Genetic Basis Of Hybrid Sterility Between Indica And Japonica Subspecies In Asian Cultivated Rice

Oryza sativa L.is mainly made up of indica and japonica two subspecies,but there are abundant variations within the species,and it is one of the most important food crops in the world.Rice yield’s increase plays an important role in guaranteeing the food security and improving people’s living standard in China and even in the world.However,rice yield has been stagnating for a long time.At present,the utilization of intersubspecific hybrid heterosis is still an important approach for super high yield breeding of rice.However,the existence of reproductive barriers between indica and japonica subspecies seriously hindered the utilization of intersubspecific hybrid vigor.Considering that Asian cultivated rice is mainly divided into two subspecies indica and japonica as well as the rich genetic variation within Asian cultivated rice,20 Asian cultivated rice selected by The International Rice Research Institute（IRRI）representing five different ecological types were selected as the male parents,and temperate japonica cultivar Dianjingyou 1 and indica cultivar IR64 were selected as the female parents.The fertility of F₁hybrid produced by crossing the different ecological types of Asian cultivated rice with indica and japonica was studied comprehensively and systematically.On this basis,Dianjingyou 1 and IR64 were used to construct mapping populations under indica and japonica backgrounds to study the genetic basis of intersubspecific hybrid sterility in Asian cultivated rice.The main results are as follows:（1）The study of the fertility of hybrid produced by crossing the different ecological types of Asian cultivated rice with indica and japonica found that:hybrid sterility existed generally between indica and japonica subspecies,but the degree of hybrid sterility was varied with different cross combination hybrids,hybrid sterility also was observed between temperate japonica and tropical japonica,and some combinations between indica and indica.The degree of hybrid sterility of combinations between Aus type and Dianjingyou 1 or IR64 were varied depending on the varieties of Aus varieties,even the degree of hybrid sterility between indica and japonica was the most obvious.（2）In the F₁hybrid of Dianjingyou 1/IR64,pollen and spikelet fertility were all semi-sterile.In the BC₁F₁,BC₂F₁and BC₃F₁populations which Dianjingyou 1 as recurrent parent and IR64 as the donor,with the recurrent backcross,pollen fertility gradually changed from continuous bimodal distribution to bimodal distribution which could be divided into fertile and sterile obviously,and some families showed discontinuous bimodal distribution.The results showed that the genes controlling pollen fertility were separated by continuous backcross and gradually dissected into a quality trait controlled by a few Mendel factors.Spikelet fertility changed from continuous single-peak distribution to continuous double-peak distribution and then to continuous distribution,and some families showed three-peak distribution,indicating that spikelet fertility is a more complex trait,controlled by multiple fertility genes,and susceptible to environmental effects.（3）In the Dianjingyou 1/IR64//Dianjingyou 1 BC₁F₁population,seven QTL controlling hybrid sterility were detected.Five of them affected spikelet fertility,q SSF4located on chromosome 4,LOD value was 9.40,the explained variation rate was16.65%,analysis of its mapping location found that the localization interval is the same as the known S9 locus;q SSF4.1 on chromosome 4,LOD value was 3.12,the explained variation rate was 5.07%;The LOD value of q SSF5 on chromosome 5 was 5.01 and the explained variation rate was 27.60%,analysis of its mapping location showed that the localization interval was the same as the known S31（t）locus.The LOD value of q SSF6on chromosome 6 was 7.21 and the explained variation rate was 18.81%,analysis of its mapping location showed that the mapping localization interval is the same as the cloned S5.The LOD value of q SSF10 on chromosome 10 was 7.46,the explained variation rate was was 13.57%.q SSF4.1 and q SSF10 were the newly discovered QTL.There were two QTL affecting pollen fertility,q SPF10 on chromosome 10 with LOD value of 4.52 and the explained variation rate was 2.98%;q SPF12 on chromosome 12 has a LOD value of 29.61 and the explained variation rate was 34.34%,analysis of its mapping location shows that the localization interval was the same as the known gene S25.Four QTL