Cycles Of Selection Research Articles

Screening of Aptamers that Bind to the Multivalent Aminoglycoside Amikacin

Increased awareness of the multiple roles of RNA molecules has led to the realization that, in addition to their structural and functional roles, RNAs can be drug targets for small molecular therapy. Amikacin, a member of the aminoglycoside group of antibiotics, binds to specific sites in bacterial 16S ribosomal RNAs (rRNAs) and interferes with protein synthesis leading to cell death. Here, we used the systemic evolution of ligands by exponential enrichment (SELEX) method to isolate high affinity RNA fragments (aptamers) that bind to amikacin. After five rounds of SELEX selection, in which a linear N25 DNA template was used for the first selection cycle, the resulting RNA was cloned and sequenced. Among the 38 clones generated, five groups of sequences (groups A through E) containing nine conserved motifs were identified. The sequences of groups A and B were almost identical, indicating that the selected RNA was enriched. Subsequently, the Basic Local Alignment Search Tool program was used to search for the conserved motifs in bacterial 16S rRNA sequences. Strikingly, no sequence homology was observed, suggesting that the conserved sequences (motifs) identified in this studymay be novel target sites for amikacin. Bangladesh Pharmaceutical Journal 26(2): 134-143, 2023 (July)

An efficient and accurate droplet digital PCR method for rapid transgene copy number detection and homozygous identification in cotton (Gossypium hirsutum)

Transgene copy number characterization and homozygous identification are important steps for both the scientific research and transgenic breeding in plant. Traditionally, Southern blotting, quantitative PCR, genome sequencing and genetic analysis are the major methods to achieve these objectives. These methods are laborious, time-consuming and cost-expensive. Digital PCR is a breakthrough technology that is able to provide sensitive and absolute nucleic acid quantification. However, its application is rarely studied in transgene copy number detection and homozygous identification, especially in cotton, an important industrial crop worldwide. Here, we developed a droplet digital PCR (ddPCR) system to rapidly and efficiently detect the transgene copy number in cotton. The GhTPS, a single gene in per subgenome of Gossypium hirsutum, and NPT II, a marker gene in the transgenic T-DNA, were selected as the endogenous reference gene and exogenous insertion sequences, respectively. Results showed that probe concentration at 150 nM and DNA concentration at 5.0 ng/µL were the best conditions to perform ddPCR in tetraploid cotton. The detected T-DNA copy numbers obtained from the developed ddPCR system were in consistent with the results from Southern blotting analysis and genome resequencing results, respectively, demonstrating that the developed ddPCR system is accurate and efficient in identifying the T-DNA copy number. Moreover, the homozygous and heterozygous T1 offspring plants from a T0 plant were efficiently distinguished using this approach, greatly shortening the selection cycles of homozygous plants. The ddPCR system was further applied to detect the transgenic Bt copy numbers in commercialized cotton varieties, indicating the suitability and convenience in identifying transgenic cotton with different origins. This study provides a novel, rapid and efficient method for copy number and homozygous determination in transgenic cotton.

Food quality profile of pounded yam and implications for yam breeding.

Assessment of the key preferred quality traits in pounded yam, a popularly consumed yam food product in West Africa, is often done through sensory evaluation. Such assessment is time-consuming and results may be biased. Therefore, there is a need to develop objective, high-throughput methods to predict the quality of consumer-preferred traits in pounded yam. This study focused on how key quality traits in pounded yam proposed to yam breeders were determined, measured by biophysical and biochemical methods, in order to shorten the breeding selection cycle through adoption of these methods by breeders. Consumer tests and sensory quantitative descriptive analysis (QDA) validated that preferred priority quality traits in pounded yam were related to textural quality (smooth, stretchable, moldable, slightly sticky and moderately hard) and color (white, cream or light yellow). There were significant correlations between sensory textural quality attributes cohesiveness/moldability, hardness, and adhesiveness/stickiness, with textural quality measurements from instrumental texture profile analysis (TPA). Color measurement parameters (L*, a*, and b*) with chromameter agreed with that of sensory evaluation and can replace the sensory panel approach. The smoothness (R2 = 1.00), stickiness (R2 = 1.00), stretchability (R2 = 1.00), hardness (R2 = 0.99), and moldability (R2 = 0.53) of pounded yam samples can be predicted by the starch, amylose, and protein contents of yam tubers estimated by near-infrared spectroscopy. TPA and Hunter colorimeter can be used as medium-high throughput methods to evaluate the textural quality and color of pounded yam in place of the sensory panelists. © 2023 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Molecular characterization of doubled haploid lines derived from different cycles of the Iowa Stiff Stalk Synthetic (BSSS) maize population.

