Dosage compensation is transcriptionally regulated in autosomal trisomies of Drosophila

One Hundred Years of Gene Balance: How Stoichiometric Issues Affect Gene Expression, Genome Evolution, and Quantitative Traits

A century ago experiments with the flowering plant Datura stramonium and the fruit fly Drosophila melanogaster revealed that adding an extra chromosome to a karyotype was much more detrimental than adding a whole set of chromosomes. This phenomenon was referred to as gene balance and has been recapitulated across eukaryotic species. Here, we retrace some developments in this field. Molecular studies suggest that the basis of balance involves stoichiometric relationships of multi-component interactions. This concept has implication for the mechanisms controlling gene expression, genome evolution, sex chromosome evolution/dosage compensation, speciation mechanisms, and the underlying genetics of quantitative traits.

Effect of triploidy on liver gene expression in coho salmon (Oncorhynchus kisutch) under different metabolic states

BACKGROUND

Triploid coho salmon are excellent models for studying gene dosage and the effects of increased cell volume on gene expression. Triploids have an additional haploid genome in each cell and have fewer but larger cells than diploid coho salmon to accommodate the increased genome size. Studying gene expression in triploid coho salmon provides insight into how gene expression may have been affected after the salmonid-specific genome duplication which occurred some 90 MYA. Triploid coho salmon are sterile and consequently can live longer and grow larger than diploid congeners in many semelparous species (spawning only once) because they never reach maturity and post-spawning mortality is averted. Triploid fishes are also of interest to the commercial sector (larger fish are more valuable) and to fisheries management since sterile fish can potentially minimize negative impacts of escaped fish in the wild.

RESULTS

The vast majority of genes in liver tissue had similar expression levels between diploid and triploid coho salmon, indicating that the same amount of mRNA transcripts were being produced per gene copy (positive gene dosage effects) within a larger volume cell. Several genes related to nutrition and compensatory growth were differentially expressed between diploid and triploid salmon, indicating that some loci are sensitive to cell size and/or DNA content per cell. To examine how robust expression between ploidies is under different conditions, a genetic/metabolic modifier in the form of different doses of a growth hormone transgene was used to assess gene expression under conditions that the genome has not naturally experienced or adapted to. While many (up to 1400) genes were differentially expressed between non-transgenic and transgenic fish, relatively few genes were differentially expressed between diploids and triploids with similar doses of the transgene. These observations indicate that the small effect of ploidy on gene expression is robust to large changes in physiological state.

CONCLUSIONS

These findings are of interest from a gene regulatory perspective, but also valuable for understanding phenotypic effects in triploids, transgenics, and triploid transgenics that could affect their utility in culture conditions and their fitness and potential consequences of release into nature.

The influence of triploidy on gene expression in the silkworm, bombyx mori

In Bombyx mori, it is well established that polyploids are easily induced when newly laid eggs are exposed to a variety of conditions, such as high or low temperature, centrifugal force, or chemicals like colchicine. To investigate gene dosage effects by varying the ploidy, the transcription levels of six genes expressed in various tissues were analysed in the diploid and two different genetically produced triploids (PPC and CCP). In the PPC triploid, the transcription level per cell of two genes was directly proportional to the structural gene dosage, whereas two other genes showed the mRNA level expected if compensation occurred. In the CCP triploid, three genes displayed dose-dependent levels of expression, whereas one gene showed the same expression level as the diploid strains. In both triploids, exceptional cases showed a negative correlation of expression with ploidy or a positive correlation greater than expected from the structural gene dosage. Interestingly, the transcription levels of most tested genes were significantly different from the strains which were used as parents of the triploids, and also widely divergent expression patterns were found for some genes in the diploid offspring. In this study, the cause of the unexpected expression patterns observed in the euploid series is discussed in relation to the difference between the two parental strains in expression level of genes and in trans-acting regulatory effects on their target genes.

Characterization of a sex-influenced modifier of gene expression and suppressor of position-effect variegation in Drosophila

Modifier of white (Mow), a dominant transacting gene, has been identified through a mutagenic screen for second-site loci that alter the level of expression of the white eye color locus. Mow reduces the expression of white in most developmental stages, but enhances its expression in the pupal stage, the time at which the major contribution to the adult phenotype is made. Tests with an Alcohol dehydrogenase promoter-white reporter and a series of white truncation constructs have shown that Mow fails to affect the reporter; cis-regulatory mutations of white also show no response, suggesting a requirement for white regulatory domains for interaction with Mow. A quantitative analysis of steady-state transcript levels reveals that the white mRNA level decreases in the presence of one dose of Mow in larvae and adults, but the reduction is greater in females than males. Two other functionally related genes, brown and scarlet, also exhibit a similar sexually dimorphic alteration in expression, mediated by Mow. In the mid-pupal stage, by contrast, the level of white and brown mRNA is increased by Mow. In addition, Mow acts as a weak suppressor of position effect variegation (PEV). These observations suggest a connection between dosage modulation of gene expression and suppression of position-effect variegation.

Gene dosage compensation in Drosophila melanogaster

The comparative lack of sexual dimorphism of apricot and other [sex-linked] genes studied is due to "sex-limitation", i.e., to a compensatory influence of the dosage difference between the sexes in respect to other genes in the X-chromosome. The facts are of particular interest from an evolutionary standpoint.