Recent and old duplications in crustaceans "Internal Transcribed Spacer 1″: structural and phylogenetic implications.

Abstract

Internal Transcribed Spacer structures are important in preserving accessibility to specific enzymes for the maturation of rRNAs. ITS1 sequences reported in the literature in Crustaceans range between 182and 820bp and are characterized by the absence of repeats or the presence of only a limited number of microsatellites. Here, we sequenced ITS1 for a range of shrimp families (infraorder Caridea) and show that most taxa have much larger ITS1 sequences. We find a high number of microsatellites in Alpheushebes and Crangoncrangon and we report repeat units in Pandalidae, Palaemonidae and mainly in Alpheidae species. Up to four repeats were found in A.vanderbilti (1915bp), A.rostratus (1635bp) and A.lottini (1625bp). In general, four helices were found in ITS1. Repeat units led to extra hairpins and loops. No conserved positions occurred except in helix 4. Three clades were defined in A. lottini for the first time. We estimated the ITS1 divergence rate for the three clades of A.lottini collected in French Polynesia using existing calibrations of substitution rates. Rates of sequence evolution are largely influenced by repeat units, which likely evolve separately. By comparison with COI marker, we estimated the divergence rate of the whole ITS1 sequence to range from 0.5 to 1.4% Pmy and between 0.12 and 0.5% for the 3' end of ITS1 located outside the repeat units. Given the degree of identity between repeats, we suggest that a duplication event recently occurred in A. floridanus (98% identity) whereas an ancient duplication happened in A.sulcatus (50% identity) early at the origination of the group Alpheidae, approximately 50mya ago. In conclusion, our results highlight an over representation of shorter ITS1 sequences in public repositories, and underlines the importance to further understand patterns of molecular evolution of this functionally important gene.