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

Abstract

The central region of Argentina is occupied by a karyotypically stable group of Ctenomys species that form the "mendocinus" complex (C. australis, C. azarae, C. mendoci-nus, C. porteousi and C. chasiquensis), which contacts with populations of C. talarum, C. pundti and intermediate karyomorphs between both species called the "pundti" complex. In this paper, eight populations of the region of reference from La Pampa province were chromosomally studied, to help resolve the systematic and evolutionary problems posed by these 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 autosomes. 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 respectively. The NOR is carried by B5. The 2n=47 karyotype was originated by a complex rearrangement that involved the standard Al and B2 autosomes, producing a novel large metacentric. This hypothesis was based on G banding comparisons and in this paper was corroborated by observation on male meiosis of heterozygotes. Additionally a series of polymorphisms involving heterochromatic short arm variation of several autosomal pairs was observed in most populations. The sperm was of the asymmetric type. From the observed karyotypes and sperm morphology the studied populations may be divided into two groups: a) the El Guanaco population, which can be assigned to the subspecies C. talarum talarum, and b) the rest of the populations, which clearly belong to the "mendocinus" complex, being closely related to C. azarae and C. porteousi. Nevertheless, the geographic boundaries of the putative species of the "mendocinus" group are poorly defined and reproductive studies between these populations are needed to assess their specific 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, suggests the existence of positive chromosomal heterosis. Athough it is clear that the homozygous karyotype is not le-tal, it is possible that fitness differences are involved in the maintenance of the polymorphism perhaps through heterozygote superiority.

Key words:

• chromosomal rearrangements
• Ctenomys
• karyotype
• male meiosis