In addition, pairwise sequence differences (p-distances) within the cox3 data of L. marina was also calculated using MEGA v.6 (Tamura et al. 2013).
All in vitro culture used autoclaved seawater enriched with 10 mL Á L À1 Provasoli medium (Provasoli 1968). Light irradiance of 25 µmol photons Á m À2 Á s À1 was provided to the cultures by cool white fluorescent tubes and monitored using a quantum flux meter (Apogee MQ-200, Logan, UT, USA). Fertile sections of 0.5 mm 2 obtained from 10 globose macrothalli from each geographic location were used to initiate cultures. Accordingly, 10 cultures from meiospores were obtained from unilocular sporangia and 10 cultures from mitospores were obtained from plurilocular sporangia. Each clean fragment containing mature unilocular sporangia (82 AE 18, mean AE SD) or plurilocular sporangia (138 AE 26, mean AE SD) was incubated in a sterile tube with a coverslip on the bottom for spore settlement. Ten replicates of each culture type were subjected to two light and temperature regimens: long day (LD) with 16:8 h (light: dark) at 20°C and short day (SD) with 8:16 h (light:dark) at 8°C to simulate summer and winter conditions, respectively.
Morphological and anatomical observations of the globose macrothalli were done and the size of the thalli was estimated through its height (mm), diameter (mm), and drained wet weight (g). The development of the different stages of the life cycle after the germination of meiospores, mitospores, and zygotes, as well as the morphologies of the thalli obtained were monitored on coverslips under a Nikon Eclipse TE 300 microscope (Tokyo, Japan) equipped with a Nikon FDX 35 camera. In all, 20 visualizations were performed for each replica by photographing and quantifying the development of individuals in culture. All microscopic measurements were estimated from the images using the software ImageJ v. 1.46 (National Institutes of Health, Bethesda, MD, USA).
The nuclear ploidy of each phase of the life cycle was estimated by epifluorescence microscopy and flow cytometry. The DNA of individual nuclei was stained with the fluorescent dye 4 0 , 6-diamidino-2-phenylindole (DAPI). Samples from different stages of the life cycle were fixed in Carnoy's solution (ethanol: 100% acetic acid = 3:1) for at least 24 h at 5°C and were then transferred to Eppendorf tubes containing 100% ethanol and kept at 4°C until the analysis. The nuclear ploidy level was estimated based on the correlation between DNA content and nuclear size (Kapraun and Nguyen 1994). The nuclear size was estimated by measuring the relative area of DAPI-stained DNA in nuclei using ImageJ software. To assess the differences in DNA contents of individual nuclei stained with DAPI, analyses of variance (ANOVA) were conducted to establish the nuclear size differences. Each dataset was examined for homoscedasticity using Bartlett's test and normality using the Shapiro-Wilk test with a 0.05 significance level using the statistical program R Studio (R CoreTeam 2016, R version 3.5.1). Statistical analyses were performed using a "WRS2" (Mair and Wilcox 2019) and "multcomp" (Hothorn et al. 2008) packages in R.
To perform flow cytometry, the nuclei were isolated from mature thalli of different morphology corresponding to the different life cycle phases. Small portions of thallus were chopped with a razor blade in ice-cold buffer (30 mM MgCl, 120 mM trisodium citrate, 120 mM sorbitol, 55 mM 4-[2-hydroxyethyl] piperazine-1-ethanesulfonic acid [HEPES], 5 mM EDTA supplemented with 0.1% [v/v] Triton X-100 and 5 mM sodium bisulfite; pH 8.0), following the methods of Peters et al. (2004). The chopped thalli were filtered through a 30 µm and 10 µm mesh nylon filter. An aliquot of 100 µL of PI-RNase solution (propidium iodide [PI], 50 µg Á mL À1 , RNase 50 µg Á mL À1 in distilled water) was added to the suspension of nuclei, and after 20 min of incubation their DNA content was measured in a flow cytometer FACS Calibur equipped with an Argon LASER (emission at 488 nm) of INBIOSUR-CONICET. The nuclear population measurements were identified and separated from noise measurements in biplots of side-scattered light (SSC) vs. green fluorescence (FL2). Data were processed using FlowJo V.10.0.7r2 software (Tree Star, Inc., Ashland, OR, USA).