Molecular characterization of a given set of maize germplasm could be useful for understanding the use of the assembled germplasm for further improvement in a breeding program, such as analyzing genetic diversity, selecting a parental line, assigning heterotic groups, creating a core set of germplasm and/or performing association analysis for traits of interest. In this study, we used single nucleotide polymorphism (SNP) markers to assess the genetic variability in a set of doubled haploid (DH) lines derived from the unselected Iowa Stiff Stalk Synthetic (BSSS) maize population, denoted as C0 (BSSS(R)C0), the seventeenth cycle of reciprocal recurrent selection in BSSS (BSSS(R)C17), denoted as C17 and the cross between BSSS(R)C0 and BSSS(R)C17 denoted as C0/C17. With the aim to explore if we have potentially lost diversity from C0 to C17 derived DH lines and observe whether useful genetic variation in C0 was left behind during the selection process since C0 could be a reservoir of genetic diversity that could be untapped using DH technology. Additionally, we quantify the contribution of the BSSS progenitors in each set of DH lines. The molecular characterization analysis confirmed the apparent separation and the loss of genetic variability from C0 to C17 through the recurrent selection process. Which was observed by the degree of differentiation between the C0_DHL versus C17_DHL groups by Wright's F-statistics (FST). Similarly for the population structure based on principal component analysis (PCA) revealed a clear separation among groups of DH lines. Some of the progenitors had a higher genetic contribution in C0 compared with C0/C17 and C17 derived DH lines. Although genetic drift can explain most of the genetic structure genome-wide, phenotypic data provide evidence that selection has altered favorable allele frequencies in the BSSS maize population through the reciprocal recurrent selection program.

New vegetable varieties of Brassica rapa and Brassica napus with modified glucosinolate content obtained by mass selection approach.

Glucosinolates (GSLs) constitute a characteristic group of secondary metabolites present in the Brassica genus. These compounds confer resistance to pests and diseases. Moreover, they show allelopathic and anticarcinogenic effects. All those effects are dependent on the chemical structure of the GSL. The modification of the content of specific GSLs would allow obtaining varieties with enhanced resistance and/or improved health benefits. Moreover, the attainment of varieties with the same genetic background but with divergent GSLs concentration will prompt the undertaking of studies on their biological effects. The objective of this study was to evaluate the efficacy of two divergent mass selection programs to modify GSL content in the leaves of two Brassica species: nabicol (Brassica napus L.), selected by glucobrassicanapin (GBN), and nabiza (Brassica rapa L.), selected by gluconapin (GNA) through several selection cycles using cromatographic analysis. The response to selection fitted a linear regression model with no signs of variability depletion for GSL modification in either direction, but with higher efficiency in reducing the selected GSL than in the increasing. The selection was also effective in other parts of the plant, suggesting that there is a GSL translocation in the plant or a modification in their synthesis pathway that is not-organ specific. There was an indirect response of selection in other GSL; thus this information should be considered when designing breeding programs. Finally, populations obtained by selection have the same agronomic performance or even better than the original population. Therefore, mass selection seems to be a good method to modify the content of specific GSL in Brassica crops.

Population improvement via recurrent selection drives genetic gain in upland rice breeding.

