RESUMEN
As arguably the simplest free-living animals, placozoans may represent a primitive metazoan form, yet their biology is poorly understood. Here we report the sequencing and analysis of the approximately 98 million base pair nuclear genome of the placozoan Trichoplax adhaerens. Whole-genome phylogenetic analysis suggests that placozoans belong to a 'eumetazoan' clade that includes cnidarians and bilaterians, with sponges as the earliest diverging animals. The compact genome shows conserved gene content, gene structure and synteny in relation to the human and other complex eumetazoan genomes. Despite the apparent cellular and organismal simplicity of Trichoplax, its genome encodes a rich array of transcription factor and signalling pathway genes that are typically associated with diverse cell types and developmental processes in eumetazoans, motivating further searches for cryptic cellular complexity and/or as yet unobserved life history stages.
Asunto(s)
Genoma/genética , Invertebrados/genética , Invertebrados/fisiología , Animales , Adhesión Celular , Secuencia Conservada , Matriz Extracelular/genética , Regulación del Desarrollo de la Expresión Génica , Células Germinativas , Humanos , Invertebrados/anatomía & histología , Invertebrados/clasificación , Filogenia , Reproducción/genética , Análisis de Secuencia de ADN , Sexo , Transducción de Señal , Sintenía , Factores de Transcripción/genéticaRESUMEN
The first sequenced mitochondrial genome of a placozoan, Trichoplax adhaerens, challenged the conventional wisdom that a compact mitochondrial genome is a common feature among all animals. Three additional placozoan mitochondrial genomes representing highly divergent clades have been sequenced to determine whether the large Trichoplax mtDNA is a shared feature among members of the phylum Placozoa or a uniquely derived condition. All three mitochondrial genomes were found to be very large, 32- to 37-kb, circular molecules, having the typical 12 respiratory chain genes, 24 tRNAs, rnS, and rnL. They share with the Trichoplax mitochondrial genome the absence of atp8, atp9, and all ribosomal protein genes, the presence of several cox1 introns, and a large open reading frame containing an intron group I LAGLIDADG endonuclease domain. The differences in mtDNA size within Placozoa are due to variation in intergenic spacer regions and the presence or absence of long open reading frames of unknown function. Phylogenetic analyses of the 12 respiratory chain genes support the monophyly of Placozoa. The similarities in composition and structure between the three mitochondrial genomes reported here and that of Trichoplax's mtDNA suggest that their uncompacted state is a shared ancestral feature to other nonmetazoans while their gene content is a derived feature shared only among the Metazoa.
Asunto(s)
Genómica , Invertebrados/citología , Invertebrados/genética , Mitocondrias/genética , Secuencia de Aminoácidos , Animales , Mapeo Cromosómico , ADN Mitocondrial/genética , Exones/genética , Genoma/genética , Intrones/genética , Datos de Secuencia Molecular , Filogenia , Proteínas/químicaRESUMEN
We here address placozoan distribution and phylogeography in five locations in the Caribbean Sea. We performed a coarse-resolution presence/absence survey of placozoans in Belize, Bermuda, Grenada, Jamaica, and Panama and a fine-resolution study of the distribution of placozoans in Twin Cays, Belize. Placozoans were recovered in every country sampled. Animals were sequenced at the mitochondrial 16S rDNA locus, and our analysis identified four of the five previously identified clades present in the Caribbean. In addition, we discovered two new haplotypes within one of these clades, and we found sympatric clades in Belize, Bermuda, Jamaica, and Panama. These studies provide further molecular evidence for species diversity within the Phylum Placozoa.
Asunto(s)
Invertebrados/genética , Animales , Secuencia de Bases , Belice , Región del Caribe , ADN Mitocondrial/química , ADN Mitocondrial/genética , Datos de Secuencia Molecular , Filogenia , Reacción en Cadena de la Polimerasa , ARN Ribosómico 16S/química , ARN Ribosómico 16S/clasificación , ARN Ribosómico 16S/genética , Análisis de Secuencia de ADNRESUMEN
Placozoans, the simplest free-living animals, have never been observed to reproduce sexually. Here, we describe molecular evidence for sexual reproduction within one clade of the Placozoa. In a population sample of 10 individuals, within-individual and overall nucleotide diversity were similar to each other and consistent with levels observed in sexually reproducing species. Intergenic recombination as well as the sharing of alleles between heterozygous and homozygous individuals was also observed. These hallmarks of sexual reproduction establish that sex is indeed present in this phylum.