PARAPATRIC HYBRIDIZATION BETWEEN CHROMOSOME RACES OF THE SCELOPORUS-GRAMMICUS COMPLEX (PHRYNOSOMATIDAE) - STRUCTURE OF THE TULANCINGO TRANSECT

Genetic and morphological studies were carried out on a total sample o f 108 individuals of the Sceloporus grammicus complex, collected from eight sites along a transect in eastern Hidalgo, Mexico. Samples were collected from both sides and through the middle of a zone of parapatr ic hybridization between F5 (2n = 34) and FM2 (2n = 46) chromosome rac es of this group to assess general patterns of gene flow across the zo ne for independent sets of markers. Karyotypic and morphological data were collected from almost all lizards; and allozyme, mitochondrial-DN A and ribosomal-DNA restriction site data were collected from a subset of the total. Three unambiguous diagnostic chromosome markers defined the zone by showing sharp, concordant changes from one parental race to the other over a distance of about 2 km. Virtually no introgression of any of these markers occurred outside of localities immediately ad jacent to the point of contact, and this transition was also reflected in diagnostic classes of mtDNA haplotypes, albeit slightly asymmetric al in the direction of F5 introgression. Ribosomal-DNA genotypes intro gressed more extensively in both directions. No fixed genic difference s were found between these cytotypes in a sample of 42 loci, but alloz yme frequencies in five polymorphic loci displayed frequency shifts ac ross the transect, as did the distribution of mtDNA haplotypes. Morpho logically, chromosomally pure individuals in the zone could be correct ly assigned to their appropriate parental populations outside of the z one with a high degree of confidence. F1 and backcross individuals wer e morphologically intermediate in multivariate analyses, although they overlapped with both groups of parental phenotypes, and very few sing le traits displayed intermediacy. The structure and shape of the diagn ostic character clines are discussed with respect to the origin and ma intenance of this zone.