RESULTS
Phylogeny of Leathesia marina from Patagonia. A total of nine new DNA sequences were generated in this study from eight Leathesia specimens, eight cox3 (two from the microscopic gametophytes obtained in culture and six from the macroscopic sporophytes collected in the field) and one rbcL. Based on our cox3 and concatenated trees ( Figs. 1 and 2), the Argentinian L. marina formed a well-supported clade with its conspecifics from New Zealand, Germany, and Japan. The specimens from both collection sites were genetically similar and were closely related to two specimens from New Zealand, having only 0.3% sequence divergence in cox3. Intraspecific sequence divergence in cox3 was up to 7.7%. The identity of the gametophyte was confirmed as LIFE CYCLE OF LEATHESIA MARINA L. marina by the phylogeny of cox3. The phylogeny inferred from the concatenated rbcL and cox3 revealed that the clade of L. marina is a sister clade of Elachista tenuis Yamada and Cladosiphon okamuranus Tokida.
Morphology of diploid macrothalli. The macrothalli of Leathesia marina were found predominantly in the low intertidal zone, only between late winter and early autumn (i.e., from August to April in Las Grutas and from September to April in Puerto Madryn). In both populations, L. marina was often found as an epiphyte in beds of Corallina officinalis Linnaeus (Fig. 3a).
Macrothalli of Leathesia marina had a diameter of 1.7-4.5 cm, a height of 0.8-1.2 cm, and a drained wet weight of 0.3-3.7 g. In cross-section, the medulla was composed of three layers of colorless, thin-walled cells that were smaller and dichotomously branched toward the surface (Fig. 3b). The lowest medullary layer cells were relatively larger, 60.8-72.2 9 37.8-45 lm in size, irregularly shaped, and anastomosing (Fig. 3c). Directly above the latter, the cells were oblong to broadly oblong, 38.2-49 9 27.6-32.7 lm and are basal to two smaller oblong cells (27-32 9 18-19.5 lm), forming a firstorder dichotomy (Fig. 3b). Two smaller subsurface cells were also borne on each of the latter cells, 13.3-17.3 lm long, and 9.6-11 lm wide, forming the second-order dichotomy. Each subsurface cell bore cortical filaments, and the fertile specimens also have either uni-or plurilocular sporangia. Cortical filaments were solitary or in groups of up to three. Each filament possessed an inflated ovoid terminal cell containing numerous phaeoplasts (Fig. 3b). The hyaline hairs were scattered on the macrothalli surface, developing from the outermost medullary cells, and were 32.5-45.3 lm long and 8-9 lm wide (Fig 3d). Plurilocular sporangia (24.5-35 9 5.2-6 lm) were linear and uniseriate, usually of 3-6 locules, and were borne solitary or in groups of three (Fig 3d). Unilocular sporangia were ovoid and sessile, 19.4-35.2 lm long and 12.4-16.7 lm wide (Fig 4a).
Life cycle of Leathesia marina under culture conditions. Diploid generation produced from plurilocular sporangia: In both LD and SD regimes, the plurilocular sporangia on the globose macrothallus released mitospores of 1.8-2.3 lm long after 12 h of incubation. The majority of these mitospores settled after 12 h of swimming. Once settled, they exhibited bipolar germination, forming two germ tubes. Some divided profusely in different directions forming a disc from the central cell, whereas others formed loose discs (Fig. 3, e-g). Young germlings developed into minute thalli with the same morphology as the macrothalli. They never exceeded 1.5 mm in diameter constituting a generation of small diploid thalli (Fig. 3, h-j). This diploid generation was called globular microthalli. These thalli had a medullar structure formed by three layers of colorless cells, decreasing in size toward the surface (Fig. 3k). The lower cells were large, oblong, measuring 24-32 9 13-17 lm; the cells of the second layer were also oblong and 16-21 9 13-17 lm in size; and the superficial cells were oblong to broadly oblong and smaller 9-12 9 9-10 µm. Phaeophycean hairs were associated with the cortical layer. The cortical layer was typically composed of short assimilatory filaments formed from two to three cells, 5-7 lm in length. After 20 d of incubation, the globular microthalli developed plurilocular sporangia (15-30 lm long 9 3-5 lm wide) located among cortical filaments, which were linear to linear--lanceolate and had one to three rows of loculi (Fig. 3k). Unilocular sporangia were never observed in these thalli under LD and SD conditions. Settled spores from plurilocular sporangia of globular microthalli showed similar developmental patterns as the original mitospores. These grew into new globular microthalli (Fig. 3l) and repeated the same asexual cycle indefinitely.
