ABSTRACT
Cross-pollination is a major factor determining the demographic dynamics of mixed-ploidy populations. Typically, rare cytotypes are suppressed due to reduced female fertility by losing gametes in heteroploid crosses (i.e., through minority cytotype exclusion). In species with reproductive differentiation into sexual and apomictic cytotypes, sexuals might be reproductively suppressed by apomicts (or transformed due to introgression of apomixis genes). Pollen precedence potentially acts as a post-pollination pre-fertilization barrier protecting sexuals against their apomictic counterparts. We estimated the role of pollen precedence as a barrier against cross-fertilization of tetraploid sexuals by penta- and heptaploid gametophytic apomicts in Potentilla puberula (Rosaceae) by means of controlled crosses, and inference of the paternity through DNA ploidy estimation of embryos. Individuals from five regions spanning an elevational and biogeographic gradient were used to account for the variation in the relative frequencies of reproductive modes across the study area. We tested (1) whether the application of heteroploid pollen (sexual × apomictic) causes a reduction of seed yield compared to homoploid crosses (sexual × sexual), and (2) if so, whether pollen precedence recovers the seed yield in simultaneous applications of pollen from sexuals and apomicts (mixed-ploidy). Seed yield was significantly lower in hetero- than in homoploid crosses. We found clear evidence for precedence of homoploid pollen, despite a 13% to 15% of embryos experienced a change in ploidy due to heteroploid fertilizations. Thus, our study indicates that pollen precedence operates as a barrier against intercytotype fertilization in P. puberula, promoting the integrity of the sexual cytotype and their co-existence with apomictic individuals.
ABSTRACT
Apomixis - asexual reproduction via seeds - might arise de novo following polyploidisation events, or via reproductive transfer of apomixis. Both processes can be obtained within species or via hybridisation. We aimed to determine the origin of apomictic genotypes in Potentilla puberula, a rosaceous species showing reproductive differentiation with ploidy: sexual tetraploids and apomictic penta- to octoploids, which regularly co-occur in sympatry. The study is based on 726 individuals, comprising all cytotypes, collected from 138 populations in the Eastern European Alps. We established relationships of cytotypes based on AFLP fingerprinting and cpDNA sequencing to test (1) whether the apomicts are of recurrent allopolyploid origin or originated from within the species via autopolyploidy, and (2) whether there are indications for reproductive transfer versus de novo origin of apomixis. Three principal pathways were identified which explain the origin of new apomictic genotypes, all involving at least one apomictic parent and thus compatible with the idea of reproductive transfer of the apomictic trait to the progeny: (1) self-fertilisation of unreduced egg cells in apomicts; (2) cross-fertilisation among apomicts; and (3) occasionally, heteroploid crosses among sexuals and apomicts. Autopolyploids derived from tetraploid sexuals were repeatedly observed, but did not express apomixis. Finally, our results suggest no role of other species in the origin of extant apomictic genotypes of P. puberula, although local hybrids with P. crantzii were identified. In conclusion, our results show that the formation of new apomictic genotypes required a genetic contribution from at least one apomictic parent. This finding is in accordance with the idea that apomixis is inheritable in P. puberula. On the contrary, lack of apomixis in penta- and hexaploids derived from sexual backgrounds did not support the hypothesis of a de novo origin of apomixis. Relatively high frequency of remnant sexuality in the apomicts involving different cytological pathways of seed formation can explain their high cytological and genotypic diversity. Finally, lack of global introgression from a third taxon is in support of P. puberula as a concise, although highly diverse, species.
