Abstract
BackgroundPlanarians are considered to be among the extant animals close to one of the earliest groups of organisms that acquired a central nervous system (CNS) during evolution. Planarians have a bilobed brain with nine lateral branches from which a variety of external signals are projected into different portions of the main lobes. Various interneurons process different signals to regulate behavior and learning/memory. Furthermore, planarians have robust regenerative ability and are attracting attention as a new model organism for the study of regeneration. Here we conducted large-scale EST analysis of the head region of the planarian Dugesia japonica to construct a database of the head-region transcriptome, and then performed comparative analyses among related species.ResultsA total of 54,752 high-quality EST reads were obtained from a head library of the planarian Dugesia japonica, and 13,167 unigene sequences were produced by de novo assembly. A new method devised here revealed that proteins related to metabolism and defense mechanisms have high flexibility of amino-acid substitutions within the planarian family. Eight-two CNS-development genes were found in the planarian (cf. C. elegans 3; chicken 129). Comparative analysis revealed that 91% of the planarian CNS-development genes could be mapped onto the schistosome genome, but one-third of these shared genes were not expressed in the schistosome.ConclusionsWe constructed a database that is a useful resource for comparative planarian transcriptome studies. Analysis comparing homologous genes between two planarian species showed that the potential of genes is important for accumulation of amino-acid substitutions. The presence of many CNS-development genes in our database supports the notion that the planarian has a fundamental brain with regard to evolution and development at not only the morphological/functional, but also the genomic, level. In addition, our results indicate that the planarian CNS-development genes already existed before the divergence of planarians and schistosomes from their common ancestor.
Highlights
Planarians are considered to be among the extant animals close to one of the earliest groups of organisms that acquired a central nervous system (CNS) during evolution
The RNA interference technique can be applied in planarians to identify gene functions [16], and single-cell level gene profiling is possible based on a combination of fluorescence activated cell sorting (FACS) and quantitative reverse transcription polymerase chain reaction [17,18]
Based on the hypothesis that proteins with many substitutable amino acids could be candidates for members of gene groups that are able to undergo changes to accommodate to different environments, we developed a new method for detecting the fraction of substitutable amino acids within individual genes
Summary
Planarians are considered to be among the extant animals close to one of the earliest groups of organisms that acquired a central nervous system (CNS) during evolution. Planarians have robust regenerative ability and are attracting attention as a new model organism for the study of regeneration. Planarians are members of the phylum Platyhelminthes, a group of organisms that are thought to have acquired a central nervous system (CNS) with a simple brain structure early during evolution [2,3,4]. The constitution of the planarian CNS and behavior and learning/memory [10,11,12]. Planarians are attracting attention as new model organism for regeneration, including brain regeneration, research. The RNA interference technique can be applied in planarians to identify gene functions [16], and single-cell level gene profiling is possible based on a combination of fluorescence activated cell sorting (FACS) and quantitative reverse transcription polymerase chain reaction (qRT-PCR) [17,18]
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