Cycles Of Divergent Selection Research Articles

Signatures of divergent selection for cold tolerance in maize

Divergently selected genotypes can be used for detecting the genomic regions affecting the selected trait (selection signature). Moreover, the genetic distances (GDs) among divergently selected lines can be correlated with the agronomic performances of the crosses among them. Using as source the maize F2 of B73 × IABO78, we previously conducted four cycles of divergent recurrent selection and three cycles of divergent selection in inbreeding for cold tolerance at germination. We finally obtained 10 lines selected for low (L) and 10 lines selected for high (H) cold tolerance, which exhibited a notable divergence for both the selected and associated traits. Herein, we investigated the 20 lines and the 28 single diallel crosses among eight random lines (four L and four H); the main objectives were to identify the putative regions controlling the selected and associated traits and to study the relationships between crosses performances and GDs among their parental lines. Allele frequencies at 932 recombination blocks based on 19,220 polymorphic SNPs were obtained for the two lines’ groups; the F ST calculated across sliding windows indicated 18 regions highly divergent between groups. The increasing alleles for cold tolerance were contributed by both parents, consistently with the transgressive segregations previously found. Several regions associated to DG also affected various agronomic traits. The cross performances showed some relationships with the genetic distances among parental lines for traits affected by dominance, provided that all crosses were considered, while these relationships vanished when only L × H crosses were examined.

Dynamics of technological and baking qualities of winter rye grain depending on flour yield and water extract viscosity

Dynamics of technological and baking qualities of winter rye grain were studied depending on flour yield and water extract viscosity (WEV). Initial material consisted in six populations of winter rye obtained after ten cycles of divergent selection for WEV, which was carried out on the basis of cultivars Al’fa and Moskovskaya 12. For each cultivar, three populations contrastingly differing for WEV were identified: low viscosity ones (LV) obtained in minus selection, mean viscosity ones (MV) obtained on the basis of the original populations without selection, and high viscosity (HV) ones obtained in plus selection. For each population, three grades of flour were obtained: wholemeal flour (100% yield), medium flour (87% yield), and pure flour (63% yield). Comparative testing was carried out in 2015 and 2016: in a field experiment on 8-m2 plots in three repetitions. The following traits were studied: weight of 1000 grains, nature of grain, glassiness, protein and starch contents, flour whiteness, falling number, amylogram height, starch gelatinization temperature, H/D ratio, bread volume, and crumb porosity. A prominent feature of LV-populations is the trend towards small grains, low grain nature, and lowered starch contents. For most traits, HV-populations were at the level of MV-populations. It was concluded that, during production of pure rye flour, almost all pentosans together with bran are removed from external layers of caryopsis. As a result, the share of pentosans within endosperm essentially increases. These pentosans belong to the water-soluble fraction, which is proven to be true by a sharp increase in WEV in pure rye flour in comparison with wholemeal flour. An increase in WEV in the grades of thin grinding flour occurs due to water-soluble pentosans, which are contained in bran, in deep layers of endosperm. As compared to rough grinding flour (wholemeal flour), thin grinding flour (pure flour) differed by higher whiteness (by 46.6 units), better amylogram (by 46 a. units), higher H/D ratio (by 0.04), volumetric yield (by 38 cm3), and porosity of crumb of a square loaf (by 9.9%). WEV also had significant influence, which, at high expression, positively affected flour whiteness, falling number, amylogram height, starch gelatinization temperature, and shape stability of toppling bread.

