FUNCTIONAL NONEQUIVALENCE OF SPERM IN DROSOPHILA-PSEUDOOBSCURA

We report on a form of sperm polymorphism, termed polymegaly, that occ urs is species of the Drosophila obscura group. Individual males of sp ecies in this group characteristically produce more than one discrete length of nucleated, motile sperm. Hypotheses suggested to explain the evolutionary significance of sperm polymorphism have been either nona daptive or adaptive, with the latter focusing on sperm competition or nutrient provisioning. These hypotheses assume all sperm types fertili ze eggs; however, no data have been gathered to test this assumption. We found that two size classes of sperm are produced and transferred t o females in approximately equal numbers by the male; only long sperm persist in significant numbers in female sperm storage organs. Further more, we used a DNA-specific dye (bisbenzimide) and sperm-specific ant ibodies to ask if both sperm types fertilize eggs in Drosophila pseudo obscura. Confocal microscopy and immunofluorescent analyses of fertili zed eggs using anti-sperm polyclonal antisera demonstrated that only l ong sperm participate in fertilization. These data falsify those hypot heses in which all sperm types are assumed to be functionally equivale nt (fertilize eggs). Any remaining or new hypotheses for the evolution ary significance of polymegaly must incorporate these findings. Severa l new areas of research are suggested.