Fine Mapping of the Maize Cross‐Incompatibility Locus Gametophytic Factor 1 (ga1) Using a Homogeneous Population

Abstract

873 RESEARCHM aize (Zea mays L.) is a cross-pollinating crop and the level of cross-fertilization among maize plants of neighboring fields is very high due to a heavy volume of pollen produced by the maize plant and factors such as wind and insects that facilitate pollen traveling. Many field experiments have been performed to study the cross-fertilization rate of maize (Henry et al. ABSTRACTPhenotyping of a mapping population is usually the bottleneck that limits the size of the mapping population and mapping resolution. A homogeneous population mapping approach was used for mapping maize (Zea mays L.) gametophytic factor 1 (ga1) that could completely eliminate phenotyping during the mapping process. The strong allele of maize ga1 (Ga1-S), from popcorn inbred line SDGa25, showed a 100% cross-incompatibility with the majority of Chinese dent and flint maize. A homogeneous mapping population was developed by crossing an (SDGa25/Jing66) F 1 male back to an SDGa25 female. During the pollination process, pollen grains of ga1 were completely excluded from fertilization, and a homogeneous BC 1 F 1 (SDGa25//SDGa25/Jing66) population was created in which only the Ga1-S/Ga1-S genotype existed, making phenotyping unnecessary. A total of 2245 individuals of this population were genotyped with SD9 and SD12 markers and 20 recombinants were identified. The Ga1-S locus was quickly delineated to a 100 Kb region between markers dCS1 and insertion deletion ID7 at position 9,491,422 and 9,591,946 bp based on the B73 RefGen_v2 sequence. By markerassisted selection, Ga1-S was introgressed into parental lines of an elite white waxy maize hybrid by six generations of backcrossing and one generation of selfing. The homozygous Ga1-S/Ga1-S hybrid showed full cross-incompatibility to ga1 maize. The Ga1-S allele could be used as a biological reproductive barrier in reducing crosspollination between different types of maize such as waxy and non-waxy, genetically modified (GM) and non-GM maize.

A PECTIN METHYLESTERASE gene at the maize Ga1 locus confers male function in unilateral cross-incompatibility

“…To optimize heterosis in hybrid seed production, avoiding undesired pollen is required, which is also important for organic and specialty maize productions. Multiple efforts, including temporal and spatial isolations, have been made to exclude undesired pollen; however, the technique remains challenging given that maize is a cross-pollinating crop with a large volume of pollen that can travel long distances via wind and insects 2 , 3 . Unilateral cross incompatibility (UCI) may offer an effective method to manipulate pollen flow in commercial maize production 4 – 6 .…”

“…However, in contrast to SI, which requires a suite of polymorphic haplotypes to distinguish individuals within a population, the Ga1 system is composed of three different haplotypes: the Ga1-S haplotype possesses both male and female determinants, the Ga1-M haplotype only harbors the male determinant, and the ga1 haplotype (wild type) lacks both. During fertilization, Ga1-S plants completely reject the pollen of ga1 due to the female barrier but accept pollen of Ga1-S and Ga1-M haplotypes, though the selfing and reciprocal crosses are compatible 2 , 3 . Many efforts have been made to identify the male and female determinants 3 , 11 , 15 .…”

“…During fertilization, Ga1-S plants completely reject the pollen of ga1 due to the female barrier but accept pollen of Ga1-S and Ga1-M haplotypes, though the selfing and reciprocal crosses are compatible 2 , 3 . Many efforts have been made to identify the male and female determinants 3 , 11 , 15 . Recently, ZmPME3 in the Ga1 region was found to be specifically expressed in the silks of Ga1-S plants, serving as a likely candidate for the female function 15 .…”

Genome wide analysis ofGa1-smodifiers in maize

“…The precise placement of the Ga1 locus in the B73 reference genome remains somewhat ambiguous. Multiple groups who have attempted to map the Ga1 region have only been able to narrow it down to a large interval 20,21,31,33 , likely as a result of structural differences between the popcorn derived Ga1-s and dent corn derived ga1 haplotypes at this locus. Consistent with this explanation, three large gaps containing no SNP markers of size 429kb, 826kb, and 454kb were observed within the estimated position of the Ga1-s locus, in comparison to 1 SNP every 12kb in the whole dataset ( Figure 3).…”

“…Mapping receptitivity to ga1 pollen in a popcorn by dent corn recombinant inbred line population (B73 x Hp301) identified only a single locus corresponding to Ga1 21 . Fine mapping estimated that locus to be slightly larger than 1MB 33 . In contrast to the result from the B73 x Hp301 population, apparent modifiers of ga1 function have been reported in the popcorn variety Supergold 34 , from which many of the inbreds from one the three major heterotic groups in commercial hybrid popcorn production are derived 35 .…”