Hybrid Parental Lines Research Articles

Strategies for enhancing productivity, resilience, nutritional quality, and consumption of pearl millet [Pennisetum glaucum (L.) R. Br.] for food and nutritional security in India

AbstractPearl millet [Pennisetum glaucum (L.) R. Br.] is an important component of agri‐food system in areas experiencing drought and high temperature and for increasing the resilience to climatic stresses and addressing malnutrition. The purpose of this review is to examine strategies for improving productivity, stress resilience, and nutritional quality of pearl millet and to understand its consumption pattern. Genetic diversification of hybrid parental lines remains strategically important to breed diverse, disease‐resistant and drought‐tolerant hybrids. Resistance to diseases, tolerance to drought, and high temperature and greater contents of iron and zinc are targeted in improving hybrid parental lines. Lodging resistance, compact panicles, panicle exertion, and improved seed set are universal traits, whereas duration, tillering ability, seed color, and seed size have a strong regional preference. The strategy of developing high‐yielding and disease‐resistant hybrids with adaptation to challenged agro‐ecologies has led to increase in yield from 303 to 1219 kg/ha between 1960 and 2020. Yield and stress resilience are to be increased further using conventional breeding and new tools like genomic selection, speed breeding, genome editing, and precision phenotyping. Mainstreaming grain nutritional traits, viz., iron and zinc contents in genetic improvement are essential to develop high‐yielding and nutrient‐rich pearl millet. There is need to enhance the consumption of pearl millet by strengthening existing value‐chain, providing consumer a choice of diverse range of food products, creating awareness about its health benefits and promotion through government schemes.

Understanding genetic diversity in drought-adaptive hybrid parental lines in pearl millet.

Information on genetic diversity and population structure is helpful to strategize enhancing the genetic base of hybrid parental lines in breeding programs. The present study determined the population structure and genetic diversity of 109 pearl millet hybrid parental lines, known for their better adaptation and performance in drought-prone environments, using 16,472 single nucleotide polymorphic (SNP) markers generated from GBS (genotyping-by-sequencing) platforms. The SNPs were distributed uniformly across the pearl millet genome and showed considerable genetic diversity (0.337), expected heterozygosity (0.334), and observed heterozygosity (0.031). Most of the pairs of lines (78.36%) had Identity-by-State (IBS) based genetic distances of more than 0.3, indicating a significant amount of genetic diversity among the parental lines. Bayesian model-based population stratification, neighbor-joining phylogenetic analysis, and principal coordinate analysis (PCoA) differentiated all hybrid parental lines into two clear-cut major groups, one each for seed parents (B-lines) and pollinators (R-lines). Majority of parental lines sharing common parentages were found grouped in the same cluster. Analysis of molecular variance (AMOVA) revealed 7% of the variation among subpopulations, and 93% of the variation was attributable to within sub-populations. Chromosome 3 had the highest number of LD regions. Genomic LD decay distance was 0.69 Mb and varied across the different chromosomes. Genetic diversity based on 11 agro-morphological and grain quality traits also suggested that the majority of the B- and R-lines were grouped into two major clusters with few overlaps. In addition, the combined analysis of phenotypic and genotypic data showed similarities in the population grouping patterns. The present study revealed the uniqueness of most of the inbred lines, which can be a valuable source of new alleles and help breeders to utilize these inbred lines for the development of hybrids in drought-prone environments.

Development of powdery mildew (Podosphaera xanthii) resistance in cucumber (Cucumis sativus) using CRISPR/Cas9-mediated mutagenesis of CsaMLO8.

Powdery mildew (PM) diseases may severely limit the production of various crops, including members of to the Cucurbitaceae family. Successful PM infection relies on the Mildew Resistance Locus O (MLO) plant gene family, which encodes susceptibility (S) factors essential for fungus penetration into the host cell. In cucumber, natural mutations in CsaMLO8 confer resistance to the PM pathogen Podosphaera xanthii. Here, we used CRISPR/Cas9-mediated mutagenesis to generate PM resistance in the susceptible cucumber cv. 'Ilan'. Two transgene-free Csamlo8 CRISPR mutant lines (Csamlo-cr-1 and Csamlo-cr-2) were isolated, the first with a 5 bp deletion in exon 1, and the second harboring a 1280 bp deletion and 10 bp insertion between exons 1 and 5. Both lines showed high resistance to PM under semi-commercial growth conditions in the summer growing seasons of 2019 and 2021. These results provide the basis for generating transgene-free powdery mildew resistance in cucumber in any genetic background. This method can directly be employed on commercial cultivars and hybrid parental lines, and thereby substantially shorten and simplify the breeding process for PM resistance in cucumber.

