Reproductive differentiation into sexual and apomictic polyploid cytotypes in Potentilla puberula (Potentilleae, Rosaceae)

Background and Aims

Intraspecific reproductive differentiation into sexual and apomicticcytotypes of differing ploidy is a common phenomenon. However, mechanismsenabling the maintenance of both reproductive modes and integrity ofcytotypes in sympatry are as yet poorly understood. This study examined theassociation of sexual and apomictic seed formation with ploidy as well asgene flow towards sexuals within populations of purely polyploidPotentilla puberula.

Methods

The study is based on 22 populations representing various combinations offive polyploid cytotypes (tetraploid–octoploid) from East Tyrol,Austria. Embryo ploidy and the endosperm/embryo ploidy ratio obtained by aflow cytometric seed screen were used to infer reproductive modes of seedformation and to calculate the male and female genomic contributions to theembryo and endosperm. Self-incompatibility (SI) patterns were assessed and anew indirect approach was used to test for the occurrence of intercytotypematings based on the variation in the male genomic contribution to sexuallyderived embryos on the level of developed seed.

Key Results

Tetraploids formed seeds almost exclusively via sexual reproduction, whereaspenta- to octoploids were preferentially apomictic. Non-random distributionof reproductive modes within maternal plants further revealed a tendency toseparate the sexual from the apomictic mode among individuals.Self-incompatibility of sexuals indicated functionality of the gametophyticSI system despite tetraploidy of the nuclear genome. We found no indicationfor significant cross-fertilization of tetraploids by the highpolyploids.

Conclusions

The study revealed a rare example of intraspecific differentiation intosexual and apomictic cytotypes at the polyploid level. The integrity of thesexual tetraploids was maintained due to reproductive isolation from theapomictic higher polyploids. Functionality of the gametophytic SI systemsuggested that the tetraploids are functional diploids.