PHYLOGENETIC ANALYSIS OF REPTILIAN HEMOGLOBINS - TREES, RATES, AND DIVERGENCES

Phylogenetic relationships among reptiles were examined using previous ly published and newly determined hemoglobin sequences. Trees reconstr ucted from these sequences using maximum-parsimony, neighbor-joining, and maximum-likelihood algorithms were compared with a phylogenetic tr ee of Amniota, which was assembled on the basis of published morpholog ical data. All analyses differentiated alpha chains into alpha(A) and alpha(D) types, which are present in all reptiles except crocodiles, w here only alpha(A) chains are expressed. The occurrence of the alpha(D ) chain in squamates (lizards and snakes only in this study) appears t o be a general characteristic of these species. Lizards and snakes als o express two types of beta chains (beta I and beta II), while only on e type of beta chain is present in birds and crocodiles. Reconstructed hemoglobin trees for both alpha and beta sequences did not yield the monophyletic Archosauria (i.e., crocodilians + birds) and Lepidosauria (i.e., Sphenodon + squamates) groups defined by the morphology tree. This discrepancy, as well as some other poorly resolved nodes, might b e due to substantial heterogeneity in evolutionary rates among single hemoglobin lineages. Estimation of branch lengths based on uncorrected amino acid substitutions and on distances corrected for multiple subs titutions (PAM distances) revealed that relative rates for squamate al pha(A) and alpha(D) chains and crocodilian beta chains are at least tw ice as high as those of the rest of the chains considered. In contrast to these rate inequalities between reptilian orders, little variation was found within squamates, which allowed determination of absolute e volutionary rates for this subset of hemoglobins. Rate estimates for h emoglobins of lizards and snakes yielded 1.7 (alpha(A)) and 3.3 (beta) million years/PAM when calibrated with published divergence time vs. PAM distance correlates for several speciation events within snakes an d for the squamate <-> sphenodontid split. This suggests that hemoglob in chains of squamate reptiles evolved similar to 3.5 (alpha(A)) or si milar to 1.7 times (beta) faster than their mammalian equivalents. The se data also were used to obtain a first estimate of some intrasquamat e divergence times.