Doublesex controls specification and maintenance of the gonad stem cell niches in Drosophila

Abstract

Sex-specific development of the gonads is a key aspect of sexual dimorphism that is regulated by Doublesex/Mab3-related transcription factors (DMRTs) in diverse animal species. We find that in mutants for Drosophila dsx, important components of the male and female gonad stem cell niches (hubs and terminal filaments/cap cells, respectively) still form. Initially, gonads in all dsx mutants (both XX and XY) initiate the male program of development, but later half of these gonads switch to form female stem cell niche structures. One individual can have both male-type and female-type gonad niches; however, male and female niches are usually not observed in the same gonad, indicating that cells make a 'group decision' about which program to follow. We conclude that dsx does not act in an instructive manner to regulate male versus female niche formation, as these structures form in the absence of dsx function. Instead, dsx acts to 'tip the balance' between the male or female programs, which are then executed independently of dsx. We show that bric a brac acts downstream of dsx to control the male versus female niche decision. These results indicate that, in both flies and mammals, the sexual fate of the somatic gonad is remarkably plastic and is controlled by a combination of autonomous and non-autonomous cues.

Doublesex regulates fruitless expression to promote sexual dimorphism of the gonad stem cell niche

“…Conversely, if Fru expression is only a consequence of male-specific cell fate, changing Fru expression level would not alter the chances of a dsx mutant gonad developing a hub or TFs. As previously reported [ 21 ], dsx mutant gonads had a roughly equal chance of forming hubs or TFs (with another fraction forming no discernable niche structure, Fig 3C–3F ). When one copy of fru was removed in this genetic background ( dsx 1 /Df(3R)dsx 3 , fru Sat15 /+ ), we observed that the fraction of XY gonads with hubs decreased while the fraction that formed TFs increased ( Fig 3I ).…”

“…In dsx mutants, half of both XX and XY gonads remain as hubs, while the other half switch to form TFs during the L3 stage. As a result, either a hub or TFs can be found in both XX and XY gonads [ 21 ]. We examined Fru expression levels in late L3 dsx null gonads and categorized the results by chromosomal sex and niche fate (hub vs. TFs, Fig 2D ).…”

“…The fact that some dsx mutant gonads switch from having hubs to TFs during the L3 stage, at the time that the female niche normally develops, indicates that dsx is normally required in male gonads to maintain the male fate [ 21 ]. Fru is not expressed in the testis at the time of male niche formation during embryogenesis, but Fru expression initiates at the L2/L3 stage at the time that male niches must maintain hub fate, suggesting that Fru may be important for hub maintenance.…”

“…Fru is not expressed at the time of hub formation during embryogenesis, but expression is initiated during the L2/L3 larval stage. This correlates with a time period when the hub must be maintained and resist transforming into female niche structures; in dsx mutants, all gonads in XX and XY animals develop hubs, but in half of each, hubs transform into terminal filament cells and cap cells [ 21 ]. fru is not required for initial hub formation, consistent with it not being expressed at that time.…”

“…In contrast, female niche specification starts in late 3 rd larval instar when stacks of terminal filament cells are specified from cells forming the apical cap of the ovary, and continues at the larval-pupal transition with the specification of cap cells from intermingle cells [ 18 – 20 ]. Recently, we found that one important role dsx plays is to maintain the hub fate in the 3 rd instar larval (L3) stage and to prevent sex reversal [ 21 ]. In the absence of dsx , both XX and XY gonads initially follow the male path to form a hub by the end of embryogenesis, but later undergo stochastic sexual-fate reprogramming in the L3 stage in which half of both XX and XY animals form TFs in place of the hub, while the hub is maintained in the other half.…”