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Bacterial panicle blight (BPB) of rice (Oryza sativa L.) occurs when the bacterium Burkholderia glumae Kurita and Tabei infects emerging and flowering panicles, causing kernels to abort. To identify quantitative trait loci (QTLs) for BPB resistance, a population of 300 recombinant inbred lines (RILs) derived from a cross between 'Lemont' and 'TeQing' were evaluated in 2001 and 2002 in field plots spray-inoculated with B. glumae at the time of flowering. Because this RIL population had been previously used to map QTLs for three other diseases, present use of this population allowed direct comparison between the various disease resistance QTLs. Multiple interval mapping using QTL Cartographer v2.5 putatively identified 12 BPB QTLs, three of which were statistically significant in both years and found to have epistatic effects in 2002. TeQing was the source of resistance for eight QTLs; Lemont for four. Four BPB QTLs colocated with QTLs previously identified as providing resistance to one or multiple other diseases. Three BPB QTLs were also associated with late flowering. Because late flowering panicles are subjected to cooler temperatures that are less conducive to disease development during grain fill, it is possible that the genetic effects of the heading-related QTLs were biased. The present data could not distinguish between pleiotropy and close linkage of the BPB QTLs with the previously identified heading and disease resistance QTLs. doi: 10.2135/cropsci2008.07.0447

Most commercial cultivars of rice (Oryza sativa L.) are susceptible to sheath blight (SB), a devastating fungal disease causing significant losses in grain yield and quality. There are limited sources of genetic resistance adapted to U.S. growing conditions, and no commercial long grain cultivar of rice is currently available in the United States with a high level of SB resistance. Sheath blight resistance has been reported to be horizontal and quantitative. A population of 279 [F.sub.2:3] progeny rows derived from a cross between two tropical japonica U.S. rice cultivars, Rosemont (semi-dwarf, SB susceptible) and Pecos (tall, SB resistant), was used to map SB resistance. Progeny families were evaluated for disease reactions, plant height (PH), and heading date (HD) in replicated field trials for 2 yr and genotyped with 149 simple sequence repeat markers. Correlation analysis between SB ratings with PH and HD showed that both agronomic traits were significantly correlated with SB resistance. Four significant (logarithm of odds ratio [greater than or equal to] 3.6) quantitative trait loci (QTLs) were identified for SB resistance, with individual effects explaining 5.6 to 33.4% of the total phenotypic variation. Plant height appears to have a direct influence on SB resistance, with QTLs for these traits colocated on chromosome 1. Consistent results across years indicate the stability of the identified QTLs and their potential for improving rice SB resistance using marker-assisted selection.

One method for confirming the existence of quantitative trait loci (QTLs) is to identify loci of similar location and effect in multiple populations and/or environments. The literature contains two prior publications reporting the location of QTLs affecting resistance to sheath blight (SB) disease in rice (Oryza sativa L.), but lack of agreement between QTLs in the studies left all 12 unconfirmed, limiting the potential of marker-assisted selection of this trait with worldwide importance. The earlier linkage analyses were imprecise due to heterozygosity, segregation, and limited plot size and replication. We evaluated a replicated set of pure-breeding recombinant inbred lines (RILs) to increase reliability of the quantitative disease data and in turn improve accuracy of the QTL mapping. The RILs were [F.sub.2:10] descendants from an early-generation 'Lemont' x 'Teqing' population wherein SB resistance QTLs (SB-QTLs) were first identified. The present study confirmed the location and effect of six SB-QTLs, confirmed the existence but not the specific location of another, and identified eight new loci. Three of the confirmed QTLs were also found to be independent from undesirable plant height and maturity effects. This research demonstrated the importance of using replicated phenotypic data for reliably establishing the identity and effect of putative QTLs for complex traits such as SB resistance. Further marker development to facilitate marker-assisted selection of these three SB-QTLs is warranted., SHEATH BLIGHT, caused by the fungus Rhizoctonia solani Kuhn, is one of the major foliar diseases of rice worldwide that severely impairs both grain yield and quality (Ou, 1985). R. [...]

[F.sub.1] hybrid sterility and 'hybrid breakdown' of [F.sub.2] and later generations in rice (Oryza sativa L.) are common and genetically complicated. We used a restriction fragment length polymorphism linkage map and [F.sub.4] progeny testing to investigate hybrid sterility and hybrid breakdown in a cross between 'widely compatible' O. sativa ssp. japonica cultivar Lemont from the Southern U.S. and ssp. indica cultivar Teqing from China. Our results implicate different genetic mechanisms in hybrid sterility and hybrid breakdown, respectively. Hybrid sterility appeared to be due to recombination within a number of putative differentiated 'supergenes' in the rice genome, which may reflect cryptic structural rearrangements. The cytoplasmic genome had a large effect on fertility of both male and female gametes in the [F.sub.1] hybrids. There appeared to be a pair of complementary genes that behaved like 'wide compatibility' genes. This pair of genes and the 'gamete eliminator' ([S.sub.1]) or 'egg killer' (S-5) may influence the phenotypic effects of presumed supergenes in hybrids. Hybrid breakdown appeared to be largely due to incompatibilities between indica and japonica alleles at many unlinked epistatic loci in the genome. These proposed mechanisms may partly account for the complicated nature of postreproductive barriers in rice.

The genetic basis for three grain yield components of rice, 1000 kernel weight (KW), grain number per panicle (GN), and grain weight per panicle (GWP), was investigated using restriction fragment length polymorphism markers and [F.sub.4] progeny testing from a cross between rice subspecies japonica (cultivar Lemont from USA) and indica (cv. Teqing from China). Following identification of 19 QTL affecting these traits, we investigated the role of epistasis in genetic control of these phenotypes. Among 63 markers distributed throughout the genome that appeared to be involved in 79 highly significant (P < 0.001) interactions, most (46 or 73%) did not appear to have 'main' effects on the relevant traits, but influenced the trait(s) predominantly through interactions. These results indicate that epistasis is an important genetic basis for complex traits such as yield components, especially traits of low heritability such as GN and GWP. The identification of epistatic loci is an important step toward resolution of discrepancies between quantitative trait loci mapping and classical genetic dogma, contributes to better understanding of the persistence of quantitative genetic variation in populations, and impels reconsideration of optimal mapping methodology and marker-assisted breeding strategies for improvement of complex traits.

Most calories consumed by humans and livestock derive from the major cereals: rice, wheat, maize, millet, and sorghum. The cereals are members of the grass family (Poaceae) and were each [...], Independent domestication of sorghum, rice, and maize involved convergent selection for large seeds, reduced disarticulation of the mature inflorescence, and daylength-insensitive flowering. These similar phenotypes are largely determined by a small number of quantitative trait loci (QTLs) that correspond closely in the three taxa. The correspondence of these QTLs transcends 65 million years of reproductive isolation. This finding supports models of quantitative inheritance that invoke relatively few genes, obviates difficulties in map-based cloning of QTLs, and impels the comparative mapping of complex phenotypes across large evolutionary distances, such as those that separate humans from rodents and domesticated mammals.