RESUMEN
We are interested in understanding whether the annelids and arthropods shared a common segmented ancestor and have approached this question by characterizing the expression pattern of the segment polarity gene engrailed (en) in a basal annelid, the polychaete Chaetopterus. We have isolated an en gene, Ch-en, from a Chaetopterus cDNA library. Genomic Southern blotting suggests that this is the only en class gene in this animal. The predicted protein sequence of the 1.2-kb cDNA clone contains all five domains characteristic of en proteins in other taxa, including the en class homeobox. Whole-mount in situ hybridization reveals that Ch-en is expressed throughout larval life in a complex spatial and temporal pattern. The Ch-en transcript is initially detected in a small number of neurons associated with the apical organ and in the posterior portion of the prototrochophore. At later stages, Ch-en is expressed in distinct patterns in the three segmented body regions (A, B, and C) of Chaetopterus. In all segments, Ch-en is expressed in a small set of segmentally iterated cells in the CNS. In the A region, Ch-en is also expressed in a small group of mesodermal cells at the base of the chaetal sacs. In the B region, Ch-en is initially expressed broadly in the mesoderm that then resolves into one band/segment coincident with morphological segmentation. The mesodermal expression in the B region is located in the anterior region of each segment, as defined by the position of ganglia in the ventral nerve cord, and is involved in the morphogenesis of segment-specific feeding structures late in larval life. We observe banded mesodermal and ectodermal staining in an anterior-posterior sequence in the C region. We do not observe a segment polarity pattern of expression of Ch-en in the ectoderm, as is observed in arthropods.
Asunto(s)
Proteínas de Homeodominio/biosíntesis , Proteínas de Homeodominio/genética , Factores de Transcripción , Secuencia de Aminoácidos , Animales , Anélidos , Secuencia de Bases , Southern Blotting , Tipificación del Cuerpo , Clonación Molecular , ADN Complementario/metabolismo , Regulación del Desarrollo de la Expresión Génica , Biblioteca de Genes , Hibridación in Situ , Larva/fisiología , Mesodermo/metabolismo , Modelos Biológicos , Datos de Secuencia Molecular , Estructura Terciaria de Proteína , ARN Mensajero/metabolismo , Homología de Secuencia de Aminoácido , Factores de Tiempo , Distribución TisularRESUMEN
A prediction from the set-aside theory of bilaterian origins is that pattern formation processes such as those controlled by the Hox cluster genes are required specifically for adult body plan formation. This prediction can be tested in animals that use maximal indirect development, in which the embryonic formation of the larva and the postembryonic formation of the adult body plan are temporally and spatially distinct. To this end, we quantitatively measured the amount of transcripts for five Hox genes in embryos of a lophotrochozoan, the polychaete annelid Chaetopterus sp. The polychaete Hox complex is shown not to be expressed during embryogenesis, but transcripts of all measured Hox complex genes are detected at significant levels during the initial stages of adult body plan formation. Temporal colinearity in the sequence of their activation is observed, so that activation follows the 3'-5' arrangement of the genes. Moreover, Hox gene expression is spatially localized to the region of teloblastic set-aside cells of the later-stage embryos. This study shows that an indirectly developing lophotrochozoan shares with an indirectly developing deuterostome, the sea urchin, a common mode of Hox complex utilization: construction of the larva, whether a trochophore or dipleurula, does not involve Hox cluster expression, but in both forms the complex is expressed in the set-aside cells from which the adult body plan derives.
