# Escaping Constraints to Innovate: Maternal Neofunctionalization in a HoxB4 Duplicate

## TL;DR

This paper shows how a duplicated Hox gene in frogs evolved new roles in early development by escaping its usual constraints.

## Contribution

The study reveals maternal neofunctionalization of HoxB4L through cis-regulatory and structural changes in Xenopus laevis.

## Key findings

- HoxB4L acquired maternal expression and is evolving under positive selection.
- Protein-level changes in HoxB4L include altered functional regions and structural modifications.
- The gene escaped ancestral constraints through regulatory and structural divergence.

## Abstract

Transcription factors are typically thought to play a limited role in developmental evolution due to their high pleiotropic nature. However, such constraints may be relaxed following gene duplication or when proteins are organized into structural and functional modules, opening avenues for evolutionary innovation. Here, we integrate expression and genomic data to investigate the evolutionary dynamics of Hox gene duplicates in the allotetraploid frog Xenopus laevis. Despite overall conservation across the Hox clusters, we find that HoxB4L has acquired expression during maternally regulated stages and is evolving under positive selection. Protein‐level changes include the number, length, and sequence of functionally important protein regions. Our results indicate that HoxB4L has escaped ancestral constraints and is undergoing maternal neofunctionalization as a result of cis‐regulatory divergence and structural protein modifications. These findings illustrate how transcription factors can overcome developmental constraints and contribute to novel functions during early development.

Whole‐genome duplication in Xenopus laevis generated duplicated Hox genes that are largely constrained in sequence and developmental expression. However, HoxB4L uniquely acquired maternal expression through cis‐regulatory and protein structural divergence, illustrating how gene duplication enables transcription factors to escape pleiotropic constraints and evolve novel roles in early development.