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

## TL;DR

Transparent glass catfish lack a key immune gene, TLR21, and have expanded other immune genes, suggesting evolutionary adaptations in their immune system.

## Contribution

The study identifies the lineage-specific loss of TLR21 in the glass catfish and reveals structural expansions in other TLR genes.

## Key findings

- Glass catfish lack the TLR21 gene, which is present in most other fish species.
- Structural expansions in TLR5, TLR7, and TLR20 suggest compensatory immune adaptations.
- X-ray tetra also lacks TLR1, indicating a possible evolutionary trend in transparent fish.

## Abstract

Transparent fishes, such as the ornamental glass catfish, have evolved unique transparent bodies, but how their immune system adapts for defense against exogenous pathogens remains unclear. This study compared the genomes of the glass catfish and its close relative, the non-transparent North African catfish. We found that the glass catfish has lost an important immune gene called TLR21, which is present in most other fish species. Meanwhile, some of the remaining immune genes in the glass catfish have been expanded to become more complex. Our results showed that the TLR21 protein in the North African catfish can potentially recognize pathogen molecules, but its binding strength varies greatly depending on the molecular sequence from various teleosts. We also discovered that another transparent fish, the X-ray tetra, has lost another TLR gene, TLR1. These interesting findings suggest that transparent fishes may have adjusted their immune defenses during evolution by losing certain immune gene(s) while strengthening others. In summary, this research provides new insights into how transparent fishes adapt to the aquatic environment, and improves our understanding of the immune evolution in fish.

This study investigates the genomic basis of immune adaptation in the transparent glass catfish (Kv: Kryptopterus vitreolus), focusing on the loss of the Toll-like receptor 21 (TLR21) gene. Comparative genomic analysis with closely related non-transparent North African catfish (Cg: Clarias gariepinus) revealed 11 TLR genes in the latter, while only 8 TLR genes (KvTLR1, 2, 3, 5, 7, 9, 13, and 20) were retained in the glass catfish, with TLR21 specifically absent. Collinearity analysis confirmed that the genomic region containing TLR21 is conserved across eight siluriform species, with loss exclusively in the glass catfish, supporting its lineage-specific absence. Structural expansion was notable in KvTLR5, KvTLR7, and KvTLR20. 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 TLR1 gene in another transparent teleost, the X-ray tetra (Pristella maxillaris), suggests that transparent fishes may share an evolutionary trend of lineage-specific TLR 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.