Rates of movement of transposable elements on the second chromosome of Drosophila melanogaster

The rates of movement of 11 families of transposable elements of Drosophila melanogaster were studied by means of in situ hybridization of probes to p olytene chromosomes of larvae from a long-term mutation accumulation experi ment. Replicate mutation-accumulation lines carrying second chromosomes der ived from a single common ancestral chromosome were maintained by backcross es of single males heterozygous for a balancer chromosome and a wild-type c hromosome, and were scored after 116 generations. Twenty-seven transpositio ns and 1 excision were detected using homozygous viable and fertile second chromosomes, for a total of 235056 potential sources of transposition event s and a potential 252880 excision events. The overall transposition rate pe r element per generation was 1.15 x 10(-4) and the excision rate was 3.95 x 10(-6). The single excision (of a roo element) was due to recombination be tween the element's long terminal repeats. A survey of the five most active elements among nine homozygous lethal lines revealed no significant differ ence in the estimates of transposition and excision rates from those from v iable lines. The excess of transposition over excision events is in agreeme nt with the results of other in situ hybridization experiments, and support s the conclusion that replicative increase in transposable element copy num ber is opposed by selection. These conclusions are compared with those from other studies, and with the conclusions from population surveys of element frequencies.