Abstract
BackgroundHybrid weakness, a phenomenon opposite to heterosis, refers to inferior growth and development in a hybrid relative to its pure-line parents. Little attention has been paid to the phenomenological or mechanistic aspect of hybrid weakness, probably due to its rare occurrence.Methodology/Principal findingsHere, using a set of interspecific triploid F1 hybrids between Oryza sativa, ssp. japonica (genome AA) and a tetraploid wild rice species, O. alta (genome, CCDD), we investigated the phenotypic and physiological differences between the F1 hybrids and their parents under normal and nitrogen-limiting conditions. We quantified the expression levels of 21 key genes involved in three important pathways pertinent to the assayed phenotypic and physiological traits by real-time qRT-PCR. Further, we assayed expression partitioning of parental alleles for eight genes in the F1 hybrids relative to the in silico “hybrids” (parental cDNA mixture) under both normal and N-limiting conditions by using locus-specific cDNA pyrosequencing.Conclusions/SignificanceWe report that the F1 hybrids showed weakness in several phenotypic traits at the final seedling-stage compared with their corresponding mid-parent values (MPVs). Nine of the 21 studied genes showed contrasted expression levels between hybrids and parents (MPVs) under normal vs. N-limiting conditions. Interestingly, under N-limiting conditions, the overtly enhanced partitioning of maternal allele expression in the hybrids for eight assayed genes echo their attenuated hybrid weakness in phenotypes, an observation further bolstered by more resemblance of hybrids to the maternal parent under N-limiting conditions compared to normal conditions in a suite of measured physiological traits. Our observations suggest that both overall expression level and differential partitioning of parental alleles of critical genes contribute to condition-specific hybrid weakness.
Highlights
Hybrid vigor or heterosis refers to the phenomenon of superior performance in an F1 hybrid compared to its inbred parents and has been a focus of study owning to its significance in crop production [1]
We asserted that a better understanding of hybrid weakness may shed novel insights into the mechanisms of heterosis
In the course of our efforts to make wide hybrids between distantly related species in the genus Oryza, we noted that F1 hybrids between O. sativa, ssp. japonica and a tetraploid wild rice species, O. alta exhibited apparent hybrid weakness in a suite of morphological and physiological traits
Summary
Hybrid vigor or heterosis refers to the phenomenon of superior performance in an F1 hybrid compared to its inbred parents and has been a focus of study owning to its significance in crop production [1]. Heterosis has been recognized as a general phenomenon [2,3,4] and has been applied widely to maximize production in many important crops such as maize, rice and wheat [5,6,7]. Hybrid weakness, which is the opposite of heterosis, is understudied likely due to its uncommon occurrence. A phenomenon opposite to heterosis, refers to inferior growth and development in a hybrid relative to its pure-line parents. Little attention has been paid to the phenomenological or mechanistic aspect of hybrid weakness, probably due to its rare occurrence
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