Wang et al. Genetics Selection Evolution 2010, 42:24 http://www.gsejournal.org/content/42/1/24 models can be used to account for the conservation of duplicates, i.e. dosage model and buffering model. The former proposes that as the new duplicates will increase the gene dosage, they can bring about some selective advantages [20]. In contrast, the latter argues that the conserved duplicates are just used for compensation in case of the functional loss of their counterparts [34], and thus they are free from selective pressures. Given the sig- nature of positive selection (Figure 4), we propose that the dosage model is more prevalent in the fixation of tan- dem duplicates. In fact, the dosage model predicts that the fitness of dosage-sensitive genes will increase with the increase of gene copies [20], which is consistent with our observation that tandem duplicates tend to form large families (Figure 1). Another large-scale functional analy- sis has revealed that tandem duplicates are enriched in receptors and binding proteins [14], which are also dos- age-sensitive genes [20]. Interestingly, copy number vari- ants (CNV), which are strongly associated with segmental tandem duplicates [35], may also be main- tained by dosage effect and positive selection [36]. EAC effect is more prevalent in retrogenes Retrogenes and tandem duplicates display nearly oppo- site molecular properties. Since retrogenes are often dis- tant from their parental counterparts and lose the original regulatory elements, they are more likely to undergo functional divergence (Figure 2 and 3). There are also two main models available to account for the func- tional divergence, namely 'escape from adaptive conflict' (EAC) model [37] and 'duplication-degeneration-comple- mentation' (DDC) model [38]. Both of the models predict that the new duplicates will share the functions of the ancestral genes. However, the EAC model argues that duplications can release the potential benefits through functional specialization, whereas the DDC model only requires that the joint effect of the duplicates fulfil the original functions. The signature of positive selection in the retention of retrogenes votes for the prevalence of the EAC model (Figure 4). In addition to our results, the anal- ysis of gene movements has revealed that the X-linked genes are excessively transferred to autosomes via ret- ropositions in mammalian genomes [16]. These retro- genes can not only sustain essential functions during the inactivation of the male X chromosome, but also develop male-specific expression patterns [16,39]. The coexis- tence of functional divergence and selective benefits pro- vides an important evidence for the EAC model. Additional material Additional file 1 Human tandem duplicates. Additional file 2 Mouse tandem duplicates. Additional file 3 Human retrogenes. Page 5 of 6 Additional file 4 Mouse retrogenes. Competing interests The authors declare that they have no competing interests. Authors' contributions ZW conceived and performed the experiments. XD participated in the discus- sions. ZW and GD collected the data. ZW, GD and YL wrote and revised the manuscript. All authors read and approved the final manuscript. Acknowledgements This research was supported by grants from National High-Tech R&D Program (863) (2006AA02Z334, 2007DFA31040), State key basic research program (973) (2006CB910705, 2010CB529206), Research Program of CAS (KSCX2-YW-R-112, KSCX2-YW-R-190), National Natural Science Foundation of China (30900272) and Special Start-up Fund for CAS President Award Winner (to G. Ding). Author Details 1Key Lab of Systems Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, 320 Yueyang Road, Shanghai, PR China, 2Graduate School of the Chinese Academy of Sciences, 19 Yuquan Road, Beijing, PR China and 3Shanghai Center for Bioinformation Technology, 100 Qinzhou Road, Shanghai, PR China Received: 6 March 2010 Accepted: 28 June 2010 Published: 28 June 2010 TG©he2ins0eia1tsr0ctaisWncSleOanlipesgceatneivotaAniclEa;vbeiolcseunfatrisoteminecl2:eBh0idot1iM0sp,t:er/4idb2wuC:2wte4wndt.gruasnledLjotedur.rtnhael.toergm/csonftehnetC/4r2e/a1ti/v2e4CommonsAtributionLicense(htp:/creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited. References 1. Long M, Betran E, Thornton K, Wang W: The origin of new genes: glimpses from the young and old. Nat Rev Genet 2003, 4:865-875. 2. Zhang JZ: Evolution by gene duplication: an update. Trends Ecol Evol 2003, 18:292-298. 3. 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