Sperm Competition Craig LaMunyon, Assistant Research Professor Sperm competition occurs when sperm from two individuals compete to fertilize eggs. I am studying the mechanism of sperm competition and how it affects the evolution of male and female traits. Sperm competition is a widespread phenomenon, occuring in almost every species. The effects of sperm competition are varied and spectacular, ranging from post-copulatory mate guarding (many insects) and incredibly high sperm counts (birds and humans) to penises fashioned with brushes and hooks that function to remove rival sperm from the female reproductive tract (dragonflies). I study the mechanism of sperm competition in C. elegans and related nematode species. These free-living soil-inhabitants are composed in large part of self-fertilizing hermaphrodites, which can only receive sperm and not donate them to other individuals. Males are rare but readily mate with hermaphrodites. In both C. elegans and its close relative,C. briggsae, male sperm displace the hermaphrodites' own sperm and predominate in fertilization. Using an artificial insemination protocol I developed to remove seminal fluid, I found that the male sperm take precedence in the absence of seminal fluid and that the male sperm cells themselves are competitively superior to hermaphrodite sperm. The factor that turns out to determine competitiveness is sperm size: male sperm are larger than hermaphrodite sperm. My investigations have shown that larger sperm crawl faster and physically displace smaller sperm from the sites of fertilization. Further, males that make larger sperm gain sperm precedence over males that make smaller sperm. However, large sperm do not come without a cost: they take longer to produce. Finally, I have investigated the evolution of sperm size across several nematode genera including both hermaphroditic and male/female species. These data show that hermaphrodite sperm have become smaller, and that male sperm are largest in male/female species where the incidence of sperm competition is greatest. Thus, sperm competition appears to have driven the evolution of sperm size in nematodes. Many researchers feel that sperm competition brings about the evolution of small sperm as the benefits of producing ever greater numbers of sperm cause miniaturization. It appears that in nematodes this is not the case. Larger sperm, because of their increased competitiveness, have been favored. While this may seem unusual, others have found that species with more sperm competition have larger sperm than those with less sperm competition, and larger flagellated sperm from vertebrates swim faster than smaller sperm. We may find that increased sperm size is a common response to the pressures of sperm competition.
http://www.mcb.arizona.edu/wardlab/spermcomp.html All Contents Copyright © 2000. All rights reserved. Last Modified: October 13, 1998 Paul Muhlrad pmuhlrad@u.arizona.edu