Intra- and interspecific skull variation in two sister species of the subterranean rodent genusCtenomys(Rodentia, Ctenomyidae): coupling geometric morphometrics and chromosomal polymorphism

The great and rapid process of chromosomal diversification of the genus Ctenomys has highlighted these rodents as an excellent model for studying evolution. Despite their karyotypic diversity, tuco-tucos have a conservative skull form related to their subterranean habit. In this study, 221 karyotyped specimens were analyzed with geometric morphometric methods for dorsal, ventral, and lateral views of the skull in two sister species occurring in southern Brazil and Uruguay: Ctenomys torquatus (2n = 40, 44, and 46) and Ctenomys pearsoni. The geometric descriptors showed significant sexual size dimorphism and significant shape differences between sexes, species, and chromosomal groups. The differences between C. pearsoni chromosomal groups (2n = 66 and 70) were of the same order of magnitude as the interspecific differences. We detected a large unexpected morphometric difference between Brazilian and Uruguayan C. torquatus populations with 2n = 44, showing that the morphometric differentiation is not necessarily associated with karyotypic evolution. In conclusion, geometric morphometrics revealed more skull shape variation than expected, both at the intra- and interspecific levels. We propose that a combination of drift and selection on the skull may be at the origin of this diversity.