Genetic Analysis Of Two Cucumber Loci Controlling Sex Determination

Cucumber （Cucumis sativus L.） is one of the most important vegetables in the world. It is also amodel system for sex determination research because of its variety of sex phenotypes. The interactionsbetween sex determination genes and various environmental factors regulate its sex expression. It is wellaccepted that ethylene is the hormone determining sex expression in cucumber. There are three majorsex-determination related genes. M gene has been cloned and proved to be Cs-ACS2, a key gene in thebiosynthetic pathway of ethylene. The F locus promotes femaleness and controls the gynoeciousphenotype. The female specific Cs-ACS1G is thought to be the F gene. However, there is nocorresponding functional complementation proof to certify this speculation, as the efficient cucumbertransform system has not been established. We have collected3different male phenotype mutantsderived from gynoecious lines: mutantâ–³F, which is near-isogenic line of WI1983G（FFMMAA）; mutantM06, which is near-isogenic line of G06（FFMMAA）; BC3P2-hm, which is segregated from the BC3population of100（P1, FFMMAA）×98-17Jing （P2, ffMMAA）. These mutants can be used to study themolecular genetics of the F locus.The production condition and consumer habits constrained the using of gynoecious materials oncucumber breeding in China. Accelerating the using of subgynoecious cucumber in breeding to createhigh female ratio of cucumber cultivar will be a trend in the future. Previously, we have already mappedthree QTLs in the F₂and BC₁P2populations of S-2-98（subgynoecious） and95（monoecious） by using2112cucumber SRR markers, and mapped the major QTL on chromosome3. To fine mapping the majorQTL, we continued the backcross and obtained the BC₆P2and BC₇P2populations.We conducted genetic analysis about the F locus and subgynoecious major QTL on abovementioned plant materials and obtained the following results:1、The ethylene production rate was reduced and Cs-ACS1/G gene expression cannot be detected inthe apices of mutantâ–³F. We proved that Cs-ACS1G gene had already been deleted in three malecucumber mutants, which corresponds to the change of the sex phenotype. Thus we confirmed that the Flocus was resulted from the duplication of Cs-ACS1/G gene; We also detected more than two Cs-ACS1Ggene copies in sibling lines of BC₃P2-hm. The various gene copy number of Cs-ACS1G gene indicatedthat this locus could be a recombination active site.2、Using the combination of bulked segregant analysis, genome re-sequencing, and bioinformaticsanalysis, we predicted that the subgynoecious QTLs were located on chromosome3and6. This resultwas consistent with the previous mapping results with2112cucumber SSR markers. We narrowed downthe major QTL to a genomic interval of2.38Mb （12.15Mb¹4.53Mb） region of chromosome3. We alsoscreened the BC8F2population of segregated major QTL individuals and got40recombinant plants,which can be used to fine mapping the major QTL.Those results about the F locus and the major subgynoecious QTL provide new insights on sex determination mechanism of plants. Copy number variation of a single gene （Cs-ACS1G） can cause theconversion of sexual phenotype is an interesting biological phenomenon and further investigation aboutthe mechanism could be conducted. The mapping results of subgynoecious major QTL was not only aninteresting discovery but also could be used in molecular marker-assisted breeding on cucumber.