One of the main challenges of breeding programs is to identify superior genotypes from a large number of candidates. By gradually increasing the frequency of favorable alleles in the breeding population, recurrent selection improves the population mean for target traits, increasing the chance to identify promising genotypes. In rice, population improvement through recurrent selection has been used very little to date, except in Latin America. At Embrapa (Brazilian Agricultural Research Corporation), the upland rice breeding program is conducted in two phases: population improvement followed by product development. In this study, the CNA6 population, evaluated over five cycles (3 to 7) of selection, including 20 field trials, was used to assess the realized genetic gain. A high rate of genetic gain was observed for grain yield, at 215 kg.ha-1 per cycle or 67.8 kg.ha-1 per year (3.08%). The CNA6 population outperformed the controls only for the last cycle, with a yield difference of 1128 kg.ha-1. An analysis of the product development pipeline, based on 29 advanced yield trials with lines derived from cycles 3 to 6, showed that lines derived from the CNA6 population had high grain yield, but did not outperform the controls. These results demonstrate that the application of recurrent selection to a breeding population with sufficient genetic variability can result in significant genetic gains for quantitative traits, such as grain yield. The integration of this strategy into a two-phase breeding program also makes it possible to increase quantitative traits while selecting for other traits of interest.

Effect of three cycles of recurrent selection for yield in four Spanish landraces of maize

Landraces of maize (Zea mays L.) have a variability not found in the elite hybrids due to the bottleneck during selection. There is a renewed interest in using that variability, incorporating alleles for specific traits, or widening the general variability of elite germplasm. In Europe, there is also an interest in the direct use of local landraces by farmer’s associations seeking agriculture that preserves crop diversity. Pre-breeding programs are aimed to reduce the yield gap among landraces and improved materials. In the Misión Biológica de Galicia we have carried out three cycles of a S1 recurrent selection program for yield on four local landraces from Northwestern Spain with the objective of obtaining improved populations that might be cultivated by farmers and that could be used as sources of superior inbred lines. One hundred plants were selfed in each landrace and the S1 families were evaluated in a 10 × 10 simple lattice. The 20 S1 families with the highest grain yields were randomly intercrossed to form the selected population of each landrace. After three cycles of selection, each population and its cycles of selection were evaluated in three locations for two years. All the populations had a consistent yield gain in the first cycle of selection. However, the response in later cycles differed among the populations: one population had a continuous increase of 21% per cycle while two populations had a reduced yield in the last cycles. Our results highlight the need to include secondary traits to avoid unintentional indirect effects. We conclude that, in pre-breeding programs, it could be worthy to use large effective numbers to prevent inbreeding depression, particularly when the objective is the direct use of improved landraces. Another conclusion is that recurrent selection does not seem to be as efficient on European germplasm as it is on Corn Belt varieties. Finally, obtaining valuable new germplasm requires a sustained, long-term effort.

Reactions of provitamin-A-enriched maize to foliar diseases under field conditions in Nigeria

Maize is a major staple food in Sub-Saharan Africa (SSA). Vitamin A deficiency index is high in Africa and could be reduced through the consumption of provitamin-A-enriched maize. However, foliar diseases such as maize streak virus, northern corn leaf blight and common rust constrain maize production in SSA. The cultivation of host-resistant varieties is the most effective approach to mitigate their effects. Therefore, maize synthetics improved for PVA carotenoids, their selection cycles and crosses as well as a commercial disease-resistant check were assessed for resistance to maize streak virus, northern corn leaf blight and common rust at hotspots in Nigeria. The foliar diseases’ effects on the agronomic performance and carotenoid content of the maize genotypes were assessed. The Genotypes differed for most agronomic traits and foliar disease resistance. Stepwise regression revealed that, although the agronomic traits determined 93% of the grain yield, each foliar disease had effect on the yield. A unit increase in maize streak virus score increased plant aspect and husk cover scores by 0.6 and 0.4, respectively, whereas an increase in common rust score decreased plant height by 16.2 cm and increased plant aspect score by 0.7. Maize streak virus and common rust decreased genotypic variability for lutein by 36.7 and 18.7%, respectively, while northern corn leaf blight decreased genotypic variability for provitamin A by 27.1%. Most of the genotypes exhibited moderate susceptibility to northern corn leaf blight. However, three selection cycles and three crosses exhibited high tolerance to maize streak virus and moderate tolerance to common rust, thus can serve as sources of PVA-enriched, maize streak virus and common rust tolerant lines.

