ONLY LONG SPERM ARE FERTILIZATION-COMPETENT IN 6 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 D rosophila obscura group and is characterized by the production of two size classes of nucleated sperm that differ only in head and tail leng ths [1,2], Both the length and the ratio of sperm types produced is sp ecies-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 simil ar ultrastructure [6], DNA content [6], nuclear protein transition dur ing spermatogenesis [7], and storage in females [3-5]. However, a prev ious study demonstrated that only long sperm fertilize eggs [3], Here, we extend this study to examine fertilization in six obscura group sp ecies using anti-sperm antibodies and digital deconvolution microscopy . Consistent with the previous study, we found that all eggs were fert ilized by only the long sperm type, even in polyspermic eggs. Moreover , sperm entry and position during and following fertilization were sim ilar to other Drosophila groups [3,8-10]. Thus, polymegaly and its mai ntenance appear to have arisen independently of fertilization processe s per se.