Direction of the cross affects seed siring and progeny fitness in experimental homoploid crosses between two diploid Ficaria taxa

Understanding reproductive isolation mechanisms is essential for describing the origin of new species. In flowering plants, these mechanisms significantly affect hybridization rates (frequency of hybrid formation in natural populations) and crossing success (seed set and germinability in controlled crosses) and often are associated with asymmetries in hybrid fitness. Ficaria offers a valuable system for studying hybridization and reproductive isolation, as predominantly tetraploid, widespread phylogeographic taxon (lineage) verna (V) is thought to have arisen by hybridization between two parapatric diploid taxa (lineages) calthifolia (C) and fertilis (F) followed by polyploidization. To assess the extent and directionality of reproductive isolation, we conducted controlled intra-lineage and pairwise reciprocal crosses between the aforementioned diploids, evaluated reproductive assurance (autonomous apomixis, selfing) and analysed genome size, morphology and fitness of obtained F₁ progeny. No evidence of autonomous apomixis or self-fertilisation was found in the parental diploid lineages or the hybrids. The reciprocal inter-lineage crosses (F–C cross: n = 76; C–F cross: n = 45) produced viable diploid F₁ progeny whose genome size was intermediate between that of the parental lineages. The above implies that all F₁ progeny in reciprocal crosses originated from cross-pollination. No polyploid individuals were observed among the progeny resulting from any cross treatment. Progeny from inter-lineage crosses exhibited greater morphological variation than progeny from intra-lineage crosses. However, inter-lineage hybridisation was asymmetric. When C was the seed parent, the crosses yielded a lower number of seeds with lower germination rates and reduced fertility of established hybrids, in comparison to when F was the seed parent. Yet these hybrids exhibited greater stature and larger flowers than their reciprocals. These asymmetries likely reflect postzygotic cytonuclear incompatibilities between parental lineages. Our research highlights the importance of hybridisation and asymmetric reproductive barriers in shaping the evolution of Ficaria and establishes a basis for further studies on the genomic complexities that lead to polyploidisation.