Evidence for asymmetrical hybridization despite pre- and post-pollination reproductive barriers between two Silene species.

Jin-Ju Zhang,Benjamin R Montgomery,Shuang-Quan Huang

doi:10.1093/aobpla/plw032

Abstract

Interspecific hybridization is widespread among plants; nevertheless, pre- and post-zygotic isolating mechanisms may maintain species integrity for interfertile species in sympatry despite some gene flow. Interspecific hybridization and potential isolating barriers were evaluated between co-flowering Silene asclepiadea and Silene yunnanensis in an alpine community in southwest China. We investigated morphological and molecular (nuclear microsatellites and chloroplast gene sequence) variation in sympatric populations of S. asclepiadea and S. yunnanensis. Additionally, we analyzed pollinator behaviour and compared reproductive success between the putative hybrids and their parental species. Both the molecular and morphological data indicate that there were putative natural hybrids in the field, with S. asclepiadae the ovule parent and S. yunnanensis the pollen parent. Bumblebees were the primary visitors to S. asclepiadae and putative hybrids, while butterflies were the primary visitors to S. yunnanensis Pollen production and viability were significantly lower in putative hybrids than the parental species. The direction of hybridization is quite asymmetric from S. yunnanensis to S. asclepiadea Protandry combined with later peak flowering of S. yunnanensis, and pollinator preference may have contributed to the asymmetric pattern of hybridization, but putative hybrids were rare. Our results thus suggest that despite gene flow, S. asclepiadea and S. yunnanensis can maintain species boundaries, perhaps as a result of floral isolation and low fecundity of the hybrids.

Highlights

Hybridization may allow for interspecific gene flow, which at high rates can erode species integrity
The analysis indicated a high degree of purity of both parental species, with a membership proportion of the S. asclepiadea population in the S. asclepiadea cluster of 97.7 %, and a membership proportion of the S. yunnanensis population of 97.9 % in the S. yunnanensis cluster (Fig. 3)
The analysis did not resolve the hybrids into a distinct cluster; instead, all hybrid individuals were assigned to either S. asclepiadea population (28.5 %) or the S. yunnanensis population (71.5 %), 10 of 11 individuals putatively identified as being hybrids based on morphology had microsatellite genotypes strongly suggestive of contributions from both parent species; this was not the case for most individuals identified as belonging to the parent species based on morphology (Fig. 3)

Summary

Introduction

Hybridization may allow for interspecific gene flow, which at high rates can erode species integrity. VC The Authors 2016 the potential for hybridiation, a series of reproductive barriers may prevent interspecific gene flow (Baack et al 2015), making natural hybridization a transient, rare event. Flowering plants in sympatry may employ different pollinators, or the same pollinator in different flowering periods, or differ in placement of pollen on the same pollinator’s body to reduce interspecific pollen flow (Huang and Shi 2013). The role of pollinator-mediated floral isolation has been questioned given that pollinator inconstant visits may result in interspecific pollen flow, resulting in hybridization and introgression (Armbruster 2014). Studies on the coexistence of parents and hybrids can help us to understand mechanisms that maintain species boundaries

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Results

Discussion

Conclusion