As the transcriptional program that drives the developmental process of sporulation in the two yeast species has been studied with similar time-course experiments, we could performed a direct comparison of the gene expression profiles for all orthologous pairs. We used a simple approach, which consists in calculating a Pearson correlation measured between the expression values that are common to the S. pombe and S. cerevisiae time series. As a result, most of the orthologous gene pairs were positively correlated but only 63% exhibited a value higher than 0.7. This observation demonstrates different degrees of expression conservation between ortholog gene pairs, and hence one challenging question is to know whether orthologous genes occupy similar positions in the gene expression networks. Therefore, to go further and take into account all the intra and inter-species relationships between genes, we performed a comparison of the organization of the gene expression networks represented in a 3-dimensional space...

Figure legend : Histogram of the Pearson correlation measure between the expression profiles of genes that are orthologous between S. cerevisiae and S. pombe. As the two time series have different numbers of data points (7 for S. cerevisiae tn : {0, 30min, 2h, 5h, 7h, 9h, 11h} and 13 for S. pombe tm : {0, 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h}), the correlation was calculated using only the time points that are common to the two time series : {0, 2h, 5h, 7h, 9h, 11h}. Most of the orthologous gene pairs exhibits a strong correlation between their corresponding expression profiles (63% show a value higher than 0.7). But others are caracterized by a value near 0 that represents uncorrelated variables.