Structural trends in the spermatozoa of the snakes in relation to sperm competition

TOURMENTE, MAXIMILIANO; GIOJALAS, L.; CHIARAVIGLIO, MARGARITA

Madrid

Simposio; X International Symposium on Spermatology; 2006

International Spermatology Council

The evolutive significance of sperm size variations considering postcopulatory sexual selection remains poorly understood as the production of long sperm can be costly and the advantage of sperm size remains enigmatic. Theoretic considerations and previous studies suggest that selection via sperm competition may target specific aspects of sperm morphology favoring an enhancement in energy allocation to sperm motility by increasing tail length plus its associated mitochondria in the mid-piece. Although snakes show reproductive characteristics that suggest the presence of great risks of sperm competition, there are no previous analyses regarding this phenomenon in this group. The aim of this work was to analyze the structural variations in the spermatozoa of several snakes in relation to sperm competition. We used gonadosomatic index (GSI) as an adequate indirect measure of likely sperm competition. We measured the length of the head, acrosome, midpiece and principal piece of spermatozoa of three snake taxa (Boinae, Crotalinae and Xenodontinae) using phase contrast and fluorescence light microscopy and tested for correlation with the GSI. We also compared, via transmission electron microscopy images, the ultrastructure of spermatozoa of the two snake species that showed the highest (Waglerophis merremii) and lowest (Boa constrictor occidentalis) GSI and midpiece length value. Significative correlation values were only found between GSI and midpiece length. Ultrastructural comparison revealed differences in the fibrous sheath and peripheral fibers 3 and 8 in the midpiece, being W. merremii values significatively larger. The greater length observed in the midpiece (which is the responsible for generation and maintenance of motility) of species with high GSI values and the size increase in the fibrous sheath and dense fibers, which are structurally and physiologically important in sperm motility, suggest that sperm competition may have influenced the morphological evolution of snake spermatozoa.