CYTOGENETIC ANALYSIS OF CHROMOSOMAL INTERMEDIATES FROM A HYBRID ZONE BETWEEN 2 CHROMOSOME RACES OF THE SCELOPORUS-GRAMMICUS COMPLEX (SAURIA, PHRYNOSOMATIDAE)

Underdominance for chromosomal rearrangements is the central assumptio n of several models of chromosomally based speciation including the ca scade model, proposed for the Sceloporus grammicus complex. Several cy totypes of the S. grammicus complex hybridize at localities in central Mexico. A hybrid zone between two of the most chromosomally divergent races (= cytotypes) of S. grammicus (F5, 2n = 34 and FM2,2n = 44-46) was examined to assess the meiotic effects of heterozygosity at multip le chromosomes. Meiosis was examined in males heterozygous for ''simpl e'' Robertsonian fissions at chromosomes 1, 3, 4, and 6 and/or a peric entric inversion at-chromosome 4. Analysis of synaptonemal complexes a nd chromosomal configurations at diakinesis showed trivalent formation in fission heterozygotes and heterosynapsis (lack of reverse-loop for mation) in an inversion heterozygote. Analysis of metaphase II configu rations revealed primarily balanced segregation and low levels of nond isjunction regardless of chromosomal background. The lack of underdomi nance associated with ''simple'' fission heterozygosity in this narrow hybrid zone contradicts the key premise of most chromosomally based m odels of speciation.