Abstract
Hox genes code for transcription factors and are evolutionarily conserved. They regulate a plethora of downstream targets to define the anterior-posterior (AP) body axis of a developing bilaterian embryo. Early work suggested a possible role of clustering and ordering of Hox to regulate their expression in a spatially restricted manner along the AP axis. However, the recent availability of many genome assemblies for different organisms uncovered several examples that defy this constraint. With recent advancements in genomics, the current review discusses the arrangement of Hox in various organisms. Further, we revisit their discovery and regulation in Drosophila melanogaster. We also review their regulation in different arthropods and vertebrates, with a significant focus on Hox expression in the crustacean Parahyale hawaiensis. It is noteworthy that subtle changes in the levels of Hox gene expression can contribute to the development of novel features in an organism. We, therefore, delve into the distinct regulation of these genes during primary axis formation, segment identity, and extra-embryonic roles such as in the formation of hair follicles or misregulation leading to cancer. Toward the end of each section, we emphasize the possibilities of several experiments involving various organisms, owing to the advancements in the field of genomics and CRISPR-based genome engineering. Overall, we present a holistic view of the functioning of Hox in the animal world.
Highlights
The development of an animal progresses three-dimensionally across anterior-posterior (AP), dorsal-ventral (DV), and left-right (LR) axes
Homeotic genes or Hox are one of the significant contributors to bilaterian development and are evolutionarily conserved. They are often present in clusters and code for transcription factors (HOX) that act on the downstream genes to provide identity to developing segments along the AP axis of a bilaterian embryo (Akam et al, 1988; Akam, 1998; Lewis, 1998)
Deep sequencing of flanking regions of Hox loci in multiple organisms along with a Bag-of-Motif analysis to understand protein-DNA interactions can shed light on putative regulatory mechanisms associated with the clustering of cisregulatory modules (CRMs)
Summary
The development of an animal progresses three-dimensionally across anterior-posterior (AP), dorsal-ventral (DV), and left-right (LR) axes. They are often present in clusters and code for transcription factors (HOX) that act on the downstream genes to provide identity to developing segments along the AP axis of a bilaterian embryo (Akam et al, 1988; Akam, 1998; Lewis, 1998). The arrangement of these genes on the chromosome followed an order that was collinear to the segments affected in mutants from anterior to posterior direction.
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