Revisiting the Hetero-Fertilization Phenomenon in Maize

Development of a seed DNA-based genotyping system for marker-assisted selection (MAS) has provided a novel opportunity for understanding aberrant reproductive phenomena such as hetero-fertilization (HF) by observing the mismatch of endosperm and leaf genotypes in monocot species. In contrast to conventional approaches using specific morphological markers, this approach can be used for any population derived from diverse parental genotypes. A large-scale experiment was implemented using seven F2 populations and four three-way cross populations, each with 534 to 1024 individuals. The frequency of HF within these populations ranged from 0.14% to 3.12%, with an average of 1.46%. The highest frequency of HF in both types of population was contributed by the pollen gametes. Using three-way crosses allowed, for the first time, detection of the HF contributed by maternal gametes, albeit at very low frequency (0.14%–0.65%). Four HF events identified from each of two F2 populations were tested and confirmed using 1032 single nucleotide polymorphic markers. This analysis indicated that only 50% of polymorphic markers can detect a known HF event, and thus the real HF frequency can be inferred by doubling the estimate obtained from using only one polymorphic marker. As expected, 99% of the HF events can be detected by using seven independent markers in combination. Although seed DNA-based analysis may wrongly predict plant genotypes due to the mismatch of endosperm and leaf DNA caused by HF, the relatively low HF frequencies revealed with diverse germplasm in this study indicates that the effect on the accuracy of MAS is limited. In addition, comparative endosperm and leaf DNA analysis of specific genetic stocks could be useful for revealing the relationships among various aberrant fertilization phenomena including haploidy and apomixis

Exploitation of Heterosis in Pearl Millet: A Review

The phenomenon of heterosis has fascinated plant breeders ever since it was first described by Charles Darwin in 1876 in the vegetable kingdom and later elaborated by George H Shull and Edward M East in maize during 1908. Heterosis is the phenotypic and functional superiority manifested in the F1 crosses over the parents. Various classical complementation mechanisms gave way to the study of the underlying potential cellular and molecular mechanisms responsible for heterosis. In cereals, such as maize, heterosis has been exploited very well, with the development of many single-cross hybrids that revolutionized the yield and productivity enhancements. Pearl millet (Pennisetum glaucum (L.) R. Br.) is one of the important cereal crops with nutritious grains and lower water and energy footprints in addition to the capability of growing in some of the harshest and most marginal environments of the world. In this highly cross-pollinating crop, heterosis was exploited by the development of a commercially viable cytoplasmic male-sterility (CMS) system involving a three-lines breeding system (A-, B- and R-lines). The first set of male-sterile lines, i.e., Tift 23A and Tift18A, were developed in the early 1960s in Tifton, Georgia, USA. These provided a breakthrough in the development of hybrids worldwide, e.g., Tift 23A was extensively used by Punjab Agricultural University (PAU), Ludhiana, India, for the development of the first single-cross pearl millet hybrid, named Hybrid Bajra 1 (HB 1), in 1965. Over the past five decades, the pearl millet community has shown tremendous improvement in terms of cytoplasmic and nuclear diversification of the hybrid parental lines, which led to a progressive increase in the yield and adaptability of the hybrids that were developed, resulting in significant genetic gains. Lately, the whole genome sequencing of Tift 23D2B1 and re-sequencing of circa 1000 genomes by a consortium led by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) has been a significant milestone in the development of cutting-edge genetic and genomic resources in pearl millet. Recently, the application of genomics and molecular technologies has provided better insights into genetic architecture and patterns of heterotic gene pools. Development of whole-genome prediction models incorporating heterotic gene pool models, mapped traits and markers have the potential to take heterosis breeding to a new level in pearl millet. This review discusses advances and prospects in various fronts of heterosis for pearl millet. Keywords

Heterosis as Investigated in Terms of Polyploidy and Genetic Diversity Using Designed Brassica juncea Amphiploid and Its Progenitor Diploid Species

Fixed heterosis resulting from favorable interactions between the genes on their homoeologous genomes in an allopolyploid is considered analogous to classical heterosis accruing from interactions between homologous chromosomes in heterozygous plants of a diploid species. It has been hypothesized that fixed heterosis may be one of the causes of low classical heterosis in allopolyploids. We used Indian mustard (Brassica juncea, 2n = 36; AABB) as a model system to analyze this hypothesis due to ease of its resynthesis from its diploid progenitors, B. rapa (2n = 20; AA) and B. nigra (2n = 16; BB). Both forms of heterosis were investigated in terms of ploidy level, gene action and genetic diversity. To facilitate this, eleven B. juncea genotypes were resynthesized by hybridizing ten near inbred lines of B. rapa and nine of B. nigra. Three half diallel combinations involving resynthesized B. juncea (11×11) and the corresponding progenitor genotypes of B. rapa (10×10) and B. nigra (9×9) were evaluated. Genetic diversity was estimated based on DNA polymorphism generated by SSR primers. Heterosis and genetic diversity in parental diploid species appeared not to predict heterosis and genetic diversity at alloploid level. There was also no association between combining ability, genetic diversity and heterosis across ploidy. Though a large proportion (0.47) of combinations showed positive values, the average fixed heterosis was low for seed yield but high for biomass yield. The genetic diversity was a significant contributor to fixed heterosis for biomass yield, due possibly to adaptive advantage it may confer on de novo alloploids during evolution. Good general/specific combiners at diploid level did not necessarily produce good general/specific combiners at amphiploid level. It was also concluded that polyploidy impacts classical heterosis indirectly due to the negative association between fixed heterosis and classical heterosis

Different Responses of Two Genes Associated with Disease Resistance Loci in Maize (Zea mays L.) to 3-allyloxy-1,2-benzothiazole 1,1-dioxide

