Maternal or zygotic: Unveiling the secrets of the Pancrustacea transcription factor zelda

1 Laboratório Integrado de Bioquı́mica Hatisaburo Masuda, Núcleo em Ecologia e Desenvolvimento SócioAmbiental de Macaé (NUPEM), Rio de Janeiro, Brazil, 2 Instituto Nacional de Ciência e Tecnologia em Entomologia Molecular—INCT-EM, Brazil, 3 Laboratório de Quı́mica e Função de Proteı́nas e Peptı́deos, Centro de Biociências e Biotecnologia, Universidade Estadual do Norte Fluminense Darcy Ribeiro, Rio de Janeiro, Brazil

acts as a maternal repressive domain during fly embryogenesis in vivo, being particularly involved in the transcriptional regulation of zygotic zld targets and maternal mRNA clearance. Collectively, Hamm et al. [13] show for the first time that Zld is a modular TF, with different ZnF domains playing distinct roles in transcriptional regulation during early embryogenesis (Fig 1).
The work by Hamm et al. also poses intriguing questions. The observation that point mutations in the acidic patch and ZnF1 did not alter fly viability suggests at least three scenarios: 1) the amino acid changes did not impair the activity of the domains; 2) these domains are dispensable for Zld-mediated transcriptional activation in D. melanogaster (as hypothesized by the authors) or; 3) these domains are not essential in flies kept under standard laboratory conditions.
We believe that the former two possibilities are unlikely because Zld is a master TF, and the most important (and conserved) residues of the acidic patch and ZnF1 were precisely mutated. The latter hypothesis seems more appealing, as these domains have been conserved for over 300 million years in various insect lineages [12]. Detailed functional analysis of these mutant flies in different environments (e.g., under natural stress conditions) or genetic backgrounds may unveil other modular roles (like those of the ZnF2 JAZ-finger domain) and possible redundancies of these domains. Since the amount of zld expression has been reported to be critical for correct activation of target genes in time and space [4,13], it will also be important to investigate if any of the mutant zld lines display altered gradients of key TFs (e.g., Dorsal and bicoid) by live imaging. Further, analysis of TAD boundaries by Hi-C in zld ZnF2 mutants could provide new hints of Zld's function in the establishment of chromatin architectures [10].
Another interesting finding from Hamm et al. was that a subset of homozygous zld ZnF2 male and female adults displayed small eyes [13], strongly supporting the idea that zld also performs post-embryonic roles, as previously reported in beetles [12]. Importantly, such roles are also supported by zld expression in eye and wing imaginal discs [13,14]. Profound changes in cell proliferation, differentiation, and morphology take place during imaginal disc development, and the potential roles of zld in such processes are yet to be elucidated. Recently, zld was also described as an important factor for neuronal progenitor formation in flies, and its The ZnF-Novel domain is present in most insect orders but has been eroded in Diptera [12]. Zld is a multi-domain ZnF TF, and at least one of its zinc-fingers (i.e., the ZnF2 JAZ-finger) is required for viability via regulation of maternal mRNA clearance and zygotic genome activation [13]. TF, transcription factor; Zld, zelda; ZnF, zinc-finger.
https://doi.org/10.1371/journal.pgen.1007201.g001 inhibition by the tumor suppressor TRIM-NHL protein Brain tumor (Brat) appears to be essential to avoid neuronal tumor progression [15]. Hence, unveiling whether zld gene regulatory networks are similar among these developmental processes will be of great value.
Finally, Hamm et al. open new perspectives in the elucidation of Zld functions. Comparative functional studies in other insect species may provide a unique opportunity to address the roles of zld in early transcriptional reprogramming and unlock further secrets about evolution of this master TF throughout the Pancrustacea.