Chromosome painting in three-toed sloths: a cytogenetic signature and ancestral karyotype for Xenarthra

Abstract

BackgroundXenarthra (sloths, armadillos and anteaters) represent one of four currently recognized Eutherian mammal supraorders. Some phylogenomic studies point to the possibility of Xenarthra being at the base of the Eutherian tree, together or not with the supraorder Afrotheria. We performed painting with human autosomes and X-chromosome specific probes on metaphases of two three-toed sloths: Bradypus torquatus and B. variegatus. These species represent the fourth of the five extant Xenarthra families to be studied with this approach.ResultsEleven human chromosomes were conserved as one block in both B. torquatus and B. variegatus: (HSA 5, 6, 9, 11, 13, 14, 15, 17, 18, 20, 21 and the X chromosome). B. torquatus, three additional human chromosomes were conserved intact (HSA 1, 3 and 4). The remaining human chromosomes were represented by two or three segments on each sloth. Seven associations between human chromosomes were detected in the karyotypes of both B. torquatus and B. variegatus: HSA 3/21, 4/8, 7/10, 7/16, 12/22, 14/15 and 17/19. The ancestral Eutherian association 16/19 was not detected in the Bradypus species.ConclusionsOur results together with previous reports enabled us to propose a hypothetical ancestral Xenarthran karyotype with 48 chromosomes that would differ from the proposed ancestral Eutherian karyotype by the presence of the association HSA 7/10 and by the split of HSA 8 into three blocks, instead of the two found in the Eutherian ancestor. These same chromosome features point to the monophyly of Xenarthra, making this the second supraorder of placental mammals to have a chromosome signature supporting its monophyly.

Chromosome Evolution in Xenarthra: New Insights from an Ancient Group

“…In addition, based on literature data, the chromosome segments that would be homologous between human and B . tridactylus could be deduced [Svartman et al, 2006;Yang et al, 2006, Liu et al, 2011Azevedo et al, 2012]. A summary of the results of these studies is presented in table 2 .…”

“…Nevertheless, its absence in all the other studied species of the group led Azevedo et al [2012] to conclude that it may be a result of convergence, instead of a Xenarthra synapomorphy.…”

“…torquatus and possibly also in B. tridactylus led to the suggestion that this would be a chromosome signature of Xenarthra [Yang et al, 2006;Liu et al, 2011;Azevedo et al, 2012]. This association was not detected in C. hoff-manni and D .…”

“…Besides these 2 chromosome associations, the splitting of HSA8 into 3 blocks was proposed as a further synapomorphy of the group [Svartman et al, 2006;Azevedo et al, 2012]. The other syntenic associations found in Xenarthra are also part of the putative AEK and would thus constitute symplesiomorphies.…”

“…The maned sloth B. torquatus presents the smallest diploid number among sloths (2n = 50) and, although its diploid number is close to those of the other 2 Bradypus species studied, its karyotype presents some morphological chromosome differences [Jorge and Pinder, 1990;Goldschmidt et al, 1995;Azevedo et al, 2012], which is especially interesting in view of its uncertain taxonomic status [Wetzel and Ávila-Pires, 1980;Gardner, 2008] In addition, 2 distinct populations of B . torquatus have been identified by the use of molecular markers [Moraes- Barros, 2006;Lara-Ruiz et al, 2008], but all the karyotyped individuals can be traced to the southernmost population and further sampling from different areas would be interesting to verify if chromosome variations exist between the populations.…”