Molecular evolution and phylogeny of the buzzatii complex (Drosophila repleta group): A maximum-likelihood approach

The buzzatii complex of the mulleri subgroup (Drosophila repleta group) con sists of three clusters of species whose evolutionary relationships are poo rly known. We analyzed 2,085 coding nucleotides from the xanthine dehydroge nase (Xdh) gene in the 10 available species of the complex and Drosophila m ulleri and Drosophila hydei. We adopted a statistical model-fitting approac h within the maximum-likelihood (ML) framework of phylogenetic inference. W e first modeled the process of nucleotide substitution using a tree topolog y which was reasonably accurate. Then we used the most satisfactory descrip tion so attained to reconstruct the evolutionary relationships in the buzza tii complex. We found that a minimally realistic description of the substit ution process of Xdh should allow six substitution types and different subs titution rates for codon positions. Using this description we obtained a st rongly supported, fully resolved tree which is congruent with the already-k nown (yet few) relationships. We also analyzed published data from three mi tochondrial cytochrome oxidases (CO I, II, and III). In our analyses, these relatively short DNA sequences failed to discriminate statistically among alternative phylogenies. When the data of these three gene regions are comb ined with the Xdh sequences, the phylogenetic signal emerging from Xdh beco mes reinforced. All four of the gene regions evolve faster in the buzzatii and martensis clusters than in the stalkeri cluster, paralleling the amount of chromosomal evolution.