The 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 longstyled 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 selfpollination. 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 selfpollination 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.
Adaptation to local pollination regimes and secondary contact of allopatrically differentiated populations with respect to pollination ecology may result in geographical variation in floral traits. We examined the contributions of these two processes in Isodon shikokianus, which showed remarkable geographical variation in corolla tube length in western Japan. Corolla tube length varied among 17 study populations located within a relatively narrow area, and covaried with altitude and distribution of two bumblebee pollinators with different tongue lengths: the longer corolla was found at lower altitudes where the long‐tongued pollinator was more abundant, and vice versa. Additionally, bumblebee species preferentially visited flowers that fit their tongue lengths. Population genetic analysis based on 11 microsatellite loci revealed that populations with long and short corolla tubes constituted genetically distinct groups. Migration rates were low between the groups, but high within each group. These results indicate that two genetically differentiated groups made secondary contact and hybridized, and gene flow between the groups was limited. Thus, the geographical variation in corolla tube length in I. shikokianus may be a result of past allopatric differentiation and subsequent secondary contact of populations with different corolla tube lengths. The variation in corolla tube length within a narrow area may be maintained by selection owing to the altitudinally structured pollinator assemblages. Altitudinal differences in relative abundance of two pollinators and their assortative visitation with respect to corolla tube length may contribute to reproductive isolation between the two groups.
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