Indirect Effects of Ploidy Suggest X Chromosome Dose, Not the X:A Ratio, Signals Sex in Drosophila

Abstract

In the textbook view, the ratio of X chromosomes to autosome sets, X:A, is the primary signal specifying sexual fate in Drosophila. An alternative idea is that X chromosome number signals sex through the direct actions of several X-encoded signal element (XSE) proteins. In this alternative, the influence of autosome dose on X chromosome counting is largely indirect. Haploids (1X;1A), which possess the male number of X chromosomes but the female X:A of 1.0, and triploid intersexes (XX;AAA), which possess a female dose of two X chromosomes and the ambiguous X:A ratio of 0.67, represent critical tests of these hypotheses. To directly address the effects of ploidy in primary sex determination, we compared the responses of the signal target, the female-specific SxlPe promoter of the switch gene Sex-lethal, in haploid, diploid, and triploid embryos. We found that haploids activate SxlPe because an extra precellular nuclear division elevates total X chromosome numbers and XSE levels beyond those in diploid males. Conversely, triploid embryos cellularize one cycle earlier than diploids, causing premature cessation of SxlPe expression. This prevents XX;AAA embryos from fully engaging the autoregulatory mechanism that maintains subsequent Sxl expression, causing them to develop as sexual mosaics. We conclude that the X:A ratio predicts sexual fate, but does not actively specify it. Instead, the instructive X chromosome signal is more appropriately seen as collective XSE dose in the early embryo. Our findings reiterate that correlations between X:A ratios and cell fates in other organisms need not implicate the value of the ratio as an active signal.

Author Summary

In the fruit fly, Drosophila, chromosomal signals determine sex. Diploid flies with two X chromosomes are female, whereas those with one X are male. Conventionally, it is thought that the ratio of the number of X chromosomes to autosomes (X:A) constitutes the signal, because triploid flies bearing two X chromosomes and three sets of autosomes (XX;AAA) are intersexual. Under this model, the X:A signal is defined as the balance between a set of X-linked “numerator” proteins that promote female development and autosomally encoded “denominator” proteins that counteract the numerator elements. Although the X:A signal is a textbook standard, only one strong denominator element exists, and it cannot account for the effects of altered chromosome number (ploidy) on sex. To understand how X and autosome doses influence sex, we examined haploids (1X;1A) and triploids during the brief embryonic period when sex is determined. We found that ploidy affects sex indirectly by increasing in haploids, or decreasing in triploids, the number of embryonic cell cycles in which chromosomal sex is assessed. Our findings indicate that the fly sex-determination signal is more accurately viewed as a function of the number of X chromosomes rather than as a value of the X:A ratio.

Abstract

Sex is determined by X chromosome number rather than by the value of the X:autosome ratio in Drosophila.