controlling hybrid sterility were detected in the Dianjingyou 1/IR64/2/Dianjingyou 1 BC₂F₁population.Two of them affected spikelet fertility,q SSF4 on chromosome 4,with LOD value of 5.13 and the explained variation rate was 18.05%,analysis of its mapping location revealed that the localization interval is the same as the known S9 locus.The LOD value of q SSF6 on chromosome 6 was 20.77,with an explained variation rate 33.04%,analysis of its mapping location showed that the localization interval was the same as the cloned S5.There were two QTL affecting pollen fertility,q SPF10 on chromosome 10,with LOD value of 4.39 and the explained variation rate was 1.70%,which was the same as those detected in last season.q SPF12on chromosome 12 has a LOD value of 68.68 and a contribution rate of 39.02%,analysis of its location shows that the localization interval was the same as the known gene S25.In the BC₂F₁population,compared with the results of the previous season,four identical QTL were detected,q SSF4,q SSF6,q SPF10 and q SPF12.At the same time,three QTL were not detected,q SSF4.1 on chromosome 4,q SSF5 on chromosome5 and q SSF10 on chromosome 10.In the Dianjingyou 1/IR64/3/Dianjingyou 1 BC₃F₁population,q SSF4 on chromosome 4,q SSF6 on chromosome 6 and q SPF12 on chromosome 12 were detected,respectively.In a Dianjingyou 1/IR64/2/Dianjingyou 1 BC₂F₂population,spikelet fertility was normal and fertile,while pollen fertility showed a bimodal distribution which could be divided into semi-sterile and fertile two types.In this population,the gene S25,which affects pollen fertility,was detected on chromosome 12.The locus was mapped flanked by RM1208 and RM8215,at genetic distance of 0.3 and 0.90c M,respectively.Gamete segregation analysis showed that the gametes from japonica was eliminated.The mapping results of BC₁F₁,BC₂F₁,BC₃F₁and BC₂F₂populations showed that with the increase of selection and backcross generations,the loci controlling fertility gradually evolved from multiple QTL to quality traits controlled by a few Mendel factors.（4）In the BC₁F₁population which IR64 as the recurrent parent and Dianjingyou 1as the donor,pollen fertility presented a continuous bimodal distribution,but there were few spikelet sterile plants and spikelet fertility presented a continuous single-peak distribution.In the IR64/Dianjingyou 1//IR64 BC₁F₁population,four QTL controlling hybrid sterility were detected,and two of them affected spikelet fertility,q SSF2 on chromosome 2,with LOD value of 2.63 and the explained variation was 4.40%,analysis of its mapping locations showed that it was close to the reported S29（t）.The LOD value of q SSF9 on chromosome 9 was 3.26,and the explained variation was5.34%,no related sterility QTL had been reported at this location,and it was a new QTL related to spikelet fertility.There were two QTL affecting pollen fertility,q SPF2 on chromosome 2,with LOD value of 4.42 and the explained variation was 6.21%.The loci had not been reported before and it was a new QTL controlling pollen fertility.q SPF12.1,with the LOD value of 10.74 and the explained variation was16.02%on chromosome 12,the localization interval was the same as the hybrid fertility gene S25.（5）Comparing the mapping results of the Dianjingyou 1/IR64//Dianjingyou 1 and IR64/Dianjingyou 1//IR64 BC₁F₁population showed that the some same QTL can be detected from different backgrounds,such as q SPF12 and q SPF12.1.Different QTL were also detected,too.For example,q SSF4,q SSF4.1,q SSF5,q SSF6,q SSF10 and q SPF10 were detected in the Dianjingyou 1/IR64//Dianjingyou 1 BC₁F₁population,but not in the IR64/Dianjingyou 1//IR64 BC₁F₁population.Accordingly,q SSF2,q SSF9and q SPF2 detected in the IR64/Dianjingyou 1//IR64 BC₁F₁population were not detected in the Dianjingyou 1/IR64//Dianjingyou 1 BC₁F₁population.The results indicate that the occurrence of hybrid sterility is dependent on genetic backgrounds,some hybrid sterility loci can be functional under different genetic backgrounds,while some hybrid sterility loci only function under specific genetic background.The research discoveries from this study,in some degree,provide evidence for understanding the differentiation of the Asian cultivated rice,breaking the reproductive barrier between Asian cultivated rice subspecies,providing material and technical support for making full use of hybrid vigor,fostering hybrids development by using the rich genetic basis of Asian cultivated rice and providing theoretical guidance for the breeding of hybrid rice under different backgrounds（indica or japonica）.