In 2003, I had the opportunity to attend the biennial Biology of Spermatozoa Conference. This is a conference report that I wrote that was published on biomednet.com
Seminal Advances in Sexual Selection
Some of the best examples of the power of sexual selection in modifying structures and creating diversity can be found in the study of spermatozoa. Far from simple haploid genome delivery systems, sperm show an extraordinary diversity in morphology, numbers and performance. Understanding the selective pressures that resulted in this diversity, and the importance that the variation of sperm characteristics has in determining the outcome of post-copulatory processes, has attracted researchers from both medicine and biology.
Historically, however, these two groups of researchers often worked separately on parallel problems, and important advances made in one field did not always diffuse across to the other. A need for increased communication was one of the reasons that first inspired Tim Birkhead and Harry Moore of the University of Sheffield, UK, to organize a multi-disciplinary conference where scientists from varied research fields could interact and compare notes. Now in its seventh biennial incarnation, the Biology of Spermatozoa meeting, held recently at Losehill Hall in Derbyshire, UK, attracted delegates from fifteen different countries.
Human fascination with the study of sperm is not a recent phenomenon, according to Bob Montgomerie at Queen's University in Kingston, Ontario, Canada. Although Aristotle had long ago deduced that the male's semen played a role in reproduction, it was not until 1677, when Antoni Leeuwenhoeck first described the appearance of 'animacules' in his own ejaculate, that the scientific study of spermatozoa began, notes Montgomerie. Although the study of sperm was initially impeded by societal acceptance, curiosity eventually outweighed embarrassment, and a lengthy debate raged through the eighteenth and nineteenth centuries on the role and mechanism by which fertilization was achieved. In the past 30 years, the amount of scientific attention focused on sperm competition alone has risen an exponentially, and shows no sign of slowing down.
This expanding interest is reflected in the many of the facets of current spermatozoa research. Relative to the female's reproductive tract, sperm cells are very, very small, yet they still manage to find their way to the site of fertilization, due to an incredible sense of direction. Michael Eisenbach at the Weizmann Institute of Science in Israel has found that sperm are extremely sensitive to both chemical and thermal clines, and that it is this sensitivity that aids successful sperm navigation.
When the sperm eventually reach the ova, a complex series of proteins must successfully interact before the sperm can penetrate. These proteins, studied by Willie Swanson at the University of Washington, USA), are diverse, species-specific, and have an unusually high rate of evolution. This specificity prevents accidental mismatches between gametes from different species and may contribute to speciation events, he says.
Despite the progress that has been made at understanding the nature of sperm dynamics, it is still hard to define sperm 'quality'. Sperm motility and the size of the sperm heads are two characteristics that are usually measured when assessing the percentage of normal sperm cells in a male's ejaculate. The problem is that although these two characters are probably linked, data on these variables cannot be obtained simultaneously.
Measuring sperm head size is easily done from prepared slides; but no information on motility can be gained from these dead cells. Conversely, analyses of videotaped live semen samples can yield useful motility data, but provide poor estimates of sperm head volume. The solution, according to Alan Pacey of the Central Sheffield University Hospitals Trust, UK, may come from the aerospace industry. Using particle image velocimetry software initially developed for tracking high-speed aircraft, it is possible to simultaneously capture data on both individual sperm size and speed from high quality digital imaging. Ultimately, this technique will find applications in both medical studies of infertility and evolutionary studies of sperm competition by providing comprehensive data on sperm performance, he says.
Sperm morphology is also a hot topic in the field. At the extreme end of the sperm size spectrum lies Drosophila bifurca, a species in which males produce sperm with tails up 6 cm in length (20 times the length of their own body!). In a series of experiments, Nathalie Luck and colleagues of Laboratoire Populations, Génétique et Evolution, CNRS in France sought to understand the remating propensity, levels of sperm storage, and fertilization success of females of the species.
Females rarely remated if they still possessed sperm from an earlier mate in their storage organs, the team found, so direct competition between rival ejaculates is quite rare. Instead, it appears that the potential to fertilize eggs is greatly determined by the likelihood of sperm being retained by the female, with 25% of mated females failing to store sperm obtained from the male. In these cases, sperm rejection appears to be due to specific incompatibilities between pairs of mates, rather than to the simple presence of "studs and duds," said Luck, which suggests that cryptic female choice may be involved.
While it is easy to consider the competition between the ejaculates of rival males for the fertilization of a female's ova as mostly a post-copulatory process, there are many earlier steps in reproduction that are involved in deciding the outcome of fertilization. Paula Stockley of the Department of Veterinary Sciences at the University of Liverpool, UK has been investigating how the local social environment affects a male's mating behavior, using the house mouse, Mus musculus, as a model species. Since females may copulate with several mates, given the opportunity, theory predicts that a male's mating behaviour will depend on the local risk of being cuckolded. By experimentally manipulating the presence of potential mating rivals, Stockley's team was able to test how a focal male changes his reproductive behaviour in order to presumably increase his chances of being a successful father. Stockley found that males increased the rate of ejaculations per intromission in the presence of other males, most probably to increase the quantity of his sperm transferred to the female. Her study thus provides empirical support for theoretical models of how individuals should behave under varying risks of sperm competition.
The female's reproductive tract provides the arena in which sperm from rival males compete, and can itself evolve in response to the intensity of this sexual selection. In a comparative study, Matt Anderson of the Center for Reproduction of Endangered Species at San Diego Zoo, USA, collected data on the oviduct length of female mammals with different mating systems. Species with more monogamous mating systems had significantly shorter oviducts than did species that were polyandrous or promiscuous. This correlation remains positive even after controlling for potentially confounding effects of allometry as well as phylogeny. Thus, increasing the distance that must be traveled to reach the site of fertilization might be an evolutionary adaptation by females to weed out inferior sperm or to reduce the risk of polyspermy.
A new tool for the description and study of sperm, proteomic analysis, is being developed by Tim Karr of the Department of Biology and Biochemistry at the University of Bath, UK. Although resolving an entire organism's proteome (the expressed proteins of the genome) is far too complex for current analyses, the relatively simple nature of sperm makes it an ideal subject for this type of analysis. Working on Drosophila melanogaster sperm, Karr and associates have been able to identify a few hundred different proteins that are expressed in sperm cells. Ultimately, identifying the composition of sperm cells should prove an important means by which to understand the diversity and evolution of sperm characteristics, and may prove useful in clinical applications, he predicts.
Tristan Long [was at the time] a PhD candidate in the Department of Biology at Queen's University, Kingston, Ontario, Canada
Drs Tim Birkhead and Geoff Parker walking the hills of Derbyshire