Gene amplification and functional diversification of melanocortin 4 receptor at an extremely polymorphic locus controlling sexual maturation in the platyfish.

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.

A novel marker for the platyfish (Xiphophorus maculatus) W chromosome is derived from a Polinton transposon.

A consensus sequence, encoding a putative DNA polymerase type B derived from a Polinton transposon, was assembled from the sex determination region of Xiphophorus maculatus. This predicted protein, which is 1,158 aa in length, contains a DNA_pol_B_2 domain and a DTDS motif. The DNA polymerase type B gene has about 10 copies in the haploid X.

Molecular analysis of the sex chromosomes of the platyfish Xiphophorus maculatus: Towards the identification of a new type of master sexual regulator in vertebrates.

In contrast to mammals and birds, fish display an amazing diversity of genetic sex determination systems, with frequent changes during evolution possibly associated with the emergence of new sex chromosomes and sex-determining genes. To better understand the molecular and evolutionary mechanisms driving this diversity, several fish models are studied in parallel. Besides the medaka (Oryzias latipes Temminck and Schlegel, 1846) for which the master sex-determination gene has been identified, one of the most advanced models for studying sex determination is the Southern platyfish (Xiphophorus maculatus, Günther 1966).

Tissue-specific expression of dmrt genes in embryos and adults of the platyfish Xiphophorus maculatus.

The vertebrate dmrt gene family encodes transcription factors with a characteristic DNA-binding motif called the DM domain. The best studied member is dmrt1, which is involved in sexual development in fish and tetrapods. The cloning of dmrt5 from the platyfish Xiphophorus maculatus and the expression pattern of dmrt1, dmrt2a, dmrt4, and dmrt5 in adults and embryos are reported.

Molecular analysis of the sex-determining region of the platyfish Xiphophorus maculatus.

Due to the presence of genetically well-defined sex chromosomes, with a relatively restricted sex-determination region containing markers identified at the molecular level, the platyfish Xiphophorus maculatus is one of the best models for the positional cloning of a master sex-determining gene in fish. Both male and female heterogametes and three different types of sex chromosomes have been described in the platyfish, with several loci involved in pigmentation, melanoma formation, and sexual maturity closely linked to the master sex-determining locus. Using the melanoma-inducing oncogene Xmrk, its protooncogenic counterpart egfrb, as well as other X- and Y-linked molecular markers, bacterial artificial chromosome (BAC) contigs have been assembled for the sex-determining region of X.

Transposable elements as drivers of genomic and biological diversity in vertebrates.

Comparative genomics has revealed that major vertebrate lineages contain quantitatively and qualitatively different populations of retrotransposable elements and DNA transposons, with important differences also frequently observed between species of the same lineage. This is essentially due to (i) the differential evolution of ancestral families of transposable elements, with evolutionary scenarios ranging from complete extinction to massive invasion; (ii) the lineage-specific introduction of transposable elements by infection and horizontal transfer, as exemplified by endogenous retroviruses; and (iii) the lineage-specific emergence of new transposable elements, as particularly observed for non-coding retroelements called short interspersed elements (SINEs). During vertebrate evolution, transposable elements have repeatedly contributed regulatory and coding sequences to the host, leading to the emergence of new lineage-specific gene regulations and functions.

Helitron Transposons on the Sex Chromosomes of the Platyfish Xiphophorus maculatus and Their Evolution in Animal Genomes.

The sex-determining region of the sex chromosomes of the platyfish Xiphophorus maculatus contains several copies of a recently described class of DNA transposons called Helitrons, which probably transpose through a mechanism involving rolling circle replication. The unique open reading frame of platyfish elements encodes a 2816 amino-acid protein with helicase and replication initiator (Rep) domains, which are hallmarks of Helitrons. Like previously described elements from zebrafish but unlike sequences from plants, insects and nematodes, platyfish Helitrons also encode a C-terminal apurinic-apyrimidinic-like endonuclease probably captured from a non-long-terminal- repeat retrotransposon.

Evolution of melanocortin receptors in teleost fish: the melanocortin type 1 receptor.

The melanocortin type 1 receptor (Mc1r) belongs to a family of G-protein-coupled receptors involved in various physiological processes in vertebrates. Melanocortins, the Mcr natural agonists, are pituitary peptide hormones including adrenocorticotropin and melanocyte-stimulating hormones. In mammals and birds, Mc1r is involved in pigmentation and expressed in melanocytes and melanoma.

Transposable elements as a source of genetic innovation: expression and evolution of a family of retrotransposon-derived neogenes in mammals.

A family of functional neogenes called Mart, related to the gag gene of Sushi-like long terminal repeat retrotransposons from fish and amphibians, is present in the genome of human (11 genes) and other primates, as well as in mouse (11 genes), rat, dog (12 genes), cat, and cow. Mart genes have lost their capacity of retrotransposition through non-functionalizing rearrangements having principally affected long terminal repeats and pol open reading frame. Most Mart genes are located on the X chromosome in different mammals.

The Xmrk oncogene can escape nonfunctionalization in a highly unstable subtelomeric region of the genome of the fish Xiphophorus.

The Xmrk oncogene involved in melanoma formation in the fish Xiphophorus was formed relatively recently by duplication of the epidermal growth factor co-orthologue egfrb. In the platyfish X. maculatus, Xmrk is located close to the major sex-determining locus in a subtelomeric region of the X and Y sex chromosomes that frequently undergoes duplications and other rearrangements.