Ancestral Origin and Functional Expression of a Hyaluronic Acid Pathway Complement in Mussels

Area of Science:

• Biochemistry
• Evolutionary Biology
• Marine Biology

Background:

• Hyaluronic acid (HA) is a vital vertebrate extracellular matrix component involved in cell adhesion and immune regulation.
• Its presence in invertebrates is debated due to the absence of key molecular pathway components.
• The extracellular link protein (XLINK) is crucial for HA function in vertebrates.

Purpose of the Study:

• To investigate the presence and functional potential of hyaluronic acid (HA) and its associated genes in marine mussels.
• To analyze the conservation, structure, and evolutionary origin of the extracellular link protein (XLINK) gene in mussels.
• To determine the expression patterns of XLINK genes during mussel development.

Main Methods:

• Genomic and transcriptomic data mining to identify and analyze XLINK gene loci.
• Structural and phylogenetic analyses to compare mussel XLINK with vertebrate orthologs.
• Biochemical assays to quantify HA levels in *Mytilus galloprovincialis* tissues.
• Gene expression analysis during developmental stages in three mussel species.

Main Results:

• Marine mussels (*Mytilus* spp.) possess HA, with levels up to 1.02 ng/mg in mantle tissue.
• Mussel genomes encode two conserved XLINK gene loci with a complex evolutionary history.
• XLINK genes are expressed during development, ranking among the top genes in *M. trossulus* at 17 h post-fertilization.

Conclusions:

• The findings confirm that mussels can produce HA and possess functional XLINK genes.
• This suggests a potential role for HA synthesis and utilization in mussels, similar to vertebrates.
• Mussels represent a rare invertebrate group encoding genes for the HA pathway, offering a model for studying its evolution and function.