Novel 12S mtDNA findings in sloths (Pilosa, Folivora) and anteaters (Pilosa, Vermilingua) suggest a true case of long branch attraction

Abstract

We sequenced 12S RNA mtDNA for the majority of the extant species of sloths and anteaters and compared our results with previous data obtained by our group using 16S RNA mtDNA in the same specimens and to GenBank sequences of the extinct giant sloth Mylodon. Our results suggest that pigmy-anteaters may be a case of the long-branch attraction phenomenon and also show the large genetic difference between the Amazonian and Atlantic forest three-toed sloths, contrasting with the small differences observed between the two non-Atlantic forest forms of sloths. These results have important implications for the taxonomy of sloths and anteaters and strongly suggest the placement of pigmy anteaters in their own family (Cyclopidae) and raising the taxonomic status of Bradypus torquatus to a genus.Key words: anteaters, molecular evolution, mylodont, sloths, Xenarthra. In the past, xenarthrans (armadillos, anteaters and sloths) were one of the most diverse groups in South America with more than 200 known fossil genera (MacKenna and Bell, 1997) but there are now only 13 genera and 30 extant species. Almost all xenarthrans endemic to South and Central America, the exception being the armadillo (Dasypus novemcinctus), which occurs in North America.Sloths almost became extinct about ten thousand years ago, with only two genera and five species still surviving in Central and South America. MacKenna and Bell (1997) placed sloths into two infraorders consisting of the extinct Mylodon and the Megatheria, the latter being considered as an assemblage of extinct families, the Megatheriidae, and the extant families Bradypodidae, containing the genus Bradypus composed of only three species (B. tridactylus, B. variegatus and B. torquatus), and Megalonychidae, containing the genus Choloepus with only two species (C. didactylus and C. hoffmanni).The anteaters, the closest partner to the sloths, show great variation in size between genera, ranging from 16 kg to 23 kg for the strictly terrestrial giant anteater Myrmecophaga tridactyla, to 3.8 kg to 8.5 kg for the medium-sized semi-arboreal anteater Tamandua tetradactyla and 155 g to 275 g for the diminutive arboreal pigmy anteater Cyclopes didactyla, all of which are included in the family Myrmecophagidae.Despite distinctive morphology and specialization, anteaters and sloths are considered as a monophyletic assemblage, the Pilosa clade, as supported by morphological (Engelmann, 1985;Gaudin, 2004;MacKenna and Bell, 1997;Paterson et al., 1992) and molecular evidence (Delsuc et al., 2001). Delsuc et al. (2004) applied Bayesian analysis to three nuclear genes (5130 sites) and obtained a molecular time scale for the evolution of extant xenarthrans and other mammals indicating that the age of the Pilosa node is around 55 megaannum (Ma, equal to 10 6 years) before the present (BP), with node ages of 40 Ma BP for anteaters and 21 Ma BP for sloths.The majority of sloths became extinct about ten thousand years ago, so obtaining an understanding of the phylogenetic relationships between extinct ...

Morphology, molecular phylogeny, and taxonomic inconsistencies in the study ofBradypussloths (Pilosa: Bradypodidae)

“…The inferred split time between B. tridactylus and B. variegatus was between 6.6 and 4.8 mya, depending on the calibration point (21-18 mya). This date differs from the 0.4 mya estimated by Barros et al (2003Barros et al ( , 2008 using 16S and 12S mtDNA sequences. Considering the results presented here, we assume that the date obtained by Barros et al (2003Barros et al ( , 2008 applies to the divergence between 2 mitochondrial lineages of B. variegatus.…”

“…The observed divergences do not impose errors in the phylogenetic discussion of Stanhope et al (1998), because the DNA sequence was used only to represent a threetoed sloth, which is correct. However, the assumption that any of these DNA sequences represent an mtDNA lineage of B. tridactylus is mistaken.According to our analysis of the control data set of concatenated Cytb and 16S, the split between mtDNA lineages of B. torquatus and those of the remaining Bradypus occurred about 12 mya, a date corresponding to that obtained by Barros et al (2003Barros et al ( , 2008. The inferred split time between B. tridactylus and B. variegatus was between 6.6 and 4.8 mya, depending on the calibration point (21-18 mya).…”

“…Analyses of population genetics data have been reported for only 2 sloth species, B. torquatus and B. variegatus (Lara-Ruiz et al 2008; MoraesBarros et al 2006 MoraesBarros et al , 2007. The remaining molecular data generated for three-toed sloths are mostly from studies describing the placement of Xenarthra in eutherian phylogeny (Arnason et al 1997;Eizirik et al 2001;Murphy et al 2007;Prasad et al 2008) or on sloth phylogeny (Barros et al 2003(Barros et al , 2008Greenwood et al 2001;Poinar et al 2003). In GenBank ,11% of the total nonprimate eutherian DNA data available (until February 2010) are from xenarthrans.…”

“…We estimated divergence times for the main Bradypus lineages to compare our data with published studies on split times between B. variegatus and B. tridactylus. Barros et al (2003Barros et al ( , 2008 used mitochondrial 16S and 12S genes to estimate a split between B. variegatus and B. tridactylus of about 400,000 years ago. We used the maximum-likelihood phylogeny based on control data set of concatenated Cytb (471 bp) and 16S (498 bp) gene segments.…”

“…Estimates of divergence times were obtained using the penalized likelihood method (Sanderson 2002) implemented with program R8s 1.7.1 (Sanderson 2003). Not having fossil records of arboreal sloths or the mitochondrial DNA (mtDNA) substitution rate for Bradypus, we used the same criteria of Barros et al (2003Barros et al ( , 2008 to calibrate the tree and estimate divergence times. Our calibration point was the split between Choloepus and Bradypus, which occurred 21-18 million years ago (myaDelsuc et al 2001(myaDelsuc et al , 2004.…”