Ugradnja Ga1-S gena u roditeljske linije hibrida kukuruza belog zrna pomoću molekularnih markera

Maize Research Institute Zemun Polje has a marker-assisted backcross breeding (MABB) program aimed to incorporate the strong allele of incompatibility gene Ga1 into parental components of the hybrids with specific traits (white kernel). The main objective of this research was marker-assisted development of the white maize hybrid's parental lines with complete cross-incompatibility to foreign pollens. Ga1-S-tightly-linked molecular markers successfully identified heterozygous plants with a percentage of approximately 50% after all three backcrossings. Regarding the background selection, 69% of progenies had recurrent parent's genome (RPG) above the theoretical value for BC3 generation. Also, a few individuals even had 99%, the value theoretically achieved in the BC6 generation, showing that MABB made a genetic gain in RPG recovery. Foreground selection in this generation aimed to identify homozygous dominant individuals. Out of 264 BC3 F2 plants, 70 (26.5%) were dominant homozygotes, which is in accordance with the expected Mendelian ratio. Progenies of the BC3 F2 homozygous dominant plants were planted alternatively with yellow-kernel maize of the same maturity to check cross-incompatibility. Unfortunately, none of the dominant progenies had a 100% white kernel, most likely due to the presence of modifier genes that increase/decrease the effectiveness of pollen exclusion or that, alternatively, modifiers are lost during the backcrossing. It could be expected that successfully employed functional markers (developed from the sequence polymorphisms present within a functional gene(s) associated with phenotypic trait variations) would outbalance the noted impediments and enhance MABB efficiency to transfer the desired gene(s) controlling simple or complex trait(s) into cultivated varieties.

Genomic Basis of Defining Heterotic Pools in Pigeonpea (Cajanus cajan L.): A Review

Hybrid breeding strategy leading to define heterotic pools expedited execution of hybrid technology for amelioration of yield, quality, wide adaptability and resistance to biotic and abiotic stresses. In case of pigeonpea conventional hybrid breeding programme has yielded good dividends, yet it can be further honed up by understanding the genome wide variations in the hybrid parental lines for defining heterotic pools. Also, combining ability basis of defining heterotic pools offer enormous opportunities for pragmatic exploitation of pigeonpea hybrid technology with high yield advantages realized in farmers’ fields.

Addressing Iron and Zinc Micronutrient Malnutrition Through Nutrigenomics in Pearl Millet: Advances and Prospects.

Iron (Fe) and zinc (Zn) micronutrient deficiencies are significant health concerns, particularly among the underprivileged and resource-poor people in the semi-arid tropics globally. Pearl millet is regarded as a climate-smart crop with low water and energy footprints. It thrives well under adverse agro-ecologies such as high temperatures and limited rainfall. Pearl millet is regarded as a nutri-cereal owing to health-promoting traits such as high grain Fe and Zn content, metabolizable energy, high antioxidant and polyphenols, high proportion of slowly digestible starches, dietary fibers, and favorable essential amino acid profile compared to many cereals. Higher genetic variability for grain Fe and Zn content has facilitated considerable progress in mapping and mining QTLs, alleles and genes underlying micronutrient metabolism. This has been made possible by developing efficient genetic and genomic resources in pearl millet over the last decade. These include genetic stocks such as bi-parental RIL mapping populations, association mapping panels, chromosome segment substitution lines (CSSLs) and TILLING populations. On the genomics side, considerable progress has been made in generating genomic markers, such as SSR marker repository development. This was followed by the development of a next-generation sequencing-based genome-wide SNP repository. The circa 1,000 genomes re-sequencing project played a significant role. A high-quality reference genome was made available by re-sequencing of world diversity panel, mapping population parents and hybrid parental lines. This mini-review attempts to provide information on the current developments on mapping Fe and Zn content in pearl millet and future outlook.

