Genetic Analysis And Fine Mapping Of The Ga1Gene Conferring Unilateral Cross-incompatibility In Maize

Maize (Zea mays L.) is both a self and cross compatible species. However, many popcornstrains carry unilateral cross incompatibility genes and cannot be fertilized by pollen of dentand flint maize strains although the reciprocal crosses are successful. These genes are namedgametophyte factors (Ga) because the cross incompatibility are governed by the genotypes ofindividual pollen grains rather than the parental sporophytes. A Chinese popcorn strainSDGa25carrying the strongest allele of Ga1(Ga1-s) was chosen as material for genetic studyand fine mapping. The main results were as follows：1. One hundred and forty-eight dent and flint maize inbred lines were crossed as male withSDGa25by hand pollination in two years. Only3lines, ZD12F, a foreign line, JKN2000F, awaxy line, and HN178, a dent strain, showed cross compatible to SDGa25. The rest of themshowed complete cross incompatible to SDGa25.2. Ga1-s and Ga1-m were dominant genes. Ga1-m alleles from different strains were notallelic.3. The Ga1locus was fine mapped into a100kbp interval based on the B73RefGen_v2sequence between markers dCAPS1on BAC AC208523on the telomere side and ID7onBAC AC204382on the centromere side. The mapping interval was2.1Mbp shorter thanpreviously reported and was achieved by integrating results from three different mappingpopulations. Thirteen tightly linked markers to Ga1-s were developed in this study.4. SDGa25BAC library consisted of300000clones, and more than99%of the clonescontain maize nuclear DNA inserts with an average size of70kb. The library containedgenomic DNA that was equivalent of10×haploid genome. Two positive BAC clonescovering the whole mapping interval were sequenced.5. Based on the sequences of the positive BAC clones and B73RefGen_v2sequence,3genes was predicted. GRMZM2G027021and GRMZM2G039983had14and6exons, respectively. GRMZM2G027021was GTP-binding protein hflX. Compared to that of ga1, thethird exons of Ga1-s and Ga1-m were deleted. The deletion may be involved in pollen-pistilinteractions. GRMZM2G039983had homology to WDL1of Arabidopsis, which regulatedanisotropic growth of cells, and perhaps could inhibit ga1pollen tube growth on Ga1-s pistil.Compared to that of ga1, Ga1-s and Ga1-m alleles had insertions at the same site of3’untranslated region. Only Ga1-s allele had a CACTA transposon element in the insertionbetween the first and the second exon.6. The present paper reports introgression of Ga1-s into elite waxy corn hybrids JKN2000by marker assisted selection and the possibility of using Ga1-s as biological reproductivebarrier for the containment of gene flows between different types of corns. Ga1-s/Ga1-shybrids showed complete cross-incompatible to foreign pollen, whereas46%cross-compatible rate occurred in the ga1/ga1hybrids.