Study On The Genetic Effect Of Rice Wide Compatibility Gene S₅～n And Its Fine-mapping

The rice wide compatibility gene S₅ⁿ was transferred from various wide compatibility varieties into the indica variety Nanjing 11 and japanica variety Balilla, and the allele S5i from a typical Indica variety Nantehao (NTH) into Balilla by continuously backcrossing. A number of near-isogenic lines (NILs) were obtained after self-crossing of 2 or 3 generations following backcrossing of 7 to 10 generations. Spikelet fertility of test population was scored in NILs construction processing. Then, some NILs were crossed with testing variety Nanjing 11 and Balilla to get the three-crossed populations. And the spikelet fertility was also scored. Based on the data, the genetic effect of the allele S₅ⁿ was estimated. Wide compatibility gene S₅ⁿ has previously been mapped in a 50kb region on BAC AP003458 between marker J13 and J17. In this study, NILs and populations derived from three-way cross based on near isogenic lines of S₅ⁿ were assayed by the 7 markers distributed in this region. The primary results are as follows:(1) The WCG S₅ⁿ can increase the spikelet fertility of indica/japanica hybrid F1 by 30%.(2) The wide compatibility gene S₅ⁿ can improve the spikelet fertility of F1 derived from indica variety Nanjing11 and japonica variety Balilla to the normal level.(3) Recombinants were not detected between J13 and J17 through molecular assay of NILs, but the result reassured the previous mapping.(4) Four recombinant individuals were detected in a three-way cross population, F1299. By analyzed the recombinant events, the S₅ⁿ was located in two probable region, J13～Z3 or J16～J17.(5) Gene prediction analysis of the two fragment using FGENESH identified two and four ORFs, respectively. In the fragment between J13 and Z3, two genes were identified, one located on negative strand, and the other on positive strand. The one on the negative strand encodes a hypothetical protein; and the other encodes an aspartyl protease. In the fragment between J16 and J17, four genes were found, they all located on negative strand. The first one encodes a protein that has highly homology with Tic22; the second encodes a hypothetical protein; the third encodes an unknown protein; the fourth encodes a cellulose synthase, which functioned in the process of cellulose biosynthesis. And the gene which encodes aspartyl protease is most probably the candidate gene of S₅ⁿ.This result will be very useful in molecular cloning of the S₅ⁿ allele and marker-assisted transferring of the wide compatibility gene S₅ⁿ in rice breeding programs.