Résumé
Extant xenarthrans (armadillos, anteaters and sloths) are among the most
derived placental mammals ever evolved. South America was the cradle of
their evolutionary history. During the Tertiary, xenarthrans experienced
an extraordinary radiation, whereas South America remained isolated from
other continents. The 13 living genera are relics of this earlier
diversification and represent one of the four major clades of placental
mammals. Sequences of the three independent protein-coding nuclear markers
alpha2B adrenergic receptor (ADRA2B), breast cancer susceptibility
(BRCA1), and von Willebrand Factor (VWF) were determined for 12 of the 13
living xenarthran genera. Comparative evolutionary dynamics of these
nuclear exons using a likelihood framework revealed contrasting patterns
of molecular evolution. All codon positions of BRCA1 were shown to evolve
in a strikingly similar manner, and third codon positions appeared less
saturated within placentals than those of ADRA2B and VWF. Maximum
likelihood and Bayesian phylogenetic analyses of a 47 placental taxa data
set rooted by three marsupial outgroups resolved the phylogeny of
Xenarthra with some evidence for two radiation events in armadillos and
provided a strongly supported picture of placental interordinal
relationships. This topology was fully compatible with recent studies,
dividing placentals into the Southern Hemisphere clades Afrotheria and
Xenarthra and a monophyletic Northern Hemisphere clade (Boreoeutheria)
composed of Laurasiatheria and Euarchontoglires. Partitioned likelihood
statistical tests of the position of the root, under different character
partition schemes, identified three almost equally likely hypotheses for
early placental divergences: a basal Afrotheria, an Afrotheria + Xenarthra
clade, or a basal Xenarthra (Epitheria hypothesis). We took advantage of
the extensive sampling realized within Xenarthra to assess its impact on
the location of the root on the placental tree. By resampling taxa within
Xenarthra, the conservative Shimodaira-Hasegawa likelihood-based test of
alternative topologies was shown to be sensitive to both character and
taxon sampling.