THE GENETIC-STRUCTURE OF A HYBRID ZONE BETWEEN 2 CHROMOSOME RACES OF THE SCELOPORUS-GRAMMICUS COMPLEX (SAURIA, PHRYNOSOMATIDAE) IN CENTRAL MEXICO

The F5 (2n = 34) and FM2 (2n = 44-46) chromosome races of the Scelopor us grammicus complex form a parapatric hybrid zone in the Mexican stat e of Hidalgo, characterized by steep concordant dines among three diag nostic chromosome markers across a straight-line distance of about 2 k m. Here, we show that this zone is actually structured into local patc hes in which hybridization extends over an extremely irregular front. The distribution of hybrid-index (HI) scores across the transect revea ls some hybridization at almost all localities mapped in a central 7 k m x 3 km area. Pooling the central samples produces both a strong hete rozygote deficit for all diagnostic markers and strong linkage disequi libria between all pairwise combinations of these (unlinked) markers. Moreover, a highly significant association exists between the habitat on which each individual was caught and its karyotype (F5 chromosomes are more likely to be found on oak). Analysis of genotype frequencies over a range of spatial scales shows that there is no significant hete rozygote deficit or habitat association within local areas of less tha n about 200 m; however, there is significant linkage disequilibrium ov er the smallest scales (R = D (pquv)(1/2) = 0.29. support limits, 0.18 -0.36) over 100 m. These patterns suggest that lizards mate and choose habitats randomly within local patches. This conclusion is supported by mark-recapture estimates of dispersal (approximate to 80 m in a gen eration) and by inference of matings from embryo and maternal karyotyp es. Closer examination of the two-dimensional pattern reveals a convol uted cine for all three markers, with a width of 830 m (support limits 770 m-930 m). This dine width, combined with the strength of local li nkage disequilibrium, implies a dispersal rate of sigma = 160 m in a g eneration and an effective selection pressure of 30% on each chromosom e marker. The proportion of inviable embryos is greater in females fro m the center of the hybrid zone; this is caused by effects associated with both karyotype and location. The hybrid zone is likely to be main tained by selection against chromosomal heterozygotes, by other kinds of selection against hybrids, and by selection adapting the chromosome races to different habitats. The structure of the contact may be caus ed by both random drift and by selection in relation to habitat.