Characterization of heterosis and genomic prediction-based establishment of heterotic patterns for developing better hybrids in pigeonpea.

Whole-genome resequencing (WGRS) of 396 lines, consisting of 104 hybrid parental lines and 292 germplasm lines, were used to study the molecular basis of mid-parent heterosis (MPH) and to identify complementary heterotic patterns in pigeonpea [Cajanus cajan (L.) Millsp.] hybrids. The lines and hybrids were assessed for yield and yield-related traits in multiple environments. Our analysis showed positive MPH values in 78.6% of hybrids, confirming the potential of hybrid breeding in pigeonpea. By using genome-wide prediction and association mapping approaches, we identified 129 single nucleotide polymorphisms and 52 copy number variations with significant heterotic effects and also established a high-yielding heterotic pattern in pigeonpea. In summary, our study highlights the role of WGRS data in the study and use of heterosis in crops where hybrid breeding is expected to boost selection gain in order to ensure global food security.

Genetic Gains in Pearl Millet in India: Insights Into Historic Breeding Strategies and Future Perspective.

Pearl millet (Pennisetum glaucum R. Br.) is an important staple and nutritious food crop in the semiarid and arid ecologies of South Asia (SA) and Sub-Saharan Africa (SSA). In view of climate change, depleting water resources, and widespread malnutrition, there is a need to accelerate the rate of genetic gains in pearl millet productivity. This review discusses past strategies and future approaches to accelerate genetic gains to meet future demand. Pearl millet breeding in India has historically evolved very comprehensively from open-pollinated varieties development to hybrid breeding. Availability of stable cytoplasmic male sterility system with adequate restorers and strategic use of genetic resources from India and SSA laid the strong foundation of hybrid breeding. Genetic and cytoplasmic diversification of hybrid parental lines, periodic replacement of hybrids, and breeding disease-resistant and stress-tolerant cultivars have been areas of very high priority. As a result, an annual yield increase of 4% has been realized in the last three decades. There is considerable scope to further accelerate the efforts on hybrid breeding for drought-prone areas in SA and SSA. Heterotic grouping of hybrid parental lines is essential to sustain long-term genetic gains. Time is now ripe for mainstreaming of the nutritional traits improvement in pearl millet breeding programs. New opportunities are emerging to improve the efficiency and precision of breeding. Development and application of high-throughput genomic tools, speed breeding, and precision phenotyping protocols need to be intensified to exploit a huge wealth of native genetic variation available in pearl millet to accelerate the genetic gains.

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.

Genome-Wide Association Studies and Genomic Selection in Pearl Millet: Advances and Prospects.

Pearl millet is a climate-resilient, drought-tolerant crop capable of growing in marginal environments of arid and semi-arid regions globally. Pearl millet is a staple food for more than 90 million people living in poverty and can address the triple burden of malnutrition substantially. It remained a neglected crop until the turn of the 21st century, and much emphasis has been placed since then on the development of various genetic and genomic resources for whole-genome scan studies, such as the genome-wide association studies (GWAS) and genomic selection (GS). This was facilitated by the advent of sequencing-based genotyping, such as genotyping-by-sequencing (GBS), RAD-sequencing, and whole-genome re-sequencing (WGRS) in pearl millet. To carry out GWAS and GS, a world association mapping panel called the Pearl Millet inbred Germplasm Association Panel (PMiGAP) was developed at ICRISAT in partnership with Aberystwyth University. This panel consisted of germplasm lines, landraces, and breeding lines from 27 countries and was re-sequenced using the WGRS approach. It has a repository of circa 29 million genome-wide SNPs. PMiGAP has been used to map traits related to drought tolerance, grain Fe and Zn content, nitrogen use efficiency, components of endosperm starch, grain yield, etc. Genomic selection in pearl millet was jump-started recently by WGRS, RAD, and tGBS (tunable genotyping-by-sequencing) approaches for the PMiGAP and hybrid parental lines. Using multi-environment phenotyping of various training populations, initial attempts have been made to develop genomic selection models. This mini review discusses advances and prospects in GWAS and GS for pearl millet.

