The effects of inbreeding depression and pollinator visitation on the maintenance of herkogamy in Oxalis corniculata, a species derived from a heterostylous ancestor

Abstract

AbstractThe evolutionary transition from outcrossing in heterostylous morphs to selfing in homostylous morphs has occurred in many plant lineages. Homostylous variants with a low degree of herkogamy may increase the reproductive advantage of autonomous self‐pollination when there is little inbreeding depression and/or low pollinator visitation. Oxalis corniculata, a self‐compatible perennial herb derived from a tristylous ancestor, is reported to have two floral morphs. Homostyled plants are broadly distributed in Japan, whereas long‐styled plants are reported to occur only in coastal areas. We examined the hypothesis that a reproductive advantage in avoiding inbreeding depression and/or frequent pollinator visits enables long‐styled plants to live in some areas. Studies on floral variation revealed that the degree of herkogamy varied widely and continuously within and among populations. Homostyled and long‐styled morphs were functionally defined based on the ability of autonomous self‐pollination. Homostyled plants were widely distributed, whereas long‐styled plants were found in and around the two coastal populations and relatively inland populations. Pollination experiments provided no evidence of inbreeding depression in either homostyled or long‐styled plants. Pollinator visitation frequency was higher in the two coastal populations than in the other populations. Thus, reproductive disadvantage in long‐styled plants lacking autonomous self‐pollination has reduced in the two coastal populations, and long‐styled plants may be maintained due to a competitive advantage of large seed size. In contrast, we recorded no comparable frequencies of pollinator visitation in other populations containing long‐styled plants, indicating that there are other factors contributing to the maintenance of long‐styled plants in these areas.