RESUMEN
Invertebrate species possess one or two Na+ channel genes, yet there are 10 in mammals. When did this explosive growth come about during vertebrate evolution? All mammalian Na+ channel genes reside on four chromosomes. It has been suggested that this came about by multiple duplications of an ancestral chromosome with a single Na+ channel gene followed by tandem duplications of Na+ channel genes on some of these chromosomes. Because a large-scale expansion of the vertebrate genome likely occurred before the divergence of teleosts and tetrapods, we tested this hypothesis by cloning Na+ channel genes in a teleost fish. Using an approach designed to clone all of the Na+ channel genes in a genome, we found six Na+ channel genes. Phylogenetic comparisons show that each teleost gene is orthologous to a Na+ channel gene or gene cluster on a different mammalian chromosome, supporting the hypothesis that four Na+ channel genes were present in the ancestors of teleosts and tetrapods. Further duplications occurred independently in the teleost and tetrapod lineages, with a greater number of duplications in tetrapods. This pattern has implications for the evolution of function and specialization of Na+ channel genes in vertebrates. Sodium channel genes also are linked to homeobox (Hox) gene clusters in mammals. Using our phylogeny of Na+ channel genes to independently test between two models of Hox gene evolution, we support the hypothesis that Hox gene clusters evolved as (AB) (CD) rather than [D[A(BC)]].
Asunto(s)
Evolución Molecular , Variación Genética , Filogenia , Canales de Sodio/genética , Vertebrados/genética , Animales , Mapeo Cromosómico , Peces , Genes Duplicados , Genes Homeobox , Humanos , Modelos Moleculares , Datos de Secuencia Molecular , Especificidad de Órganos , Estructura Secundaria de Proteína , Reacción en Cadena de la Polimerasa de Transcriptasa Inversa , Salamandridae , Canales de Sodio/química , Vertebrados/clasificaciónRESUMEN
Gonadal steroid hormones act upon specific areas of the vertebrate brain to affect the reproductive physiology and behavior of the animal. Steroid receptors are members of a superfamily of ligand-dependent transcription factors that mediate the effects of steroid hormones by modulating gene expression in the cells containing the receptors. The neuroanatomical distributions of steroid receptor-containing cells have been described for several species by using steroid autoradiography, immunocytochemistry, and more recently in situ hybridization. We have used the polymerase chain reaction to amplify and clone fragments of the estrogen receptor, progesterone receptor, and androgen receptor of whiptail lizards (genus Cnemidophorus). These clones were used to produce probes for use in in situ hybridization assays and to map the neuroanatomical distribution of all three sex steroid hormone receptors in the forebrains of unisexual (C. uniparens) and sexual (C. inornatus) species of whiptail lizards. The distribution of receptor-expressing cells reported here is in general agreement with previous reports in other species with receptor-containing cells concentrated in septal, amygdaloid, cortical, preoptic, and hypothalamic nuclei.