Fig. 5
From: The evolutionary arms race between transposable elements and piRNAs in Drosophila melanogaster
The evolutionary arms race between TEs and piRNAs revealed by simulations. a A schematic diagram illustrating the process and consequence of TE:piRNA interactions. Three possible consequences of TE:piRNA interactions depend on TE replication rate, the repressive strength of piRNAs on TEs, and the strength of purifying selection against TEs: 1) Excessive TEs. When TE replication rate is high and the repressive strength of piRNA is weak (TEs jumping into piRNA cluster and become piRT producing piRNAs), TEs soon become excessive in the genome, disrupt coding genes and have detrimental effects on the genome. 2) Arms race. When more piRTs produce more piRNAs and have stronger repression on TE, TE replication rate becomes lower and less TE exists in the genome, but the piRNA also alleviate detrimental effects of TEs on the genome. 3) Excessive piRNAs. If piRNA repression is very strong, TE activity becomes quite low and hardly jumps in the genome. Note that excessive dosage of piRNAs might cause off-target effects on the normal mRNAs and hence reduce the fitness of the host organism (dashed lines). The width of the lines represents the repression strength of piRNAs. b-c The numbers (y-axis) of TEs (blue), piTEs (pink), effective TEs (cyan) accumulated in one chromosome along with the generations (x-axis) in the simulations. Under the same selection scaling factor (s = 2 for b and s = 5 for c), higher numbers of TEs, piTEs, and the effective TEs carried by one chromosome were observed when the repressiveness of piRNAs (R) on TEs gets stronger. d Stronger repression of piRNA on the activities of TEs cause a positive correlation between piRNAs and TEs. The thick red lines are the mean Spearman’s Rho (y-axis) between the abundance of piRNAs and TEs along generations (x-axis) in the simulations under R = 12 (left) or R = 20 (right). The thin dashed red lines are the 2.5 to 97.5% quantiles obtained in simulations. The black lines are Spearman’s Rho under R = 0. Since in both cases, the median (thick black) and the 2.5% (thin black) quantiles are both zero, and the 97.5% (thin black) quantile is displayed. e Escaping of TEs from piRNA repression (e = 0.001, green compared with e = 0, red) decreases the positive correlation between the copy numbers of TEs and matched piRNAs. In all of these simulations, the following parameters are used: u = 0.03, Ne = 5000, d = 0.003, i = 0.001, r = 10− 8, p = 0.5, a = 10− 3, b = 5 × 10− 4, f = 0.2, e = 0 in b-d. The R and s values are displayed on each panel. The correlation was calculated in 1000 sampled chromosomes that have at least one TE from the populations. All simulations were performed for 200 replicates