Abstract: Catfishes (order Siluriformes) inhabit diverse aquatic environments worldwide, ranging from tropical lowlands to fast-flowing mountain rivers. However, the contribution of genomic variation to ecological diversification in catfishes, particularly within high-altitude lineages, remains poorly understood. Here, we investigated genome evolution in an extreme environment by focusing on Glyptosternum maculatum, the highest-altitude-dwelling catfish known and a basal lineage of Glyptosternoid fishes. We generated a high-quality, haplotype-resolved genome assembly using PacBio HiFi and Hi-C sequencing. The two phased haplotypes span 741.8 Mb and 753.7 Mb, with contig N50 values >4 Mb, representing a substantial improvement in continuity and accuracy over previous assemblies. Comparative genomic analyses revealed significant contraction of gene families associated with G protein-coupled receptor activity, particularly olfactory receptor (OR) and trace amine-associated receptor (TAAR) genes. Water-soluble OR subfamilies showed a pronounced reduction relative to other catfishes, suggesting lineage-specific remodeling of chemosensory repertoires that may reflect ecological specialization in high-altitude river systems. Population genetic analyses identified chromosome 11 as the sex chromosome, harboring an ~0.97 Mb sex-determining region (SDR) consistent with an XX/XY system. The X and Y chromosomes are highly similar and lack evolutionary strata, indicating a young, homomorphic pair. Despite limited sequence divergence, the SDR shows an enrichment of LINE retrotransposons, with evidence of recent transposable element accumulation extending into adjacent regions. Our results provide a chromosome-level genomic resource for Glyptosternoid fishes and offer new insights into sensory gene evolution and the early dynamics of sex chromosome differentiation in a high-altitude teleost.

Key words: catfish, chemosensory gene, genome evolution, sex chromosomes