Intraindividual flower variation in distylous plants

Abstract

Plant individuals respond to distinct environmental conditions, which can drive morphological variation. Conversely, intraindividual floral variation affects interactions with pollinators and plant fitness. In distyly, population-level variation has been seen as negative since morph reciprocity in stigma and anther heights promotes precise cross-pollination. However, although natural selection occurs at the individual level, we still do not know how sexual structures vary within individuals in the floral morphs of distylous populations. We used typically distylous populations of Neotropical Rubiaceae family as model systems to study intraindividual variation. Based on the morphology of sexual structures, we calculated the coefficients of variation of heights and size (lengths) as a measure of intraindividual variation (IIV). Stigmas of thrum morph (stigma below anthers) had higher IIV in height when compared to their respective anthers and the stigmas and anthers of pin morph (stigma above anthers). On the other hand, the IIV in size was higher in the stigmas of both morphs when compared to anthers. Higher variation/imprecision in anthers and stigmas affect reciprocal herkogamy and hinder precise pollination both at population and individual levels. However, we suggest that higher variations in stigma heights and sizes may be an intraindividual strategy to occupy a greater positional range, which would increase the chances of cross-pollen deposition among its flowers. This would adjust flowers to the different pollinator guilds, which probably exert different selective pressures. Our study offers a new perspective on distylous flowers, suggesting that IIV may increase pollen reception and explain imprecision at the population level.