Haploid generation produced from unilocular sporangia. Meiospores were released from unilocular sporangia (Fig. 4a) on the globose macrothallus after 12 h of incubation. They were 1-1.3 lm long and they settled after 12 h. They had unipolar germination (Fig. 4b) and developed a germ tube with transverse divisions forming uniseriate branched filaments (Fig. 4c). These meiospores formed two types of microthalli with different growth patterns. Some formed a basal structure before developing into erect branched microthalli (Fig 4, d-g), whereas others formed prostrate branched microthalli (Fig 4, h-j).
The erect branched microthalli grew up in LD and SD regimes, representing 100% and 37.8% of the individuals in the LD and SD regimes, respectively. These were characterized by upright filaments with lateral branches of 9-11 9 2-4 lm, that emerged from rounded basal cells of 6-4 9 2-4 lm (Fig. 4e). After 4-5 weeks, the basal or intercalary cells of lateral branches differentiated into plurilocular reproductive structures (10-20 9 4-5 lm) with one to three rows of loculi (Fig. 4g). This morphology generated itself during successive generations.
Prostrate branched microthalli only grew under the SD regime, representing 62.2% of the individuals from germling in SD regimes and had lateral branches extending horizontally with cells of 9-13 9 4-6 lm, forming a dense matrix of interwoven prostrate filaments (Fig. 4h). These prostrate microthalli did not form any reproductive structures, remaining latent for 18 months. However, when these microthalli were transferred to LD conditions, they formed plurilocular gametangia. The gametangia were linear, 20-30 9 6-8 lm and possessed one to three rows of loculi (Fig. 4j). Gametogenesis occurred 2 weeks after the thalli were transferred from SD to LD conditions. Gametes (1-2 lm diameter) were released after 24 h of incubation and later 1352 the prostrate microthalli were detached from the substrate. Relatively larger female gametes settled first, becoming spherical, and were subsequently fertilized by persistently motile male gametes. The resulting zygotes were larger (4-5 lm diameter; Fig. 4k). Zygotes developed into diploid macrothalli under LD conditions, which in young stages exhibited bipolar germination. Successive divisions in different directions resulted in symmetrical disc; later, this developed into the typical macroscopic globose structure (Fig. 4, l-o). Both mitospore-producing plurilocular sporangia and meiospore-producing unilocular sporangia developed on each mature macrothallus upon reaching a size of 0.8-1.2 cm. 6.3% of unfertilized female gametes developed parthenogenetically to produce new prostrate branched microthalli.
All Leathesia marina specimens from both collection localities exhibited similar development and life cycles patterns in in vitro culture. They completed the cycle in 6 months developing different morphologies. During the haploid generation, two types of microthalli that developed from meiospores were observed: one erect and branched and other prostrate. The prostrate branched microthalli behaved like true gametophytes, since they produced gametes that developed into macrothalli after fertilization. Meanwhile, the erect morphology (i.e., erect branched microthalli) only regenerated itself through mitospores. The diploid generation was also represented by two other morphologies: a globular microthallus derived from mitospores, which remained in a dwarf state and repeated the same asexual cycle indefinitely, and a globose macrothallus, which is a product of fertilization and commonly found in nature (Fig. 5).
Nuclear ploidy of the different phases of Leathesia marina life cycle. When determining the ploidy by flow cytometry, the presence of one peak represents the cells in the G1 phase of the cell cycle, followed by a plateau or S phase, whereas the occurrence of a second peak (with twice the channel value) corresponds to post-DNA replication cells in the G2/M phase.
The nuclei isolated from the macrothalli presented three populations of cells, producing one 2C peak in G1 cellular phase, a smaller 4C peak in G2/ M and a small 1C peak, possibly caused by haploid spores produced by meiosis (Fig. 5a).
Young germlings produced from mitospores had a nuclear cross-sectional area of 3.85 AE 1.2 µm 2 (mean AE SD; Fig. 6c). The nuclei isolated from the globular microthalli were 2C, followed by S phase and a 4C peak (Fig. 5b)  area of 3.21 AE 0.7 lm 2 (Fig. 6, j and l). Young germlings from the meiospores had a nuclear crosssectional area of 1.64 AE 0.16 µm 2 (mean AE SD; Fig. 6a). The nuclei isolated from the erect microthalli had a 1C peak and a smaller 2C peak, suggesting active cell growth (Fig. 5c). Their nuclear crosssectional area was 1.48 AE 0.07 µm 2 (Fig. 6, e-h). On the other hand, the nuclei from prostrate microthalli under latency conditions (SD), were only in phase G1, without cell division (Fig. 5c) and their nuclear area was 1.24 AE 0.10 µm 2 (Fig. 6, m-p). The nuclei of the erect microthalli were significantly larger (F 1,197 = 33.64; P = 0.001) than those of the prostrate microthalli, possibly due to active cell growth.

