Abstract
BackgroundChaetognaths, or arrow worms, are small marine, bilaterally symmetrical metazoans. The objective of this study was to analyse ribosomal protein (RP) coding sequences from a published collection of expressed sequence tags (ESTs) from a chaetognath (Spadella cephaloptera) and to use them in phylogenetic studies.ResultsThis analysis has allowed us to determine the complete primary structures of 23 out of 32 RPs from the small ribosomal subunit (SSU) and 32 out of 47 RPs from the large ribosomal subunit (LSU). Ten proteins are partially determined and 14 proteins are missing. Phylogenetic analyses of concatenated RPs from six animals (chaetognath, echinoderm, mammalian, insect, mollusc and sponge) and one fungal taxa do not resolve the chaetognath phylogenetic position, although each mega-sequence comprises approximately 5,000 amino acid residues. This is probably due to the extremely biased base composition and to the high evolutionary rates in chaetognaths. However, the analysis of chaetognath RP genes revealed three unique features in the animal Kingdom. First, whereas generally in animals one RP appeared to have a single type of mRNA, two or more genes are generally transcribed for one RP type in chaetognath. Second, cDNAs with complete 5'-ends encoding a given protein sequence can be divided in two sub-groups according to a short region in their 5'-ends: two novel and highly conserved elements have been identified (5'-TAATTGAGTAGTTT-3' and 5'-TATTAAGTACTAC-3') which could correspond to different transcription factor binding sites on paralog RP genes. And, third, the overall number of deduced paralogous RPs is very high compared to those published for other animals.ConclusionThese results suggest that in chaetognaths the deleterious effects of the presence of paralogous RPs, such as apoptosis or cancer are avoided, and also that in each protein family, some of the members could have tissue-specific and extra-ribosomal functions. These results are congruent with the hypotheses of an allopolyploid origin of this phylum and of a ribosome heterogeneity.
Highlights
Chaetognaths, or arrow worms, are small marine, bilaterally symmetrical metazoans
Bioinformatic analysis of the chaetognath ribosomal protein (RP) cDNAs (DNA complementary to RNA) sequences revealed the candidate clones for 28 small ribosomal subunit (SSU) RP genes and for 37 large ribosomal subunit (LSU) RP genes, with respectively between 1 to 45 clones and between 1 to 42 clones for each of the RP gene family (Figure 1 and Table 1)
In approximately half of the cases, for a RP gene family, several mRNAs are found in this expressed sequence tags (ESTs) database, these mRNAs can be divided in two or more types and each type encodes the same isoform; generally, within each type, the mRNA sequences can be divided in two subtypes, which differ principally by a short sequence in their 5'-ends ("than the other (TTT)" and "TAC" sites in Figures 1 and 2)
Summary
Chaetognaths, or arrow worms, are small marine, bilaterally symmetrical metazoans. The objective of this study was to analyse ribosomal protein (RP) coding sequences from a published collection of expressed sequence tags (ESTs) from a chaetognath (Spadella cephaloptera) and to use them in phylogenetic studies. Chaetognaths are a small marine phylum, only comprising about 120 species; they live in various habitats, but most of them are planktonic [1] They play a key role in marine food webs and are considered to be the second phylum, after copepods in terms of plankton biomass [2]. One of us (JPC) described about a quarter of the known species and reported many original observations, such as the progressive stages of acquisition of one pair of appendages on the posterior half of the tail by modifying a part of their balancing fins [4] He reported astonishing variations of the secondary muscle, one of the locomotory muscles [5], exhibiting two forms; one is unique in the animal Kingdom (alternation of two sarcomere types) and the other, found only in more or less benthic species, functions by supercontraction
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