Sexual isolation in two bee-pollinated Costus (Costaceae)

Examining reproductive isolating barriers is essential for understanding processes of speciation. Sexual isolation has been shown to contribute to speciation in many sympatric taxa; however, its role in parapatric taxa with reduced interspecific gene flow is poorly understood. I investigated the extent of sexual isolation in two closely related Neotropical understory species, Costus allenii and C. villosissimus, that occur in adjacent habitats within flight distance of their shared pollinators, euglossine bees. Pollination arrays were used to test whether individual pollinators travel between species, to estimate the proportion of hetero- and conspecific pollen deposited on the stigmas, and to examine the proportion of hybrid progeny. In comparison to C. allenii, C. villosissimus produces flowers with larger labella, longer stamen–labellum distances, and longer styles. Pollinators visited both species but preferred C. villosissimus. This preference caused pollinator isolation in C. villosissimus. In C. allenii, the frequency of heterospecific pollinator transitions was not less common than that of conspecific transitions, but floral mechanical isolation greatly reduced the likelihood of heterospecific pollen deposition. The contribution of gametic isolation was not strong in either species. Based on data for pollinator isolation, floral mechanical isolation, and gametic isolation, it appears that sexual isolation is weak in C. allenii, restricting heterospecific gene flow by 25 %, but moderate in C. villosissimus, where gene flow from C. allenii is reduced by 70 %. Further research will estimate the magnitude of other isolation barriers to determine the relative contribution of sexual isolation to total isolation in this parapatric species pair.