Three new satellite sequences and a mobile element found inside HSP70 introns of the Mediterranean mussel (Mytilus galloprovincialis)

Abstract

We report the characterization of 3 new repetitive sequences from the bivalve mollusc Mytilus galloprovincialis, designated Mg1, Mg2, and Mg3, with monomer lengths of 169, 260, and 70 bp, respectively. The 3 repeats together constitute approximately 7.8% of the M. galloprovincialis genome and were found, together with ApaI-type 2 repeats, inside the introns of 2 genes of the HSP70 family, hsc70 and hsc71. Both the monomer length and the genomic content of the repeats indicate satellite sequences. The Mg1 repetitive region and its flanking sequences exhibit significant homology to CvE, a member of the Pearl family of mobile elements found in the eastern oyster (Crassostrea virginica). Thus, the whole homologous region is designated MgE, the first putative transposable element characterized in M. galloprovincialis. The ApaI, Mg2, and Mg3 repeats are continuously arranged inside the introns of both the hsc70 and hsc71 genes. The presence of perfect inverted repeats flanking the ApaI-Mg2-Mg3 repetitive region, as well as a sequence analysis of the repeats, indicates a transposition-like insertion of this region. The genes of the HSP70 family are highly conserved, and the presence of repetitive DNA or of mobile elements inside their introns is reported here for the first time.

Structural and functional liaisons between transposable elements and satellite DNAs

“…Repeats are variable in DNA sequence, copy number (2-10), and monomer length (50-500 bp) when compared among species (Yang and Barbash 2008). According to classification in Thomas et al (2014), elements called PERI from seven species of the Drosophila buzzatii cluster (Kuhn and Heslop-Harrison 2011), MINE-1 and MINE-2 from Lepidoptera (Coates et al 2010(Coates et al , 2011, DTC84 from the clam Donax trunculus (Šatović and Plohl 2013), pearl from the oyster (Gaffney et al 2003), MgE from Mytilus galloprovincialis (Kourtidis et al 2006a), and Tsp from the sea urchin (Cohen et al 1985) are all DINE-1 like elements. Recently, two different MITE elements (terMITE1 and terMITE2) were described in termites, both also containing variable number of internal tandem repeats which are 16 and 114-bp long, respectively (Luchetti 2015).…”

“…Tandem repeats of DINE-1-like element of the pearl family were found inserted into introns of hsc70 and hsc71 genes, in the Mediterranean mussel M. galloprovincialis (Kourtidis et al 2006a). These genes are members of the highly conserved HSP70 (heat shock protein) family, characterized in this organism (Kourtidis et al 2006b).…”

“…The event is interpreted as a rare case of insertion in introns of these genes, indicating a transposition-like mechanism in which the whole repetitive region is inserted as a single unit. Based on comparison with the Crassostrea gigas orthologues genes that lack inserted repetitive elements, authors conclude that insertions might induce divergences in gene functioning and also contribute to accumulation of differences between species (Kourtidis et al 2006a). Copies of TCAST satDNA of the beetle T. castaneum were found embedded in a transposon-like structure with long TIRs and are statistically overrepresented in the relative vicinity of nine immunoglobulin-like genes (Brajković et al 2012).…”

Helitrons , the Eukaryotic Rolling-circle Transposable Elements

“…However, recent studies suggest that the pattern of distribution is shaped by selection because young insertions are evenly distributed towards the chromosome arms and in the euchromatic regions (22,39,49). Indeed, HINE insertions are also found in genic regions in multiple drosophilids (39), lepidopterans (31), and molluscs (34,35).…”

RUDI, a short interspersed element of the V-SINE superfamily widespread in molluscan genomes

“…Despite the large degree of mollusc diversity and the wider evolutionary and economic interest in molluscs, their genomes have been poorly explored regarding repetitive DNAs and TEs in particular. In addition to studies of satellite DNAs (for example, Clabby et al 1996;Biscotti et al 2007;Passamonti et al 1998;Petrović et al 2009;Plohl et al 2010;Petraccioli et al 2015), specific analyses of mollusc transposable elements are surprisingly limited (Gaffney et al 2003;Kourtidis et al 2006;Šatović and Plohl 2013) and only one SINE has been isolated in the cephalopod genome (Akasaki et al 2010). Here, we report the discovery and analysis of a new HCD-SINE element named RUDI, identified in bivalves, gastropods, chitons and solenogasters.…”