The virtues of gaps: Xenarthran (Edentate) monophyly supported by a uniquedeletion in alpha A-crystallin

Shared insertions or deletions (indels) in protein-coding DNA can be strong indicators of the monophyly of a taxon. A three-amino acid deletion had pr eviously been noted in the eye lens protein alpha A-crystallin of ho specie s of sloths and two species of anteaters, which represent the Pilosa, one o f the two infraorders of Xenarthra (Edentata). This deletion has not been o bserved in 55 spades from 16 other eutherian orders. or in 2 species of mar supials, or in 34 nonmammalian vertebrates, from birds to shark At the geno mic level, we have now detected this deletion in two species of armadillos of the second xenarthran infraorder, Cingulata, as well as in an additional species of anteater. Phylogenetic trees were constructed from a 145-bp seq uence of the alpha A-crystallin gene of 39 tetrapod species, supporting xen arthran monophyly with values from 76% to 90%. To quantify the additional s upport for xenarthran monophyly, as given by die three-residue deletion, we computed the probabilities for the occurrence of this deletion per evoluti onary time unit for alternative hypothetical tree topologies. In the estima tes obtained, the sb trees in which the xenarthran subgroups are unresolved or paraphyletic give an increasingly lower likelihood than do the two tree s that assume xenarthran monophyly. For the monophyletic trees, the probabi lity that the deletion observed in the xenarthrans is due to a single event is > 0.99. Thus, this deletion in alpha A-crystallin gives strong molecula r support for the monophyly of this old and diverse order.