Reexamination of diversifying selection of polymorphic allozyme genes by using population cages in Drosophila melanogaster

Adaptation to environmental heterogeneity in populations ofDrosophila melanogaster

For 29 generations, populations ofDrosophila melanogasterwere offered one favourable (standard) and one suboptimal (salt-supplemented) medium, either singly or simultaneously. Egg-to-adult viability, fecundity and choice of oviposition medium were measured at regular intervals on both resources up to 17 generations after initiation of the salt treatment. Except for a decrease in viability on salt medium in the single-resource populations (SRPs) maintained on the optimal medium, these fitness components remained unchanged. Estimation of a more inclusive measure of fitness, productivity, obtained at generations 27–29, revealed that: (1) the SRPs maintained on salt medium were more adapted to salt medium; (2) the mixed-resource populations (MRPs) were intermediate in their adaptation to salt medium between either type of single-resource population. These results support Levins' model of optimal strategy for populations living in a coarse-grained environment when the fitness set is convex. Family selection for increased and decreased resistance to salt in the medium, carried out for the viability component at generations nine and 19, showed that: (1) genetic variation with respect to this component was present in all populations; (2) the SRPs maintained on salt medium had responded to the salt treatment by eliminating sensitive genotypes; (3) in the first selection experiment, the MRPs had a greater amount of additive genetic variance with respect to viability than either type of SRP; in the second experiment, this difference was not significant, but it was in the predicted direction. The latter finding provides some evidence in favour of the hypothesis repeatedly presented in the literature that environmental heterogeneity could promote the maintenance of genetic variability in populations.

Norms of reaction and diversifying selection

The numbers of progeny produced by comparable numbers of female Drosophila melanogaster of 26 geographic strains on nine different culture media are examined in the context of norms of reaction. Having emphasized that diversifying selection is seldom discussed simultaneously with its seemingly related topic, norms of reaction, I present the following argument: diversifying selection has generally been viewed as involving sub-populations inhabiting separate localities and subject to different patterns of selection, norms of reaction as variation whose weighted average determines the relative fitnesses of different genotypes within individual sub-populations. Should environmental challenges frequently involve life or death (including sterility) outcomes, norms of reaction involving components of fitness engender diversifying selection within local populations (demes).

Soft selection and quantitative genetic variation: a laboratory experiment

The effect of environmental heterogeneity on the genetic variation of different quantitative characters was studied in two laboratory and two recently captured populations of Drosophila melanogaster. Two different culture media (habitats R and G) were used. Coarse-grained heterogeneity with independent density control in each habitat (R + G), and fine-grained (R/G) heterogeneity were simulated in population cages. Control populations in both R and G habitats were also maintained. Genetic differences for oviposition-site preference, larval preference and/or within-habitat viability were found between subpopulations sampled from different media. This happened in all four populations maintained on R + G, two populations maintained on R/G, and one control population. Thus, environmental heterogeneity seems to protect genetic variability responsible for between-habitat genetic differentiation, particularly when such heterogeneity corresponds to the 'soft selection' model (R + G). However, for the quasi-neutral trait sternopleural bristle number, no genetic between-habitat differentiation, nor increased heritability were observed in populations maintained under any kind of environmental heterogeneity. Hence, although soft selection seems to be a real force in determining adaptation to heterogeneous environments, the genetic variability maintained may be small in relation to the whole genome.

Allozyme-associated heterosis in Drosophila melanogaster

Two large experiments designed to detect allozyme-associated heterosis for growth rate in Drosophila melanogaster were performed. Heterosis associated with allozyme genotypes may be explained either by functional overdominance at the allozyme loci, or closely linked loci; or by genotypic correlations between allozyme loci and loci at which deleterious recessive alleles segregate. Such genotypic correlations would be favored by consanguineous mating, small effective population size, population mixing and strong natural or artificial selection. D. melanogaster is outbred, has large effective population size and there is little evidence for genotypic disequilibria. Therefore it would be unlikely to show allozyme heterosis due to genotypic correlations. In the first experiment I estimated the genotypic values of 97 replicated genotypes. In the second experiment, 500 individuals were raised in a fluctuating, stressful environment. In neither experiment was there any consistent evidence for allozyme heterosis in size or development rate, fluctuating asymmetry for size or in tendency to deviate from the population mean. In the first experiment, heterosis explained less than 5.6% of the genetic variance in growth characters. In the second, heterosis explained less than 0.1% of the phenotypic variance in growth characters. Outside of the molluscs, species which show allozyme heterosis have population structures or histories which tend to promote genotypic correlations. There is little evidence that functional overdominance is responsible for observations of allozyme-associated heterosis.

Nucleotide variation and divergence in the histone multigene family in Drosophila melanogaster

Nucleotide differences in the histone H3 gene family in Drosophila melanogaster were studied on three levels: (1) within a chromosome, (2) within a population and (3) between species (D. melanogaster and Drosophila simulans). The average difference within the H3 gene within a chromosome was 0.0040 per nucleotide site, about 52% of that within a population (0.0077). The proportion of divergent sites between the two species was 0.0575, which is about 8.5 times the difference within a species. The distribution of divergence between species was similar to that of variation within a species. Divergence and variation were noted to be greatest in the 3' noncoding region and least in the coding region. Values intermediate between these were found for the 5' noncoding region. Divergence and variation in silent sites exceeded those in the total coding region, thus indicating possible purifying selection for amino-acid-altering change. Phylogenetic relations among H3 genes and genetic differences on these three levels are evidence for the concerted evolution of the histone gene family. The molecular mechanism by which variation is produced and maintained is discussed.

