Characterization of a highly conserved satellite DNA from the parasitoid wasp Trichogramma brassicae

An EcoRI satellite DNA has been isolated, cloned and sequenced from Trichogramma brassicae, a minute parasitic wasp. This repeated family represents 16% of the genome. The monomer is 385 base pairs (bp) long and has an A+T content of 64.5%. The average nucleotide sequence variability among 12 randomly chosen monomers is extremely low (0.5%), suggesting that the amplification of the monomer into a high-copy-number family occurred recently. An EcoRI satellite DNA probe has been developed and used, at high stringency, as an identification tool to unambiguously discriminate T. brassicae from nine other Trichogramma species. However, at a lower stringency, a hybridization signal can be detected in two closely related Trichogramma species, and, using PCR assay, the presence of the T. brassicae EcoRI monomer has been detected in several other species of Trichogramma. These results argue in favor of the 'library' model of satellite DNA evolution that predicts that related species share a number of low-copy satellite sequences, some of which could be amplified into a major satellite family in each of the species. Furthermore, this T. brassicae EcoRI satellite DNA sequence exhibits particular internal features such as a long inverted repeat that can form a dyad structure. Such sequence motifs seem to be a common characteristic of satellite DNAs, suggesting that they could result from selective forces acting on repetitive DNA.