Mitochondrial DNA Sequence Divergence and Phylogenetic Relationships among Eight Chromosome Races of the Sceloporus Grammicus Complex (Phrynosomatidae) in Central Mexico

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Abstract
A 2,479-base pair mitochondrial DNA fragment was sequenced for eight chromosome races of Sceloporus grammicus from central Mexico to estimate their phylogenetic relationships. The species S. poinsetti and S. olivaceus were used separately as alternative outgroups. A total of 795 positions varied in three complete protein-coding genes examined (ND3, ND4L, ND4), and 52 of 292 positions varied across five transfer RNAs examined (glycine, argenine, histidine, serine, leucine). Sequence divergence values ranged from 0.0 to 0.23 among the ingroup taxa, and a maximum of 0.26 was observed between ingroup and outgroup taxa. Alternative analyses based upon equally weighted characters and several alternative character-weighting options were used to obtain phylogenetic hypotheses for the complex, and a single most-parsimonious tree was selected from among these on the basis of a new character-weighting method that takes into account the observed frequencies of all 12 possible substitutions for protein genes. The most-parsimonious cladogram showed that chromosomal evolution in this complex has been more complicated than previously hypothesized. Several rearrangements (Robertsonian fissions) have evolved independently on two or more occasions, which corroborates evidence from other studies showing that single rearrangements are not underdominant upon their origin, and their fixation probabilities are enhanced by repeated origins. These observations refute expectations of some general models of chromosome evolution. The same phylogenetic hypothesis was used to test the minimum-interaction model of chromosome evolution and a specific model for the evolution of macrochromosome 2. A clear distinction was also possible among alternative hypotheses of relationship for three chromosome races involved in hybridization, and the consequences for the role of chromosomal rearrangements in reducing gene flow are discussed in this context.

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