The accumulation of a transposable element inside chromosomal inversions is examined theoretically by a mathematical model, and empirically by counts of P elements associated with inversion polymorphisms in natural populations of Drosophila melanogaster. The model demonstrates that, if heterozygosity for an inversion effectively reduces element associated production of detrimental chromosome rearrangements, a differential accumulation of elements is expected, with increased copy number inside the minority inversion. Several-fold differential accumulations are possible with certain parameter values. We present data on P element counts for inversion polymorphisms on all five chromosome arms of 157 haploid genomes from two African populations. Our observations show significantly increased numbers of elements within the regions associated with the least common, or minority arrangements, in natural inversion polymorphisms.

At the white eye colour locus, there are a number of alleles that have altered expression between males and females. To test these regulatory mutations of the white eye colour locus for their phenotypic expression in metafemales (3X; 2A) compared to diploid females and males, eleven alleles or transduced copies of white were analysed. Two alleles that exhibit dosage compensation between males and females (apricot, blood) also exhibit dosage compensation in metafemales. White-ivory and white-eosin, which fail to dosage compensate in males compared to females, but that are distinct physical lesions, also show a dosage effect in metafemales. Two alleles with greater expression in males than females (spotted, spotted-55) exhibit even lower expression in metafemales. Lastly, five transduced copies of white carrying three different lengths of the white promoter, but that all exhibit higher expression in males, show reduced expression in metafemales, exhibiting an inverse correlation between the level of expression and the dosage of the X chromosome. Because these alleles of white respond to dosage compensation in metafemales as a continuum of the male and female responses, it is concluded that the same basic mechanism of dosage compensation is involved and that the dosage of the X chromosome conditions the sexually dimorphic expression.

The Drosophila melanogasterecd1 mutation causes a severe temperature-sensitive deficiency in the titre of the steroid hormone eedysone. This mutation was used to investigate the role of eedysone in both the transcription of the genes mapped at the 3C11–12 intermoult puff region and the puff formation. Thoroughly synchronized ecd1 larvae were shifted to the non-permissive temperature at various times of the development; after 24 or 48 h, the levels of the transcripts derived from Sgs-4, Pig-1 and ng-1, the three genes located at the 3C11–12 polytene bands, were determined. The results showed that the levels of the transcripts encoded by Pig-1 and ng-1 are unaffected by the drop in the ecdysone titre occurring in non-permissive conditions whereas the amount of Sgs-4 mRNA is greatly reduced. These data clearly indicate that transcription of the three genes mapped within the puff region is affected differently by the hormone. Furthermore, ecd1 larvae cultured at the non-permissive temperature show a prominent puff at the 3C11–12 polytene bands, indicating that eedysone is not essential for puff induction and that puff size is not simply correlated with high-level Sgs-4 transcription.

An allele of the mouse brachyury locus, T22H, had been shown previously to involve a deletion of several markers in the proximal part of chromosome 17, and almost certainly includes deletion of the t-complex distorter gene Tcd-1. The effects of T22H on transmission ratio distortion and male sterility caused by the t-complex were compared with those of a partial t-haplotype th51, which carries the t-form of the distorter Tcd-1t. In combination with the complete haplotypetf32, T22H caused severe impairment of male fertility, but males of genotype T22H/t6 or T22H/thSl were normally fertile. These results were very similar to those obtained when th51 was in combination with the same haplotypes. In effect on transmission ratio T22H was again similar to thSI, in that it produced a marked increase in the transmission of the haplotype t6. To test whether the effects of T22H were due to deletion of elements other than Tcd-1, the effect of T22H on transmission of the partial haplotype th2 was compared with that of the deletion Thv. Again T22H markedly increased transmission of the t-haplotype and the effect was significantly greater than the small effect produced by Thp.

It is concluded that deletion of the distorter Tcd-1 has an effect like that of the t-form of this distorter, Tcd-1t, and hence that Ted-11 must be an amorph or hypomorph. It is speculated that other t-complex distorters, Tcd-2t and Tcd-3t may also be amorphs or hypomorphs. Thus, the phenomena of transmission ratio distortion and male sterility due to the t-complex appear to be brought about by differential susceptibility of wild-type and t-responder alleles, Tcr+ and Tcrt, to a shortage of distorter gene product.

Current models predict that large increases over wild-type in the activity of one enzyme will not alter an organism's fitness. This prediction is tested in Saccharomyces cerevisiae through the use of a high copy plasmid that bears one of the following: hexokinase B (HEXB), phosphoglucose isomerase (PGI), phosphofructokinase (PFKA and PFKB), or pyruvate kinase (PYK). Transformants containing these plasmids demonstrate a four to ten-fold increase in enzyme specific activity over either the parent strain or transformants containing the plasmid alone. Haploid and diploid transformants derived from independent backgrounds were grown on both fermentable and non-fermentable carbon sources and evaluated for several components of fitness. These include growth rate under non-limiting conditions, maximum stationary phase density, and viability in extended batch culture. Cell viability is not affected by overproduction of these enzymes. Growth rate and stationary phase density do not differ significantly among strains that overexpress HEXB, PGI or contain the vector alone. PFKA, B transformants show reduced growth rate on glucose in one background only. For these loci the current model is confirmed. By contrast, when grown on glucose, yeast overexpressing PYK demonstrate reduced growth rate and increased stationary phase density in both backgrounds. These effects are abolished in cells containing plasmids with a Tn5 disrupted copy of the PYK gene. Our results are consistent with reports that the PYK locus may exert control over the yeast cell cycle and suggest that it will be challenging to model relations between fitness and activity for multifunctional proteins.

Using a stochastic model of a finite population in which there is mutation to partially recessive detrimental alleles at many loci, we study the effects of population size and linkage between the loci on the population mean fitness and inbreeding depression values. Although linkage between the selected loci decreases the amount of inbreeding depression, neither population size nor recombination rate have strong effects on these quantities, unless extremely small values are assumed. We also investigate how partial linkage between the loci that determine fitness affects the invasion of populations by alleles at a modifier locus that controls the selfing rate. In most of the cases studied, the direction of selection on modifiers was consistent with that found in our previous deterministic calculations. However, there was some evidence that linkage between the modifier locus and the selected loci makes outcrossing less likely to evolve; more losses of alleles promoting outcrossing occurred in runs with linkage than in runs with free recombination. We also studied the fate of neutral alleles introduced into populations carrying detrimental mutations. The times to loss of neutral alleles introduced at low frequency were shorter than those predicted for alleles in the absence of selected loci, taking into account the reduction of the effective population size due to inbreeding. Previous studies have been confined to outbreeding populations, and to alleles at frequencies close to one-half, and have found an effect in the opposite direction. It therefore appears that associations between neutral and selected loci may produce effects that differ according to the initial frequencies of the neutral alleles.