Functional equivalence of Hox gene products in the specification of the tritocerebrum during embryonic brain development ofDrosophila

Abstract

Hox genes encode evolutionarily conserved transcription factors involved in the specification of segmental identity during embryonic development. This specification of identity is thought to be directed by differential Hox gene action, based on differential spatiotemporal expression patterns, protein sequence differences, interactions with co-factors and regulation of specific downstream genes. During embryonic development of the Drosophila brain, the Hox gene labial is required for the regionalized specification of the tritocerebral neuromere; in the absence of labial, the cells in this brain region do not acquire a neuronal identity and major axonal pathfinding deficits result. We have used genetic rescue experiments to investigate the functional equivalence of the Drosophila Hox gene products in the specification of the tritocerebral neuromere. Using the Gal4-UAS system, we first demonstrate that the labial mutant brain phenotype can be rescued by targeted expression of the Labial protein under the control of CNS-specific labial regulatory elements. We then show that under the control of these CNS-specific regulatory elements, all other Drosophila Hox gene products, except Abdominal-B, are able to efficiently replace Labial in the specification of the tritocerebral neuromere. We also observe a correlation between the rescue efficiency of the Hox proteins and the chromosomal arrangement of their encoding loci. Our results indicate that, despite considerably diverged sequences, most Hox proteins are functionally equivalent in their ability to replace Labial in the specification of neuronal identity. This suggests that in embryonic brain development, differences in Hox gene action rely mainly on cis-acting regulatory elements and not on Hox protein specificity.

Diversification and Functional Evolution of HOX Proteins

“…Similarly, Drosophila Hox genes have been also shown to have redundant activity in specification of tritocerebrum identity, endocrine ring gland development, dorsal DA3 muscle lineage specification and head repression etc. (Hirth et al, 2001;Coiffier et al, 2008;Enriquez et al, 2010;Sanchez-Higueras et al, 2014). The redundant role of Hox genes is not limited to Drosophila, vertebrate HOX proteins have also been shown to have overlapping or redundant functions during development of several tissue types (Young et al, 2009;Lacombe et al, 2013;Denans et al, 2015).…”