Karyotypic variation in populations of Ctenomys (Rodentia, Ctenomyidae) from La Pampa Province (Argentina)

The central region of Argentina is occupied by a karyotypically stable grou p of Ctenomys species that form the "mendocinus" complex (C. australis, C. azarae, C. mendocinus, C. porteousi and C. chasiquensis), which contacts wi th populations of C. talarum, C. pundti and intermediate karyomorphs betwee n both species called the "pundti" complex. In this paper, eight population s of the region of reference from La Pampa province were chromosomally stud ied, to help resolve the systematic and evolutionary problems posed by thes e entities. In one population (El Guanaco) a 2n=48, FN=81-82 karyotype was found, which comprised 18 pairs of biarmed and 5 pairs of telocenctric auto somes. The NOR was carried by a small metacentric and the sperm was of the symmetric type. In the rest of the populations, 2n=47 and 2n=48 karyotypes were observed with FNs varying between 71-74. The 2n=48 karyotypes include 14 pairs of biarmed (A complement) and 9 pairs of telocentric (B complement ) autosomes. One exceptional individual had 15 A pairs and 8 B pairs respec tively. The NOR is carried by B5. The 2n=47 karyotype was originated by a c omplex rearrangement that involved the standard A1 and B2 autosomes, produc ing a novel large metacentric. This hypothesis was based on G banding compa risons and in this paper was corroborated by observation on male meiosis of heterozygotes. Additionally a series of polymorphisms involving heterochro matic short arm variation of several autosomal pairs was observed in most p opulations. The sperm was of the asymmetric type. From the observed karyotypes and sperm morphology the studied populations m ay be divided into two groups: a) the El Guanaco population, which can be a ssigned to the subspecies C. talarum talarum, and b) the rest of the popula tions, which clearly belong to the "mendocinus" complex, being closely rela ted to C. azarae and C. porteousi Nevertheless, the geographic boundaries o f the putative species of the "mendocinus" group are poorly defined and rep roductive studies between these populations are needed to assess their spec ific status. The high frequency of 2n=47 heterozygotes for the complex rearrangement (ca . 50%) coupled to the extreme rarity of the homozygous 2n=46 karyotype, sug gests the existence of positive chromosomal heterosis. Although it is clear that the homozygous karyotype is not letal, it is possible that fitness di fferences are involved in the maintenance of the polymorphism perhaps throu gh heterozygote superiority.