Size isn't everything

I n his second major book, The Descent of Man and Selection in Relation to Sex, Charles Darwin showed that when animals compete for mates, sexual selection can drive many types of evolutionary change (Darwin, 1871). However, he did not notice that this competition continued even after animals had finished mating. Sperm competition was only described a century later when groundbreaking work showed that if two (or more) males mate with a single female, their sperm compete for the chance to fertilize her egg cells or ‘oocytes’ (Parker, 1970). Since this process is carried out inside her reproductive tract, the female can influence the outcome, and the males can too, via chemicals released in their ejaculates. This competition can be lengthy and intense, particularly in animal species where females store sperm for later use. The sperm of nematode worms resemble those of other animals in many ways, but they move by crawling rather than by swimming. The nematode Caenorhabditis elegans is ideal for studying sperm competition because its short lifespan and large brood size make the animals easy to work with in the laboratory. Furthermore, this worm’s genetics have been well studied. Now, in eLife, Jody Hansen, Daniela Chavez and Gillian Stanfield—who are all at the University of Utah—have used the power of nematode genetics to transform our understanding of sperm competition (Hansen et al., 2015). C. elegans has two sexes, males and hermaphrodites, both of which make sperm. Thus, several types of competitive interactions can be studied using these animals—the most important of which involves sperm from a male outcompeting sperm from a hermaphrodite to fertilize the hermaphrodite’s own oocytes. A series of beautiful studies showed that the competitive advantage of male sperm can be explained, in part, because they are larger than hermaphrodite sperm (LaMunyon and Ward, 1995, 1998). Furthermore, comparisons between related species supported the idea that competition favored larger sperm in nematodes, and this model was confirmed by laboratory experiments (LaMunyon and Ward, 1999, 2002). As a result, a simple model for how nematode sperm compete was proposed (Figure 1A; Ellis and Scharer, 2014). Inside a hermaphrodite, sperm are stored in the two structures called spermathecae and fight for positions near the maturing oocytes, where they will have the best chance of fertilization. Since each newly fertilized oocyte passes through the spermatheca, it dislodges many of the sperm, setting up a new round of competition to move into a good spot. Copyright Ellis and Wei. This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.