Transcriptome Analysis of ESTs from a Chaetognath Reveals a Deep-Branching Clade of Retrovirus-Like Retrotransposons

Abstract

Chaetognaths constitute a small marine phylum exhibiting several characteristic which are highly unusual in animal genomes, including two classes of both rRNA and protein ribosomal genes. As in this phylum presence of retrovirus-like elements has never been documented, analysis of a published expressed sequence tag (EST) collection of the chaetognath Spadella cephaloptera has been made. Twelve sequences representing transcript sections of reverse transcriptase domain of active retrotransposons were isolated from~11,000 ESTs. Five of them are originated from Gypsy retrovirus-like elements, whereas the other are transcripts from a Bel-Pao LTR-retrotransposon, a Penelope-like element and LINE retrotransposons. Moreover, a part of a putative integrase has also been found. Phylogenetic analyses suggest a deep-branching clade of the retrovirus-like elements, which is in agreement with the probably Cambrian origin of the phylum. Moreover, retrotransposons have not been found in telomeric-like transcripts which are probably constituted by both vertebrate and arthropod canonical repeats.

Careful with understudied phyla: The case of chaetognath

“…Moreover, in their S. cephaloptera EST collection, Marletaz et al [ 9 ] have found that some sequences are similar to those of reverse transcriptase genes or of parts of retrotransposons; similarly, LINE-like and Gypsy -like retrotransposons have been found in Sagitta sp . [ 57 ] and in S. cephaloptera [ 58 ], showing evidence of the presence of functional reverse transcriptase proteins in chaetognath cells. Interestingly, SINE elements are retrosequences; this group contains all the sequences arisen by reverse transcription of ribosomal, messenger and small stable RNAs [ 59 , 60 ].…”