Domino is potentially required for various processes in Drosophila development

of Dom partly differ between AMPs and NBs, it is not surprising that Dom also influences Drosophila embryogenesis and imaginal disc development. Like AMPs, imaginal disc cells are differentially influenced by dom knockdown or overexpression (3.2). The role of Myc in imaginal discs cells is quite well understood. Myc levels influence cell size, whereas proliferation rates are not markedly changed (Pierce et al., 2004). However, when Myc is manipulated in a mosaic, cellular competition induces apoptosis of the cells with lower Myc levels (de la Cova et al., 2004; Moreno et al., 2004). Considering the phenotype of imaginal discs with dom knockdown in the posterior compartment, it will be interesting to analyze the role of apoptosis for the given effect (Figure 20). It can be hypothesized that upon the lack of dom, Myc target genes might not be expressed, which then could lead to the induction of apoptotic cell death in these cells by the surrounding wild type cells. In previous studies on dom knockdown or overexpression of dom in the complete disc led to morphological defects in the wing but not to a complete loss of cells. Hence, the elimination of dom knockdown cells by neighbouring wild type cells appears even more likely (Eissenberg et al., 2005; Ellis et al., 2015; Hall et al., 2004; Kwon et al., 2013).

In contrast to the knockdown phenotype, overexpression of DomE, the major isoform of dom expressed in wing discs (Ruhf et al., 2001) in the posterior compartment leads to morphologically defective wing discs, thus rather not pointing towards a cell competition mediated loss of cells (Figure 22). The morphological effects of dom manipulation in complete wing discs have been shown to depend on the ability of Dom to modulate Notch signaling (Eissenberg et al., 2005; Hall et al., 2004). Notch signaling regulates imaginal disc growth and patterning, thus the DomE overexpression phenotype could depend on misregulation of Notch signaling (Estella and Baonza, 2015). Taken together, the different phenotypes upon dom knockdown and overexpression in imaginal discs might be results of the interaction with two different cofactors: Myc and Notch signaling.

Dom null mutants embryos display a very remarkable nuclear phenotype with nuclear membrane invaginations. This phenotype resembles overexpression phenotypes of proteins that structure the nuclear envelope (Brandt et al., 2006; Brandt et al., 2008; Pilot et al., 2005). Interestingly, the nuclear envelope is linked to chromatin and morphological changes in the nuclear membrane influence gene expression, a mechanism which is for example used during maternal to zygotic transition (Hampoelz et al., 2011). The link between the nuclear envelope and chromatin is established by the heterochromatin protein 1 (HP1) and the Brahma associated factor (BAF) (Polychronidou et al., 2010; Ye et al., 1997). As the incorporation of H2Av by Dom into chromatin is required to prevent heterochromatin spreading and acts upstream to HP1 incorporation, excess HP1 could theoretically lead to the membrane invaginations (Baldi and Becker, 2013). Remarkably, the chromatin state has never been reported to influence the nuclear membrane organization, but informational flow is thought to work in the opposite direction (Polychronidou and Großhans, 2011). Although the nuclear phenotype observed in dom null mutant embryos will require additional thorough analyses, the results of this study embryogenesis (Johnston et al., 1999b; Pierce et al., 2004), Dom might interact with other key regulators of cell cycle progression in the embryo. An obvious candidate for this would be the E2F family, as they have been shown to be vital for the cell cycle in the Drosophila embryo and because the Tip60 complex has been linked to the E2F network in mammals and Drosophila (Duronio et al., 1995; Lu et al., 2007; Taubert et al., 2004).

Alternatively, considering the nuclear phenotype and the bright staining of nuclei by Hoechst in dom mutant embryos (Figure 10, Figure 17), it is possible that Dom is required in embryonic nuclei to prevent heterochromatin spreading by H2Av incorporation, which would be crucial for genome maintenance (Rong, 2008). Misregulation of this could lead to the activation of cell cycle checkpoints, thus reduced or totally stalled cell division and consequently bigger embryonic cells.

Dom mutant embryos further display polarity defects in the epithelium. Although electron microscopy demonstrated that AJs are formed, the localization of polarity determinants appears to be affected, especially in processes requiring extensive cell shape changes, like dorsal closure. This also explains the inability of dom mutant embryos to form head and dorsal cuticle (3.1.3). So far no maternal dom null mutants have been investigated for epithelial polarity, thus one could speculate that the importance of dom in epithelial polarity could be even more fundamental as the maternally provided Dom protein might be sufficient for the establishment of AJs and maintenance of parts of the polarity. I did not detect extraordinary amounts of apoptotic cells and blocking apoptosis did not restore the dom mutant phenotype (Figure 18, Figure 19). Therefore, the cells observed to leave the tissue might reorient due to polarity defects and might be unable to maintain appropriate positioning of AJs. Considering the role of Dom in the maintenance of larval NB polarity, Dom might have a central function in regulating polarity. It will also be interesting to investigate the function of dom for the polarity of imaginal disc epithelial cells, as misregulation leads to defects. However, the intrinsic polarity of embryonic NBs is surprisingly unaffected, suggesting that the impact of Dom on cellular polarity might be cell-type and thus cofactor specific (Figure 11, Figure 13). The NB cell orientation is most likely defective as a secondary effect due to epithelial misorganization and subsequent defects in communication between the epithelium and the NBs (Yoshiura et al., 2012).

Notably, embryonic NBs, in contrast to larval NBs, get smaller with each division (Ito and Hotta, 1992). The importance of Dom to enable cell growth in NBs is therefore restricted to larval NBs and not required in embryonic NBs. Therefore, the growth inducing capability of Myc is also not required in embryonic NBs. Further, Myc has been shown to be dispensable for successful embryogenesis (Johnston et al., 1999b; Pierce et al., 2004).

It is thus possible that Dom is required solely in NBs that depend on Myc. Additionally, considering that Dom does not maintain embryonic NB polarity and maintenance, this strongly argues against a role of the Tip60 complex in larval NBs apart from regulation of Myc-responsive genes.

Taken together, Dom functions in diverse processes and highly dependent on the cellular context. The defects observed upon dom loss of function and overexpression in this study

could depend on various cofactors like Myc, E2F and Notch signaling. The chromatin remodeling complexes in which Dom functions have been shown to vary between different cell types. Also the functions appear to vary between activation of proliferation, self-renewal and possibly cellular competition. To completely understand the processes in which Dom is involved it will be required to study which cofactors interact with Dom and in which chromatin remodeling complexes Dom executes its diverse functions.