Hemoglobin gene repertoire in teleost and cichlid fishes shaped by gene duplications and genome rearrangements

Hemoglobin is a crucial element of the oxygen transport system in vertebrates. It exhibits remarkable gene diversity across teleost fishes, reflecting their evolutionary adaptations for thriving in various aquatic environments. In this study, we present the dynamic evolution of hemoglobin subunit genes based on a comparison of high quality long-read genome assemblies of 24 vertebrate species, including 16 teleosts (of which six are cichlids). Our findings indicate that teleost genomes contain between five (fugu) and 43 (salmon) hemoglobin genes, representing the largest hemoglobin gene repertoire among vertebrates. We find evidence that the ancestor of teleosts had at least four Hb and three or four Hb{beta} subunit genes, and that the current gene diversity emerged during subsequent teleost radiation, driven primarily by (tandem) gene duplications, genome compaction, and rearrangement dynamics. We provide insights into the genomic organization of hemoglobin clusters, revealing the parallel origin of multiple clusters in tetrapods and in teleosts. Importantly, we show that the presence of paralogous rhbdf1 genes flanking both teleost hemoglobin clusters (LA and MN) supports the hypothesis for the origin of the LA cluster by rearrangement within teleosts, rather than by the teleost specific whole-genome duplication. We specifically focus on cichlid fishes, where adaptation to low oxygen environments has been shown to play roles in species diversification. Our analysis of six cichlid genomes, including the Pungu maclareni from crater lake Barombi Mbo, for which we sequenced the representative genome, reveals 18 to 31 copies of the Hb genes, and elevated rates of non- synonymous substitutions compared to other teleosts. Overall, this work facilitates a deeper understanding of how hemoglobin genes contribute to the adaptive and diversification potential of teleosts.