Asunto(s)
Evolución Biológica , Proteínas Fetales , Regulación del Desarrollo de la Expresión Génica , Genes Homeobox , Familia de Multigenes , Poliquetos/embriología , Poliquetos/genética , Animales , Embrión no Mamífero/fisiología , Evolución Molecular , Femenino , Masculino , Datos de Secuencia Molecular , Oocitos/fisiología , Espermatozoides/fisiología , Proteínas de Dominio T Box/genética , Transcripción GenéticaRESUMEN
Expression patterns for five Hox genes were examined by whole-mount in situ hybridization in larvae of Chaetopterus, a polychaete annelid with a tagmatized axial body plan. Phylogenetic analysis demonstrates that these genes are orthologs of the Drosophila genes labial, proboscipedia, zen, Deformed, and Sex combs reduced and are termed CH-Hox1, CH-Hox2, CH-Hox3, CH-Hox4, and CH-Hox5, respectively. Expression studies reveal a biphasic expression pattern. In early larval stages, well before any indications of segmental organization exist, a novel pattern of expression in bilateral posterior proliferating cell populations, corresponding to the teloblasts, was detected for each of the genes, with CH-Hox1 and CH-Hox2 expressed before the remaining three. In middle larval stages, all five genes are expressed in bilateral strips along the ventral midline, corresponding with the developing ventral nerve cord. In addition, CH-Hox1 and CH-Hox2 show strong expression at the foregut-midgut boundary. By late larval stages the expression is generally confined to the ventral CNS and ectoderm of the anterior parapodia. Anterior boundaries of expression are "colinear," at later larval stages, with CH-Hox2 expressed most rostrally, in the first segment, and anterior expression boundaries for CH-Hox1, CH-Hox3, CH-Hox4, and CH-Hox5 in segments 2, 3, 4, and 5, respectively. Like vertebrates and spiders, but unlike insects, CH-Hox3 participates in this colinear axial expression pattern. CH-Hox1 and CH-Hox2 have distinct posterior boundaries of expression in the ninth segment, which corresponds to a major morphological boundary, and may reflect a reorganization of Hox gene regulation related to the evolutionary reorganization of the Chaetopterus body plan.
Asunto(s)
Tipificación del Cuerpo/genética , Genes Homeobox , Proteínas de Homeodominio/genética , Poliquetos/embriología , Secuencia de Aminoácidos , Animales , Clonación Molecular , Secuencia Conservada , Sistema Digestivo/embriología , Evolución Molecular , Regulación del Desarrollo de la Expresión Génica , Proteínas de Homeodominio/clasificación , Larva , Datos de Secuencia Molecular , Sistema Nervioso/embriología , Homología de Secuencia de Aminoácido , Terminología como Asunto , Distribución TisularRESUMEN
Seven post-gastrulation larval stages are described for the sedentary polychaete Chaetopterus. Analysis of larval anatomy and morphology through ontogeny reveals significant differences in the temporal sequence of segmentation, and in the character of segments formed, from the typical embryological pattern described for other polychaete families, such as nereidids or spionids. When compared in alternative phylogenetic schemes, these differences represent significant developmental heterochrony, among other evolutionary transitions, which has arisen in the chaetopterid lineage. The heterochrony is correlated with the extreme morphological regionalization along the anterior-posterior body axis, a feature that is also characteristic of chaetopterids.
Asunto(s)
Evolución Biológica , Larva/crecimiento & desarrollo , Poliquetos/crecimiento & desarrollo , Animales , Gástrula/fisiología , Hibridación in Situ , Filogenia , Poliquetos/genética , Sondas ARN , ARN Mensajero/análisis , Factores de Tiempo , Tubulina (Proteína)/genéticaRESUMEN
A survey of genomic DNA from the polychaete Chaetopterus variopedatus was conducted using the polymerase chain reaction. Twelve unique homeobox-containing gene fragments were recovered. Phylogenetic analysis indicates that seven of the fragments are from genes belonging to Hox homeobox classes. Other fragments show orthology with Xlox, caudal, and Prh homeobox classes, with two fragments not definitely assignable to a homeobox class by our analysis. Orthology with gene sequences reported for the polychaete Ctenodrilus serratus, by Dick and Buss (1994), was calculated and indicated that at least eight of the C. variopedatus fragments are homologous to these previously reported sequences. Tabulation of the Hox gene relationships suggest that polychaetes have representative genes of each of the Hox cognate groups except Abd-B. This conclusion further suggests that the Hox cluster in the basal protostome ancestor had already undergone the gene duplications leading to the complete complement of homeotic genes known in Drosophila, with the possible loss of Abd-B in the polychaete lineage.