Saccharum spontaneum, a Novel Source of Resistance to Root-Lesion and Root-Knot Nematodes in Sugarcane.

Root-lesion nematode (Pratylenchus zeae) and root-knot nematode (Meloidogyne javanica) are two important pathogens of sugarcane (Saccharum hybrid). No commercial cultivars are resistant to these nematodes in Australia. Twenty accession lines of S. spontaneum, a wild relative of sugarcane, were tested against these two nematode species. S. spontaneum lines were tested twice for resistance to root-lesion nematode and three times for root-knot nematode. Reproduction (final population/starting population) of root-lesion nematodes was significantly lower in 17 of the 20 S. spontaneum accession lines tested in two experiments compared with two commercial cultivars. Four S. spontaneum lines supported a significantly lower number of root-lesion nematodes per gram of root than that of two commercial sugarcane cultivars. Reproduction of root-knot nematodes was significantly lower in 16 S. spontaneum lines compared with two commercial cultivars. Fourteen of the S. spontaneum lines tested supported significantly fewer eggs per gram of root compared with two commercial cultivars. This study showed that S. spontaneum lines possessed resistance for root-lesion and root-knot nematodes. Targeted crossing with commercial hybrid parental lines should be conducted to introduce nematode resistance into sugarcane cultivars for the Australian sugar industry.

QTL mapping and validation of fertility restoration in West African sorghum A1 cytoplasm and identification of a potential causative mutation for Rf2

Key messageMajor A1cytoplasm fertility restoration loci,Rf2andRf5, were found in the West African sorghum. A potential causative mutation forRf2was identified. KASP markers were validated on independent material.To accelerate the identification and development of hybrid parental lines in West African (WA) sorghum, this study aimed to understand the genetics underlying the fertility restoration (Rf) in WA A1 cytoplasmic male sterility system and to develop markers for a routine use in WA breeding programs. We genotyped by sequencing three F2 populations to map the Rf quantitative trait loci (QTL), validated the molecular KASP markers developed from those QTL in two F2:3 populations, and assessed the most promising markers on a set of 95 R- and B-lines from WA breeding programs. Seven QTL were found across the three F2 populations. On chromosome SBI-05, we found a major fertility restorer locus (Rf5) for two populations with the same male parent, explaining 19 and 14% of the phenotypic variation in either population. Minor QTL were detected in these two populations on chromosomes SBI-02, SBI-03, SBI-04 and SBI-10. In the third population, we identified one major fertility restorer locus on chromosome SBI-02, Rf2, explaining 31% of the phenotypic variation. Pentatricopeptide repeat genes in the Rf2 QTL region were sequenced, and we detected in Sobic.002G057050 a missense mutation in the first exon, explaining 81% of the phenotypic variation in a F2:3 population and clearly separating B- from R-lines. The KASP marker developed from this mutation stands as a promising tool for routine use in WA breeding programs.

The Application of Molecular Markers to Accelerate the Recovery of Cytoplasmic and Nuclear Male Sterility in South African Onion (Allium cepa L.) Hybrid Parental Lines

Male sterility is important to prevent self-pollination and loss of the onion hybrid genotype. Classic methods require 4-8 years of progeny testing before the cytoplasm type can be determined. An accurate and time-saving method was needed. Various types of markers were tested for application to South African onion parental lines of hybrid cultivars, and which could determine male sterility and maintainer genotypes accurately and easily with large numbers of samples. Five cytoplasmic (5’cob, orfA501, orf725, IGS and cob-type 2) and four nuclear markers (jnurf13, isotig34671_610, isotig30856_1351 and isotig29186_1830) were sourced. Genomic DNA was isolated from onion seedlings and young leaves growing from bulbs in the Agricultural Research Council (ARC) research field. PCR marker amplification products were separated by agarose or denaturing polyacrylamide gel electrophoresis (PAGE) gels. Real-time polymerase chain reaction (PCR) was performed with custom TaqMan® SNP genotyping assays containing primer/probe pairs designed to detect single nucleotide polymorphisms (SNPs) linked to the nuclear Ms locus. OrfA501 proved useful as a presence/absence marker for cytoplasmic male sterility, while TaqMan® SNP genotyping assays were superior to the jnurf13 nuclear marker in terms of rapid throughput. PCR molecular markers and custom TaqMan® SNP genotyping assays were efficient in screening the onion lines rapidly and accurately for their cytoplasmic and nuclear male sterility genotype. These methods reduced the time to identify the correct genotype of male sterile and maintainer lines, gave accurate genotypic information and proved to be useful on a larger scale. These molecular marker methods will facilitate the production of the correct seed for commercialization of onion lines worldwide.

