Hox expression in the direct-type developing sand dollar Peronella japonica

Area of Science:

• Developmental Biology
• Evolutionary Biology
• Genomics

Background:

• Echinoderms, including sea urchins and sand dollars, are deuterostomes with a unique pentameral body plan, distinct from their hemichordate relatives.
• Hox gene complexes are crucial for axial patterning in bilaterians, typically organized in chromosomal clusters.
• The organization of the Hox cluster in sea urchins is atypical compared to hemichordates, with limited data on Hox expression in developing echinoderms.

Purpose of the Study:

• To comprehensively investigate the expression patterns of Hox genes in the adult rudiment of a sand dollar echinoid.
• To understand the roles of different Hox genes in axial patterning and morphological development within echinoderms.
• To explore the evolutionary implications of Hox gene co-option in the transition from bilateral ancestors to radially symmetric echinoderms.

Main Methods:

• Comprehensive examination of Hox gene expression in the adult rudiment of a sand dollar.
• Classification of expressed Hox genes based on their spatial expression patterns (linear vs. radial).
• Analysis of the correlation between Hox gene expression and specific developmental processes and morphological features.

Main Results:

• Nine Hox genes are expressed in the sand dollar adult rudiment, falling into two main expression groups: linear and radial.
• Hox11/13b exhibits both linear expression in the coelomic mesoderm and radial expression around the adult mouth.
• Linear Hox gene expression may guide digestive tract transformation, while radial expression correlates with echinoderm/sea urchin novelties; radial gene recruitment appears linked to loss of ancestral axial roles.

Conclusions:

• The study provides a detailed map of Hox gene expression in a sand dollar rudiment.
• Findings suggest that the co-option of Hox genes into distinct linear and radial expression patterns is key to echinoderm evolution.
• This comprehensive analysis offers insights into the molecular basis of echinoderm evolution from a bilateral ancestor.