Comparative sperm morphometry in three South American snake taxa: considerations on sperm competition theories

TOURMENTE, MAXIMILIANO; GUIDOBALDI, A.; GIOJALAS, LAURA C.; CHIARAVIGLIO, MARGARITA

Tampa Bay

Congreso; 21st Joint Meeting of Ichthyologists and Herpetologists; 2005

Society for the Study of Amphibians and Reptiles

The extensive variation in sperm size, even among close related species, is probably a reflection of both the risk of sperm competition and the environment in which it takes place. The evolutive significance of such variation considering postcopulatory sexual selection remains poorly understood as the production of long sperm can be costly and the advantage of sperm size remains enigmatic. An increment in sperm size may either increase its longevity if size signifies energy reserve, or trade it off with swimming speed by increasing tail length plus its associated mitochondria in the mid-piece. Sperm competition may also select for bigger sperm because they are able to displace smaller competing sperm or serve as an indicator of male genetic quality. Snakes show reproductive characteristics like promiscuous mating systems, long and short term sperm storage and unique structural spermatic features, being excellent study subjects to test sperm competition hypotheses. The objective of this work was to detect variations in sperm morphometry across snake taxa and to relate it to differential sperm competition pressures. The length of the head, acrosome, midpiece and principal piece of spermatozoa of three snake taxa: Boinae, Crotalinae and Xenodontinae, was measured using phase contrast and fluorescence light microscopy. Data were controlled for normality and a parametric analysis of variance (ANAVA) was performed for each variable. Pairwise differences among means were tested a posteriori. Although head length did not varied significatively among taxa, the acrosome, principal piece and particularly midpiece length showed significative variation. The high variation degree detected in midpiece length would have an important influence in the metabolic features of these cells, as this region is the responsible for generation and maintenance of motility. These snake species also show differences in life histories, reproductive cycles and mating systems. Thus, it would be possible that variations in sperm length, especially in the midpiece and tail regions, were influenced by sperm competition risk derived from such differences.