The chromosomes of the Didelphidae (Marsupialia) and their evolutionary significance

Abstract

One hundred and seventy-seven specimens of American didelphids, representing 9 genera and 22 species have been studied for their chromosomal constitution. Didelphids are very conservative in chromosomal complements. All of the studied species can be sorted into one of three kinds of karyotypes: 2n = 14 (three species of Didelphis, one of Lutreolina, two of Philander, and one of Chironectes), 2n = 14 (eight species of Marmosa. one of Metachims, three of Caluromys, and one of Dromiciops), and 2n = 18 (three species of Monodelphis). These karyotypes are stable, showing only minor variations within each basic pattern. I t is concluded that chromosornals evolution in the Didelphidae proceededs from low numbers to higher numbers by a process of centromeric fissioning complemented by some pericentric inversions and/or translocations. The pattern of karyotypic stability is consistent with bradytely at the organismic level of evolution. This is explained by a low rate of regulatory genetic evolution promoted by epistatic selection favouring the retention of chromosomal arrangements highly advantageous for overall adaptation. 191 192 0. A. RElG ET AL.

Karyotype composition of some rodents and marsupials from Chapada Diamantina (Bahia, Brasil)

“…Cytogenetic analyses can also show the diversity existing in each chromosome complement and corroborate studies of geographic variation among populations. Marsupials normally do not present variation in their autosomal chromosomes, classified in the patterns of 14, 18 or 22 chromosomes (Paresque et al, 2004;Reig et al, 1977;Souza et al, 1990;YonenagaYassuda et al, 1982). On the other hand, Neotropical rodents (Cricetidae, Sigmodontinae in particular) present chromosomal variation within and among populations, species and genera (e.g.…”

Allozyme genetic distances and evolutionary relationships in marsupials of North and South America

“…All differences in electrophoretic mobility were assumed to be of genetic origin and inherited in a Mendelian fashion. All differences in mobility were confirmed in side-by-side comparisons Reig et al (1977) with electromorphs of similar mobility. Therefore, genotypes of each sample were directly determined from differential electromobility.…”

“…Living members of the Didelphidae comprise some 76 species classified in about a dozen genera, which are a prominent element of the Neotropical fauna and represent a wide range of niches within the small carnivore adaptive zone (Gardner 1973, Reig et al 1977, 1985. Therefore, didelphids are an excellent material for the study of evolutionary patterns in a primitive group of mammals.…”

“…In addition to this difficulty, there is the karyotypic homogeneity. All the species of marsupials of the American superfamily Didelphoidea have been found to fall into one of three basically similar karyotypes (Reig et al 1972(Reig et al , 1977. These three different karyotypes are characterized by particular chromosome numbers in the diploid set (14, 18 or 22) and chromosome morphology.…”

“…Therefore, in the present work we report an allozymic study by means of electrophoretic techniques including five South American and one North American didelphid species whose phylogenetic relationships remain unclear. Representatives from the Didelphidae family were also used as a model to test the relationships between the bradytelic condition, the karyotype stability found in this group (Reig et al 1977), and genetic variability. On the other hand, the existence of species with brief life spans such as Lestodelphys halli and Monodelphis dimidiata (Marmosini tribe) and species with long life spans (Didelphini tribe) allowed us to test the hypothesis which correlates generation-time with genetic variability.…”