Hox genes in Annelid Capitella
Elaine C. Seaver’s lab(Kewalo Marine Laboratory) is interested in understanding the origins of body plan novelty during evolution and to answer some questions uses Capitella sp. I , a polychaete annelid, as a developmental model. One of the primary interest of Seaver’s lab is decipher the mechanism of evolution of the segmented body plan in the Metazoa. All the segmented animals both marine and terrestrial are contained in 3 major clades: chordates, arthropods, and annelids. It is likely that if 2 species share a common segmented ancestor the mechanisms by which their segmented body plan is generated during development will also be shared.This will help to solve the long standing controversy about the number of times segmentation has arisen in the Metazoa.
Out of the three clades involving segmented species ,annelids are the least understood ones when it comes to molecular mechanisms of how they generate segments during development.Seaver’s lab has been focusing to develop Capitella as developmental model system for studies of body plan evolution within the lophotrochozoans. Capitella serves some great advantages :
1) The ability to culture in laboratory conditions
2) Embryos and larvae can be obtained through out the year.
3) Capitella genome has been recently sequenced by Joint Genome Institute,which provides a wonderful resource.
polychaetes exhibit some great variation of body plan making them ideal choice for studying the origin of differences along the anterior posterior axis like segment identity and levels of tagmatization.
Hox genes are homeodomain containing transcription factors , implicated in giving various identities to cells along AP axis of animals and it performs this function by regulating different downstream targets.
So to gain more insights into annelid body plan organization, Seaver’s lab looked into the expression and organisation of Hox complex in Capitella.The results of their study was published in recent issue of PlosOne. Its nice to see plosone coming out with so many exciting articles related to evolution and especially Hox genes, just to remind you it was not long ago that articles on Trichoplax and Nematostella Homeodomain proteins were published in the same journal.
It is quite well known now that Hox genes display some very unique characteristics like present in clusters and follow colinearity. Today we know that Hox genes are present in all major clades of bilaterians
and cnidarians .Some great work by many scientists and their students in last two decades helped us understand many intrinsic details pertaining to these master genes in development.Details of species-specific repertoire, genomic organization, presence or absence of clusters and, in the case of vertebrates, numbers of clusters, and their deployment have formed the basis of models of animal body plan, evolution and diversification.
Caption: A total of 11 Capitella sp. I Hox genes are distributed among three scaffolds. Black lines depict two scaffolds, which contain 10 of the Capitella sp. I Hox genes. The eleventh gene, CapI-Post1, is located on a separate scaffold surrounded by ORFs of non-Hox genes (unpublished data). No predicted ORFs were identified between adjacent linked Hox genes. Transcription units are shown as boxes denoting exons, connected by lines that denote introns. Transcription orientation is denoted by arrows beneath each box. Color coding is the same as that used in Figure 1 for each ortholog.
It should be noticed that all these models are based on the results obtained from studies involving deuterostomes and ecdysozoans, hence limiting the inferences that can be made about Hox genes in the protostome/deuterostome ancestor.So till now we didnot have good understanding about Hox genes in other major bilaterian clade “Lophotrochozoans” before this work by Seaver’s lab.(Bilaterains are divided into protostomes [Ecdysozoans and Lophotrochozoans ] and deuterostomes [Xenoturbella,Echinodermata,Hemichordata and Chordata]Expression patterns of few hox genes were known before in lophotrochozoans but expression for the full Hox gene complement within a single species has not been determined and details of genomic arrangement data of hox genes was also limited.
In this study, authors present detailed genomic linkage data of the first lophotrochozoan Hox cluster and expression patterns for these Hox genes from the polychaete annelid Capitella sp. I during larval and juvenile stages.
The Capitella sp. I genome possesses definitive members of all four classes of Hox genes proposed to be present in the bilaterian ancestor. Total genome searches identified 11 Hox genes in Capitella, representing 11 distinct paralog groups thought to represent the ancestral lophotrochozoan complement. 8 of the 11 Capitella Hox genes are genomically linked in a single cluster, have the same transcriptional orientation, and lack interspersed non-Hox genes. Authors also looked into the expression patterns of Hox genes by means of Insitu hybridization and results show that except PostI Hox genes all others are expressed in ectoderm during larval stages.These genes also show spatial and temporal colinearity.Work which involves studying hox gene expression in different species with different life histories and body plan play an important role in by providing valuable insights into how Hox genes contribute to animal body plan evolution.
References:
Segmentation: mono- or polyphyletic?
Seaver EC.
Int J Dev Biol. 2003;47(7-8):583-95. Review.
2) Fro¨ bius AC, Matus DQ, Seaver EC (2008) Genomic Organization and Expression Demonstrate Spatial and Temporal Hox Gene Colinearity in the
Lophotrochozoan Capitella sp. I. PLoS ONE 3(12): e4004. doi:10.1371/journal.pone.0004004
Image Credit :
1) Littoraria/ FlickR
2 & 3) Frobius AC, Matus DQ, Seaver EC (2008) Genomic Organization and Expression Demonstrate Spatial and Temporal Hox Gene Colinearity in the Lophotrochozoan Capitella sp. I. PLoS ONE 3(12): e4004. doi:10.1371/journal.pone.0004004
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