CYTONUCLEAR GENETIC-STRUCTURE OF A HYBRID ZONE IN LIZARDS OF THE SCELOPORUS-GRAMMICUS COMPLEX (SAURIA, PHRYNOSOMATIDAE)

Lizards of the Sceloporus grammicus complex are comprised of multiple chromosome races that form several zones of parapatric hybridization i n central Mexico. We scored diagnostic mitochondrial DNA (mtDNA) haplo types and autosomal chromosome markers in a sample of 342 lizards from one well-defined zone between 2n = 34 and 2n = 46 races. A two-part a nalysis was performed on this data set in an attempt to infer the pred ominant evolutionary forces shaping the cytonuclear structure of this zone. The complications posed by its spatial structure were addressed by analysing a hierarchical series of smaller subsamples chosen to app roximate single mating units. Two critical conclusions were drawn from this first-stage analysis. First and foremost, the three chromosomes have largely concordant cytonuclear disequilibrium patterns within eac h subsample with adequate numbers of individuals for detecting nonrand om cytonuclear associations. This suggests that the cytonuclear struct ure of this zone is predominantly a result of deterministic genome-wid e forces rather than genetic drift or deterministic forces specific to individual chromosomes or loci. Second, the fit of a series of migrat ion models to the data shows that the cytonuclear structure of the sub samples is well accounted for by continued gene flow from the two pare ntal races alone, with random mating with respect to cytonuclear genot ype and no other evolutionary forces. These results motivate several f urther empirical and theoretical investigations to refine our understa nding of the relative roles of migration and other potentially importa nt forces such as natural selection and genetic drift, in this and oth er hybrid zones.