Divergent selection for timing of vegetative phase change

AbstractVegetative phase change is a key trait in plant development and marks the transition from juvenile to adult growth phases. In maize (Zea mays L.), juvenile plants and plants with a late phase change can be identified by the increased production of tillers, aerial roots, and distinctive epicuticular wax. Resistance to agronomically significant pathogens can also differ between juvenile and adult plants. Using last leaf with juvenile wax as the target of selection, the maize population Minnesota 11 underwent 16 cycles of divergent selection. The objectives of this research were to identify emergent trends for each trait and to determine the magnitude and direction of phenotypic changes over long‐term selection compared to the source population. Phenotypic data on plant architecture traits, ear traits, and common rust resistance were collected in 2020 and 2021 by concurrently growing plants from selection cycles representing both directions of selection. Last leaf with juvenile wax, the direct target of selection increased to leaf 17.8 in the late phase change population and decreased to leaf 5 in the early phase change population from leaf 9 in the source population. Late vegetative phase change was positively correlated with increased common rust infection and plant height, and negatively correlated with ear size traits. This study shows that the observed phenotypic changes follow patterns of genetic variation consistent with divergent selection, but with possible effects of inbreeding.

A Selective Nano Cell Cycle Checkpoint Inhibitor Overcomes Leukemia Chemoresistance (Small 25/2023)

Nano Cell Cycle Checkpoint Inhibitors In article number 2300736, Daishun Ling, Fangyuan Li, and co-workers demonstrate that ultra-small iron oxide nanoparticles (IONPs) can serve as a selective nano cell cycle checkpoint inhibitor (NCCI) to catalyze the intracellular H2O2 into abundant ·OH, cleave heat shock protein 90 (Hsp90), and thus induce the degradation of the client protein ATR/CHK1, leading to the dysfunction of the G2/M checkpoint in acute myeloid leukemia (AML) cells. As a result, NCCI can interfere with the chemotherapy-induced DNA damage repair, which effectively reverses the chemoresistance of acute myeloid leukemia.

A Simplified Virtual-Vector-Based Model Predictive Control Technique With a Control Factor for Three-Phase SPMSM Drives

Compared with the single vector-based model predictive control (MPC) technique, the multiple vector-based MPC technique gains interests in improving the total harmonic distortion (THD) of the stator currents and reducing electromagnetic torque and stator flux ripples for a three-phase surface permanent magnet synchronous motor (SPMSM). However, the complicated enumeration and operation in vector selection and duty cycle calculation increase the computational burden and limit its peneration. Hence, in this paper, a simplified virtual vector-based MPC with a control factor for a three-phase SPMSM is proposed. Without sector identification and enumeration operations, the proposed technique utilizes the control factor to automatically generate a large number of virtual vectors to achieve equivalent performance effectiveness with multiple vector-based MPC technique with lower sampling frequency. Subsequently, regardless of the number of virtual vectors, the candidate virtual vectors are directly identified in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$x-y$</tex-math></inline-formula> coordinate system to greatly reduce the computational burden. In addition, the duration of the selected optimal virtual vector and the resultant switching sequences are easily obtained and arranged to even the switching losses among the switching devices and further optimize the THD of the stator current. The proposed technique is simplified and flexible. Its control effectiveness is verified by experimental results.

