Synapsis, recombination, and meiotic segregation in the mesquite lizard, Sceloporus grammicus, complex. II. Fission heteromorphism of the FM2 cytotype and evolution of chromosome 2

Somatic and meiotic chromosomal and synaptonemal complex techniques were used to characterize the chromosomal complement and to study the fission heteromorphism of chromosome 4 in the FM2 cytotype of Sceloporus grammicus. Analysis of silver-stained somatic metaphases revealed that the nucleolar organizer region in this cytotype is located at the distal end ofa pair of medium-sized acrocentric chromosomes, rather than on the largest acrocentric chromosomal pair, as previously reported. This condition is hypothesized to be the result of at least two sequential rearrangements. Analysis of surface-spread zygotene and pachytene nuclei indicated that the components of the chromosome 4 trivalent initiated synapsis at their distal telomeric regions. Although synapsis of the fission trivalent was synchronous with that of the homomorphic autosomal pairs, completion of synapsis was delayed in the trivalent. Associations between the fission trivalent and other autosomal or sex-chromosomal elements occurred in approximately one third of the pachytene nuclei examined. Analysis of secondary spermatocytes (metaphase II configurations) revealed low levels of nondisjunction in fission heterozygotes. These analyses indicate that FM2 individuals heterozygous for the fission rearrangement of chromosome 4 suffer no meiotic deficit.