Abstract
BackgroundSelf-splicing introns are present in the mitochondria of members of most eukaryotic lineages. They are divided into Group I and Group II introns, according to their secondary structure and splicing mechanism. Being rare in animals, self-splicing introns were only described in a few sponges, cnidarians, placozoans and one annelid species. In sponges, three types of mitochondrial Group I introns were previously described in two demosponge families (Tetillidae, and Aplysinellidae) and in the homoscleromorph family Plakinidae. These three introns differ in their insertion site, secondary structure and in the sequence of the LAGLIDADG gene they encode. Notably, no group II introns have been previously described in sponges.ResultsWe report here the presence of mitochondrial introns in the cytochrome oxidase subunit 1 (COI) gene of three additional sponge species from three different families: Agelas oroides (Agelasidae, Agelasida), Cymbaxinellapverrucosa (Hymerhabdiidae, Agelasida) and Axinella polypoides (Axinellidae, Axinellida). We show, for the first time, that sponges can also harbour Group II introns in their COI gene, whose presence in animals’ mitochondria has so far been described in only two phyla, Placozoa and Annelida. Surprisingly, two different Group II introns were discovered in the COI gene of C. verrucosa. Phylogenetic analysis indicates that the Group II introns present in C. verrucosa are related to red algae (Rhodophyta) introns.ConclusionsThe differences found among intron secondary structures and the phylogenetic inferences support the hypothesis that the introns originated from independent horizontal gene transfer events. Our results thus suggest that self-splicing introns are more diverse in the mitochondrial genome of sponges than previously anticipated.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0556-1) contains supplementary material, which is available to authorized users.
Highlights
Self-splicing introns are present in the mitochondria of members of most eukaryotic lineages
Mitochondrial introns are mobile genetic elements that form self-splicing RNA molecules. They are divided into Group I and Group II introns depending on their secondary structure and splicing mechanism [1]
The PCR led to products longer than expected in three species: A. oroides (2,755 bp), A. polypoides (3,032 bp) and C. verrucosa (7,188 bp)
Summary
Self-splicing introns are present in the mitochondria of members of most eukaryotic lineages. These protein-coding genes can be mitochondrial genes involved in the Huchon et al BMC Evolutionary Biology (2015) 15:278 soft corals, sea anemones) (e.g., [2, 8,9,10,11]) and Porifera (sponges) [12,13,14,15,16], while Group II introns have been found in Placozoa [6, 7] and Annelida (a catworm of the genus Nephtys) [17] Within these phyla, they are sporadically encountered in unrelated families. This patchy distribution and the fact that the intron phylogeny does not fit the species’ phylogeny support the view that they can be horizontally transmitted [10,11,12,13, 15, 18]
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