Molecular mapping and inheritance of restoration of fertility (Rf) in A4 hybrid system in pigeonpea (Cajanus cajan (L.) Millsp.)

Key messageWe report molecular mapping and inheritance of restoration of fertility (Rf) in A4 hybrid system in pigeonpea. We have also developed PCR-based markers amenable to low-cost genotyping to identify fertility restorer lines.Commercial hybrids in pigeonpea are based on A4 cytoplasmic male sterility (CMS) system, and their fertility restoration is one of the key prerequisites for breeding. In this context, an effort has been made to understand the genetics and identify quantitative trait loci (QTL) associated with restoration of fertility (Rf). One F2 population was developed by crossing CMS line (ICPA 2039) with fertility restorer line (ICPL 87119). Genetic analysis has shown involvement of two dominant genes in regulation of restoration of fertility. In parallel, the genotyping-by-sequencing (GBS) approach has generated ~ 33 Gb data on the F2 population. GBS data have provided 2457 single nucleotide polymorphism (SNPs) segregating across the mapping population. Based on these genotyping data, a genetic map has been developed with 306 SNPs covering a total length 981.9 cM. Further QTL analysis has provided the region flanked by S8_7664779 and S8_6474381 on CcLG08 harboured major QTL explained up to 28.5% phenotypic variation. Subsequently, sequence information within the major QTLs was compared between the maintainer and the restorer lines. From this sequence information, we have developed two PCR-based markers for identification of restorer lines from non-restorer lines and validated them on parental lines of hybrids as well as on another F2 mapping population. The results obtained in this study are expected to enhance the efficiency of selection for the identification of restorer lines in hybrid breeding and may reduce traditional time-consuming phenotyping activities.

English

Downy mildew (DM) epidemics have repeatedly caused huge grain and straw production losses on pearl millet single-cross hybrids in India. The present study was undertaken to identify the QTL positions controlling DMR on the linkage map for a pearl millet mapping population of 295 F7 RIL population derived from a single F1 plant from plant × plant cross 81 B-P13 (susceptible) × AIMP 92901-P03 (resistant) using three isolates of the pathogen from Gujarat, Haryana and Rajasthan. A skeleton genetic linkage map of 536.8 cM (Haldane) was constructed using MapMaker/Exp version 3.0b at LOD threshold value of 3.0 using 39 scorable polymorphic loci. A total of four major DMR QTLs were identified on linkage groups of 1,3 and 4. The inheritance of of these QTLs showed AIMP 92901-P03 providing the resistant alleles. At least one DMR QTL was detected and mapped for each of the three DM isolates. After validation, marker-assisted selection (MAS) can now be used for improving DMR of elite pearl millet hybrid parental lines using polymorphic flanking markers from donor parent AIMP 92901-P03.

General and specific combining abilities in a maize (Zea mays L.) test-cross hybrid panel: relative importance of population structure and genetic divergence between parents.

