Only long sperm are fertilization-competent in six sperm-heteromorphic Drosophila species

Males of many species exhibit sperm heteromorphism, in which multiple morphologies of sperm are produced in a common testis (see references in [1]). Polymegaly, a form of sperm heteromorphism, is found in the Drosophila obscura group and is characterized by the production of two size classes of nucleated sperm that differ only in head and tail lengths [1,2]. Both the length and the ratio of sperm types produced is species-specific [1,3-5], and each sperm type develops in its own bundle deriving from a single stem cell [4]. Previous studies suggested that both types of sperm are fertilization-competent on the basis of similar ultrastructure [6], DNA content [6], nuclear protein transition during spermatogenesis [7], and storage in females [3-5]. However, a previous study demonstrated that only long sperm fertilize eggs [3]. Here, we extend this study to examine fertilization in six obscura group species using anti-sperm antibodies and digital deconvolution microscopy. Consistent with the previous study, we found that all eggs were fertilized by only the long sperm type, even in polyspermic eggs. Moreover, sperm entry and position during and following fertilization were similar to other Drosophila groups [3,8-10]. Thus, polymegaly and its maintenance appear to have arisen independently of fertilization processes per se.