RESUMEN
Anthropogenic introduction of species has resulted in a breakdown of geographical barriers and hybridization in previously allopatric species. Thus, examining hybridization proneness of exotic species contributes to revealing its potential threat. Moreover, reproductive barriers may be strengthened or weakened due to long-term geographical isolation for these newly sympatric species. However, few studies have evaluated multiple barriers between alien and native species. In this study, we quantified the importance of four pre-pollination barriers (phenological, floral traits, pollen production, and floral constancy) and four post-pollination barriers (pollen-pistil incompatibility, seed set, seed viability, and seedling survival) between two introduced and five native Sagittaria species. Results showed that introduced S. platyphylla was cross-compatible with two native species, whereas introduced S. montevidensis was incapable of hybridizing with any native species. Different barriers were asymmetric within species pairs and multiple barriers acted in concert to maintain species boundaries. Post-pollination barriers contributed more to total reproductive isolation in native species, whereas pre-pollination barriers played a stronger role in total reproductive isolation for two introduced species. Seed set was the only barrier that was positively correlated with genetic distance. Our results provide a perspective to better understand reproductive barriers for secondary contact species. We highlight the importance of monitoring hybridization events before human introduction and the possible conservation strategies to remove invasive species with hybridization proneness.
RESUMEN
PREMISE: Pollinator sharing of co-flowering plants may result in interspecific pollen receipt with a fitness cost. However, the underlying factors that determine the effects of heterospecific pollen (HP) are not fully understood. Moreover, the cost of stigma closure induced by HP may be more severe for plants with special touch-sensitive stigmas than for plants with non-touch-sensitive stigmas. Very few studies have assessed HP effects on stigma behavior. METHODS: We conducted hand-pollination experiments with 10 HP donors to estimate HP effects on stigma behavior and stigmatic pollen germination in Campsis radicans (Bignoniaceae) at low and high pollen loads. We assessed the role of phylogenetic distance between donor and recipient, pollen size, and pollen aperture number in mediating HP effects. Additionally, we observed pollen tube growth to determine the conspecific pollen-tube-growth advantage. RESULTS: Stigma behavior differed significantly with HP of different species. Pollen load increased, while pollen size decreased, the percentage of permanent closure and stigmatic germination of HP. Stigmatic HP germination increased with increasing aperture number. However, HP effects did not depend on phylogenetic distance. In addition, conspecific pollen had a pollen-tube-growth advantage over HP. CONCLUSIONS: Our results provide a good basis for understanding the stigma-pollen recognition process of plant taxa with touch-sensitive stigmas. We concluded that certain flowering traits drive the HP effects on the post-pollination period. To better understand the impact of pollinator sharing and interspecific pollen transfer on plant evolution, we highlight the importance of evaluating more factors that determine HP effects at the community level.
