Relative size of orthologous groups of B. tryoni , C. capitata and D. melanogaster. The gene models within each orthologous group produced by OrthoMCL were classified according to species of origin. For each species pair, the ratio of the number of gene models was then calculated on a pairwise basis. The histogram shows the counts for the ratio indicated on the X-axis. For each species pair, the most common ratio was 1:1 indicating a strong correspondence of gene models between species. However, the ratios for D. melanogaster in particular are biased downward due to the greater number of gene models available for that species.