Quantifying reciprocity in distylous and tristylous plant populations

Reciprocal herkogamy (heterostyly) is an example of extreme relevance of accuracy of the location of the sexual organs within some floral systems. It involves the reciprocal positioning of anthers and stigmas in flowers of different plants within the same population, and the accuracy of this positioning is important to promote out-cross pollination, prevent self-interference, or both. Hence, several indices have been proposed to quantify reciprocity, most of them for populations with two different morphs (distylous). Here, we propose an extension of our index of reciprocity for distylous populations to be applied also to populations with three morphs (tristylous), allowing effective comparisons of reciprocity between tristylous, but also distylous populations. As for the distylous version, the index is based on comparison of the position of every single sexual organ in the sample with each and every organ of the opposite sex, for each of the three possible organ levels. Due to the massive amount of calculations required, a macro was developed that is available as Supplementary Information and at the website of the authors. The index and macro were tested on several hypothetical tristylous and distylous populations with predetermined mean and dispersion of sexual organs at each level, as well as on several actual tristylous and distylous populations. The index proposed is a solid tool for the study of reciprocity in distylous and tristylous populations. Comparisons between distylous and tristylous populations are easily performed and can be readily interpreted. The applicability of the index is facilitated through the software provided.