The evolution of molluscs.

Area of Science:

• * Evolutionary biology
• * Developmental biology
• * Phylogenomics

Background:

• * Molluscs are a diverse phylum with significant ecological and economic roles.
• * Their evolutionary history and phenotypic diversification have been historically challenging to resolve.
• * Traditional hypotheses suggested a gradual increase in complexity from simple to complex forms.

Purpose of the Study:

• * To investigate the evolutionary origins and phenotypic diversification of molluscs.
• * To re-evaluate traditional hypotheses using modern phylogenomic and developmental data.
• * To understand the conservation and variation of developmental pathways within molluscs.

Main Methods:

• * Phylogenomic analyses to determine the higher-level relationships within Mollusca.
• * Comparative gene expression studies, focusing on developmental regulatory genes like Hox genes.
• * Analysis of conserved signaling pathways such as bone morphogenetic protein (BMP)/decapentaplegic (Dpp) and genes involved in chirality.

Main Results:

• * Phylogenomic studies support a dichotomy: Aculifera (aplaco- and polyplacophorans) and Conchifera (other molluscs).
• * This contradicts the view of gradual evolution of complexity, suggesting secondary simplification in some groups (e.g., aplacophorans).
• * Hox genes lost anterior-posterior patterning roles in conchiferans, being co-opted for novelties. BMP/Dpp and chirality pathways show conservation with variations.

Conclusions:

• * Molluscan phylogeny is characterized by a fundamental split, not a simple progression of complexity.
• * Developmental pathways are conserved across metazoans but exhibit unique modifications within molluscan lineages.
• * Genome editing tools like CRISPR/Cas9 can test novel hypotheses on molluscan evolution derived from developmental variations.