Population dynamics of an Ac-like transposable element in self- and cross-pollinating arabidopsis

Theoretical models predict that the mating system should be an important fa ctor driving the dynamics of transposable elements in natural populations d ue to differences in selective pressure on both element and host. We used a PCR-based approach to examine the abundance and levels of insertion polymo rphism of Ac-III, a recently identified Ac-like transposon family, in natur al populations of the selfing plant Arabidopsis thaliana and its close outc rossing relative, Arabidopsis lyrata. Although several insertions appeared to be ancient and shared between species, there is strong evidence for rece nt activity of this element family in both species. Sequences of the region s flanking insertions indicate that all Ac-III transposons segregating in n atural populations are in noncoding regions and provide no evidence for loc al transposition events. Transposon display analysis suggests the presence of slightly higher numbers of insertion sites per individual but fewer tota l polymorphic insertions in the self-pollinating A. thabiana than A. lyrata . Element insertions appear to be segregating at significantly lower freque ncies in A, lyrata than A. thaliana, which is consistent with a reduction i n transposition rate, reduction in effective population size, or reduced ef ficacy of natural selection against element insertions in selfing populatio ns.