| Since their discovery, transposable elements (TEs) have been regarded either as useful building blocks of genomes, or as "selfish DNA": genetic parasites that exploit the sexual cycle to spread in copy number within populations to the detriment of their hosts. If the "selfish DNA" hypothesis is correct, TEs are expected to deteriorate and be lost from asexual populations. This thesis tests the predictions of the "selfish DNA" hypothesis in four asexual plant species, focusing on patterns of nucleotide diversity and nucleotide substitution. Sequences bearing strong resemblance to known TE families including Ty1/copia, Ty3/gypsy, and LINE-like elements were successfully isolated from all four plant species, and showed patterns of nucleotide substitution consistent with a long history of purifying selection. Stochastic simulations were also conducted, and suggested that this result is expected if the host species has been asexual for less than tens of thousands of generations. |