General and specific combining abilities of maize hybrids between 288 inbred lines and three tester lines were highly related to population structure and genetic distance inferred from SNP data. Many studies have attempted to provide reliable and quick methods to identify promising parental lines and combinations in hybrid breeding programs. Since the 1950s, maize germplasm has been organized into heterotic groups to facilitate the exploitation of heterosis. Molecular markers have proven efficient tools to address the organization of genetic diversity and the relationship between lines or populations. The aim of the present work was to investigate to what extent marker-based evaluations of population structure and genetic distance may account for general (GCA) and specific (SCA) combining ability components in a population composed of 800 inter and intra-heterotic group hybrids obtained by crossing 288 inbred lines and three testers. Our results illustrate a strong effect of groups identified by population structure analysis on both GCA and SCA components. Including genetic distance between parental lines of hybrids in the model leads to a significant decrease of SCA variance component and an increase in GCA variance component for all the traits. The latter suggests that this approach can be efficient to better estimate the potential combining ability of inbred lines when crossed with unrelated lines, and limits the consequences of tester choice. Significant residual GCA and SCA variance components of models taking into account structure and/or genetic distance highlight the variation available for breeding programs within structure groups.

Kultur Antera untuk Percepatan Perakitan Varietas Padi di Indonesia

<p>Rice is a staple<br />food crop in Indonesia, while the need is increasing due to<br />high rice consumption as well as population increase. The<br />problems can be solved through increase of national rice<br />production. Productivity of lowland and upland should be<br />increased intensively and other potential dry area outside<br />Java and Bali Islands should be considered for extending the<br />area of production. Recently, high yielding variety such as<br />semi dwarf variety, hybrid rice, and new plant type of rice<br />were being developed by Indonesian breeders. However,<br />new method is needed to complement conventional<br />breeding method in order to accelerate rice breeding.<br />Anther culture is one of in vitro culture techniques that can<br />be used to accelerate the obtainment of pure lines through<br />doubled-haploids (DHs) regenerated at first generation of<br />culture for less than one year. Thus, application of anther<br />culture in conventional breeding will increase the efficiency<br />of selection process as well as reducing the cost for labour,<br />land and breeder’s time. The obtainment of green plantlets<br />derived from anther culture of indica rice subspecies has<br />been improved by the addition of 1 mM putrescine into<br />induction and regeneration media. Recently, several upland<br />rice lines tolerant to abiotic stresses (i.e. low light intensity<br />and aluminum toxicity) and biotic stresses (i.e. leaf and<br />neck blast), several lowland rice/paddy lines tolerant to<br />biotic stresses (i.e. bacterial leaf blight and blast), and<br />several hybrid parental lines (i.e. male sterile, maintainer<br />and restorer) were obtained in 2-3 years from several rice<br />breeding program involving anther culture. However,<br />potential use anther culture to provide unique genetic<br />material for mapping populations for use in functional<br />genomics and molecular breeding has not been explored.<br />The results indicated that anther culture is a feasible<br />technology that can be used for accelerating rice breeding<br />program in Indonesia.</p>

Conventional and molecular marker-assisted selection and pyramiding of genes for multiple disease resistance in tomato

Tomato (Solanum lycopersicum L.) is widely grown in the tropics but production is subject to high losses from diseases. AVRDC—The World Vegetable Center initiated a program to develop fresh market tomato lines resistant to begomoviruses causing tomato yellow leaf curl disease, Phytophthora infestans causing late blight, Ralstonia solanacearum causing bacterial wilt, Stemphyllium spp. causing gray leaf spot, Fusarium oxysporum f. sp. lycopersici race 2, and Tobacco mosaic virus. This work provides greenhouse, field, molecular marker, and laboratory protocols used in the screening and selection process that were applied to segregating populations during generation advance over three years to develop five multiple disease resistant F7 fresh market tomato lines. Resistance of the five lines to the abovementioned diseases was confirmed in subsequent evaluations. Average yields of the five lines exceeded 100t/ha under optimal temperatures in a dry season trial, but yields were reduced in a second trial under higher temperatures and rainfall. Seed of three multiple disease resistant F7:8 lines is available from AVRDC (http://avrdc.org/seed/improved-lines/); these lines have potential for release as inbred line cultivars, hybrid parental lines, or breeding stock.

Patterns of Molecular Diversity in Current and Previously Developed Hybrid Parents of Pearl Millet [<i>Pennisetum glaucum</i> (L.) R. Br.

