Chromosome, allozyme, and morphometric variation in three cytoypes of the Sceloporus grammicus complex

Abstract

The iguanid lizard Sceloporus qrammicus consists of at least six chromosome "races" (=cytotypes) distributed throughout most of Mexico. Three of these cytotypes (2n=32, 34, and 36) were studied in detail in that part of the range extending from the Mexican states of Queretaro and Guanajuato north into southern Texas. Studies of chromosomal variation among the cytotypes revealed high frequencies of both fusion/fission and pericentric inversion polymorphisms. These results extend earlier findings and confirm the presence of several fusion/fission polymorphisms in all cytotypes. The occurrence of these polymorphisms throughout ancestral and derived cytotypes suggests that chromosomal change in S. qrammicus is very recent, and that the polymorphisms are not negatively heterotic. Limited meiotic data indicate that crossing over does not occur within the regions of pericentric inversions. The distribution of each cytotype includes several habitat types, ranging from thorn scrub communities near sea level to montane forest zones on some of the highest peaks in Mexico. Starch-gel electrophoresis was used to assay 19 presumptive genetic loci encoded by 14 enzymatic and non-enzymatic proteins across 13 samples of S. grammicus representing all cytotypes. Results showed high levels of heterozygosity and minimal genetic divergence associated with the chromosomal change. At most loci the same allele was either fixed or dominant across all samples regardless of cytotype. No genic area effects were demonstrated. Geographic variation was assessed in 6 multistate, 17 meristic, and 14 morphometric characters among 18 samples of S. qrammicus representing all three cytotypes. Univariate and multivariate statistical treatments showed no appreciable morphological divergence among the cytotypes. No morphological area effects were evident. The data are used to falsify predictions of the stochastic models of chromosomal evolution. The chromosome data may also falsify one prediction of the canalization model. Alternative modes of chromosome evolution in S. qrammicus are discussed.