Maternal Haploid Induction Research Articles

A diallel analysis of a maize donor population response to in vivo maternal haploid induction: II. Haploid male fertility

AbstractDoubled haploid (DH) lines are used in maize (Zea mays L.) breeding to accelerate the breeding cycle and create homogenous inbred lines in as little as two seasons. These pure inbred lines allow breeders to quickly evaluate new cross combinations. There are two important steps in creating DH lines: (a) generation and selection of haploid progeny, and (b) genome doubling to create fertile, diploid inbreds. Colchicine is widely used to artificially double genomes in haploid plants, which is hazardous, expensive, and time consuming. In this study, three public inbred lines—A427, A637, and NK778—were found to have substantial haploid male fertility (HMF). A six‐parent full diallel between these three HMF lines and three non‐HMF lines was created and HMF was scored. Diallel analysis revealed significant general combining ability (GCA) estimates of up to 17% for HMF, as well as significant specific combining ability (SCA) effects of up to 25%. No significant reciprocal effects were found. Haploid male fertility is promising to be incorporated into elite maize breeding programs to potentially overcome the need of using colchicine treatments for genome doubling. Colchicine aided doubling success rates varying from almost 0 to 30%. Haploid male fertility has an advantage over artificial genome doubling, in terms of both increased success rates and decreased costs for DH line production.

A Diallel Analysis of a Maize Donor Population Response to In Vivo Maternal Haploid Induction: I. Inducibility

The maize (Zea mays L.) in vivo maternal doubled haploid (DH) system is an important tool used by maize breeders and geneticists around the world. The ability to rapidly produce DH lines of maize for breeding allows breeders to quickly respond to new selection criteria based on the ever‐changing biotic and abiotic stresses that maize is subjected to across its growing area. There are two important steps in the generation of DH lines using the in vivo maternal DH system: (i) the production and identification of haploid progeny, and (ii) the doubling of genomes to create fertile, diploid inbred lines that can be used for topcross and per se evaluation. For this study, the focus is the first step, the production and identification of haploid progeny. A diallel mating between six inbred lines of maize, three highly inducible lines (CR1HT, PA91HT1, and WF9) and three lines with low inducibility (NK778, A427, and A637) was produced to study the genetic makeup of inducibility in temperate maize germplasm. A maximum estimated rate of inducibility was found in A427/A637 at 14.6%. Significant general combining ability (GCA), specific combining ability (SCA), reciprocal, environmental, and GCA × environment and SCA × environment interaction effects were found. Misclassification rates ranged from 0 to 45.2% in the 30 hybrids considered. This study supports the use of germplasm with improved inducibility for breeding to improve rates of inducibility in germplasm that has low induction rates.

Dissection of a major QTL qhir1 conferring maternal haploid induction ability in maize

Key messageAmong the qhir11 and qhir12 sub-regions of a major QTL qhir1, only qhir11 has significant effect on maternal haploid induction, segregation distortion and kernel abortion.In vivo haploid induction in maize can be triggered in high frequencies by pollination with special genetic stocks called haploid inducers. Several genetic studies with segregating populations from non-inducer x inducer crosses identified a major QTL, qhir1, on chromosome 1.04 contributing to in vivo haploid induction. A recent Genome Wide Association Study using 51 inducers and 1482 non-inducers also identified two sub-regions within the qhir1 QTL region, named qhir11 and qhir12; qhir12 was proposed to be mandatory for haploid induction because the haplotype of qhir11 was also present in some non-inducers and putative candidate genes coding for DNA and amino acid binding proteins were identified in the qhir12 region. To characterize the effects of each sub-region of qhir1 on haploid induction rate, F2 recombinants segregating for one of the sub-regions and fixed for the other were identified in a cross between CML269 (non-inducer) and a tropicalized haploid inducer TAIL8. To quantify the haploid induction effects of qhir11 and qhir12, selfed progenies of recombinants between these sub-regions were genotyped. F3 plants homozygous for qhir11 and/or qhir12 were identified, and crossed to a ligueless tester to determine their haploid induction rates. The study revealed that only the qhir11 sub-region has a significant effect on haploid induction ability, besides causing significant segregation distortion and kernel abortion, traits that are strongly associated with maternal haploid induction. The results presented in this study can guide fine mapping efforts of qhir1 and in developing new inducers efficiently using marker assisted selection.

Gametophytic and zygotic selection leads to segregation distortion through in vivo induction of a maternal haploid in maize

Production of maternal haploids via a male inducer can greatly accelerate maize breeding and is an interesting biological phenomenon in double fertilization. However, the mechanism behind haploid induction remains elusive. Segregation distortion, which is increasingly recognized as a potentially powerful evolutionary force, has recently been observed during maternal haploid induction in maize. The results present here showed that both male gametophytic and zygotic selection contributed to severe segregation distortion of a locus, named segregation distortion 1 (sed1), during maternal haploid induction in maize. Interestingly, analysis of reciprocal crosses showed that sed1 is expressed in the male gametophyte. A novel mapping strategy based on segregation distortion has been used to fine-map this locus. Strong selection for the presence of the sed1 haplotype from inducers in kernels with haploid formation and defects could be detected in the segregating population. Dual-pollination experiments showed that viable pollen grains from inducers had poor pollen competitive ability against pollen from normal genotypes. Although defective kernels and haploids have different phenotypes, they are most probably caused by the sed1 locus, and possible mechanisms for production of maternal haploids and the associated segregation distortion are discussed. This research also provides new insights into the process of double fertilization.

Selection of Haploid Maize Kernels from Hybrid Kernels for Plant Breeding Using Near-Infrared Spectroscopy and SIMCA Analysis

Samples of haploid and hybrid seed from three different maize donor genotypes after maternal haploid induction were used to test the capability of automated near-infrared transmission spectroscopy to individually differentiate haploid from hybrid seeds. Using a two-step chemometric analysis in which the seeds were first classified according to genotype and then the haploid or hybrid status was determined proved to be the most successful approach. This approach allowed 11 of 13 haploid and 25 of 25 hybrid kernels to be correctly identified from a mixture that included seeds of all the genotypes.

Effect of source germplasm and season on the in vivo haploid induction rate in tropical maize

For in vivo production of doubled haploid (DH) lines in maize, the rate of haploid induction is of crucial importance. Maternal haploid induction depends primarily on the inducer used as a pollinator. However, the source germplasm used as a maternal parent and the environmental conditions for induction may also influence haploid induction and these aspects have not been examined in tropical maize so far. The objectives of our study were to (i) monitor the variation for haploid induction rate (HIR) among diverse source germplasm in tropical maize, (ii) determine the relative importance of general (GCA) and specific (SCA) combining abilities for HIR, and (iii) investigate the influence of summer and winter seasons and genotype × season interactions on this trait. Ten inbreds were mated in a half diallel design. The resulting 45 F1 single crosses were pollinated with the haploid inducer hybrid RWS × UH400 during the summer 2008 and winter 2009 seasons in a lowland tropical environment in Mexico. HIR of the single crosses averaged over seasons ranged from 2.90 to 9.66% with an overall mean of 6.74%. Mean HIR was significantly (P < 0.01) higher during the winter (7.37%) than summer season (6.11%). Significant (P < 0.01) variation was observed due to GCA effects of parental inbreds of single crosses but not for SCA, GCA × season and SCA × season interactions. Our study underpins that a higher HIR in tropical maize can be obtained by selecting appropriate source germplasm and undertaking pollination under favorable environmental conditions.

Morphological and molecular evidences for DNA introgression in haploid induction via a high oil inducer CAUHOI in maize

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.