ICRISAT’s pearl millet (Pennisetum glaucum (L.) R. Br.) breeding program at Patancheru, India, has developed genetically diverse hybrid parents since 1980s. The present study investigated genetic diversity pattern between two groups of parents in this program, bred till 2004 and developed during 2004-2010. Combined analysis of 379 hybrid parents (current 166 parents and 213 previously developed hybrid parents) carried out using a set of highly polymorphic 28 SSRs detected 12.7 alleles per locus. An average of 8.5 and 8.7 SSR alleles per locus were found in previously developed and current parents, respectively, indicating marginal improvement in the levels of genetic diversity of hybrid parents in this program. Distance matrix differentiated these current and previously developed hybrid parents into 2 separate clusters, indicating infusion of new genetic variability over time as reflected by development of more genotype-specific alleles. Also, the seed and restorer parents were found clearly separated from each other in both the sets with few crossovers, indicating existence of two diverse and broad-based pools in hybrid parents of pearl millet. Restorer parents (R-lines) were found more diverse than seed parents (B-lines), as higher average gene diversity was detected among R-lines (0.70) than B-lines (0.56), though variation between B- and R-lines was found reduced in newly developed lines to 9.22% from 16.98% in previously developed lines. Results suggested that newly developed lines were as much divergent when compared with previously developed lines, indicating that current ICRISAT pearl millet breeding program was moving towards development of diverse new hybrid parental lines. The study suggested use of trait-specific donors in B- and R-lines separately to maintain sufficient genetic distance between seed and restorer breeding lines. It was pointed out to cross parents having higher genetic distance within the seed (B-lines) and restorer (R-lines) breeding programs to derive diverse and productive hybrid parental lines in future.

Seed set variability under high temperatures during flowering period in pearl millet (Pennisetum glaucum L. (R.) Br.)

Pearl millet has recently emerged as a significant irrigated summer season cereal crop in north-western India. But its flowering coincides with air temperatures of ≥40°C, leading to reduced seed set and poor grain yield in most of the available hybrids, although a few hybrids with good seed set and high yield potential are widely cultivated. Under a recent initiative to diversify the genetic base of heat tolerant hybrids, field screening of 221 hybrid parental lines (both B- and R-lines), 53 germplasm accessions and 4 improved populations over four-year period revealed large genetic variability in seed set at daily maximum air-temperature of ≥42°C during flowering. Two locations data on 46 medium maturing genotypes screened during summer 2009 showed that seed set in pearl millet started declining when maximum air temperatures reached 42°C and decreased in curvilinear fashion to 20 percent at 46°C. Similar relationship of seed set with minimum and mean temperature was observed with threshold values of 26.4°C and 34.2°C, respectively. Similarly, the relationship of percent seed set with vapor pressure deficit (VPD) showed threshold value of 6.2kPa for maximum VPD, 1.2kPa for minimum VPD and 3.7kPa for mean VPD. Seed set on 2 each of heat tolerant and susceptible genotypes fitted well on the seed set-temperature response curve for the maximum, minimum and mean air temperatures. Based on 3 to 4 year field screening (2009–2012), five hybrid seed parents (ICMB 92777, ICMB 05666, ICMB 00333, ICMB 02333 and ICMB 03555) and a germplasm accession IP 19877 with 61 to 69% seed set as compared to 71% seed set in a heat tolerant commercial hybrid 9444 (used as a control) was identified. Intra-population variability for heat tolerance was observed in four populations, and highly heat tolerant progenies from two of them were identified. Evaluation of six hybrid parents under controlled environment (maximum temperature of 43°C and minimum temperature of 22°C) revealed boot-leaf stage of pearl millet plant to be more heat sensitive than panicle-emergence stage, and investigations on 6 A-/B-pairs under controlled environment (max. temperature of 44°C and min. temperature of 22°C) revealed female reproductive system of pearl millet to be more heat sensitive than pollen. Comparison of 23 hybrids and their parents for seed set at high air temperature (>42°C) showed heat tolerance as a dominant trait, implying heat tolerance in one parent would be adequate to produce heat tolerant hybrids in pearl millet. Heat tolerant composite developed using identified lines showed high mean seed set under high air temperatures during flowering.