DISCUSSION
Leathesia marina populations from the two sites in the South Atlantic Ocean exhibited the same life cycle, development, and morphologies. This is corroborated by our molecular data confirming that populations at both macroscopic (obtained from nature) and microscopic (obtained in culture) stages are conspecifics. The inferred phylogeny revealed that L. marina from Atlantic Patagonia is closely related to L. marina from New Zealand.
Many organisms with heteromorphic life cycles exhibit marked morphological, physiological, and ecological divergence between the different haploid-diploid phases (Couceiro et al. 2015). The present study revealed that four different morphologies are involved in the life history of Leathesia marina under in vitro culture. That is, diploid phases occur as globular macrothalli or globular microthalli, whereas haploid morphologies occur as branched erect microthalli and branched prostrate microthalli.
Other authors have reported the occurrence of these morphologies in separate studies of Leathesia marina originated from meiospores and mitospores under in vitro culture. However, our current study is the first to report gamete fusion and the development of resulting zygote into the globose macrothalli that we observe in the field. Prostrate microthalli in Leathesia marina (as L. difformis) cultures have been found by Dangeard (1965) and Cole et al. (1968) generated from unilocular sporangia at low temperatures. Damman (1930) observed that spores released by unilocular sporangia develop into asexual microscopic thalli that bear plurilocular sporangia. This reproductive behavior coincides with the erect branched microthalli found in this study. Kylin (1933) and Sauvageau (1925) reported globular microthalli that regenerated for several generations through plurilocular sporangia. Kylin (1933) considered that the globular microthalli as an early macroscopic state, although he could not prove it; while Sauvageau (1925) suggested that it was a different morphology, since never re-generated new macrothalli from plurilocular sporangia.
The globular microthalli that we obtained in culture from plurilocular sporangia of the globose macrothallus remained in a dwarf state for several generations, never reaching the size and shape of the globose macrothalli in culture (originated by fecundation). In natural populations of Leathesia marina from Puerto Madryn, the globular macrothalli appear in spring, constituting a single cohort developing through the growing season without recruiting any new thalli (Poza et al. 2017). This suggests the absence of successive generations of macrothalli (originated from plurilocular sporangia) in this Patagonian population. However, it is likely that Poza et al. (2017) were not able to detect globular microthalli as new recruitment in their study populations due to the small size. Thus, we consider that globular microthalli in nature are as small as those that grew in culture. Careful search is probably required to find globular microthalli in nature. Studies on natural L. marina populations in Nahant, Massachusetts reported the presence of dwarf morphotypes that were growing as epiphytes on Zostera marina Linnaeus (Webber 1981). These dwarf morphotypes that grow to 2 mm in diameter were recognized as Leathesia nana-type. Webber (1981) alluded to the possibility that L. nana-type may be a growth form of L. marina (as L. difformis) based on the morphological, anatomical, and seasonal distribution data. In another species of the family Chordariaceae, Litosiphon laminariae (as Litosiphon pusillus) were also observed to have dwarf sporophytes that originated from plurilocular sporangia under winter culture conditions (Nygren 1975).
Leathesia marina displayed a haplodiplontic life cycle with marked differences between the two generations in terms of size, morphology, and reproductive behavior, which appears to be determined by temperature and photoperiod, as observed in our current culture studies. Some authors hypothesize that the different generations are adapted to different niches, allowing these species to survive in a fluctuating environment differing in temperature, day length, competitors, or herbivore pressures (Lubchenco and Cubit 1980, Zupan and West 1990, Cunningham et al. 1993. The sporophytic generation was dominant in size and is more conspicuous. This prevalence of the diploid phase should not be interpreted as an evolutionary advantage because in many taxa the haploiddiploid life cycles are an evolutionarily stable strategy, with the genetic benefits of both phases (Klinger 1993). The genetic advantage of the diploid phase is that nearly all deleterious mutations within the genome are masked, as the vast majority of mutations that adversely affect fitness are partially recessive (Crow and Kimura 1965). However, mutations are more efficiently eliminated in haploids. As haploid populations tend to carry fewer mutations, they tend to have a higher average fitness at equilibrium than diploid populations (Otto 1996, Mable andOtto 1998).
