Despite the rediscovery of heterosis about a century ago and the suggestion of various genetic models to explain this phenomenon, little consensus has yet been reached about the genetic basis of heterosis. Following the genome organization variation and gene effects, an understanding of gene differential expression in hybrids and its parents provides a new opportunity to speculate on mechanisms that might lead to heterosis. Investigation on allele-specific gene expression in hybrid and gene differential expression between hybrids and its parents might contribute to improve our understanding of the molecular basis of heterosis and eventually guide breeding practices. In this review, we discussed the recent researches on allelic-specific expression in hybrid which was frequently observed in recent studies and analyzed its regulatory mecha-nism. All possible modes of gene action, including additivity, high- and low-parent dominance, underdominance, and over-dominance, were observed when investigating gene differential expression between hybrids and its parents. Data from tran-scriptomic studies screened several heterosis-associated genes and highlighted the importance of certain key biochemical pathways that may prove to be quintessential for the manifestation of heterosis. So far, no uniform global expression pat-terns were observed in these gene expression studies. Most heterosis-associated gene expression analyses have not revealed a predominant functional category to which differentially expressed genes belong. However, these gene expression profiling studies represent a first step towards the definition of the complex gene expression networks that might be relevant in the context of heterosis. New technique on gene expression profile and advancements in bioinformatics will facilitate our un-derstanding of the genetic basis of heterosis at the gene-expression level.
[1] Shull GH. The composition of a field of maize. J Hered, 1908, 4(1): 296-301.
[2] Birchler JA, Auger DL, Riddle NC. In search of the mo-lecular basis of heterosis. Plant Cell, 2003, 15(10): 2236-2239.
[3] Birchler JA, Yao H, Chudalayandi S. Unraveling the ge-netic basis of hybrid vigor. Proc Natl Acad Sci USA, 2006, 103(35) 12957-12958.
[4] Birchler JA, Yao H, Chudalayandi S, Vaiman D, Veitia RA. Heterosis. Plant Cell, 2010, 22(7): 2105-2112.
[5] Troyer AF. Adaptedness and heterosis in corn and mule hybrids. Crop Sci, 2006, 46(2): 528- 543.
[6] Lippman ZB, Zamir D. Heterosis: revisiting the magic. Trend Genet, 2007, 23(2): 60-66.
[7] Springer NM, Stupar RM. Allelic variation and heterosis in maize: How do two halves make more than a whole? Genome Res, 2007, 17(3): 264-275.
[8] Hochholdinger F, Hoecker N. Towards the molecular basis of heterosis. Trends Plant Sci, 2007, 12(9): 427-432.
[9] Goff SA. A unifying theory for general multigenic hetero-sis: energy efficiency, protein metabolism, and implica-tions for molecular breeding. New Phytol, 2011, 189(4): 923-937.
[10] Sanghera GS, Wani SH, Hussain W, Shafi W, Harib-hushan A, Singh NB. The magic of heterosis: New tools and complexities. Nat Sci, 2011, 9(11): 42-53.
[11] Baranwal VK, Mikkilineni V, Zehr UB, Tyagi AK, Kapoor S. Heterosis: emerging ideas about hybrid vigour. J Exp Bot, 2012, 63(18): 6309-6314.
[12] Kaeppler S. Heterosis: Many genes, many mechanisms- End the search for an undiscovered unifying theory. ISRN Bot, 2012, doi: 10.5402/2012/682824.
[13] Davenport CB. Degeneration, albinism and inbreeding. Science, 1908, 28(718): 454-455.
[14] East EM. Inbreeding in corn. Rep Connecticut Agric Exp Stn, 1908, 1907: 419-429.
[15] Yu SB, Li JX, Xu CG, Tan YF, Gao YJ, Li XH, Zhang QF, Saghai Maroof MA. Importance of epistasis as the genetic basis of heterosis in an elite rice hybrid. Proc Natl Acad Sci USA, 1997, 94(17) 9226-9231.
[16] Li ZK, Luo LJ, Mei HW, Wang DL, Shu QY, Tabien R, Zhong DB, Ying CS, Stansel JW, Khush GS, Paterson AH. Overdominant epistatic loci are the primary genetic basis of inbreeding depression and heterosis in rice. I. Biomass and grain yield. Genetics, 2001, 158(4): 1737-1753.
[17] Knight JC. Allele-specific gene expression uncovered. Trends Genet, 2004, 20(3): 113- 116.
[18] Kliebenstein DJ, West MA, van Leeuwen H, Kim K, Doerge RW, Michelmore RW, St Clair DA. Genomic survey of gene expression diversity in Arabidopsis thaliana. Genetics, 2006, 172(2): 1179- 1189.
[19] Brem RB, Yvert G, Clinton R, Kruglyak L. Genetic dis-section of transcriptional regulation in budding yeast. Science, 2002, 296(5568): 752-755.
[20] Romagnoli S, Maddaloni M, Livini C, Motto M. Relationship between gene expression and hybrid vigor in primary root tips of young maize (Zea mays L.) plantlets. Theor Appl Genet, 1990, 80(6): 769-775.
[21] Guo M, Rupe MA, Zinselmeier C, Habben J, Bowen BA, Smith OS. Allelic variation of gene expression in maize hybrids. Plant Cell, 2004, 16(7): 1707-1716.
[22] Cowles CR, Hirschhorn JN, Altshuler D, Lander ES. Detection of regulatory variation in mouse genes. Nat Genet, 2002, 32(3): 432-437.
[23] Yan H, Yuan W, Velculescu VE, Vogelstein B, Kinzler KW. Allelic variation in human gene expression. Science, 2002, 297(5584): 1143.
[24] Springer NM, Stupar RM. Allele-specific expression pat-terns reveal biases and embryo- specific parent-of-origin ef-fects in hybrid maize. Plant Cell, 2007, 19(8): 2391-2402.
[25] Paschold A, Jia Y, Marcon C, Lund S, Larson NB, Yeh CT, Ossowski S, Lanz C, Nettleton D, Schnable PS, Hochholdinger F. Complementation contributes to transcriptome complexity in maize (Zea mays L.) hybrids relative to their inbred parents. Genome Res, 2012, 22(12): 2445-2454.
[26] Zhuang Y, Adams KL. Extensive allelic variation in gene expression in Populus F1 hybrids. Genetics, 2007, 177(4): 1987-1996.
[27] Adams KL, Wendel JF. All
