The dynamics of the roo transposable element in mutation-accumulation lines and segregating populations of Drosophila melanogaster

Abstract

We estimated the number of copies for the long terminal repeat (LTR) retrotransposable element roo in a set of long-standing Drosophila melanogaster mutation-accumulation full-sib lines and in two large laboratory populations maintained with effective population size approximately 500, all of them derived from the same isogenic origin. Estimates were based on real-time quantitative PCR and in situ hybridization. Considering previous estimates of roo copy numbers obtained at earlier stages of the experiment, the results imply a strong acceleration of the insertion rate in the accumulation lines. The detected acceleration is consistent with a model where only one (maybe a few) of the approximately 70 roo copies in the ancestral isogenic genome was active and each active copy caused new insertions with a relatively high rate ( approximately 10(-2)), with new inserts being active copies themselves. In the two laboratory populations, however, a stabilized copy number or no accelerated insertion was found. Our estimate of the average deleterious viability effects per accumulated insert [E(s) < 0.003] is too small to account for the latter finding, and we discuss the mechanisms that could contain copy number.

Figure 1.—

General design of the mutation-accumulation experiment.

Figure 2.—

Empirical distribution for the QPCR estimate of the number of roo inserts in the 64 MA2 lines (open bars) together with the corresponding expected distribution in the absence of new insertions obtained by simulation using resampled residuals (shaded bars).

Figure 3.—

Average number of roo copies per gamete in the mutation-accumulation lines through the whole MA1–MA2 experiment. Solid dots are experimental values and the lines give the prediction from Equation 2 fitted assuming different x₀ values (solid thick line, x₀ = 1, ν = 0.0087; solid thin line, x₀ = 2, ν = 0.0067; dashed thick line, x₀ = 5, ν = 0.0044; dashed thin line, x₀ = 25, ν = 0.0015; solid shaded line, x₀ = 50, ν = 0.0009. In all cases n₀ = 75.7; see text for explanation).