Fertilization competence and sperm size variation in sperm-heteromorphic insects

Between species, variation in sperm size has been related to male–female coevolution and male–male competition. In contrast, variation within species is poorly understood. A particular case of intraspecific sperm-size variation occurs in sperm-heteromorphic species, where males produce distinct sperm morphotypes, usually only one of which is fertile. This allows to investigate sperm size variation under different selection regimes. Nonfertile morphotypes, whose role is aside from fertilization, may have other functions, and this may be reflected by changes in developmental processes and a different phenotype compared to fertile sperm. We show that the intraspecific coefficient of variation in sperm length is up to four times lower for fertile than nonfertile morphotypes across 150 sperm-heteromorphic species (70 butterfly, 71 moth, 9 diopsid fly species). This is in agreement with a previous study on 11 species in the <i>Drosophila obscura</i> group. Significantly lower variation in fertile than nonfertile sperm morphometry may result from fertilization-related selection for optimal sperm size, novel functions of nonfertile sperm, or from tighter control of fertile sperm development. More data are needed to clarify the consequences and adaptive significance of within-morph variation, and its consistent pattern across sperm-heteromorphic insects.