Sex-Specific Pattern Formation During Early Drosophila Development

Abstract

The deleterious effects of different X-chromosome dosage in males and females are buffered by a process called dosage compensation, which in Drosophila is achieved through a doubling of X-linked transcription in males. The male-specific lethal complex mediates this process, but is known to act only after gastrulation. Recent work has shown that the transcription of X-linked genes is also upregulated in males prior to gastrulation; whether it results in functional dosage compensation is not known. Absent or partial early dosage compensation raises the possibility of sex-biased expression of key developmental genes, such as the segmentation genes controlling anteroposterior patterning. We assess the functional output of early dosage compensation by measuring the expression of even-skipped (eve) with high spatiotemporal resolution in male and female embryos. We show that eve has a sexually dimorphic pattern, suggesting an interaction with either X-chromosome dose or the sex determination system. By manipulating the gene copy number of an X-linked transcription factor, giant (gt), we traced sex-biased eve patterning to gt dose, indicating that early dosage compensation is functionally incomplete. Despite sex-biased eve expression, the gene networks downstream of eve are able to produce sex-independent segmentation, a point that we establish by measuring the proportions of segments in elongated germ-band embryos. Finally, we use a whole-locus eve transgene with modified cis regulation to demonstrate that segment proportions have a sexdependent sensitivity to subtle changes in Eve expression. The sex independence of downstream segmentation despite this sensitivity to Eve expression implies that additional autosomal gene-or pathway-specific mechanisms are required to ameliorate the effects of partial early dosage compensation. IN Drosophila, dosage compensation occurs by an upregulation of transcription from the X chromosome in males (Mukherjee and Beermann 1965;Belote and Lucchesi 1980;Straub et al. 2005). The best-characterized mechanism of upregulation depends on the male-specific activity of the malespecific lethal (MSL) complex (Gelbart and Kuroda 2009), which binds to sites on the X chromosome and enhances the rate of transcript elongation (Larschan et al. 2011). MSL activity is inhibited in females by the translational repression of Msl2 protein, which is necessary for complex formation, by the protein product of the sex-determination gene Sex-lethal (Sxl) (Kelley et al. 1995). The canonical MSL-dependent mechanism does not appear to be active prior to gastrulation as msl2 transcript is not detected until cleavage cycle 14 (Lott et al. 2011) and the earliest expression of Msl3 protein in male embryos is detected only at stage 6 (gastrulation) (Rastelli et al. 1995;Franke et al. 1996;Cline 2005).Despite the lack of MSL-dependent dosage compensation during the blastoderm stage, the transcription of X-linked genes expressed before gastrulation is, in fact, elevated in males (Lott et al. 2011). With the exc...

Consequences of Eukaryotic Enhancer Architecture for Gene Expression Dynamics, Development, and Fitness

“…One of them is the loss of viability in MSE (or INV_MSE) hemizygotes that is largely masked in homozygotes, which we interpret as a manifestation of the segmentation system's capacity to buffer certain stripe 2 defects. Developmental canalization acting downstream of eve may also explain the buffering of a sex-dependent difference in Eve stripe 2 expression that we also discovered in the course of this study [73]. Contrary to the general belief that segmentation is sex-independent, Eve stripe 2 expression is in fact sex-specific during cycle 14 in both WT and eve + embryos [73].…”

Naturally Occurring Deletions of Hunchback Binding Sites in the Even-Skipped Stripe 3+7 Enhancer

“…Recent technical and analytical improvements have enabled quantitative analyses of enhancer function and logic [87], [88], [90]–[92] and dosage compensation [77], [93]. Developmental networks must cope with variation due to chance, the internal and external environment, and in the relevant genetic components.…”

Adaptation of the length scale and amplitude of the Bicoid gradient profile to achieve robust patterning in abnormally large Drosophila melanogaster embryos

“…Embryos were stained by both a fluorescently labeled probe for detecting eve mRNA and Sytox Green (Invitrogen) for nuclear counterstain, mounted in 90% glycerol + n-Propyl galate, and imaged on the Zeiss AxioPlan 2 microscope (Carl Zeiss) with a Vti Infinity 3 confocal (Visitech International). Captured images were then processed and eve expression boundary positions (defined as the trough-to-peak midpoint positions) recorded for individual embryos as previously described (Manu et al, 2013).…”