Duplication of the southern white rhinoceros (Ceratotherium simum simum) luteinizing hormone β subunit gene

Abstract

Luteotropic glycoprotein hormones (LGH) include luteinizing hormone (LH) and chorionic gonadotropin (CG). The order Primates is the only phylogenetic clad known to exhibit more than one LGHβ subunit gene per haploid genome. In the present study, we report the discovery of a second case of LGHβ gene replication, in the white (w) rhinoceros (r or rhino). The presence of more than one gene was strongly suggested by a complex banding pattern observed on Southern blots of DNA prepared from two unrelated white rhinos. The existence of two LGHβ genes per haploid genome was estimated by genomic equivalence assay. However, genomic restriction-site mapping studies, together with other findings, suggested that the replicates are probably not tandemly arranged as occurs in primates. A simple band pattern was observed in Southern blots of four other perissodactyl species, indicating that a single-copy LHβ gene is the consensus condition. Two distinct white rhino LHβ genomic clones (wrLHβ1 and wrLHβ2) were isolated. The nucleotide sequence of wrLHβ1 was identical with that of wrLHβ2, except that the latter lacked the consensus mammalian LGHβ second intron. Sequences of the TATA-containing proximal 5'- flanking regions of the two genes were homologous to at least -57 relative to the site of pituitary transcriptional initiation. We conclude that wrLHβ1 is the extant form of the ancestral perissodactyl LHβ gene, whereas wrLHβ2 is a randomly integrated cDNA element (processed gene) reverse transcribed from a partially spliced ancestral wrLHβ1 mRNA. That wrLHβ2 was heritable demonstrates that wrLHβ1 was transcribed in gametes or early conceptus cells contributing to the germline at some point in time since the divergence of white rhinos from other members of the family Rhinocerotidae. Furthermore, because homologous proximal (pituitary) promoter sequence is present in wrLHβ2, it can be concluded that the wrLHβ1 mRNA template from which wrLHβ2 is derived was transcribed from a secondary promoter located upstream of the consensus TATA-regulated pituitary promoter.