Untitled

Abstract

It has been suggested that the karyotype of the marsupials derived from a low diploid number (2n = 14) which originated, through fissions of biarmed chromosomes, the karyotypes with a higher 2n. The telomeric sequence (T2AG3)n was in situ hybridized to the chromosomes of Gracilinanus microtarsus and G. emiliae, Micoureus demerarae and Marmosa murina, species with 2n = 14, in Monodelphis sp., M. domestica, M. kunsi and M. brevicaudata with 2n = 18, and in Lutreolina crassicaudata, Didelphis albiventris, Chironectes minimus, Philander opossum and P. frenata, all of them with 2n = 22. The probe hybridization occurred in the telomeric regions of both arms, short and long, of all chromosomes of the complement of all individuals of all species analysed. However, in some pairs of the karyotypes of Gracilinanus microtarsus and Micoureus demerarae (with 2n = 14), and in Monodelphis sp., M. domestica, M. kunsi and M. brevicaudata (2n = 18) ectopic signs of hybridization were detected proximal to the centromeres, suggesting the retention of this telomeric sequence in the centromeric regions of some chromosomes of these species. Based on these results, it is proposed that the karyotype of marsupials evolved from a 2n = 22 to a 2n = 14, by means of chromosomal fusions.

American marsupials chromosomes: why study them?

“…The finding of interstitial telomeric sequences (ITS) in the autosomes of Didelphidae species with 2n = 14 and 2n = 18, but not in those with 2n = 22, led to the suggestion that the karyotype of South American marsupials would have proceeded through centric fusions leading to the reduction in diploid number from an ancestral complement with at least 22 chromosomes (Svartman and Vianna-Morgante, 1998; Carvalho and Mattevi, 2000). This hypothesis defied the idea of the “ancestral” 2n = 14 marsupial karyotype which had prevailed for the previous three decades.…”

“…The results in the eight 2n = 14 species analyzed were more variable: four species had no ITS ( Caluromys philander , Metachirus nudicaudatus ; Marmosa murina and Gracilinanus emiliae ), Gracilinanus microtarsus had ITS on pair 1, Micoureus demerarae had a maximum of eight signals on the four largest autosome pairs, Marmosops incanus had ITS on pairs 1-5 and M . parvidens presented ITS in all of its six autosomal pairs (Svartman and Vianna-Morgante, 1998; Carvalho and Mattevi, 2000; Pagnozzi et al , 2000, 2002). …”

“…Although the fusion hypothesis was clearly advanced to account for the karyotypic evolution of the three diploid numbers in American marsupials (Svartman and Vianna-Morgante, 1998; Carvalho and Mattevi, 2000) Metcalfe et al (2004) studied the distribution of ITS and of constitutive heterochromatin in nine species of Australian marsupials to test this view. Three of the species analyzed presented the presumed “ancestral” 2n = 14 karyotype and the other six species were Macropodinae with varying diploid numbers which had been previously extensively studied.…”

Reconstruction of the ancestral marsupial karyotype from comparative gene maps

“…If the marsupial ancestor had a 2n = 22 chromosome complement, it would probably resemble karyotypes of members of Family Didelphidae with 22 chromosomes, since the American marsupials are at the base of the marsupial phylogenetic tree [13,18]. The strongest evidence for the higher ancestral number is the presence of interstitial telomere signals in members of Didelphidae with 2n = 18 or 2n = 14 karyotypes, suggesting that lower diploid number karyotypes were derived by chromosome fusions, ultimately leading to the 2n = 14 karyotype commonly found amongst marsupials, and recognised as the basal karyotype of Australidelphia [12,19]. However, Pagnozzi et al [20,21] observed that these interstitial signals coincide with constitutive heterochromatin, and proposed that they actually represent satellite DNA rather than telomeric sequence, as was also concluded for an Australian species with a 2n = 14 karyotype [22].…”

Tracing the evolution of amniote chromosomes

“…Cross-species chromosome painting demonstrated that, despite the commonality of the 2 n  = 22 diploid number across different marsupial families, the 18 autosomal segments detected were arranged differently between 2n = 22 species (O’Neill et al 1999; Rens et al 2003). The main evidence to support the 2 n  = 22 hypothesis was the observation of interstitial telomere signals in members of the Family Didelphidae, a family representing one of the earliest offshoots of the marsupial lineage, with 2 n  = 14 or 2 n  = 18 karyotypes, suggesting that chromosome fusions were responsible for these lower diploid numbers (Carvalho and Mattevi 2000; Svartman and Vianna-Morgante 1998). The inability to compare marsupial chromosome arrangement to outgroup species using chromosome painting prevented further testing of either hypothesis.…”