Reproductive barriers in Serapias x kelleri.

Background: Understanding the reproductive barriers in natural plant hybrids is fundamental for comprehending mechanisms of speciation and evolutionary divergence. Most studies focus on reproductive barriers and gene exchange material between parental species, whereas fewer emphasize the biological significance of hybrid viability and their ability to interbreed with each other or with the parental species. To address this gap, the present study focuses on the hybrid Serapias x kelleri and its parental species, Serapias vomeracea and Serapias cordigera to evaluate reproductive success and the role of prezygotic and postzygotic barriers. Specifically, the purpose of this study was to investigate and evaluate the reproductive success, the biological and evolutionary implications of Serapias x kelleri, in terms of fruit production and seeds produced through hand pollination of any possible bidirectional cross combinations between hybrid plants and both parental species.

Results: Controlled manual crosses between hybrids and parental species to assess fruit and viable seed production were conducted. Results revealed no prezygotic barriers, as all pollinated flowers formed fruit. However, postzygotic barriers were evident, with significant variability in viable seed production. Hybrid self-crosses showed the lowest seed viability (0.6%), indicating strong postzygotic isolation and possible inbreeding depression. Crosses between hybrids and parental species exhibited a variable embryo production, with average values ​​ranging from 5,6% to 29,1%. Remarkable is the fact that when the hybrid acts as a pollen donor, the percentage of viable seeds with embryos is higher, with average values ​​ranging from 18.9% to 29.1%. Opposite, when the hybrid receives pollen from one of the parental species, the values ​​of viable seeds decrease dramatically, with averages that do not exceed 10%. These data clearly show an asymmetric reproductive capacity of the hybrid, with significantly higher success when acting as the pollen parent compared to the maternal parent.

Conclusions: This study emphasizes the critical role of reproductive barriers in maintaining species integrity within the Serapias genus. Natural hybrids such as S. x kelleri can facilitate gene flow between parental species. However, strong postzygotic barriers significantly limit their fertility, thereby driving evolutionary divergence and preserving the genetic identity of each species. These barriers consequently reduce the likelihood of speciation.