Fine Mapping Of The Cross-incompatibility Gene Ga1-s In Maize

As a typical cross-pollinated plant,maize(Zea mays L.)is an important crop all over the world.Due to gametophyte factors(Ga)distributed in the genome,it's prevalent that popcorn strains cannot be fertilized by pollen from regular maize(dent or flint maize)while the reciprocal crosses are successful,which is known as cross incompatibility(Nelson 1952).To date,the genetic basis of cross incompatibility still remains unclear.In the present study,we used a recombinant inbred line(RIL)population derived from the cross between ZHENG58 and SK to perform gene mapping.The essential results are summarized as follows:1.By using Maize SNP50 Bead Chip(Ganal et al 2011),three cross-incompatible loci located on chromosome 4,9 and 10 were identified with significantly distorted segregations in ZHENG58/SK RIL population(at the significance level of P<0.01).Of which,the locus on chromosome 4 distorted toward to SK but the loci on chromosome 9 and 10 distorted to ZHENG58.Based on the physical location,we found the locus on chromosome 4 showed consistency as Ga1-s,the well-known locus that was previously reported to cause cross-incompatibility(Zhang et al 2012;Bloom and Holland 2012).2.To verify the reliability of such three loci,three F2 populations from ZHENG58/SK,B73/SK and Mo17/SK were screened by the flanking markers surrounding the three cross-incompatible loci to check the segregating pattern.It was consistently found the Ga1-s locus on chromosome 4 significantly distorted to SK in all three F2 populations(at the significance level of P<0.01),which indicated its reliability and no interaction with genetic backgrounds.3.Ga1-s locus was delimited to a 1.8Mb region on chromosome 4,which harbored 10 candidate genes based on the Zm B73 Ref Gen_v2 by testing the segregating pattern of the target region in recombinant progenies of ZHENG58/SK F2 population.Then we constructed the local physical map of SK by anchoring SK BAC clones based on 11 markers covering the 1.8Mb region.Presently,eight positive BAC clones were obtained and verified the credibility by sequencing.4.In order to assess the cross-compatibility between popcorn and regular maize at the broad scale,we crossed 42 maize inbred lines with diverse pedigree and geographic distribution to two cross-incompatible strains,SK and CML304.It was found 22 maize inbred lines showed strong cross-inccompatibiliy(< 10%)and 20 inbred lines with weak cross-incompatibility(? 90%).Moreover,it was revealed the wide cross-compatibility of the elite maize inbred line P178 as female parent,by crossing it with two cross-incompatible strains and 42 inbred lines.The findings provide the promising clues to utilize exotic germplasm such as tropical lines and accelerate the progress of pyramiding favorable alleles in maize breeding.