Abstract
Reproductive isolation (RI) plays an important role for speciation, but assessing reproductive barriers at all life-cycle stages remains challenging. In plants, most studies addressing the topic have been focusing on herbs with short generation times. The present study attempted to quantify several reproductive barriers between a hybridizing species pair of long-lived woody rhododendrons. Consistent with findings of previous studies, pre-zygotic reproductive barriers contributed more to total RI than post-zygotic reproductive barriers. Especially in the more widespread species geographic isolation was an important barrier, and pollinator constancy contributed exceptionally to RI in both species. Additionally to strong pre-zygotic reproductive barriers, post-zygotic reproductive barriers were considerable, and had asymmetric tendencies favoring one of the species as maternal parent. Overall, despite occasional hybridization, the present study provides evidence for strong RI between R. cyanocarpum and R. delavayi.
Highlights
In sympatry the highest contribution to reproductive isolation (RI) for both species comes from pollinator constancy, with other pre-zygotic barriers being very weak in R. cyanocarpum but considerable in R. delavayi
Post-zygotic barriers are strong, and seem to be slightly asymmetric, indicating higher likelihood for occurrence of hybrid seedlings originating from R. delavayi mothers
With pre-zygotic barriers contributing considerably more to overall RI than post-zygotic barriers, the data fits the pattern already observed by Lowry et al.[23]
Summary
The specific goal of the present study was to conduct a comprehensive assessment of diverse barriers that can contribute to RI
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