Atypical tristyly and generalist pollination system in a population of Oxalis psoraleoides (Oxalidaceae)

Abstract

Heterostyly is a polymorphism of reproductive structures, which can manifest as distyly or tristyly, depending on whether there are two or three floral morphs in the population. Such system commonly exhibits a heteromorphic incompatibility system, reciprocal heights in the sexual organs between floral morphs, and a balanced ratio of the morphs in a population. Deviations in heteromorphic incompatibility systems can result in alterations in the heights of reproductive structures and illegitimate crossing between stamens and styles of different heights and between individuals of the same morph, representing atypical heterostyly. This study aimed to detect the variation of the floral morph ratios in a tristylous population of Oxalis psoraleoides (Oxalidaceae) and to analyze the possible mechanisms involved, such as pollinator frequency, reciprocal herkogamy and the presence of heteromorphic incompatibility. We investigated variation of the morph ratio (1:1:1), stigma-anther reciprocity estimated by the floral inaccuracy index, frequency of visitors/pollinators, and evaluated the reproductive system. The population of O. psoraleoides is characterized as anisopletic with the mid-styled morph being most frequent. Its flowers were pollinated mainly by bees and lepidopterans, the latter being more frequent. In the analysis of reciprocal herkogamy, the lower organs show lower values of floral inaccuracy, thus representing the level with the highest degree of reciprocal herkogamy, followed by the medium and the upper level. Oxalis psoraleoides is self-incompatible, despite forming fruits in illegitimate cross-pollinations. We consider the study population of O. psoraleoides as tristylous due to the disproportion among the morphs and the deviations in the degrees of compatibility. Supported by the fact that fruits only form in legitimate and illegitimate intermorph crosses, we suggest that the mixed pollination system may be contributing to the presence of atypical characteristics in the Oxalis clusters of the population studied.