Genic variation in natural populations of Drosophila simulans was surveyed using allozymic and two-dimensional electrophoretic techniques. Consistent with some previous reports, allozymic heterozygosity appeared lower than in the sibling species D. melanogaster (0.07 vs. 0.16). No variation was detected by two-dimensional electrophoresis of 19 lines scored for 70 abundant proteins. This is consistent with reported reductions in estimates of genic heterozygosity by two-dimensional electrophoresis in D. melanogaster, Mus musculus, and man. Although the amount of intraspecific variation detected in abundant proteins was lower than that detected for allozymes in D. simulans and D. melanogaster, the genetic distances between the sibling species calculated from the two data sets are not significantly different (0.35 and 0.20). The allozyme and two-dimensional electrophoresis data confirmed the impression from other measures of genetic variation (mitochondrial DNA restriction maps and inversion polymorphisms) that D. simulans is substantially less variable than D. melanogaster.

We have sampled a London population of Drosophila melanogaster for null alleles at twenty-five allozyme loci. The same loci and biochemical techniques were used as in our previous survey of a North Carolina population (Voelker et al. 1980). This second survey is completely concordant with the first. No nulls were detected among the five X-linked loci. The mean frequency of nulls at the twenty autosomal loci was 0.0023. Although there is significant interlocus heterogeneity, the two populations appear to have the same frequencies at each locus. This suggests that null alleles at these allozyme loci are in mutation-selection balance, and we estimate the average heterozygous effect of an allozyme null to be 0.0015. Consideration of allozyme null-allele frequencies, the effects of allozyme null alleles on viability and fertility and the generally greater amount of genetic variability at allozyme loci determined by electrophoresis lead us to doubt the validity of generalizing from allozyme data to the whole genome.

A Raleigh, NC, population of Drosophila melanogaster was sampled for the presence of enzyme null alleles at 25 loci. No nulls were found at any of five X-linked loci. Nulls were recovered at 13 of 20 autosomal loci; the weighted mean frequency for all 20 autosomal loci was estimated to be 0.0025. A consideration of the effects of these null alleles on viability strongly suggests that, although they may contribute to so-called polygenic variation, they are not representative of the entire genome.

We have found the two-dimensional electrophoretic technique of O'Farrell to be highly efficient in the detection of charge-change substitutions in a large number of proteins. We have applied this method to determine the level of heterozygosity of the most abundant proteins in Drosophila melanogaster adults from a natural population. The estimate of per-locus heterozygosity obtained from approximately 54 loci screened was 4% with 6 loci polymorphic. This is much lower than overall estimates obtained by standard gel electrophoresis but is not different from estimates for "Group I" enzymes--i.e., those utilizing a narrow spectrum of substrates of intracellular origin. We consider these data to throw open the question of the level of genetic variability in nature.

GENIC heterozygosity at enzyme loci has been examined for correlation with parameters as diverse as substrate specificity1, physiological function2 and quaternary structure3. However, none of these has provided an adequate explanation for allozymic variation in general. One report4 suggested a positive relationship for Drosophila enzymes between heterozygosity and subunit molecular weight (MW), which can be equated to the size of the structural gene element. Various models, both neutral and selective, would predict a correlation between gene size and variability5–7. Recently, a similar correlation has been found within three classes of vertebrates8,31, and between the number of rare alleles and subunit size in human populations9,10. We report here that we have tested the relationship between heterozygosity and subunit size in Drosophila species with a new selection of enzymes. Our results indicate that the correlation is indeed general. The relationship is quasi-linear and does not differ significantly from several models. The results support the hypothesis that there are constraints on the number of sites available to charge substitution within an enzyme molecule.