Prepollination barriers prevent gene flow between co-occurring bat-pollinated bromeliads in a montane forest

Reproductive isolation mechanisms in flowering plants are fundamental to preserving species' evolutionary independence and to enabling the local coexistence of closely related species. These reproductive barriers are expected to contribute to maintaining local diversity of highly diverse plant guilds, such as bromeliads in neotropical ecosystems. We evaluated how strong and effective these barriers are by analyzing different mechanisms that act before and after pollination in a guild of four epiphytic bromeliads from the genus Werauhia (Tillandsioideae) pollinated by bats in a Costa Rican montane forest.

We employed several reproductive isolation indices proposed in the literature to estimate the effect of flowering phenology, floral morphology, interspecific compatibility, production, and viability of hybrid seeds as barriers to gene flow between species pairs.

The overall reproductive isolation between species was complete or nearly so. We found that temporal isolation due to different flowering schedules between species significantly contributed to preventing interspecific gene flow. However, flowering data from four reproductive seasons showed interannual variation in the intensity of this temporal barrier due to fluctuations in the species' blooming patterns. For species with overlapping flowering, mechanical isolation caused by differences in flower size and position of reproductive organs was significant, and such differences in flower architecture are thought to influence pollen deposition on different areas of the pollinator's body. Postpollination barriers showed varying intensity, from full to partial interspecific incompatibility. When hybrid progeny was produced, the number of seeds and their germination capacity were lower compared to progeny from intraspecific crosses.

Overall, prepollination mechanisms (phenology and floral design) were of great importance to eliminate pollen transfer between species and, when present, postpollination barriers had a redundant effect. Our results contradict previous reports that suggested a weak effect of premating barriers among bromeliad species. Additional studies involving other pollination guilds are required to gain a better understanding of the prevalence of different reproductive isolation mechanisms in the highly diverse Bromeliaceae family.