Estrogen receptors in Xenopus: duplicate genes, splice variants, and tissue-specific expression

Abstract

The estrogenic steroid hormones, acting primarily through the nuclear estrogen receptors ERα and ERβ, regulate sexual differentiation in a wide variety of vertebrates. In the frog Xenopus laevis, estrogen regulates the strength of vocal neuromuscular synapses and contributes to the physiological basis of sexually differentiated songs. To understand the mechanisms by which estrogen produces these effects, we have characterized the ERs of X. laevis and their expression in laryngeal muscle and other tissues. We found a remarkable molecular diversity in the estrogen receptor population within individuals. First, we have identified two distinct ERα genes, xlERα1 and xlERα2, which represent, to our knowledge, the first discovery of retained duplicates of the ERα gene in any species. These two genes are highly conserved at the amino acid level but have distinct nucleotide sequences; moreover, ERα2 has no N-terminal domain. Cloning of ERα and ERβ in the related species Xenopus tropicalis and phylogenetic analysis indicate that the two xlERα loci were generated by a duplication specific to the X. laevis lineage—most likely the genome duplication that led to a doubling of the X. laevis chromosome number about 30 million years ago. The primary ER expressed in X. laevis laryngeal muscle is the novel gene xlERα2; ERα1 is primarily expressed in liver, forebrain, and oviduct. Alternatively spliced transcripts of both xlERα1 and xlERα2 are also expressed in a tissue-specific manner. We propose that complementary spatial expression of these two genes and their alternatively spliced transcripts contributes to their conservation over such a long period of time, consistent with the subfunctionalization model for evolution after gene duplication.