Good things come in small packages

Abstract

Planarians are small freshwater flatworms widely used for years in developmental and morphogenetic studies owing to their ability to regenerate completely from just the tiniest pieces of their body. Genetically, planarians are also very interesting because different strains of one species can reproduce sexually, asexually or employ both sexual and asexual reproduction types.In a recent study, a Russian team led by Sergey Lukyanov isolated a new family of C-type lectin-like domain-containing genes (CTLDs) from the planarian Girardia tigrina [1xIdentification and characterization of a new family of C-type lectin-like genes from planaria Girardia tigrina. Shagin, D.A. et al. Glycobiology. 2002; 12: 463–472Crossref | PubMedSee all References[1]. Previously known animal CTLD-containing proteins comprise a large superfamily of proteins, in which lectin domains are accompanied by nonlectin domains. The genes isolated from G. tigrina planarian encode proteins containing varying amounts of CTLDs without any other types of domain. Lukyanov and his team conclude from this unique genomic architecture that these genes encode a previously unknown type of lectin-like protein. The new genes are expressed in differentiated gland cells and the proteins are excreted as mucus.In another recent study, the same group isolated and sequenced the first virus-like extrachromosomal element, dubbed PEVE (planarian extrachromosomal virus-like element) [2xComplete genome sequence of a novel extrachromosomal virus-like element identified in planarian Girardia tigrina. Rebrikov, D.V. et al. BMC Genomics. 2002; 3: 15Crossref | PubMed | Scopus (4)See all References[2]. PEVE differs from all known viruses and viral-like elements and it is thought to encode two helicases resembling proteins from distant viral classes. The authors suggest that there are two variant forms of PEVE, both having a hairpin–loop structure but with different arrangements of single- and double-stranded DNA regions. The PEVE element was detected only in one asexual planarian strain and was isolated with the use of a sophisticated variant of subtractive hybridization. There is no doubt that there are many more surprises hidden in the planarian genome.