FEMALE FECUNDITY IN A HYBRID ZONE BETWEEN 2 CHROMOSOME RACES OF THE SCELOPORUS-GRAMMICUS COMPLEX (SAURIA, PHRYNOSOMATIDAE)

Individuals of the F5 and FM2 cytotypes of the Sceloporus grammicus co mplex form a narrow zone of parapatric hybridization near Tulancingo, Hidalgo, Mexico. Reproductive parameters were examined among chromosom ally parental and hybrid females to assess the degree to which reduced clutch size is correlated with the level of chromosomal heterozygosit y. Although clutch size in the two parental groups was highly correlat ed with female body size, this was not the case for females with inter mediate karyotypes. These females displayed increased levels of infert ility manifested as smaller clutches and as inviable embryos. F-1 fema les produced the smallest average clutches and suffered the most preci pitous fecundity loss (up to 75%). The number of heterozygous marker c hromosomes and heterozygosity at chromosome 2 had significant effects on the number of viable embryos. Analysis of embryo karyotypes reveale d the production of triploid offspring and an excess number of embryos heterozygous at chromosome 1. Differences in viability, among females heterozygous for the same number of chromosomes, suggest that genetic background of the female and/or sire may be an important factor in de termining reproductive success.