MITOCHONDRIAL-DNA SEQUENCE DIVERGENCE AND PHYLOGENETIC-RELATIONSHIPS AMONG 8 CHROMOSOME RACES OF THE SCELOPORUS-GRAMMICUS COMPLEX (PHRYNOSOMATIDAE) IN CENTRAL MEXICO

A 2,479-base pair mitochondrial DNA fragment was sequenced for eight c hromosome races of Sceloporus grammicus from central Mexico to estimat e their phylogenetic relationships. The species S. poinsetti and S. ol ivaceus 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 div ergence values ranged from 0.0 to 0.23 among the ingroup taxa, and a m aximum of 0.26 was observed between ingroup and outgroup taxa. Alterna tive analyses based upon equally weighted characters and several alter native character-weighting options were used to obtain phylogenetic hy potheses for the complex, and a single most-parsimonious tree was sele cted from among these on the basis of a new character-weighting method that takes into account the observed frequencies of all 12 possible s ubstitutions for protein genes. The most-parsimonious cladogram showed that chromosomal evolution in this complex has been more complicated than previously hypothesized. Several rearrangements (Robertsonian fis sions) have evolved independently on two or more occasions, which corr oborates evidence from other studies showing that single rearrangement s are not underdominant upon their origin, and their fixation probabil ities are enhanced by repeated origins. These observations refute expe ctations of some general models of chromosome evolution. The same phyl ogenetic hypothesis was used to test the minimum-interaction model of chromosome evolution and a specific model for the evolution of macroch romosome 2. A clear distinction was also possible among alternative hy potheses of relationship for three chromosome races involved in hybrid ization, and the consequences for the role of chromosomal rearrangemen ts in reducing gene now are discussed in this context.