Morphology and Rubisco turnover characteristics of perennial ryegrass breeding populations after two and four cycles of divergent selection for long or short leaf length

Perennial ryegrass populations previously subjected to two or four cycles of selection for short or long leaf length were studied to determine the response of morphological traits to selection and interaction to determine yield. Measured morphological traits were leaf length, leaf appearance interval, ligule appearance interval, leaf elongation duration, leaf elongation rate, tiller number, tiller dry weight, and herbage dry matter. Additionally, Rubisco concentration during leaf development was measured to determine the association of Rubisco turnover with morphological characteristics and yield. Rubisco was measured and modelled as a three-parameter (D, peak Rubisco concentration; G, time of D; and F, curve width measure), log-normal curve. Leaf length, leaf elongation rate, tiller weight, and plant dry matter diverged after two cycles of selection and further divergence occurred, with these traits being, respectively, 35, 28, 53, and 61% greater in the long- than the short-leaved plants after four cycles of selection. Opposite trends were displayed by Rubisco turnover, with selection for long leaves co-selecting for increased Rubisco turnover time and selection for short leaves resulting in increased leaf Rubisco concentration. There was indication of coupling of leaf appearance with Rubisco turnover. Across populations, multivariate analysis indicated that plant yield was associated with Rubisco concentration rather than Rubisco turnover. The association between higher yield and lower Rubisco concentration could be targeted in the breeding of high-yielding, nitrogen-efficient forage grasses. Plant yield was mainly associated with increased leaf area, indicating that yield could be improved by selecting for longer leaves and faster rates of leaf expansion.

Divergent selection for ester-linked diferulates in maize pith stalk tissues. Effects on cell wall composition and degradability

Cross-linking of grass cell wall components through diferulates (DFAs) has a marked impact on cell wall properties. However, results of genetic selection for DFA concentration have not been reported for any grass species. We report here the results of direct selection for ester-linked DFA concentration in maize stalk pith tissues and the associated changes in cell wall composition and biodegradability. After two cycles of divergent selection, maize populations selected for higher total DFA (DFAT) content (CHs) had 16% higher DFAT concentrations than populations selected for lower DFAT content (CLs). These significant DFA concentration gains suggest that DFA deposition in maize pith parenchyma cell walls is a highly heritable trait that is genetically regulated and can be modified trough conventional breeding. Maize populations selected for higher DFAT had 13% less glucose and 10% lower total cell wall concentration than CLs, suggesting that increased cross-linking of feruloylated arabinoxylans results in repacking of the matrix and possibly in thinner and firmer cell walls. Divergent selection affected esterified DFAT and monomeric ferulate ether cross link concentrations differently, supporting the hypothesis that the biosynthesis of these cell wall components are separately regulated. As expected, a more higher DFA ester cross-coupled arabinoxylan network had an effect on rumen cell wall degradability (CLs showed 12% higher 24-h total polysaccharide degradability than CHs). Interestingly, 8-8-coupled DFAs, previously associated with cell wall strength, were the best predictors of pith cell wall degradability (negative impact). Thus, further research on the involvement of these specific DFA regioisomers in limiting cell wall biodegradability is encouraged.

Divergent selection for water-extract viscosity in winter rye

Five cycles of divergent selection for water-extract viscosity in winter rye are carried out. Divergence between the populations depends both on the genotype of the variety and on the direction of selection. Selection in the plus direction is more effective than in the minus direction. As a result of selection, subpopulations from variety Al’fa differ in extract viscosity by 4.0 times and from cultivar Moskovskaya 12 by 5.7 times. The average genetic trend of the investigated varieties for plus selection is respectively 43.3 and 88.2% and for minus selection −10.5 and −13.3%. Skewness of the response occurred already in the first selection cycle and persisted in subsequent cycles. The value of realized heritability (h 2) varied within 0.70–0.74 for plus selection and 0.33–0.44 for minus selection. Selection for extract viscosity correlates with other valuable breeding traits. Populations with low viscosity differ from those with high viscosity by a lower yield, long and less lodging-resistant stem, and smaller 1000 grain weight and had worse baking qualities of grain. The causes of skewness and ways to increase the effectiveness of selecting rye for special purpose use are discussed.

