Selection efficiencies for improving drought/salt tolerances and yield using introgression breeding in rice(Oryza sativa L.)

The backcross(BC) breeding strategy has been increasingly used for developing high yielding varieties with improved abiotic stress tolerances in rice. In this study, 189Huang-Hua-Zhan(HHZ) introgression lines(ILs) developed from three different selection schemes were evaluated for yield related traits under drought stress and non-stress conditions in the target and off-season winter nursery environments to assess the selection efficiency of BC breeding for improving different complex traits, and led us to five important results. The first result indicated that the primary target traits should be selected first in the target environments(TEs) in order to achieve the maximum genetic gain. Secondly, BC breeding for drought tolerance(DT) in rice was almost equally effective by strong phenotypic selection in the main target environments and in the winter-season of Hainan.Thirdly, exploiting genetic diversity in the subspecific gene pools is of great importance for future genetic improvement of complex traits in rice. Fourthly, considerable genetic gain can be effectively achieved by selection for secondary target traits among the ILs with the primary traits. Finally, the developed ILs provide useful materials for future genetic/genomic dissection and molecular breeding of complex traits.

Dominant early heading without yield drag in a sister-line BC breeding progeny DEH_229 is controlled by multiple genetic factors with maineffect loci

Dominant early heading(DEH)in rice(Oryza sativa L.)is of interest in both breeding and genetics.The genetic mechanisms underlying DEH have remained largely unclear.We have developed a near-isogenic DEH line without yield drag named DEH_229 by sister-line backcross(BC)breeding with MH63,a restorer,as the genetic background.We conducted a pilot genetic investigation under both short-day(SD)and long-day(LD)conditions.The DEH line harbored only 1.06%variation in the genome sequence relative to MH63.The variants were distributed throughout the genome.Using QTL mapping by sequencing(QTL-seq)on an F_(2) population derived from a cross of MH63×DEH_229,57 loci were detected under the SD condition.Joint mapping employing a genome-wide association study with accessions from the 3000 rice genome sequencing project(3K-RG),reduced the number of QTL by 43.9%.Using Rice Functional Genomics&Breeding(RFGB)database,the number of SNP cluster regions within the QTL regions reduced by 27.3%.Further comparison of the genome variation between DEH_229 and MH63 in addition to gene annotation information revealed a new DEH allele of DTH3 with multiple variable sites as a possible major factor underlying the early-heading phenotype of DEH_229.An InDel marker,ZMEH_1,was designed based on the variation between DEH_229 and MH63 within this region.It accounted for 86.0%of heading date variation under both SD and LD conditions in 109 randomly chosen progeny derived from extreme lines of the MH63×DEH_229 population.This study reveals the genetic complexity of DEH in the near-isogenic line and may provide useful material and marker information for plant molecular breeding.