On the morphology of spermatozoa of tuco‐tucos, Ctenomys (Rodentia: Ctenomyidae): New data and its implications for the evolution of the genus

Abstract

Sperm morphology was studied in 10 species of the caviomorph rodent Ctenomys. Ctenomys argentinus, C. conoueri, C. dorbigny and C.perrensis had symmetric spermatozoa with paddle‐like heads. Ctenomys australis, C. mendocinus, C. porteousi, C. rionegrensis and Ctenomys sp., on the other hand, had spermatozoa with paddle‐like heads but with the tail inserted at one side of the central axis and a nuclear caudal extension originating from the base of the head at the opposite side of the insertion of the tail and running parallel to the flagellum; these spermatozoa are referred to as simple‐asymmetric. In C. yolandae, a complex‐asymmetric morphological type not previously described for the genus was found. This type is characterized by the presence of two nuclear caudal extensions. Symmetric spermatozoa (total length = 52 pm) were shorter than asymmetric (both simple and complex) ones (total length = 87 pm). In spite of these differences, the relative size of heads, midpieces and tails were maintained in the three groups, representing 12%, I I YO and 88% of the average total length, respectively. Within each group of species bearing the same sperm type, a low interspecific variability both in morphological patterns and dimensions of sperm cells was observed. This low interspecific variability associated with the north to south geographical distribution of species having, respectively, symmetric and asymmetric spermatozoa, suggests that these characters appeared at an early stage in evolution of the group, and probably played an important role in the first steps of speciation by promoting reproductive isolation.

Species groups and the evolutionary diversification of tuco-tucos, genusCtenomys(Rodentia: Ctenomyidae)

“…This supports the suggestion of D 'Elía et al (1999) and Slamovits et al (2001) that the asymmetric morph appeared more than once in the evolutionary history of Ctenomys. The uncertain position of C. maulinus, a species with the asymmetric sperm, and the lack of sequence data for C. yolandae, which has a third sperm morphology (Vitullo et al 1988), hinders the recovery of the evolutionary history of this variable trait. Similarly, additional work is needed to understand the chromosomal evolution of the genus and the general pattern of diversification.…”

“…More recent attempts were based on allozyme frequencies (Sage et al 1986), karyotypes (Reig and Kiblisky 1969), penial morphology (Altuna and Lessa 1985;Balbontin et al 1996), and sperm morphology (Feito and Gallardo 1982). Vitullo et al (1988) proposed that different sperm variants appeared early in the radiation, possibly indicating a natural subdivision of the genus. D 'Elía et al (1999) noted that the group of species with asymmetric sperm is diphyletic in a phylogeny based on mitochondrial (mt)DNA sequences.…”

Geographical variation of heterochromatin in <i>Ctenomys flamarion</i><i>i</i> (Rodentia-Octodontidae) and its cytogenetic relationships with other species of the genus

“…Ctenomys talarum is distinct from this group because only 44% of its G-bands are homologous with the G-bands of the mendocinus species, its skull morphology differs (Massarini et al, 1991), and its sperm are symmetrical (Vitullo et al, 1988). Ctenomys rionegrensis (from Uruguay) was not studied in great detail, but its diploid number (2n = 48), constitutive hetero chromatin pattern, and asymmetrical sperm show that it be longs to the mendocinus group ( .…”

“…In the Bolivian species C. conoveri. the centromeric C-bands are small (Anderson et al, 1987) and the sperm symmetrical (Vitullo et al, 1988).…”

The dental evidence on the differentiation of the ctenomyine rodents (Caviomorpha, Octodontidae, Ctenomyinae)

“…Spotorno et al (1995) recently suggested that the Octodontoidea (exclusive of the Echimyidae) might have originated in the central Andes. To support this hypothesis for the ctenomyine, Spotorno et al (1995) cite the more primitive sperm morphology of the Ctenomys species of this area, in comparison to those living southward (see Feito and Gallardo 1982, Vitullo et al 1988, Vitullo and Cook 1991, Gallardo 1991, Roldan et al 1992. However, Ctenomys is the most recent genus in the subfamily (Verzi 1994), recorded only since the Upper Marplatan (Early Pleistocene, 2.4 Ma; Cione and Tonni 1995).…”