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Multiple and asymmetrical origin of polyploid dog rose hybrids (Rosa L. sect. Caninae (DC.) Ser.) involving unreduced gametes

Background and Aims
Polyploidy and hybridization are important factors for generating diversity in plants. The species-rich dog roses ( Rosa sect. Caninae ) originated by allopolyploidy and are characterized by unbalanced meiosis producing polyploid egg cells (usually 4 x ) and haploid sperm cells (1 x ). In extant natural stands species hybridize spontaneously, but the extent of natural hybridization is unknown. The aim of the study was to document the frequency of reciprocal hybridization between the subsections Rubigineae and Caninae with special reference to the contribution of unreduced egg cells (5 x ) producing 6 x offspring after fertilization with reduced (1 x ) sperm cells. We tested whether hybrids arose by independent multiple events or via a single or few incidences followed by a subsequent spread of hybrids.


Methods
Population genetics of 45 mixed stands of dog roses across central and south-eastern Europe were analysed using microsatellite markers and flow cytometry. Hybrids were recognized by the presence of diagnostic alleles and multivariate statistics were used to display the relationships between parental species and hybrids.


Key Results
Among plants classified to subsect. Rubigineae , 32 % hybridogenic individuals were detected but only 8 % hybrids were found in plants assigned to subsect. Caninae . This bias between reciprocal crossings was accompanied by a higher ploidy level in Rubigineae hybrids, which originated more frequently by unreduced egg cells. Genetic patterns of hybrids were strongly geographically structured, supporting their independent origin.


Conclusions
The biased crossing barriers between subsections are explained by the facilitated production of unreduced gametes in subsect. Rubigineae . Unreduced egg cells probably provide the highly homologous chromosome sets required for correct chromosome pairing in hybrids. Furthermore, the higher frequency of Rubigineae hybrids is probably influenced by abundance effects because the plants of subsect. Caninae are much more abundant and thus provide large quantities of pollen. Hybrids are formed spontaneously, leading to highly diverse mixed stands, which are insufficiently characterized by the actual taxonomy.