Evolution of the Chorionic Gonadotropin β Genes in Primates

Abstract

Abstract Several genes associated with reproduction have been duplicated during primate evolution and evolved under positive selection. Chorionic gonadotropin hormone (HCG) produced by trophoblast cells has emerged in primates and assists maternal physiology to establish pregnancy. CGB gene, coding for the HCG β subunit has evolved through a duplication of the ancestral lutenizing hormone β ( LHB ) gene 55–35 million years ago. Only a few sequence changes in the duplicate locus were required to result in a novel CGB gene with different tissue‐specificity and function. Evolution of the LHB / CGB genomic region has been accompanied by active genome dynamics either as series of additional duplications in Old World monkeys and apes, or as functional modifications of genes in New World monkeys. Duplicated, highly homologous genes with diverged functions are maintained through a balance between selective pressures targeted to their specific functions and evolution in consort facilitated by interlocus gene conversion. Key Concepts Several genes associated with female and male reproduction have been shown to be duplicated during primate evolution and evolve under positive selection. Chorionic gonadotropin hormone (HCG) produced by the trophoblast cells has emerged in primates and acts as a key hormone to assist maternal physiology to establish pregnancy. Primate‐specific CGB gene, coding for the β subunit of HCG has emerged through a duplication of the ancestral lutenizing hormone β ( LHB ) gene approximately 55–35 million years ago. Only a few sequence changes in the duplicate locus were required to result in a novel CGB gene with different tissue‐specificity, expression pattern and function. Evolution of the LHB / CGB genomic region in primates has been accompanied by active genome dynamics either as a series of additional duplications in the lineage to OWM and apes, or as functional modifications of the gene loci in NWM. Comparison of the human and chimpanzee LHB/CGB gene clusters identified large species‐specific parallel rearrangement events, resulting in lineage‐specific number of CGB gene copies (five versus six genes in chimpanzee compared to human). Duplicated genes tend to evolve in consort facilitated by active interlocus gene conversion increasing and preserving sequence similarity among the gene copies. Intraspecies gene conversion events have led to a higher human–chimpanzee divergence between duplicated LHB/CGB genes compared to the available data for unique genomic regions. Duplicated, highly homologous genes with diverged functions are maintained through balancing interlocus gene conversion activity with selective pressure targeting their specific functions.