This paper describes a stable chromosomal polymorphism in a species of tuco-tuco (Ctenomys minutus) and examines the dynamics of a narrow hybrid zone between chromosomal races distributed parapatrically on the coastal plain of southeastern Brazil. A cytogenetic survey studied 132 specimens of C. minutus from 22 trapping sites within the contact zone and surrounding areas. Seven polymorphic sites were identified; at all other sites, 2n was fixed for 2n = 46 or 48. A single chromosomal rearrangement is involved in the observed variation, which served as a diallelic Mendelian character in a Hardy–Weinberg analysis. Pooling polymorphic populations revealed that the mating system is random among genotypes (i.e., no mating bias is present or there is no purifying selection). Observed frequency of the metacentric chromosome follows a clinal variation. The hybrid zone is located along an ecotone, which could imply that exogenous selection is important in its maintenance. However, applying static-cline methodologies revealed weak exogenous selection, and it is possible that the hybrid zone could be the result of a relatively recent secondary contact with a width determined primarily by neutral diffusion. A historical scenario for the time, place or origin, and spread of the species and its chromosomal races is also presented.
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