Genome-wide identification of nuclear receptor (NR) genes and the evolutionary significance of the NR1O subfamily in the monogonont rotifer Brachionus spp.

Nuclear receptors (NRs) are a large family of transcription factors that are involved in many fundamental biological processes. NRs are considered to have originated from a common ancestor, and are highly conserved throughout the whole animal taxa. Therefore, the genome-wide identification of NR genes in an animal taxon can provide insight into the evolutionary tendencies of NRs. Here, we identified all the NR genes in the monogonont rotifer Brachionus spp., which are considered an ecologically key species due to their abundance and world-wide distribution. The NR family was composed of 40, 32, 29, and 32 genes in the genomes of the rotifers B. calyciflorus, B. koreanus, B. plicatilis, and B. rotundiformis, respectively, which were classified into seven distinct subfamilies. The composition of each subfamily was highly conserved between species, except for NR1O genes, suggesting that they have undergone sporadic evolutionary processes for adaptation to their different environmental pressures. In addition, despite the dynamics of NR evolution, the significance of the conserved endocrine system, particularly for estrogen receptor (ER)-signaling, in rotifers was discussed on the basis of phylogenetic analyses. The results of this study may help provide a better understanding the evolution of NRs, and expand our knowledge of rotifer endocrine systems.