Zelda and the evolution of insect metamorphosis
Authors: Ventos-Alfonso Alba , Ylla Guillem , Belles Xavier 
Published: July 2018

In the Endopterygote Drosophila melanogaster, Zelda is a key activator of the zygotic genome during the maternal-to-zygotic transition (MZT). Zelda binds cis-regulatory elements (TAGteam heptamers), and makes chromatin accessible for gene transcription. Recently, Zelda has been studied in two other Endopterygotes: Apis mellifera and Tribolium castaneum, and the Paraneopteran Rhodnius prolixus. We have studied Zelda in the cockroach Blattella germanica, a hemimetabolan, short germ-band, and Polyneopteran species. Zelda protein of B. germanica has the complete set of functional domains, which is typical of lower insects. The TAGteam heptamers of D. melanogaster have been found in the B. germanica genome, and the canonical one, CAGGTAG, is present at a similar relative number in the genome of these two species and in the genome of other insects, suggesting that, although within certain evolutionary constraints, the genome admits as many CAGGTAG motifs as its length allows. Zelda-depleted embryos of B. germanica show defects involving the blastoderm formation and the abdomen development and have genes contributing to these processes down-regulated. We conclude that in B. germanica Zelda strictly activates the zygotic genome, within the MZT, a role conserved in more derived Endopterygote insects. In B. germanica, Zelda is expressed during MZT, whereas in D. melanogaster and T. castaneum it is expressed well beyond this transition. Moreover, in these species and A. mellifera, Zelda has functions even in postembryonic development. The expansion of Zelda expression and functions beyond the MZT in holometabolan species might have been instrumental for the evolutionary transition from hemimetaboly to holometaboly. In particular, the expression of Zelda beyond the MZT during embryogenesis might have allowed building the morphologically divergent holometabolan larva.

Author summary

In early insect embryo development, the protein Zelda is a key activator of the zygotic genome during the maternal-to-zygotic transition. This has been thoroughly demonstrated in the fruit fly Drosophila melanogaster, as well as in the red flour beetle Tribolium castaneum, both species belonging to the most modified clade of endopterygote insects, showing complete (holometabolan) metamorphosis. In these species, Zelda is expressed and have functions in early embryogenesis, in late embryogenesis and in postembryonic stages. We have studied Zelda in the German cockroach, Blattella germanica, which belong to the less modified clade of polyneopteran insects, showing an incomplete (hemimetabolan) metamorphosis. In B. germanica, Zelda is significantly expressed in early embryogenesis, being a key activator of the zygotic genome during the maternal-to-zygotic transition, as in the fruit fly and the red flour beetle. Nevertheless, Zelda is not significantly expressed, and presumably has no functions, in late embryogenesis and in postembryonic stages of the cockroach. The data suggest that the ancestral function of Zelda in insects with hemimetabolan metamorphosis was to activate the zygotic genome, a function circumscribed to early embryogenesis. The expansion of Zelda expression and functions to late embryogenesis and postembryonic stages might have been a key step in the evolutionary transition from hemimetaboly to holometaboly. In hemimetabolan species embryogenesis produces a nymph displaying the essential adult body structure. In contrast, embryogenesis of holometabolan species produces a larva that is morphologically very divergent from the adult. Expression of Zelda in late embryogenesis might have been a key step in the evolution from hemimetaboly to holometaboly, since it would have allowed the building the morphologically divergent holometabolan larva.