Northern Wild Rice (Zizania palustris L.) breeding, genetics, and conservation

AbstractCultivated Northern Wild Rice (NWR; Zizania palustris L.) is a high‐value, small commodity crop grown in irrigated paddies, primarily in Minnesota and California. Domestication of the species began ∼60 years ago as demand for the nutritional grain outpaced hand‐harvesting efforts from lakes and rivers in the Great Lakes region of the United States and Canada. Cultivated NWR cultivars are open‐pollinated and highly heterogeneous and have primarily been bred for seed retention, yield, and seed size. As a lowland crop, NWR's life cycle, particularly its unique seed physiology, poses challenges to breeding efforts, limiting selection cycles per year, and requiring annual grow‐outs of all germplasm. Recent efforts have increased the genomic resources available to NWR researchers, including a reference genome assembly and methodology optimization for genotyping‐by‐sequencing technologies. The species’ close phylogenetic relationship with white rice (Oryza sativa) also provides a unique opportunity to utilize comparative genomic approaches to identify genes conferring agronomic traits of interest in NWR, particularly domestication traits such as seed retention. Z. palustris is an enigmatic species with regional ecological, cultural, and agricultural significance in the Great Lakes. As NWR is grown in both the centers of origin and diversity, it is important for NWR plant breeders to be good stewards of our domesticated plants and include a diversity of stakeholders in our decision‐making processes. In this work, we have aimed to unpack some of the disputes regarding the breeding of cultivated NWR and the science behind our work. Additionally, we have discussed conservation efforts for natural stands of NWR which will help preserve the many ecosystem, nutritional, spiritual, and economic services provided by this important species.

Recurrent Selection of Echinochloa crus-galli with a Herbicide Mixture Reduces Progeny Sensitivity.

Herbicide mixtures are used to increase the spectrum of weed control and to manage weeds with target-site resistance to some herbicides. However, the effect of mixtures on the evolution of herbicide resistance caused by enhanced metabolism is unknown. This study evaluated the effect of a fenoxaprop-p-ethyl and imazethapyr mixture on the evolution of herbicide resistance in Echinochloa crus-galli using recurrent selection at sublethal doses. The progeny from second generations selected with the mixture had lower control than parental plants or the unselected progeny. GR50 increased 1.6- and 2.6-fold after two selection cycles with the mixture in susceptible (POP1-S) and imazethapyr-resistant (POP2-IR) biotypes, respectively. There was evidence that recurrent selection with this sublethal mixture had the potential to evolve cross-resistance to diclofop, cyhalofop, sethoxydim, and quinclorac. Mixture selection did not cause increased relative expression for a set of analyzed genes (CYP71AK2, CYP72A122, CYP72A258, CYP81A12, CYP81A14, CYP81A21, CYP81A22, and GST1). Fenoxaprop, rather than imazethapyr, is the main contributor to the decreased control in the progenies after recurrent selection with the mixture in low doses. This is the first study reporting the effect of a herbicide mixture at low doses on herbicide resistance evolution. The lack of control using the mixture may result in decreased herbicide sensitivity of the weed progenies. Using mixtures may select important detoxifying genes that have the potential to metabolize herbicides in patterns that cannot currently be predicted. The use of fully recommended herbicide rates in herbicide mixtures is recommended to reduce the risk of this type of resistance evolution.

Thinking teacher education through military imaginaries: A diffractive and decolonising reading

ABSTRACT This article problematises Initial Teacher Education (ITE) and assessment practices to contribute to the ongoing work of decolonising higher education. It critiques the entanglement of military imaginaries and ITE via a diffractive reading of ITE discourse and policy in the Australian context through military imaginaries in academic and SF literature. In military imaginaries there exists an imperative to enact warfare as ‘target processing’; that is, the essence of warfare is understood to be the operationalisation of a continuous cycle of identification of targets, selection and implementation of pre-determined methodologies to match these targets, followed by measurement of the outcomes of the process. This article considers how current ITE expectations prioritise a similar continuous cycle of selection and implementation of standardised methodologies and measurement of effects; effectively an ideology of ‘teaching as processing’. It examines how this expectation assists in creating certain concerning expectations and disappearances within teaching and education.

Using haplotype and QTL analysis to fix favorable alleles in diploid potato breeding.

