Population frequencies of transposable elements in selfing and outcrossingCaenorhabditisnematodes

Abstract

SummaryPopulation genetics theory predicts that differences in breeding systems should be an important factor in the dynamics of selfish genetic elements, because of different intensities of selection on both hosts and elements. We examined population frequencies of transposable elements (TEs) in natural populations of the self-fertilizing nematodeCaenorhabditis elegansand its outcrossing relativeCaenorhabditis remanei. We identified a Tc1-like class of elements in theC. remaneigenome with homology to the terminal inverted repeats of theC. elegansTc1 transposon, which we name mTcre1. We measured levels of insertion polymorphism for all 32 Tc1 elements present in the genome sequence of theC. elegansN2 strain, and 16 mTcre1 elements from the genome sequence of theC. remaneiPB4641 strain. We show that transposons are less polymorphic and segregate at higher frequencies inC. eleganscompared withC. remanei. Estimates of the intensity of selection based on the population frequencies of polymorphic elements suggest that transposons are selectively neutral inC. elegans, but subject to purifying selection inC. remanei. These results are consistent with a reduced efficacy of natural selection against TEs in selfing populations, but may in part be explained by non-equilibrium TE dynamics.