Evolution of Sceloporus grammicus complex (Sauria: Iguanidae) in central Mexico II. Studies on rates of nondisjunction and the occurrence of spontaneous chromosomal mutations

Abstract

The iguanid lizard Sceloporus grammicus has a high level of karyotypic variability, and has often been cited as an example of chromosomal speciation. We examined a total of 2036 secondary spermatocytes from 30 S. grammicus males, and found that 16 of the 30 individuals (including a single lizard collected from a hybrid zone between two chromosome races) produced completely balanced spermatocytes. Fourteen of the 30 lizards (including both chromosomal heterozygotes and homozygotes) had relatively low (0.6% to 7.1°70) levels of aneuploidy. Heterozygotes had a 1.1°70 increase over homozygotes in the number of aneuploid spermatocytes observed. The frequency of aneuploidy in S. grammicus may not be high enough to cause chromosomal speciation by any of the mechanisms that have been proposed for this complex. Most individuals showed balanced segregation of the autosomal trivalents, but nearly half of the lizards had a significant excess of spermatocytes with the X 1 and X 2 rather than the Y sex chromosomes. Five lizards had spermatocytes which had fission mutations not found in the somatic cells. As many as 5.9o70 of the spermatocytes in one individual had chromosomal mutations. This chromosomal mutation rate has important implications for chromosomal evolution in S. grammicus.

Synapsis, recombination, and meiotic segregation in the mesquite lizard, <i>Sceloporus grammicus</i>, complex

“…1987). In 16 of 30 heterozygous and homozygous males, Porter and Sites ( 1987) found that all of the spermatocytes were completely balanced. In the remaining males, the frequency of aneuploidy ranged from 0.6% to 7.1 %, with heterozygotes dis playing an average increase in Mil aneuploidy of 1.1 % over the homozygotes.…”

“…The estimated frequency of nondisjunction and aneuploidy in various heterozygotes ranges from 0% to 34% (Ford and Evans, 1972;Cattanach and Mosel ey, 1973;Gropp and Winking. 1981;Searle, 1986;Porter and Sites, 1987;Stewart-Scott and Bruere, 1987), and in bulls (Lo gue and Harvey, 1978) and sheep (Long, 1978) heterozygotes displayed only moderately higher frequencies of nondisjunction than did homozygotes (3.6% and 4.1%, respectively). From these observations it is obvious that no universal statement can be made regarding the fitness effects of single Robertsonian rearrangements.…”

“…The observation of two metacentric elements in a single second ary spermatocyte from a fission heterozygote could have been the result of either a spontaneous fusion mutation or a premeiotic nondisjunction. Porter and Sites (1987) found abnor mal spermatocytes and apparent spontaneous germ-line fission mutations in 5 of the 30 lizards they examined. The extensive chromosomal repatterning displayed in the S. grammicus com plex, including both interpopulational heteromorphisms and intrapopulational fixation, suggests that this species may be more susceptible to mutational events.…”

Patterns of karyotype evolution in complexes of sibling species within three genera of African murid rodents inferred from the comparison of cytogenetic and molecular data

“…It also became clear that negatively heterotic effects associated with pericentric inversions are minimal and single or double heterozygotes are therefore easily tolerated in populations. Although there are theoretically no meiotic obstacles to their accumulation, higher levels of pericentric inversions heterozygosity (three or more) remain extremely rare (Davis et al, 1986;Porter and Sites, 1987; for review see Volobouev et al, 2001). In fact it may be argued that above a certain threshold, the effect of pericentric inversions may become deleterious .…”

STAGGERED CLINES IN A HYBRID ZONE BETWEEN TWO CHROMOSOME RACES OF THE HARVESTMANGAGRELLOPSIS NODULIFERA(ARACHNIDA: OPILIONES)

“…However, very few studies of meiotic or other fitness-related consequences of chromosomal heterozygosity have been performed in natural hybrids. The level of nondisjunction in a single heterozygote is usually quite small and highly variable between individuals (Ford and Evans 1972;Gropp and Winking 1981;Searle 1986;Porter and Sites 1987;Wallace et al 1992), whereas heterozygotes for several rearrangements usually suffer dramatic fertility reductions due to extensive germ-cell death and high levels of nondisjunction (Gropp and Winking 1981;de Boer 1986;de Boer and de Jong 1989;Speed 1989;Searle 1993).…”