At present, the potato (Solanum tuberosum L.) of international commerce is autotetraploid, and the complexity of this genetic system creates limitations for breeding. Diploid potato breeding has long been used for population improvement, and because of an improved understanding of the genetics of gametophytic self-incompatibility, there is now sustained interest in the development of uniform F1 hybrid varieties based on inbred parents. We report here on the use of haplotype and quantitative trait locus (QTL) analysis in a modified backcrossing (BC) scheme, using primary dihaploids of S. tuberosum as the recurrent parental background. In Cycle 1, we selected XD3-36, a self-fertile F2 individual homozygous for the self-compatibility gene Sli (S-locus inhibitor). Signatures of gametic and zygotic selection were observed at multiple loci in the F2 generation, including Sli. In the BC1 cycle, an F1 population derived from XD3-36 showed a bimodal response for vine maturity, which led to the identification of late versus early alleles in XD3-36 for the gene CDF1 (Cycling DOF Factor 1). Greenhouse phenotypes and haplotype analysis were used to select a vigorous and self-fertile F2 individual with 43% homozygosity, including for Sli and the early-maturing allele CDF1.3. Partially inbred lines from the BC1 and BC2 cycles have been used to initiate new cycles of selection, with the goal of reaching higher homozygosity while maintaining plant vigor, fertility, and yield.

A Selective Nano Cell Cycle Checkpoint Inhibitor Overcomes Leukemia Chemoresistance.

Cell cycle checkpoint activation promotes DNA damage repair, which is highly associated with the chemoresistance of various cancers including acute myeloid leukemia (AML). Selective cell cycle checkpoint inhibitors are strongly demanded to overcome chemoresistance, but remain unexplored. A selective nano cell cycle checkpoint inhibitor (NCCI: citric acid capped ultra-small iron oxide nanoparticles) that can catalytically inhibit the cell cycle checkpoint of AML to boost the chemotherapeutic efficacy of genotoxic agents is now reported. NCCI can selectively accumulate in AML cells and convert H2 O2 to • OH to cleave heat shock protein 90, leading to the degradation of ataxia telangiectasia and Rad3-related proteinand checkpoint kinase 1, and the subsequent dysfunction of the G2/M checkpoint. Consequently, NCCI revitalizes the anti-AML efficacy of cytarabine that is previously ineffective both in vitro and in vivo. This study offers new insights into designing selective cell cycle checkpoint inhibitors for biomedical applications.

Bright red passion fruit - evaluation of colorimetry and physicochemical quality for the fresh fruit market

The species Passiflora edulis Sims has varieties that produce fruits with colors that vary yellow to reddish purple and dark purple, all of which are well adapted to Brazil's varied climatic conditions. In the foreign market, demand is greatest for the variety Passiflora edulis f. edulis Sims, or pure purple passion fruit (gulupa), whose adaptation is more restricted to cool and upland regions. The Embrapa Cassava and Fruits research unit (Embrapa Mandioca e Fruticultura) generated a variety of passion fruit derived from P. edulis with reddish-purple fruits, aiming at the identification of genotypes with lower acidity and greater fruit sweetness to supply the fresh fruit market. A half-sib population of 83 P edulis genotypes, segregated for light red to purple and yellow color, was evaluated. For previous selection, the fruits were tested based on sweetness, and 67 genotypes were identified, while the other genotypes constituted the population of origin. Fruit weight, pulp and peel weight, juice yield, soluble solids content (SS), titratable acidity (TA) and SS/TA ratio were evaluated, in addition to colorimetric evaluation CIE-LAB (L*a*b*) . For physical traits, there was no significant difference between the pre-selected and original populations. For the chemical characteristics of the pulp, variation was observed only in titratable acidity (TA), which was 38% lower in the pre-selected population, resulting in a 43% increase in the SS/TA ratio. High correlations were not observed between the colorimetric parameters (L*, a* and b*) of the peel and pulp as well as TA and SS/TA. The heritability (h2) of the evaluated traits was high (66.40% to 86.25%), and 26 genotypes were chosen (ratio from 7.0 to 15.29) with 33.12% lower acidity and 39.84% higher ratio in the next selection cycle. These selected individuals are favorable to development of a passion fruit cultivar with sweeter flavor to supply the domestic and foreign fresh fruit markets.

