CSIRO PUBLISHING www.publish.csiro.au/journals/rfd Reproduction, Fertility and Development, 2006, 18, 797–805 Variation of sperm head shape and tail length in a species ofAustralian hydromyine rodent: the spinifex hopping mouse, Notomys alexis M. Bauer A and W. G. Breed A,B A Discipline of Anatomical Sciences, School of Medical Sciences, Faculty of Health Sciences, The University of Adelaide, Adelaide, SA 5005, Australia. B Corresponding author. Email: bill.breed@adelaide.edu.au Abstract. In Australia, there are around 60 species of murid rodents that occur in the subfamily Hydromyinae, most of which produce highly complex, monomorphic, spermatozoa in which the head has an apical hook together with two ventral processes containing filamentous actin and a long tail of species-specific length. One of the few exceptions to this is the spinifex hopping mouse, Notomys alexis, whose spermatozoa have previously been shown to have pleiomorphic heads. In this study, the structural organisation of the sperm head has been investigated in more detail and the variability in length of the midpiece and total length of the sperm tail has been determined for this species. The findings confirm that pleiomorphic sperm heads are invariably present in these animals and that this variability is associated with that of the nucleus, although nuclear vacuoles were not evident. The total length of the sperm tail, as well as that of the midpiece, was also highly variable both within, as well as between, individual animals. The reason(s) for this high degree of variability in sperm morphology is not known but it may relate to a relaxation of the genetic control of sperm form owing to depressed levels of inter-male sperm competition. Extra keywords: murine rodents, sperm variation. Introduction In most murid rodents, the sperm head is falciform in shape with a bilaterally flattened nucleus, complex acrosome and large perinuclear thecal extension that makes up the per- foratorium (Friend 1936; Clermont et al. 1955; Lalli and Clermont 1981; for review see Breed 2005). Most species of Australian hydromyine rodents have a sperm head struc- ture in which two additional processes, largely composed of cytoskeletal material, the ventral processes, extend from the upper concave surface (Breed 1997). Consequently the morphology of the sperm head of these species is far more complex than that of most other murid rodents. There are, however, a few species in two of the hydromyine genera, Pseudomys and Notomys, where this is not the case. In the former genus, which contains around 25 species, P. novaehol- landiae has spermatozoa with a single apical hook, whereas in P. shortridgei the sperm head is spatulate-shaped, and in P. delicatulus it is pear-shaped (Breed 1997). Within Notomys, although two species, N. cervinus and N. mitchelli, have typical monomorphic sperm populations in which the head has two ventral processes together with a long apical hook, the spinifex hopping mouse, N. alexis, appears to produce highly divergent sperm heads that are very variable in morphology (Breed and Sarafis 1983; Suttle et al. 1988). Whether this variability of sperm head form extends to that of the sperm tail remains to be determined. The aim of the present study is, therefore, to determine in more detail the variation of the sperm head shape of these animals and also to ascertain whether there is an associated variability in the length of the sperm tail. Materials and methods Animals Captive-bred male spinifex hopping mice, Notomys alexis, were obtained from the Nature Education Centre, Norwood, South Australia. The mice were housed under regulated temperature (18–25 ◦ C) and lighting (12 h light : 12 h dark photoperiod) conditions with food and water provided ad libitum. Male spinifex hopping mice become sexually mature at around 70 days of age and, in order to obtain spermatozoa from sex- ually mature males, animals >100 days of age were used. They were killed by exposure to an overdose of carbon dioxide gas, the body mass determined, the testes removed, weighed and spermatozoa obtained from the caudae epididymides and in some cases vasa deferentia. The Animal Ethics Committee of The University of Adelaide approval number for all experiments was M-61-2001A. Transmission electron microscopy of spermatozoa For transmission electron microscopy (TEM), small pieces of cau- dae epididymides were fixed in 2.5% glutaraldehyde and 2.5% paraformaldehyde made up in 0.1 m phosphate buffer (pH 7.4). The tissue was then post fixed with 1% osmium tetroxide, dehydrated by © CSIRO 2006 10.1071/RD06045 1031-3613/06/070797