Two recombinant inbred (RI) populations having 194 and 222 lines each, derived, respectively, from a highly heterotic inter-(IJ ) and intrasubspecific (II ) hybrid, were backcrossed to their respective parents. The RI and two backcross populations along with F 1 and its two parents of each hybrid were evaluated for nine important traits, including grain yield and eight other yield-related traits. A total of 76 quantitative trait loci (QTL) for the IJ hybrid and 41 QTL for the II hybrid were detected in the RI population, midparent heterosis of two backcross populations, and two independent sets of data by summation (L 1 1 L 2 ) and by subtraction (L 1 À L 2 ) of two backcross populations (L 1 and L 2 ). The variance explained by each QTL ranged from 2.6 to 58.3%. In the IJ hybrid, 42% (32) of the QTL showed an additive effect, 32% (24) a partial-tocomplete dominant effect, and 26% (20) an overdominant effect. In the II hybrid, 32% (13) of the QTL demonstrated an additive effect, 29% (12) a partial-to-complete dominant effect, and 39% (16) an overdominant effect. There were 195 digenic interactions detected in the IJ hybrid and 328 in the II hybrid. The variance explained by each digenic interaction ranged from 2.0 to 14.9%. These results suggest that the heterosis in these two hybrids is attributable to the orchestrated outcome of partial-to-complete dominance, overdominance, and epistasis. H ETEROSIS, a term to describe the superiority of heterozygous genotypes over their corresponding parental genotypes (Shull 1908), has been under investigation for $100 years, but no consensus exists about the genetic basis underlying this very important phenomenon. Two contending hypotheses, the dominance hypothesis and the overdominance hypothesis, were proposed to explain this phenomenon about one century ago. The dominance hypothesis attributes heterosis to canceling of deleterious or inferior recessive alleles contributed by one parent, by beneficial or superior dominant alleles contributed by the other parent in the heterozygous genotypes at different loci (Davenport 1908;Bruce 1910;Jones 1917). The overdominance hypothesis attributes heterosis to the superior fitness of heterozygous genotypes over homozygous genotypes at a single locus (East 1908;Shull 1908).Molecular markers and their linkage maps have greatly facilitated the identification of individual loci conditioning heterosis and the estimation of gene action of underlying loci. Quantitative trait locus (QTL) mapping studies aiming at understanding the genetic basis of heterosis have been conducted in rice and other crops (Xiao et al. 1995;Li et al. 1997Li et al. , 2001Yu et al. 1997;Luo et al. 2001;Hua et al. 2002Hua et al. , 2003Semel et al. 2006;Frascaroli et al. 2007; Melchinger et al. 2007a,b). Evidence from such studies suggests that heterosis may be attributable to dominance (Xiao et al. 1995;Cockerham and Zeng 1996), overdominance (Stuber et al. 1992;Li et al. 2001;Luo et al. 2001), pseudooverdominance due to tightly linked loci with beneficial ...

Although activation of calcium-activated neutral protease (calpain) by the NMDA receptor has been suggested to play critical roles in synaptic modulation and neurologic disease, the nature of its substrates has not been completely defined. In this study, we examined the ability of calpain to cleave the NMDA receptor in cultured hippocampal neurons. Activation of the NMDA receptor by agonist application led to rapid calpain-specific proteolysis of spectrin and decreased levels of NR2A/2B subunits. Cleavage of the NR2A/2B subunit created a 115 kDa product that retained the ability to bind 125I-MK-801 and is predicted to be active. Increases in levels of this product appeared within 5 min of NMDA receptor activation and were stable for periods of >30 min. Subtype-specific antibodies demonstrated that the NR2B subunit was cleaved in these primary cultures, but the NR2A subunit was not. An inhibitor of calpain blocked both the decrease of intact NR2B and the increase of the low molecular weight form, whereas neither caspase nor cathepsin inhibitors had an effect on these events. Cell surface biotinylation experiments demonstrated that the 115 kDa fragment remained on the cell surface. This NR2B fragment was also found in the rat hippocampus after transient forebrain ischemia, showing that this process also occurs in vivo. This suggests that calpain-mediated cleavage of the NR2B subunit occurs in neurons and gives rise to active NMDA receptor forms present on the cell surface after excitotoxic glutamatergic stimulation. Such forms could contribute to excitotoxicity and synaptic remodeling.

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