AUTHOR=Cheng Yunying , Chen Jinlin , Mukhtar Irum , Chen Jianming 

TITLE=Genome-Wide Characterization of the Nuclear Receptor Gene Family in Macrostomum lignano Imply Its Evolutionary Diversification

JOURNAL=Frontiers in Marine Science

VOLUME=8

YEAR=2021

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2021.653447

DOI=10.3389/fmars.2021.653447

ISSN=2296-7745

ABSTRACT=<p>Nuclear receptors (<italic>NRs</italic>), a series of key transcription factors that are mostly activated by endogenous ligands or environmental xenobiotics, are reportedly good phylogenetic markers of animal genome evolution. As the early diverging class of bilaterians, however, a comprehensive view of the <italic>NR</italic> family in a marine free-living flatworm <italic>Macrostomum lignano</italic> and comparative information in flatworms are still lacking, which is of significance to address the evolutionary diversification of the <italic>NR</italic> family and imply the adaptive evolution in the early diverging Bilateria. Herein, a total of 51, 26, and 23 putative <italic>NR</italic> genes were identified in <italic>M. lignano</italic>, <italic>Sparganum proliferum</italic>, and <italic>Clonorchis sinensis</italic>, respectively, which were classified into eight subfamilies, implying an extensive expansion of the <italic>NR</italic> family in <italic>M. lignano</italic>. It is presumed that the extensive expansion was mainly attributed to the <italic>M. lignano</italic>-specific hidden polyploidy, segmental, and tandem duplication events. The duplicated <italic>NR</italic> pairs in <italic>M. lignano</italic> and the <italic>NR</italic> orthologs in flatworms all experienced the purifying selection. Phylogenetic analyses indicated the presence of <italic>NR3</italic>-like genes in <italic>M. lignano</italic>, which is first reported in flatworms. Intron loss and reduced intron size were mainly contributed to the structural divergence of <italic>NR</italic> genes in flatworms. The combined data provide indispensable information for a better understanding of the complexity and the adaptive evolution of the <italic>NR</italic> gene family in metazoans.</p>