A genome-wide phylogeny of the family Adrianichthyidae and redescription of the monotypic Horaichthys setnai

The family Adrianichthyidae comprises over 40 species distributed across East, South-East, and South Asia and represents an important system for studying evolutionary diversification in tropical and subtropical regions. However, incongruences between morphology-based taxonomy and molecular phylogenies remain unresolved, particularly regarding the systematic position of Oryzias setnai, a species endemic to western peninsular India originally described as Horaichthys setnai. Here, we reconstruct a genome-wide nuclear phylogeny of Adrianichthyidae using more than 3000 single-copy orthologous genes identified by BUSCO, incorporating a newly generated genome assembly for O. setnai. Our analyses support O. setnai as sister to all other adrianichthyids, contrary to morphology-based classification. Morphological re-examination confirms that O. setnai possesses numerous autapomorphies absent from Oryzias, including reproductive traits associated with internal fertilization, distinctive skeletal features, and a unique body morphology. Together, these data support the reinstatement of the genus Horaichthys. Accordingly, we redescribe the species under its original combination, Horaichthys setnai. Notably, this lineage exhibits a markedly elongated branch, indicating an accelerated rate of molecular evolution in a phylogenetically isolated lineage lacking close extant relatives. This combination of rapid molecular evolution and phylogenetic isolation provides insights into evolutionary processes underlying morphological and reproductive autapomorphies, as well as lineage-specific variation in diversification rates.