Genetic Differentiation of Transposable Elements under Mutation and Unbiased Gene Conversion

A model is developed to predict the extent of genetic differentiation in a family of transposable elements under the combined effects of genetic drift, transposition, mutation and unbiased gene conversion. The model is based on simplifying assumptions that are valid when transposition is always to new sites and copy number per site is low. In the absence of gene conversion, the degree of differentiation as measured by the probability of identity of different elements is the same as at a single locus with the same mutation rate but in a population of effective size Nc/2, where N is the population size and c is the number of copies per individual. The inclusion of unbiased gene conversion does not significantly change this result. If, as seems to be the case, families of transposable elements are relatively homogeneous, then the model implies either that mutation rates for transposable elements are much lower than at comparable single-copy loci or that some other force, such as natural selection or biased gene conversion, is at work. Transposition is a very ineffective force for homogenizing a family of transposable elements.