Genetic Dissection Of Heterosis For Seedling Traits And Carbon And Nitrogen Contents Of Sink Source In Shanyou 63

The shortages of food production caused by rapid population growth have been solved by the utilization of heterosis in rice.But,how the heterosis performs,how its function connects with the genetic network and molecular metabolism pathway remains a mystery.And in recent years,the traditional breeding approach,although found effective in breeding rice hybrids in China,are confronted with two severe challenges.The first is the transformation of the rice planting technique from conventional manual labor to mechanized rice transplantation with the decreasing of rural labor force.The genetic improvements of seedling traits of hybrid rice to adapt to the mechanization would be the an important direction for rice breeding in future.At meantime,misuse of nitrogen fertilizer lead to low nitrogen use efficiency in rice production,along with severe environmental pollution.We need to maintain the rice yield under the premise of improve nitrogen use efficiency,to achieve the balance of nutrition efficiency and high yield in rice development as far as possible.Superior hybrids of rice usually show heterosis at two stages: canopy development at vegetative stage and panicle development at reproductive stage resulting in heterosis in yield.However,previous studies of the genetic basis of heterosis in rice focused on yield traits at maturity stage.We know little about the genetic basis of heterosis for rice seedling traits.Thus,we want to conduct a comprehensive analysis the heterosis of seedling traits.In this study,we analyzed the genetic basis of heterosis at seedling stage making use of an “immortalized F₂”?IF₂?population composed of 105 hybrids produced by intercrossing recombinant inbred lines?RILs?derived by single seed decent from a cross between Zhenshan 97 and Minghui 63,the parents of Shanyou 63,an elite hybrid widely grown in China.A total of eight seedling traits were investigated using hydroponic culture.The distributions of the trait values showed continuous variation and obvious transgressive segregation for most of traits in the IF₂ population.We analyzed single-locus and digenic genetic effects at the whole genome level using an ultrahigh-density SNP bin map.The results revealed large numbers of heterotic effects for seedling traits.Overdominance effects were prevalent for all the traits.While dominance effects were relatively rare,digenic dominance effects also accounted for a large portion of the genetic effects for all the traits.The patterns of digenic interaction types were similar among the traits.The number of additive by additive interaction was greater than additive by dominance interaction and dominance by additive interaction,and the number of dominance by dominance interaction at least.Various levels of negative overdominance were also observed for the traits,indicating that heterozygosity was not always favorable for the expression of the trait even in such a highly heterotic hybrid.The results suggested that cumulative small advantages of the single-locus effects and two-locus interactions could explain the genetic basis of seedling heterosis of Shanyou 63.Except focusing on the genetic basis of performance of seedling traits,we also evaluated the MPH per se with IF₂ and RIL population.The heterosis of all the traits showed transgressive segregation.We found that the genetic effects of single-locus for heterosis are similar with the trait performance.Overdominance effects were prevalent at whole genome,while dominance effects were seldom detected for all the traits.The proportions of positive and negative overdominance effects were different among these traits.A total of 277 bins were involved in positive overdominance effects,while 198 bins were negative overdominant for all the eight traits.Some bins were detected simultaneously among these traits,which indicated pleiotropic effects of single-locus overdominance.Digenic interactions of heterosis showed a large number epistasis effects.The largest number of digenic interactions is root dry weight,more than 100 pairs,while only 17 pairs were detected in maximum root length.Interestingly,we found that the patterns of interaction types of heterosis were much different from trait performance.Additive by additive interaction were not the most for heterosis of all the traits,and dominance by dominance interaction also didn't hold the least proportion.Thus,we believed that the genetic effects of epistasis were distinctive between trait performance and heterosis in rice seedling.Besides,only a small number of digenic interactions were overlapped among these traits,suggesting the genetic basis of epistasis were different from each trait.The high nitrogen use efficiency in rice reflected in absorption,assimilation,distribution and transport at whole growing periods,especially during the grain filling stage,which could affect photosynthesis efficiency and accelerate leaf senescence.The relationship between the genetic mechanism of carbon and nitrogen metabolisms and yield are poorly understood.We surveyed the contents of carbon and nitrogen of grain and straw at mature stage in the RIL and IF₂ population to dissect the genetic mechanisms behind these phenomena.We found that in straw the nitrogen content and showed a strong negative heterosis,while Carbon/Nitrogen ratio showed a large positive heterosis.The carbon content in grain and straw showed a weak positive heterosis.In both grain and straw,the range of genetic variation of nitrogen content was greater than carbon content.The results suggested that rather than the carbon content,nitrogen content was the crucial factor which affected the Carbon/Nitrogen ratio in straw.The results of single-locus effect analysis suggested that overdominance effects were prevalent for all traits,while no significant dominance effects were detected.The patterns of digenic interaction types were similar with the performance of seedling trait.The number of additive by additive interaction was greater than additive by dominance interaction and dominance by additive interaction,and the number of dominance by dominance interaction was at least.Several bins were identified to have possible pleiotropic effects with the same direction in both straw and grain,which suggested a synergistic effect between grain and straw rather than trade-off.The genetic variation of carbon and nitrogen contents in Shanyou 63 could be explained by the sum of relative amounts of genetic components.