Frequency of classes of deletions sizes (in number of genes) between LCA1 and LCA2, for syntenic fragments. Open triangles, observed frequencies. Open squares, expected frequencies if losses were random and deletions sizes followed a Pascal (geometric) law of mean P equal to the frequency of genes in syntenic fragments actually lost (P = 0.438). Inbedded histogram: i) open bars represent the difference in % between observed frequencies and simulated frequencies of size classes (averages of n = 150 simulations) for a constant probability of loss (H1 hypothesis) ii) open bars represent the difference in % between simulated frequencies for a probability of loss function of the CAI (H2 hypothesis) and for simulated frequencies under H1. The class "zero" corresponds to two adjacent syntenic genes conserved in LCA2 (i.e. no deletion occurred).