Absence of Toll-like Receptor 21 (TLR21) Gene in the Genome of Transparent Glass Catfish (Kryptopterus vitreolus)

This study investigates the genomic basis of immune adaptation in the transparent glass catfish (<i>Kv</i>: <i>Kryptopterus vitreolus</i>), focusing on the loss of the Toll-like receptor 21 (<i>TLR21</i>) gene. Comparative genomic analysis with closely related non-transparent North African catfish (<i>Cg</i>: <i>Clarias gariepinus</i>) revealed 11 <i>TLR</i> genes in the latter, while only 8 <i>TLR</i> genes (<i>KvTLR1</i>, <i>2</i>, <i>3</i>, <i>5</i>, <i>7</i>, <i>9</i>, <i>13</i>, and <i>20</i>) were retained in the glass catfish, with <i>TLR21</i> specifically absent. Collinearity analysis confirmed that the genomic region containing <i>TLR21</i> is conserved across eight siluriform species, with loss exclusively in the glass catfish, supporting its lineage-specific absence. Structural expansion was notable in <i>KvTLR5</i>, <i>KvTLR7</i>, and <i>KvTLR20</i>. Molecular docking indicated that binding stability between CpG oligonucleotides and TLR21 varies significantly, with CpG-B 1681 showing the strongest interaction, which highlights sequence-dependent ligand recognition. Interestingly, absence of the <i>TLR1</i> gene in another transparent teleost, the X-ray tetra (<i>Pristella maxillaris</i>), suggests that transparent fishes may share an evolutionary trend of lineage-specific <i>TLR</i> gene loss. Together, these findings reveal a distinctive evolutionary trajectory in the innate immune receptor family of transparent fishes and provide new molecular insights into their adaptive immune strategies. These insights will benefit the academic community by improving comparative frameworks for fish innate immunity, and they may inform disease prevention and health management strategies in aquaculture and the ornamental fish trade.