The presence of different haploid and diploid microthalli with different reproductive behavior suggests high plasticity in the development of Leathesia marina. The cultured globular microthalli and erect branched microthalli of L. marina produced fertile structures after 20-30 d and were regenerated during several generations, behaving as diploid and haploid sporophytes (i.e., asexual phases), respectively, since no zooid fusions were observed under the culture conditions evaluated. However, this reproductive behavior could be fostered by laboratory conditions; therefore, it is necessary to document these morphologies in natural populations of Patagonia to reveal their true reproductive strategies. A high level of morphogenic and reproductive heterogeneity has also been noted in Ectocarpus siliculosus under controlled conditions in laboratory culture (M€ uller 1967, 1980. Architectural plasticity with variation in growth rate and branching pattern are common features in algae, especially in response to environmental changes (Le . Hence, further studies focused on exploring the occurrence and function of these microthalli in nature and its possible links in the life cycle of L. marina is needed. Meanwhile, the prostrate branched microthalli can be maintained in latency for more than 1 year and can exhibit gametogenesis, but only under higher temperatures and longer photoperiods (LD; summer conditions). The same behavior has also been reported in other widely distributed brown algae such as Dictyota kunthii (=Glossophora kunthii) and Dictyota dichotoma, where low temperatures inhibited formation of reproductive structures (Hoffmann 1988, Hwang et al. 2005. It is known that algae respond to many cues for the timing of gametogenesis, where light, photoperiod, and temperature play a major role in the induction of reproduction (Brawley and Johnson 1992).
The prostrate branched microthalli, also called "banks of microscopic forms" (Chapman 1986), would be crucial to the persistence of populations during periods when the macroscopic stages are absent (Carney and Edwards 2006). The delayed development in juvenile or alternate life stages may be an adaptation to surviving in temporally variable environments and in establishing spatial patterns in other stages (Swanson and Druehl 2000). Macroalgal microscopic stages generally tend to be more tolerant to unfavorable light, temperature, and nutrient conditions, being able to persist for extended periods (such as during winter) and promote rapid recruitment once the conditions become favorable (Chapman and Burrows 1971, Nakahara 1984, Wiencke and tom Dieck 1989, 1990, Hoffmann and Santelices 1991, Edwards 2000. In turn, the macroscopic diploid phase produced by gamete fusion, appears only under summer conditions, provides the genetic advantages of having two sets of homologous chromosomes and a high rate of beneficial mutations (Crow and Kimura 1965).
Macroscopic thalli of Leathesia marina from Patagonia have high reproductive output due to the great increase in the number of sporangia and high release of spores at the end of the macrothalli season, coinciding with the months when the prostrate branched microthalli appeared (Poza et al. 2017). The high reproductive output of the macroscopic phase could ensure massive recruitment of the haploid phases, establishing banks of microscopic stages with unrelated individuals that accrue over time. This could allow the exchange of genetic material between unrelated individuals.
The life-history characteristics of Leathesia marina that we described here appear to be an adaptive strategy of the species to the extreme climate of Patagonia, a region in the southern hemisphere with a wellmarked seasonality. This arid region is characterized by a predominance of strong west winds and yearround low humidity (Paruelo et al. 1998). These strong dry winds, combined with low rainfall, give the Patagonian intertidal zone the highest desiccation stress recorded for rocky shore communities (Bertness et al. 2006). In this way, globular macrothalli with irregular morphology, such as having numerous interstices and large assimilatory filaments, could favor water retention and thus avoid desiccation during the summer. Whereas prostrate microthalli are favored during winter as such small morphologies are more tolerant to cold temperatures and have lower energy requirements (Hurd et al. 2014).
Leathesia marina is a good example of the heteromorphic life cycle, with great morphological plasticity that involves resting states (prostrate branched microthalli). These characteristics may allow the species to survive long periods of unfavorable environmental conditions and to recover following severe disturbances. The understanding of life cycles and microscopic macroalgal development is critical for the protection of the natural populations.  Table S1. Leathesia marina specimens included in the molecular analyses, with GenBank accession numbers for their rbcL and cox3 sequences. LIFE CYCLE OF LEATHESIA MARINA