Ultrastructure and phylogenetic significance of spermatozoa in Nemertea

Abstract

Sperm ultrastructure is frequently employed as a source for phylogenetic inference due to the ease of accessing spermatozoa. Despite being unicellular, sperm cells exhibit a relatively high number of diverse characters and character states. Spermatozoa are subject to strong sexual selection as they are finely tuned for maximizing male reproductive success. Given this strong functional constraint, one might anticipate the emergence of convergent characters in line with similar modes of reproduction. As a result, it might be expected that sperm cells do not possess substantial phylogenetic signal, with functional constraints overshadowing any evolutionary heritage. To test this assumption, we conducted a study on sperm ultrastructure in 11 nemertean species, representing closely related groups and major nemertean lineages. We analyzed these data for their phylogenetic signal within the context of the most recent nemertean phylogenies. Our findings demonstrate that, at all systematic levels, functional constraints imposed by sexual selection, or the mode of reproduction do not supersede the influence of historical constraints on sperm ultrastructure.