Changes in reciprocal herkogamy during the tristyly-distyly transition in Oxalis alpina increase efficiency in pollen transfer

Although the spatial separation of sexual organs within a flower (herkogamy) has been interpreted as a mechanism that promotes efficient pollen transfer, there have been few attempts to relate variation in herkogamy to probabilities of pollen flow. Here, we used a heterostylous species with variation in reciprocal herkogamy to test this hypothesis. We measured legitimate and illegitimate pollen flow with fluorescent dyes in four selected populations of Oxalis alpina corresponding to the extremes of a previously reported evolutionary gradient from tristyly to distyly. After the breakdown of tristyly, the observed increment in reciprocal herkogamy between the long and short morphs was associated with a 30% increase in the proportion of dye received from compatible illegitimate pollinations. In all populations, the most likely effective pollen vectors were two Heterosarus bee species. Our results support the adaptive value of reciprocal herkogamy in promoting efficient pollen transfer in heterostylous species.