Sperm ultrastructure of Tarsius bancanus (Tarsiidae, Primates): implications for primate phylogeny and the use of sperm in systematics.

Abstract

The ultrastructure of the epididymal sperm of Tarsius bancanusis described. The sperm possess the typical eutherian pattern of a dorsoventrally flattened, ovate sperm head, comprising a nucleus capped by a symmetrical acrosome, and a distinct midpiece and principal piece containing a 9 + 9 + 2 arrangement of outer coarse fibres and microtubules. However, unique features are also present. The overall head length (9 μm) equals the greatest for any primate yet examined, and the subacrosomal space ("perforatorium" or "pseudoperforatorium") tilted at 30° to the sagittal axis of the sperm, is described for the first time for mammals. The acrosome extends only a short distance beyond the length of the nucleus of the mature sperm, and a significant reduction in the acrosome to nuclei ratio appears to occur during the final stages of sperm maturation. In contrast to earlier predictions based on the spermatid of Tarsius syrhicta, the mature spermatozoa of Tarsiusshows greatest morphological similarity with the sperm of the Anthropoidea, which have a short symmetrical acrosome, than with the Strepsirhini, which have a relatively long acrosome that can be either symmetrical (Lemuriformes) or asymetrical (Lorisiformes). Four proposed phylogenies of the Primates are assessed using comparative sperm ultrastructure. Placing the Tarsiidae as a sister group to the Lorisidae appears the least likely. The sperm data are consistent with the Tarsiidae being a sister group to the Anthropoidea, to the Strepsirhini, or even to the extant primate groups as a whole. Use of sperm morphology to provide characters in phylogenetic systematics of the primates is discussed, and the principle of "total evidence" is preferred to the common practice of "hanging" sperm on phylogenetic hypotheses based on other evidence.