We used k-means clustering to classify genes according to their expression profiles, into four classes named ("Nitrogen", "Early", "Middle" and "Late"). The classification was performed using all the genes significantly induced during the sporulation in each yeast, i. e. 518 genes for the S. cerevisiae genome and 724 genes for S. pombe. After the selection procedure of ortholog gene pairs, we compared the selection rate into the four temporal classes :

S. pombe Temporal classes All genes Ortholog genes Selection rate Nitrogen 99 14 14.1% Early 164 20 12.2% Middle 305 42 13.8% Late 156 22 14.1% Sum Total 724 98 13.5%

S. cerevisiae Temporal classes All genes Ortholog genes Selection rate Nitrogen 44 8 18.2% Early 118 16 13.5% Middle 328 55 16.7% Late 28 6 21.4% Sum Total 518 85 16.4%

Using the Chi2 test, we tested for each specie whether the selection rate are really different between the four temporal classes. The resulted Chi2 values (with 3 degree of freedom) were compared to the critical values with different significance levels and were not greater than any of them, meaning that while there is an apparent difference between the groups, this difference may be due to chance.