Chapter 14: Sperm evolution in Dasypodids

P. CETICA AND M. S. MERANI

Biology of the Xenarthra

University Press of Florida

Año: 2008;

The spermatozoon is an extremely specialized cell and its morphology would seem to have been modeled for fertilization of the oocyte.  In eutherian spermatozoa a high degree of uniformity is observed in sperm shape and dimensions.  Most species present an oval head and an average length of 70 mm, a pattern proposed as the basic ancestral type.  Derived spermatic shapes have been described mainly in various groups of rodents.  However, we have documented an important evolutionary diversification in dasypodid spermatozoa.  Based on analyses of sperm shape and sizes we have identified four different spermatic groups.  Sperm in group 1, represented by the genus Dasypus, present the typical ancestral mammalian sperm type.  The remaining sperm types are derived.  Sperm patterns for group 3 (Priodontes, Cabassous) and group 4 (Chaetophractus, Zaedyus, Euphractus) present a very peculiar head, in the shape of a shovel or spoon respectively.  These heads are very large and extremely thin, with length and width measurements typically greater than those of other mammals.  Sperm from group 2 (Tolypeutes) are unique compared to the others; the sperm head is shaped like a spoon but also presents a particular laterocaudal extension, giving it a notable and characteristic asymmetry.  This evolutionary diversification of dasypodid spermatozoa implies strong selective pressures affecting the morphology of sperm cells of different armadillo genera.  We observed an association between the morphology of female genital tracts and the sperm groupings described above.  We conclude that particular structural characteristics of female reproductive tracts have distinct relationships with sperm morphology in each genus of Dasypodidae.