Background and aims: Although evolution of sexual polymorphism has been traditionally analysed using discrete characters, most of these polymorphisms are continuous. This is the case of heterostyly. Heterostyly is a floral polymorphism successfully used as a model to study the evolution of the sexual systems in plants. It involves the reciprocal positioning of anthers and stigmas in flowers of different plants within the same population. Studies of the functioning of heterostyly require the quantification of the degree of reciprocity between morphs of heterostylous species. Some reciprocity indices have been proposed previously, but they show significant limitations that need to be dealt with. This paper analyses these existing indices, and proposes a new index that aims to avoid their main problems (e.g. takes into account population variability and offers a single value per population).
Methods: The new index is based on the comparison of the position of every single sexual organ in the population with each and every organ of the opposite sex. To carry out all the calculations, a macro was programmed with MS Visual Basic in MS Excel. The behaviour of the index is tested using hypothetical data to simulate different situations of dimorphic populations; the index is also tested with some actual populations of different species of the genus Lithodora.
Results and conclusions: The index of reciprocity proposed here is a sound alternative to previous indices: it compares stigma-stamen height gaps for all potential crosses in the population, it comprises stigma-stamen distance as well as dispersion, it is not skewed by the more frequent sex, and it can be meaningfully compared between populations and species. It has produced solid results for both hypothetical and natural populations.