Article

Evolution of the relaxin-like peptide family

Howard Florey Institute of Experimental Physiology and Medicine, University of Melbourne, Australia.
BMC Evolutionary Biology (Impact Factor: 3.41). 03/2005; 5:14. DOI: 10.1186/1471-2148-5-14
Source: PubMed

ABSTRACT The relaxin-like peptide family belongs in the insulin superfamily and consists of 7 peptides of high structural but low sequence similarity; relaxin-1, 2 and 3, and the insulin-like (INSL) peptides, INSL3, INSL4, INSL5 and INSL6. The functions of relaxin-3, INSL4, INSL5, INSL6 remain uncharacterised. The evolution of this family has been contentious; high sequence variability is seen between closely related species, while distantly related species show high similarity; an invertebrate relaxin sequence has been reported, while a relaxin gene has not been found in the avian and ruminant lineages.
Sequence similarity searches of genomic and EST data identified homologs of relaxin-like peptides in mammals, and non-mammalian vertebrates such as fish. Phylogenetic analysis was used to resolve the evolution of the family. Searches were unable to identify an invertebrate relaxin-like peptide. The published relaxin cDNA sequence in the tunicate, Ciona intestinalis was not present in the completed C. intestinalis genome. The newly discovered relaxin-3 is likely to be the ancestral relaxin. Multiple relaxin-3-like sequences are present in fugu fish (Takifugu rubripes) and zebrafish (Danio rerio), but these appear to be specific to the fish lineage. Possible relaxin-1 and INSL5 homologs were also identified in fish and frog species, placing their emergence prior to mammalia, earlier than previously believed. Furthermore, estimates of synonymous and nonsynonymous substitution rates (dN/dS) suggest that the emergence of relaxin-1, INSL4 and INSL6 during mammalia was driven by positive Darwinian selection, hence these peptides are likely to have novel and in the case of relaxin-1, which is still under positive selection in humans and the great apes, possibly still evolving functions. In contrast, relaxin-3 is constrained by strong purifying selection, demonstrating it must have a highly conserved function, supporting its hypothesized important neuropeptide role.
We present a phylogeny describing the evolutionary history of the relaxin-like peptide family and show that positive selection has driven the evolution of the most recent members of the family.

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    • "The first hypotheses regarding the origin and diversification of the Rln/Insl-Rxfp signaling systems in vertebrates proposed that three of the peptides (Rln3, Insl5 and Rln) were present prior to the diversification of teleosts, and that the fourth peptide (Insl3) arose in tetrapods (Wilkinson and Bathgate, 2007; Wilkinson et al., 2005a). With the identification of multiple Rln3-and Rxfp3-like molecules in teleosts (Wilkinson et al., 2005a), it was proposed that the signaling of Rln/Insl peptides in teleosts was mediated solely by Rxfp3-like receptors (Wilkinson and Bathgate, Fig. 4. Maximum Likelihood (ML) optimization of the phylogenetic relationship among rln/insl genes across vertebrates, with midpoint rooting. The ML tree is based on DNA sequences including only the first two nucleotides in each codon and employing the miHIV + G model of sequence evolution. "
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    ABSTRACT: Relaxin family peptide receptors (Rxfps) and their ligands, relaxin (Rln) and insulin-like (Insl) peptides, are broadly implicated in the regulation of reproductive and neuroendocrine processes in mammals. Most placental mammals harbour genes for four receptors, namely rxfp1, rxfp2, rxfp3 and rxfp4. The number and identity ofrxfpsin other vertebrates are immensely variable, which is probably attributable to intraspecific variation in reproductive and neuroendocrine regulation. Here, we highlight several interesting, but greatly overlooked, aspects of the rln/insl-rxfp evolutionary history: the ancient origin, recruitment of novel receptors, diverse roles of selection, differential retention and lineage-specific loss of genes over evolutionary time. The tremendous diversity of rln/insl and rxfp genes appears to have arisen from two divergent receptors and one ligand that were duplicated by whole genome duplications (WGD) throughout vertebrate evolution, although several genes, notably relaxin in mammals, were also duplicated via small scale duplications. Duplication and loss of genes have varied across lineages: teleosts retained more WGD-derived genes, dominated by those thought to be involved in neuroendocrine regulation (rln3, insl5 and rxfp 3/4 genes), while eutherian mammals witnessed the diversification and rapid evolution of genes involved in reproduction (rln/insl3). Several genes that arose early in evolutionary history were lost in most mammals, but retained in teleosts and, to a lesser extent, in early diverging tetrapods. To elaborate on their evolutionary history, we provide updated phylogenies of the Rxfp1/2 and Rxfp3/4 receptors and their ligands, including new sequences from early diverging vertebrate taxa such as coelacanth, skate, spotted gar, and lamprey. We also summarize the recent progress made towards understanding the functional biology of Rxfps in non-mammalian taxa, providing a new conceptual framework for research on Rxfp signaling across vertebrates.
    General and Comparative Endocrinology 07/2014; 209. DOI:10.1016/j.ygcen.2014.07.014 · 2.67 Impact Factor
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    • "Human relaxin-2 is the only form of circulating relaxin that is substantially increased during pregnancy [4]. Human relaxin-2 is functionally equivalent to relaxin-1 in all other mammals [5]. "
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    BioMed Research International 07/2014; 2014:836397. DOI:10.1155/2014/836397 · 2.71 Impact Factor
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    • "The identity of the RLN gene lost by New and Old World monkeys remained unclear (fig 1B and C; Arroyo, Hoffmann, Opazo 2012), as support for the relevant nodes was not significant to resolve among competing alternatives. The phylogenetic evidence presented by Arroyo, Hoffmann, Opazo (2012b) suggested an older origin than previously proposed, but it was not conclusive (Wilkinson et al. 2005; Park et al. 2008; Park, Semyonov, et al. 2008; Hoffmann and Opazo 2011). Phylogenetic analyses of paralogous members of a gene family often result in nonorthologous genes appearing more similar to each other than they are to their true orthologs. "
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    Genome Biology and Evolution 02/2014; 6(3). DOI:10.1093/gbe/evu023 · 4.53 Impact Factor
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