UC Berkeley

The diversity of animals is a topic of childlike fascination, poetic wonder, and scientific discovery; but how did such great diversity of form and function come to be? To uncover the possible mechanisms by which diverse new body plans are generated over evolutionary time, we turn to understanding the patterns and processes behind how individual bodies are constructed — and deviated — during embryonic development. Critically important to the specification of body plan diversity is the patterning of the anterior-posterior axis and establishment of regional identities by the Hox genes.

Hox genes have a revolutionizing history, complete with striking transformation of form upon their mutation. The rapid discovery of orthologous sets of Hox genes in a diversity of species led to the exciting realization that not only are individual Hox genes conserved across Bilateria, so too are their patterns of expression, mutation, arrangement, and properties in aggregate—changes to which have been implicated as important generators of evolutionary diversification. This apparent conservation enabled many useful generalizations to be gleaned from a limited number of representative species, and paradigms such as posterior prevalence became widespread. Increasingly, however, new technologies enable us to expand the scope of our study to a wider array of species and cases breaking the “rules” of these models are accumulating. This warrants the comprehensive study of Hox genes in aggregate in a greater number of representative species to resolve a fuller picture of the Hox complex and its interactions during the specification of regionalized bodies and the diversification of specialized forms.

By resolving Hox expression patterns in transformed individuals following Hox gene knockout in the appendage-diverse amphipod crustacean Parhyale hawaiensis, we have revealed a number of cross-regulatory interactions among Hox genes that influence the patterning of Hox domains. Furthermore, by amassing a fuller picture of the Hox expression underlying the appendage transformations, we make a more informed model of the functional roles and interactions of the different Hox genes in the designation of limb identities. We have shown that posterior prevalence is not sufficient to predict the appendage transformations in Parhyale and that a modular ‘Hox code’ expressed in individual segments functions to establish segmental identity. We believe this study demonstrates that interactions among Hox genes provide a mechanism for the diversification of appendage morphologies and arrangements over evolutionary time.