Optimising selection in full-sib progeny of the cashew

In Brazil, the productivity and quality of the peduncle and nut of the cashew have increased significantly due to the use of binomial hybridisation-cloning. In the present research, genetic parameters (variance components – individual REML) and genetic values (mean components – individual BLUP) were estimated in a test of full-sib segregating progenies, aiming at the selection of superior cashew progenies and individuals. A controlled-pollination, segregating population from the cashew genetic breeding program of Embrapa Agroindústria Tropical, comprising four different progenies, each represented by 48 full-sib (FS) individuals, were evaluated over six consecutive crops. The following traits were evaluated: nut yield, mean nut weight, reaction to Anacampsis phytomiella, and reaction to oidium (powdery mildew). The experimental design was of randomised blocks, with four replications and 12 plants per plot. The REML/BLUP geneticstatistical procedure was used to optimise the selection process. It seems reasonable to opt for a selection limit of 14 individuals with superior behaviour to comprise the recombination unit of the next cycle of recurrent selection using the remaining seeds from this breeding population. Plants 25 and 27 from the progeny identified as having the best performance (FS 13/3) for direct cloning were selected to comprise the local trial of the cashew genetic breeding program.

The effect of cycles of genomic selection on the wheat (T. aestivum) genome

We documented changes in the wheat genome attributed to genomic selection including loss of diversity, and changes in population structure and linkage disequilibrium patterns. We conclude that training and prediction populations need to co-evolve instead of the use of a static training population. Genomic selection (GS) is widely used in plant breeding to shorten breeding cycles. Our objective was to assess the impact of rapid cycling GS on the wheat genome. We used 3927 markers to genotype a training population (YTP) and individuals from five cycles (YC1-YC5) of GS for grain yield. We assessed changes of allele frequency, genetic distance, population structure, and linkage disequilibrium (LD). We found 27.3% of all markers had a significant allele frequency change by YC5, 18% experienced a significant change attributed to selection, and 9.3% had a significant change due to either drift or selection. A total of 725 of 3927 markers were fixed by YC5 with selection fixing 7.3% of the 725 markers. The genetic distance between cycles increased over time. The Fst value of 0.224 between YTP and YC5 indicates their relationship was low. The number LD blocks decreased over time and the correlation between LD matrices also decreased over time. Overall, we found a reduction in genetic diversity, increased genetic differentiation of cycles from the training population, and restructuring of the LD patterns over cycles. The accuracy of GS depends on the genomic similarity of the training population and the prediction populations. Our results show that the similarity can decline rapidly over cycles of GS and compromise the predictive ability of the YTP-based model. Our results support implementing a GS scheme where the training and prediction populations co-evolve instead of the use of a static training population.

Genetic Improvement for Grain Yield under Heat Stress Conditions in Bread Wheat (Triticum aestivum L.)

Extreme temperature is one of the abiotic stresses that causes significant yield reductions in wheat. Therefore, the current study aims to create heat-tolerant wheat plants with superior grain yield productivity by using traditional breeding. Two cycles of phenotypic directional selection for grain yield per plant were applied in two F2 populations. These populations were derived from crosses among four local wheat varieties. In the first cycle, a directional selection for grain yield per plant was performed under both the favorable environment (optimal sowing date) and heat stress environments (late sowing date). Under the favorable conditions, the responses of F3 plants to selection for grain yield per plant were positive and significantly higher (averaged 7.71%) than under heat stress conditions (averaged 8.20%). Moreover, the average of the concurrent response to selection for grain yield per plant in grain yield per spike had a positive and high percentage under the favorable (7.00%) and the heat stress environment (8.84%). Highly significant and positive responses were observed within the F3 plants of the two populations. The value of the average was greater under favorable (6.85%) than under the heat stress conditions (7.20%). Additionally, the 1000 kernel weight was highly significant and positive responses (5.35% and 6.11%) in both favorable and heat stress environments, respectively. Generally, the observed results indicating the F4 plants could be a promising source to introduce yield related traits to develop high-yielding wheat cultivars for heat-stressed environments.