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PROJECTS: MOLECULAR MECHANISMS & EVOLUTIONARY CONSEQUENCES OF VARIATION IN SPERM SHAPE

spermSperm morphology is one of many male reproductive traits subject to rapid evolution, leading to high levels of diversity within and among closely-related taxa. Sperm morphology varies significantly among individuals for a wide range of species and is consistent across ejaculates or spawnings of the same male. This pattern suggests that sperm morphology is under strong genetic control and is evolving under selection that promotes phenotypic variation. The goals of this research are to understand the genetic and cellular mechanisms that control variation in sperm morphology, how this variation influences reproductive success, and the evolutionary mechanisms that act to maintain variation in sperm morphology within populations.

Quantifying variation in sperm morphology

In the green sea urchin (Strongylocentrotus droebachiensis), sperm morphology varies significantly among individuals and populations. Variation among 38 individuals and three populations were tested using ANOVA. Sperm head length were significantly different among individuals and populations and had low variation among spawning events of a single male. Variation in this trait was also stable within a population across multiple years.

Candidate sperm morphology genes

To identify genes associated with variation in sperm morphology, we are using a functional genomics approach that uses a high-density oligonucleotide array that tiles across the last exon of every predicted gene in the sea urchin genome. RNA was extracted from testis of three males with long sperm heads and three males with short sperm heads, all from the same population. We have identified 1800 genes that are differentially expressed in testes of males producing long versus short sperm heads. Many of these genes are directly relevant to male reproduction and sperm, but the majority serve general metabolic, transport, signaling and regulatory functions in many tissue types. Several promising cellular pathways have been identified, including electron transport, cell cycle regulation, and vesicle-mediated protein trafficking.

Sperm morphology affects fertilization success

We are investigating the functional significance of sperm morphology on fertilization success using sperm competition experiments between long sperm head and short sperm head males. I used the same six males as in the tiling array experiment (Fig. 1), from which live sperm were obtained prior to tissue harvest. Three males with long sperm heads were competed against three males with short sperm heads as well as with each other. All sperm competition experiments were replicated over two females.

Preliminary results show that fertilization success seems to be a function of both sperm morphology and female. Sperm competitors produced different outcomes of relative paternity success with different females, and the ratio of paternity success tends to be similar within a female across different pairs of males. The ambiguity of these results is not surprising, if balancing selection is indeed acting on sperm morphology. Under this scenario, long sperm heads may be advantageous under some conditions but not others. Further experiments investigating fertilization success while varying some of these factors will be a first step toward elucidating the conditions under which long and short sperm heads are favored.

Hopkins Marine Station, Stanford University, 120 Ocean View Blvd., Pacific Grove, CA 93950