Gene Amplification and Functional Diversification of Melanocortin 4 Receptor at an Extremely Polymorphic Locus Controlling Sexual Maturation in the Platyfish

Abstract

In two swordtail species of the genus Xiphophorus, the onset of puberty has been shown to be modulated at the P locus by sequence polymorphism and gene copy-number variation affecting the type 4 melanocortin hormone receptor Mc4r. The system works through the interaction of two allelic types, one encoding wild type and the other dominant-negative receptors. We have analyzed the structure and evolution of the P locus in the platyfish Xiphophorus maculatus, where as many as nine alleles of P determining the onset of sexual maturity in males and females, fecundity in females, and adult size in males are located on both the X and Y chromosomes in a region linked to the master sex-determining locus. In this species, mc4r has been amplified to up to 10 copies on both the X and Y chromosomes through recent large serial duplications. Subsequently, mc4r paralogues have diverged considerably into many different subtypes. Certain copies have acquired new untranslated regions through genomic rearrangements, and transposable element insertions and other mutations have accumulated in promoter regions, possibly explaining observed deviations from the classical mc4r transcriptional pattern. In the mc4r-coding sequence, in-frame insertions and deletions as well as nonsense and missense mutations have generated a high diversity of Mc4r-predicted proteins. Most of these variants are expressed in embryos, adults, and/or tumors. Functional receptor characterization demonstrated major divergence in pharmacological behavior for Mc4r receptors encoded by different copies of platyfish mc4r, with differences in constitutive activity as well as binding and stimulation by hormones. The high degree of allelic and copy-number variation observed between individuals can explain the high level of polymorphism for sexual maturation, fecundity, and body size in the platyfish: multiple combinations of Mc4r variants with different biochemical properties might interact to modulate the melanocortin signaling that regulates the hypothalamus-pituitary-gonadal axis.T ELEOST fish of the genus Xiphophorus, which comprise two major and morphologically quite divergent groups, the platyfish and swordtails, are prominent models for the study of polymorphic traits such as sexual development, reproduction, pigmentation, and cancer (Kallman

Transposable elements and early evolution of sex chromosomes in fish

“…If the recombination is not completely suppressed between the X and Y, occasional transfer of blocks of TEs from the Y to the X may have occurred. In a different manner, we could think that the region was unstable before becoming a SD region and therefore more prone to become the SD region through gene duplication (sustained by the presence of many mc4R genes; Lampert et al 2010;Volff et al 2013). In this case, X and Y regions remain unstable.…”

A new model army: Emerging fish models to study the genomics of vertebrate Evo‐Devo

“…These skin cancers are caused by Xmrk , an oncogenic, sex‐linked, local gene duplicate of the receptor tyrosinase gene egfrb (Meierjohann and Schartl, ); Xmrk has been proposed to represent a Xiphophorus “speciation gene” (see Schartl [] for a critical discussion). The sex‐linked “puberty locus” determining the onset of male maturity and mating behavior in Xiphophorus appears to be caused by copy number variation of multiple melanocortin 4 receptor ( mc4r ) gene duplicates that were identified by sequencing BAC clones from the highly repetitive platyfish sex chromosomes (Lampert et al, ; Volff et al, ).…”

A multicopy Y-chromosomal SGNH hydrolase gene expressed in the testis of the platyfish has been captured and mobilized by a Helitron transposon

“…Only a few functional genes have been described in the sex-determining region, including the multicopy melanocortin receptor gene mc4r , which controls the onset of puberty and is present on both the X and the Y chromosomes [14,27]. …”