Probenazole (3-allyloxy-1,2-benzothiazole 1,1-dioxide, PBZ) is a bactericide and fungicide that acts by inducing plant defense systems. It has been shown to induce the expression of NBS-LRR genes like RPR1 (rice probenazole-response gene) in rice (Oryza sativa L.) and systemic acquired resistance (SAR)-like disease resistance. Two maize (Zea mays L.) genes Zmnbslrr1 (a NBS-LRR gene, cloned from a disease resistance analog PIC11 based) and Zmgc1 , (a putative guanylyl cyclase-like gene) have both been associated with quantitative resistance loci (QTL) for resistance to Fusarium graminearum . PIC11 was associated with Fusarium stalk rot and ZmGC1 showed resistance to Gibberella ear rot caused by F. graminearum . The objectives of the current study here were to characterize the Zmnbslrr1 gene and to determine whether it and Zmgc1 respond to the inducer PBZ. The transcript abundance of Zmnbslrr1 expression was significantly reduced in corn seedlings of the Gibberella ear rot resistant genotype CO387 48 h after PBZ treatment. In contrast, the transcript abundance of the maize Zmgc1 gene increased more than 10-fold 8h after the treatment. Therefore, the two genes do not appear to be coordinately regulated by PBZ

Genetics of Resistance to Aflatoxin Accumulation in Corn (Zea mays)

Aflatoxin accumulation in corn continues to be a major problem in all southeastern corn growing states. Development of resistant inbreds and hybrids is a sustainable approach to reduce aflatoxin contamination. Mapping of quantitative trait loci (QTL) for resistance to Aspergillus flavus infection and aflatoxin accumulation in maize and developing markers associated with them can be helpful to speed up the breeding program. An F2:3 mapping population developed from the cross between Mp715 and B73 and a genetic linkage map was constructed using 136 simple sequence repeat (SSR) markers spanning the whole genome. QTL for aflatoxin resistance were identified in both years and were located on chromosomes 3, 4, 5, 8, 9, and 10 with contribution ranging from \u3c1.0 to 9.2% individually toward resistance phenotype. A highly significant correlation was observed between husk cover and aflatoxin content in both years. A few QTL responsible for close husk cover identified in both years overlapped with the QTL region for aflatoxin resistance. Therefore, it should be possible to use markers identified in this study for selection and improvement of both traits simultaneously through MAB. A suppression subtraction hybridization (SSH) library was constructed using tissues from Mp715 and B73 to identify the differentially expressed genes in response to Aspergillus flavus. Three hundred genes related to various functions were identified from the library. Thirty differentially expressed genes were selected to study their expression pattern in seven maize inbreds through RT-PCR and showed differential expression at different time points after fungus inoculation. Higher expression of pathogenesis related protein-4, leucine rich repeat family protein, RNA binding protein, and ubiquitin C-terminal hydrolase in resistant inbreds (Mp715, Mp719, Mp420, and Mp313E) was confirmed by real-time qPCR. These genes may be responsible for resistance in these resistant inbreds. They were integrated into the linkage map generated in this study through in silico mapping. The gene encoding PR4, which was highly expressed in resistant germplasm was located in bin 4.02 where a QTL for aflatoxin resistance was identified. The genes found in the QTL regions and markers linked with them would be helpful to improve resistance to aflatoxin accumulation in corn

Identification and Mapping of Quantitive Trait Loci Conferring Disease and Insect Resistances in Maize

Molecular markers were used to identify quantitative trait loci (QTLs) conferring resistance to three diseases and three insect pests in 110 maize recombinant inbred lines (RTLs). The markers included 116 restriction fragment length polymorphisms (RFLPs) and four simple sequence repeats (SSRs). The 110 RILs were derived from a cross between Hi34 (an Antigua 2D conversion) and TZi17 (a Nigerian inbred) by single seed descent (SSD) procedure. Significant differences among the parents and significant departures from normality with regard to these diseases and pests of the RIL populations served as the basis for further analysis and QTL mapping. The RTL data were analyzed to determine the chromosomal locations of QTLs by the use of QTL Cartographer version 1.12 and single factor analysis of variance (SAS GLM). The three corn diseases evaluated include maize streak virus (MSV), head smut (Sphacelotheca reiliafia (Kiihn) Clint), and common rust (Puccinia sorghi Schw.). The three insect pests studied were the corn leaf aphid (Rhopalosiphum maidis (Fitch)), fall armyworm {Spodoptera fnigiperda (J. E. Smith)), and sugarcane borer (Diatraea saccharalis (Fabricius)). Insect and disease nurseries of the RILs were planted or had been previously planted at International Institute of Tropical Agriculture (IITA) in Nigeria, International Corn and Wheat Improvement Center (CIMMYT) in Mexico, Pioneer Co. in South Africa, and Waimanalo, Hawaii from 1992 to 1998. Composite interval mapping located a major QTL conferring resistance to MSV, previous named msvl, and a major QTL conferring resistance to Sphacelotheca reiliana (Kiihn) Clint, designed as sprl, on the short arm of chromosome 1 between asgSO and nmcl67. The two genes were about 12 cM apart and both originated from Nigerian parent TZil7. Each explained 29.6% and 10.6% of the phenotypic variations, respectively. Two QTLs, designated as qrp1 and qrp2 with general resistance to Piiccinia sorghi Schw., were mapped to chromosomes 6 and 9, respectively. A major gene conferring resistance to corn leaf aphid, designated as aph2, was mapped on short arm of chromosome 2 with about 14.3% phenotypic variation explanation. Seven and three QTLs were identified for resistance to fall armyworm and sugarcane borer, respectively