Figure 3.

Promoter divergence and sexual selection. Results of the linear regression analyses between the genetic divergences of the regulatory elements of protamines against relative testes size. The divergence unit is the mean number of nucleotide substitution per site. Note that neither the evolution of (a) the intron sequences (INT, used as a neutral marker of evolution; r²=−0.096, p=0.5562) nor that of (b) the protamine 1 (Prm 1) promoter (R²=−0.043, p=0.4322) fits a linear regression model. Only when (c) the protamine 2 (Prm 2) promoter (R²=0.707, p=0.0055) divergence is considered alone or in combination with (d) other parameters of divergence (Prm 2 promoter−Prm 1 promoter)/INT; R²=0.86, p=0.0006) are significant relationships with relative testes size found. Genetic divergences of Prm 1 promoter, Prm 2 promoter and INT sequences were estimated as the mean distribution values of branch lengths computed after a 10 000 bootstrap-based analysis (see fig. S5 in the electronic supplementary material). R² and p represent the fit and the statistical confidence of the linear regression models.