BMC Genomics 2009, 10(Suppl 1):S14 http://www.biomedcentral.com/1471-2164/10/S1/S14 evolutionary stage when massive genomic duplications and explosive increase in tissues complexity happened. Second, the expansion of gene families in the peripheral layer of the gene network may also be asymmetric. Certain small branches of the big family may get disproportional expansion in particular species at particular evolutionary stages, as exemplified by the type 2 taste receptors in human. Tissue distribution of GPCRs and the "downstream genes" of different duplication ages Duplicated genes often have divergent expression patterns in tissues. We further examined the expression profiles of human GPCRs and the "downstream genes" with respect to their duplication ages. The expression profiles were based on the EST count information from UniGene [20]. For each human gene in the GPCR model system, we identified the tissues where it was highly expressed (see Methods for details). The duplication age of a human gene was defined as the age of the most recent duplication event leading to the gene. Due to the approximate nature of age estimation, we divided the duplication time into three broad intervals, including earlier than 800 Myrs, 800–400 Myrs and 400-0 Myrs, and focused on the last two intervals. For genes that duplicated in each of the intervals, and then for each of the 45 tissues covered by UniGene, we calculated the number of genes with high expression in that particular tissue. The result was summa- rized in Figure 4 for the top tissues in number of highly expressed genes of the GPCR model system. Shown in Figure 4A was the result for GPCRs. Genes duplicated in the 400-0 Myrs interval were denoted as group 400, and genes duplicated in the 800-400 Myrs interval were denoted as group 800. The tissues were ranked by the number of group 400 genes that were highly expressed in a tissue. Our result showed that expression of the more recently duplicated genes (group 400) was enriched in blood and spleen, both of which important tissue for immune response. The chemokine receptor family contributed greatly to these two tissues. Enrichment of expression in the connective tissue was also observed, which was connected to the adhesion receptor family. On the other hand, group 800 genes were expressed in a much wider range of tissues. Interesting, most of the GPCRs expressed in the brain were in group 800. Relatively few GPCR genes expressed in brain or nerve tissues were recently duplicated. Shown in Figure 4B was the result for the "downstream genes". The data series and the ranking of the tissues were defined the same as in Figure 4A. As has been shown ear- lier, relatively fewer "downstream genes" were duplica- tion after 400 Myrs, which was reflected in the small number of genes in group 400 in Figure 4B. Similar to GPCRs, the "downstream genes" in group 800 were expressed in a wider range of tissues. Expression in brain and nerve again was most enriched in group 800. In general, these results of tissue distribution indicated that a substantial enrichment of both GPCRs and the "downstream genes" expressed in brain and nerve tissues happened during the 800-400 Myrs interval. However, similar surge were not observed during the 400-0 Myrs interval. Instead, an enrichment of the GPCRs that expressed in the immune-related tissues of blood and spleen were observed, and this again was contributed mostly by several small GPCRs subfamilies. Discussion Current theories of gene duplication and gene networks suggest that genes on the periphery of the network are preferentially retained after gene duplication, in compari- son to the genes form the backbones of the network. This is actually necessary for the gene network to remain scale- free after rounds of gene duplication. However, the data supporting these theories were mostly based on the genetic network or the PPI network in yeast and worm. Similar data on gene networks in more advanced species, including vertebrates, were not yet complete and reliable enough. Our model system of GPCRs and the "down- stream genes", took the advantage of the knowledge that the gene network roughly maps to the cellular organiza- tion [9], offered an opportunity to get some insight into the relationship of gene duplication and gene network in the context of vertebrate evolution. Our result showed that GPCRs families had significantly more continued expansion after 400 Myrs, in comparison to the "down- stream genes". Under the assumption that all genes have equal opportunities for duplication, this result confirmed that duplicated GPCR genes were preferentially fixed dur- ing the 400-0 Myrs interval, compared to the "down- stream genes". However, this preferential retention was time dependent, as during the 800-400 Myrs interval we found both GPCR and "downstream gene" families expe- rienced explosive expansion. One explanation for the result of the 800-400 Myrs interval might be that tissue complexity of the species might also experience an explo- sive increase during that interval, which might have driven fixation of duplicated genes in all the layer of the network. The expression profiles of the genes with dupli- cation ages in the 800-400 Myrs interval offered partial support for the explanation, as genes duplicated at that stage were found to be expressed broadly in a wide range of tissues in human, including brain and nerve. Our results suggested that the gene basis of tissue diversity was largely established by gene duplications in the 800-400 Myrs interval. Page 6 of 10 (page number not for citation purposes)