![]() ![]() For instance, gene expression in hybrids that is significantly different from the average of the parental inbred lines (midparent value) is described as nonadditive ( Birchler et al., 2003 Stupar and Springer, 2006). The phenomenon of heterosis is often associated with transcriptional variation between inbred lines and hybrids ( Hochholdinger and Hoecker, 2007). Hybrid vigor was monitored for traits such as biomass and grain yield but also resistance to environmental cues, such as drought stress ( Araus et al., 2010). Heterosis or hybrid vigor denotes the superior performance of heterozygous F1 plants compared with their genetically distinct, homozygous parents (for review, see Falconer and Mackay, 1996). Most of these nonsyntenic genes emerged by single-gene duplication mechanisms, most likely after the last genome duplication of maize ( Woodhouse et al., 2010). This third class of genes is characterized by a lack of syntenic orthologs in the genomes of other grass species. Thus, the gene complement of modern maize can be divided into three categories: (1) pairs of duplicate genes that are shared by both subgenomes (2) single-copy genes present in only one of the two subgenomes and (3) genes that cannot be assigned to any subgenome. ![]() Each of these subgenomes originally contained a complete set of genomic information. The synteny to sorghum allows a subdivision of the maize genome into two distinct subgenomes. Nevertheless, 3,000 to 5,000 pairs of paralogs are retained in modern maize ( Schnable et al., 2011). Comparisons of the duplicated genome of modern maize with orthologous regions of its unduplicated progenitor sorghum ( Sorghum bicolor) revealed that many of the duplicated maize genes disappeared in the course of evolution ( Haberer et al., 2005). A syntenic comparison of the genomes of grass species demonstrated that the lineage leading to maize experienced a whole-genome duplication about 5 to 12 million years ago ( Blanc and Wolfe, 2004 Swigonová et al., 2004). Syntenic genes are defined by their preserved colocalization on chromosomes of different species. Between the widely used inbred lines B73 and Mo17, thousands of structural variations ( Springer et al., 2009 Swanson-Wagner et al., 2010), differences in total genome size ( Laurie and Bennett, 1985), and content of repetitive DNA ( Kato et al., 2004) have been detected, underscoring their high degree of intraspecific variation. The divergence between different maize genotypes is the consequence of copy number variation, presence/absence variation ( PAV), single-nucleotide polymorphisms ( SNPs), and insertion-deletion polymorphisms. Maize displays a remarkable degree of structural intraspecific genomic diversity, which is likely unparalleled among higher eukaryotes ( Springer et al., 2009 Swanson-Wagner et al., 2010). Maize ( Zea mays) is a fundamental source of food, feed, and energy ( Persson et al., 2009). Hence, the significant overrepresentation of nonsyntenic genes among SPE patterns and their stability under water limitation might suggest a function of these genes during the early developmental manifestation of heterosis under fluctuating environmental conditions in hybrid progeny of the inbred lines B73 and Mo17. Nonsyntenic genes that lack syntenic orthologs in other grass species, and thus evolved late in the grass lineage, were significantly overrepresented among SPE genes. While, on average, 75% of all SPE patterns were not altered in response to polyethylene glycol treatment, only 17% of the remaining genotype-specific expression patterns were not changed by water deficit. SPE patterns were substantially more stable to expression changes by water deficit treatment than other genotype-specific expression profiles. As a consequence, the number of active genes in hybrids exceeded the number of active genes in the parental inbred lines significantly independent of treatment. In this study, 1,997 genes only expressed in B73 and 2,024 genes only expressed in Mo17 displayed SPE complementation under control and water deficit conditions. Single-parent expression ( SPE) of genes is an extreme instance of gene expression complementation, in which genes are active in only one of two parents but are expressed in both reciprocal hybrids. In this study, we monitored the transcriptomic divergence of the maize ( Zea mays) inbred lines B73 and Mo17 and their reciprocal F1 hybrid progeny in primary roots under control and water deficit conditions simulated by polyethylene glycol treatment. Heterosis is the superior performance of F1 hybrids compared with their homozygous, genetically distinct parents. ![]()
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