The evolutionary role of transposable elements (TEs) is still highly controversial. Two key parameters, the transposition rate (
u
and
w
, for replicative and non-replicative transposition) and the excision rate (
e
) are fundamental to understanding their evolution and maintenance in populations. We have estimated
u
,
w
and
e
for six families of TEs (including eight members: IS1, IS2, IS3, IS4, IS5, IS30, IS150 and IS186) in
Escherichia coli
, using a mutation accumulation (MA) experiment. In this experiment, mutations accumulate essentially at the rate at which they appear, during a period of 80 500 (1610 generations × 50 lines) generations, and spontaneous transposition events can be detected. This differs from other experiments in which insertions accumulated under strong selective pressure or over a limited genomic target. We therefore provide new estimates for the spontaneous rates of transposition and excision in
E. coli
. We observed 25 transposition and three excision events in 50 MA lines, leading to overall rate estimates of
u
∼ 1.15 × 10
–5
,
w
∼ 4 × 10
−8
and
e
∼ 1.08 × 10
−6
(per element, per generation). Furthermore, extensive variation between elements was found, consistent with previous knowledge of the mechanisms and regulation of transposition for the different elements.
Evolution of transposable elements: an IS10 insertion increases fitness in Escherichia coli.
