The indeterminate gametophyte1 mutation in maize has been known to disrupt development of the female gametophyte. Mutant embryo sacs have abnormal numbers and behavior of micropylar and central cell nuclei, which result in polyembryony and elevated ploidy levels in the endosperm of developing kernels. In this study, we confirm abnormal nuclear behavior and present novel findings. In contrast to the normal form, there is no obvious polarity in two-nucleate embryo sacs or in the micropylar cells of eight-nucleate embryo sacs. We show that the second and third mitoses are not fully synchronized and that additional mitoses can occur in all of the nuclei of the mutant embryo sac or in just the micropylar or central regions. After cellularization, individual micropylar cells can undergo mitosis. Abnormal microtubular behavior results in irregular positioning of the nuclei, asynchronous microtubular patterns in different pairs of nuclei, and abnormal phragmoplasts after the third mitotic division. These results indicate that in addition to acting primarily in controlling nuclear divisions, the indeterminate gametophyte1 gene acts secondarily in regulating microtubule behavior. This cytoskeletal activity most likely controls the polarization and nuclear migration underlying the formation and fate of the cells of the normal embryo sac.

Maize kernels inheriting the indeterminate gametophyte mutant (ig) on the female side had endosperms that ranged in ploidy level from diploid (2x) to nonaploid (9x). In crosses with diploid males, only kernels of the triploid endosperm class developed normally. Kernels of the tetraploid endosperm class were half-sized but with well-developed embryos that regularly germinated. Kernels of endosperm composition other than triploid or tetraploid were abortive.-Endosperm ploidy level resulting from mating ig/ig x tetraploid Ig similarly was variable. Most endosperms started to degenerate soon after pollination and remained in an arrested state. Hexaploid endosperm was exceptional; it developed normally during the sequence of stages studied and accounted for plump kernels on mature ears. Since such kernels have diploid maternal tissues (pericarp) but triploid embryos, the present finding favors the view that endosperm failure or success in such circumstances is governed by conditions within the endosperm itself.-Whereas tetraploid endosperm consisting of three maternal genomes and one paternal genome is slightly reduced in size but supports viable seed development, that endosperm having two maternal and two paternal chromosome sets was highly defective and conditioned abortion. Thus, development of maize endosperm evidently is affected by the parental source of its sets of chromosomes.

The phenomenon of maternal haploid induction in maize was first described many years ago, but the underlying mechanism is still unclear. In this study, the Stock-6-derived, haploid-inducing line CAUHOI with high kernel oil content (KOC), was used as the pollinator to produce maternal haploids from the maize hybrid ZD958 with low KOC. CAUHOI is homozygous for the dominant marker gene R1-nj. Haploids were identified by morphological and cytological investigations. The frequency of haploid induction from this cross was 2.21%. Unexpectedly, many haploid kernels had weakly pigmented purple color on the embryo, and some haploid kernels had high KOC. Simple sequence repeat (SSR) analysis showed that 43.18% of the haploids carried segments from CAUHOI, and a small proportion (average 1.84%) of the genome of CAUHOI was introgressed into haploids. Haploid kernels with high KOC had a higher frequency of segment introgression from CAUHOI (2.92%) than that in haploid kernels with low KOC (1.79%), showing that the marker gene R1-nj and high-oil genes from CAUHOI were expressed during the development of some haploid embryos, and confirmed that the DNA introgression from the inducer parent occurred during maternal haploid induction. Together, these results suggested that the chromosome elimination was probably responsible for haploid induction in maize, and late somatic elimination might occur. Several possible mechanisms underlying haploid formation are discussed.

In vivo haploid induction (HI) triggered by pollination with special intraspecific genotypes, called inducers, is unique to Zea maysL. within the plant kingdom and has revolutionized maize breeding during the last decade. However, the molecular mechanisms underlying HI in maize are still unclear. To investigate the genetic basis of HI, we developed a new approach for genome-wide association studies (GWAS), termed conditional haplotype extension (CHE) test that allows detection of selective sweeps even under almost perfect confounding of population structure and trait expression. Here, we applied this test to identify genomic regions required for HI expression and dissected the combined support interval (50.34 Mb) of the QTL qhir1, detected in a previous study, into two closely linked genomic segments relevant for HI expression. The first, termed qhir11(0.54 Mb), comprises an already fine-mapped region but was not diagnostic for differentiating inducers and noninducers. The second segment, termed qhir12(3.97 Mb), had a haplotype allele common to all 53 inducer lines but not found in any of the 1482 noninducers. By comparing resequencing data of one inducer with 14 noninducers, we detected in the qhir12 region three candidate genes involved in DNA or amino acid binding, however, none for qhir11 We propose that the CHE test can be utilized in introgression breeding and different fields of genetics to detect selective sweeps in heterogeneous genetic backgrounds.Copyright © 2016 by the Genetics Society of America.

Producing maternal haploids via a male inducer can greatly accelerate maize (Zea mays) breeding process. However, the mechanism underlying haploid formation remains unclear. In this study, we constructed two inducer lines containing cytogenetic marker B chromosome or alien centromeric histone H3 variant-yellow fluorescent protein vector to investigate the mechanism. The two inducer lines as the pollinators were crossed with a hybrid ZhengDan958. B chromosomes were detected in F1 haploids at a low frequency, which was direct evidence to support the occurrence of selective chromosome elimination during haploid formation. We found that most of the inducer chromosomes were eliminated in haploid embryonic cells during the first week after pollination. The gradual elimination of chromosomes was also detected in the endosperm of defective kernels, although it occurred only in some endosperm cells as late as 15 d after pollination. We also performed a genome-wide identification of single nucleotide polymorphism markers in the inducers, noninducer inbred lines, and 42 derived haploids using a 50K single nucleotide polymorphism array. We found that an approximately 44-Mb heterozygous fragment from the male parent was detected in a single haploid, which further supported the occurrence of paternal introgression. Our results suggest that selective elimination of uniparental chromosomes leads to the formation of haploid and possible defective kernels in maize as well, which is accompanied with unusual paternal introgression in haploid cells.

对影响玉米花药培养的几个因素及再生植株的倍性鉴定的结果表明、基因型是影响玉米花培效果的重要因素，杂交种的花培效果好于不稳定系，不稳定系又好于自交系。N₆与正₁₄两种培养基都适于花药培养，但正₁₄培养基优于N₆培养基。特别是液体培养基效果更好。花药接种的适宜时期为单核中期和后期。单核后期又稍好于单核中期，单核早期的花药不能诱导愈伤组织。单倍体与二倍体花粉植株之间叶片气孔保卫细胞长度差异极显著，长度小于29μm的为单倍体。大于29μm的为二倍体或多倍体。鉴定的准确性可达到95%。因此可利用测量气孔保卫细胞长度的方法鉴定花粉植株的倍性。