SELBY, M.S.*; RENO, P.L.; MCCOLLUM, M. A.; LOVEJOY, C. O; Kent State University; Kent State University; Mercer College; Kent State University: Is colobine thumb reduction pleiotropic?

Colobines typically have shorter thumbs and longer posterior digits than cercopithecines. Functional explanations for thumb reduction in Colobus include reducing thumb interference during forelimb-first landings. However, colobine digit elongation and pollical reduction may be neither developmentally independent, nor separate adaptations. In a diverse sample of cercopithecoids, we measured the third metacarpal, and proximal and intermediate phalanges. Lengths and articular breadths of the radius and humerus were also measured to assess scaling with limb length and body size. These data were analyzed using PCA and t-tests. Component 1 of the PCA loads highly for long bone lengths and provides no separation. However, the two subfamilies did show separation for component 2, where thumb length and digit length load oppositely. These results suggest that cercopithecines and colobines do not differ in relative humerus or distal radius length, but differ significantly in thumb, and posterior metacarpal and digit length. This pattern may result from differences in the expression of Hox and/ or its target genes. Hoxd11 is expressed in both the developing zeugopod and autopod, whereas Hoxa13 and Hoxd13 are restricted to only the autopod. As cercopithecoids do not differ in distal radius length, these differences may be related to Hoxa13 and/ or Hoxd13 expression. In colobines, downregulation of Hoxa13 and/ or Hoxd13 is consistent with shortened thumbs and longer posterior digits due to relaxed competition with Hoxd11 at the growth plate during elongation; indeed, murine Hoxa13 mutants largely phenocopy the colobine first digit morphology. Thus, selection may have primarily targeted posterior digit elongation, with pollical reduction representing merely a pleiotropic effect.