Evolutionary differentation in Ctenomys magellanius: Allozymic, morphological variation and biogeographic considerations of 2 chromosomal forms

The tuco tuco Ctenomys magellanicus is the unique subterranean rodent on Ti erra del Fuego. The complex comprises 2 chromosomal forms that were recogni zed as C. m. magellanicus (2n=36) and C. m. fueguinus (2n=34), whose taxono mic status is discussed. These forms are distributed in small isolates wher e genetic drift, high inbreeding and internal selection operate. Morphologi cal analysis and allozymic variation from 43 adults were used to determine biogeographic and evolutionary differentiation between 2 chromosomal forms. The analyses show karotypic variation but no morphological differentiation and also a significative sexual dimorphism within and between both chromos omal complements. The pattern of general homogeneity suggests a process of speciation with chromosomal species that have not yet reached morphological differentiation.