Analysis of spatial and temporal variation in the community structure of yeasts associated with decayingOpuntia cactus

The microbial structure within, between, and over time in decaying cladodes of the common prickly pearOpuntia stricta was studied at each of two separate localities. In general, the effective number of yeast species and yeast species diversity increased as the rot aged to the observed maximum time of 4 weeks. Yeast heterogeneity at the two localities differed in the mode of environmental influence, with spatial variability (among rots) most important at one and temporal variability (within rots over time) most important at the other. Differences in cactus density and quality (age) are most likely determinants of the differences in yeast community structure.

The genetic structure of natural populations of Drosophila melanogaster. XX. Comparison of genotype-environment interaction in viability between a northern and a southern population

In order to examine the operation of diversifying selection as the maintenance mechanism of excessive additive genetic variance for viability in southern populations in comparison with northern populations of Drosophila melanogaster, two sets of experiments were conducted using second chromosomes extracted from the Ogasawara population (a southern population in Japan) and from the Aomori population (a northern population in Japan). Chromosomal homozygote and heterozygote viabilities were estimated in eight kinds of artificially produced breeding environments. The main findings in the present investigation are as follows: (1) Significant genotype-environment interaction was observed using chromosomes extracted from the Ogasawara population. Indeed, the estimate of the genotype-environment interaction variance for heterozygotes was significantly larger than that of the genotypic variance. On the other hand, when chromosomes sampled from the Aomori population were examined, that interaction variance was significant only for homozygotes and its value was no more than one quarter of that for the chromosomes from the Ogasawara population. (2) The average genetic correlation between any two viabilities of the same lines estimated in the eight kinds of breeding environments for the chromosomes sampled from the Ogasawara population was smaller than that for the chromosomes from the Aomori population both in homozygotes and in heterozygotes, especially in the latter. (3) The stability of heterozygotes over homozygotes against fluctuations of environmental conditions was seen in the chromosomes from the Ogasawara population, but not from the Aomori population. (4) From the excessive genotype-environment interaction variance compared with the genotypic variance in heterozygotes, it was suggested for the chromosomes from the Ogasawara population that the reversal of viability order between homozygotes took place in some environments at the locus level. On the basis of these findings, it is strongly suggested that diversifying selection is operating in a southern population of D. melanogaster on some of the viability polygenes which are probably located outside the structural loci, and the excessive additive genetic variance of viability in southern populations is maintained by this type of selection.

Temporal and microgeographic variation in allozyme frequencies in a natural population of Drosophila buzzatii

Temporal variation in allozyme frequencies at six loci was studied by making monthly collections over 4 yr in one population of the cactophilic species Drosophila buzzatii. Ten sites were defined within the study locality, and for all temporal samples, separate collections were made at each of these sites. Population structure over microgeographic space and changes in population structure over time were analyzed using F-statistic estimators, and multivariate analyses of allele and genotype frequencies with environmental variables were carried out. Allele frequencies showed significant variation over time, although there were no clear cyclical or seasonal patterns. A biplot analysis of allele frequencies over seasons within years and over years showed clear discrimination among years by alleles at four loci. During the 4 yr, three alleles showed directional changes which were associated with directional changes in environmental variables. Significant associations with one or more environmental variables were found for allele frequencies at every locus and for both expected and observed heterozygosities (except those for Est-1 and Est-2). Thus, variation in allele frequencies over time cannot be attributed solely to drift. Significant linkage disequilibria were detected among three loci (Est-2, Hex and Aldox), but there was no evidence for spatial or temporal patterns. The F-statistic analyses showed significant differentiation among months within years for all loci, but the statistic used (coancestry) was heterogeneous among loci. Estimates of F (inbreeding) for all loci were significantly different from zero, with the loci in four groups, Adh-1 (negative), Pgm(small positive), Est-2 and Hex (intermediate) and Est-1 and Aldox (high positive). The correlation of genes within individuals within populations (f) for each locus in each month by site sample differed among loci, as did the (f) for each locus in each month by site sample differed among loci, as did the patterns of change in f over time (seasons). Heterogeneity in the F-statistic estimates indicates that natural selection is directly or indirectly affecting allele and genotype frequencies at some loci. However, the F-statistic analyses showed essentially no microgeographic structure (i.e., among sites), although there was significant heterogeneity in allele frequencies among flies emerging from individual rots. Thus, microspatial heterogeneity probably is most important at the level of individual rots, and coupled with habitat selection, it could be a major factor promoting diversifying selection and the maintenance of polymorphism.(ABSTRACT TRUNCATED AT 400 WORDS)

Targeted selection experiments and enzyme polymorphism: negative evidence for octanoate selection at the G6PD locus in Drosophila melanogaster

Published studies have reported significant selection with respect to the G6pd locus for Drosophila melanogaster reared on Na-octanoate food. We have reexamined the selective effects of Na-octanoate on egg to adult viability with respect to the G6pd polymorphism using specially constructed X chromosomes. Four experiments were carried out using different 6Pgd backgrounds in two recombinant sets of chromosomes segregating for the G6pd locus but constructed so as to minimize variation over most of the X chromosome. In addition, two measures of viability were used, and the size of the experiments and their associated degrees of freedom are approximately double those reported in the former studies. Our results find no evidence for differential selection on G6pd genotypes (males and females) by Na-octanoate and, therefore, do not corroborate the positive results of selection reported by other investigators. The reasons for our different results are discussed.