Evolutionary Dynamics and Transcriptional Activity of Transposons of the Pacific Oyster (Thunberg, 1793)

The DNA transposon superfamily is one of the most widespread among eukaryotes. This study investigates the evolutionary dynamics and transcriptional activity of elements in different individuals of the Pacific oyster (Thunberg, 1793). Transcriptional analysis revealed that the transposase gene of more than half of transposons is expressed, sometimes at high levels. Cases of gene activity dependence on environmental conditions, developmental stages, and tissues were also identified. Data on evolutionary dynamics demonstrate that despite the presence of potentially functional copies in over half of transposons, only a few elements have shown transpositional activity in the recent past. The following scenarios are considered: 1) transposon activity in oysters is strongly suppressed by RNA interference, requiring more extreme and/or prolonged stress exposure to overcome this repression; 2) During evolution, oyster transposons accumulated mutations that did not visibly alter the protein sequence but impaired or inactivated DNA-binding and/or catalytic domains, leaving promoters active (transcription occurs) while rendering the transposase incapable of catalyzing transposition. Since some elements remain transcriptionally active, it is plausible that a combination of favorable factors could trigger transposition events. Studying the evolutionary dynamics of transposable elements (TEs) at the intraspecific level enhances our understanding of evolutionary mechanisms and population genetic structure.