A comparison of nuclear and mitochondrial dine shapes in a hybrid zone in the Sceloporus grammicus complex (Squamata : Phrynosomatidae)

The F5 and FM2 chromosome races of the Sceloporus grammicus complex form a hybrid zone in the Mexican state of Hidalgo. Previous studies of this zone have assessed genetic structure by averaging estimates of shape and width a cross three diagnostic chromosome markers. This approach is likely to mask subtle differences in cline shape among loci (e.g. selected vs. neutral), a nd obscure any displacement of cline centres (if present). Here we use maxi mum likelihood methods to construct the best fitting individual clines for three chromosomal markers, and also add two new markers; the mitochondrial DNA (mtDNA) locus, and the nuclear ribosomal DNA (rDNA) repeat. For each lo cus, hybrid zone models were fitted by cline shape and width, and the posit ion and number of segments describing the centre of the zone. Pairwise comp arisons between all clines revealed concordance between chromosomes 2 and 6 , but significant discordance in cline structure among all other paired com binations. The concordance of chromosomes 2 and 6 suggests that these cline s are maintained by genome-wide forces. The discordance of the chromosome 1 cline suggests an influence of asymmetric introgression, while the mtDNA c line is probably influenced by selection and drift. The rDNA locus reveals a pattern best explained by either extreme asymmetric introgression or gene conversion. The structure of zone indicates that genome-wide processes and locus specific selective forces as well as drift, are operating to differe nt degrees on different loci. The locus-by-locus approach used here permits a finer discrimination among possible mechanisms responsible for the maint enance of the individual clines.