Divergent Alfalfa Root System Architecture is Maintained across Environment and Nutrient Supply

Plant root system architecture can alter, and be altered by, soil fertility and other environmental conditions. We developed alfalfa (Medicago sativa L.) populations selected for tap‐ or branch‐rooted architectures and determined responses to P supply and K placement in field experiments. In Exp. 1, populations representing the parental composite and progeny from first and second cycles of divergent selection were seeded on soils testing low to medium for available P and K, a sandy loam (Becker, MN) and a silt loam (Rosemount, MN). Experiment 2 was conducted at Becker and included only second cycle progeny. Phosphorus was added to one‐half of the plots by injection of KH2PO4 (224 kg P ha−1) into the upper 40 cm of soil. On the other plots, equal K rates (270 kg K ha−1) as KCl were topdressed. Added P did not affect herbage dry mass in the first and second years following stand establishment, but increased herbage P concentration and uptake at Becker. There was no effect of K placement, but the tap‐rooted germplasm had lower K concentration in herbage than the parent. Although progeny did not differ in total root mass, the ratio of thickened‐to‐total root mass was larger below 30 cm for the tap‐rooted selection, which also produced more fine roots. Selected germplasms did not differ in P or K uptake. Selected root system architecture characteristics were expressed under, and apparently not modified by, growing conditions that covered three site‐years on two contrasting soils with differential P supply and K placement.

Transition between vegetative phases in maize: genetic effects and variances and associated markers

SUMMARYTransition from juvenile to adult vegetative phase in maize is associated with pest and disease resistance. However, reports are not consistent on the significance and relative importance of additive or dominance genetic effects and variances. The purpose of the present research was to elucidate the genetic effects and variances and to identify molecular markers associated with phase transition. Three cycles of divergent selection were carried out in a maize synthetic accelerating and delaying phase transition. Three and four inbred lines were released from the third cycles of late phase transition (LPT) and early phase transition (EPT), respectively. Generation mean analyses were performed from two LPT×EPT crosses in order to calculate genetic effects and variances. Markers associated with vegetative phase transition were identified by contrasting simple sequence repeat (SSR) alleles between LPT and EPT inbreds and selection cycles, and by testing whether drift could explain the allelic changes observed in the respective third cycles of selection. Juvenile traits are mainly regulated by additive genetic effects and variances, whereas adult traits have a complex regulation involving dominance and epistatic effects. Based on error variances, EPT improves phenotypic stability. The SSRsphi028,phi112andumc1725were associated with selection for phase transition, suggesting that these genome regions are involved in the regulation of vegetative phase transition of maize, although none of the genes previously associated with phase transition has been located in those regions.

Segregation Among Test-Cross Progeny Suggests That Two Complementary Dominant Genes Explain the Difference Between Ascites-Resistant and Ascites-Susceptible Broiler Lines

Ascites syndrome (AS) is a major cause of economic losses to the broiler industry. The tendency of broilers to develop AS was found to be heritable, suggesting that selective breeding could provide a solution to this problem. To further elucidate the genetic control of AS, AS-susceptible (AS-S) and AS-resistant (AS-R) lines were established by 3 cycles of divergent selection on pedigree data of AS mortality under AS-inducing conditions. The rapid divergence between the lines suggested the involvement of a major gene with dominance for AS resistance. It was hypothesized that the difference between the lines is controlled by a single dominant gene, denoted A, with AA (and some Aa) individuals in the AS-R line, and aa individuals in the AS-S line. The current study was designed to test this hypothesis by test-crossing heterozygous (Aa) sires from the AS-R line and F(1) from the AS-R x AS-S cross, with recessive homozygous (aa) dams from the AS-S line. A ratio of 1:1 was expected between progeny with AS vs. healthy progeny when reared under AS-inducing conditions. Test-cross progeny of 5 sires from the AS-R line segregated 1:1, indicating that these sires were heterozygous (Aa) in the suggested major gene and thus supporting the hypothesis of a single major gene with dominance of AS resistance. There was segregation among test-cross progeny of 8 F(1) sires, but with a 3:1 ratio of AS progeny to healthy progeny. The 3:1 ratio is expected if the F(1) sires are heterozygous (AaBb) with complementary interaction between the dominant alleles in 2 unlinked major genes. The segregation among test-cross progeny of the 9 heterozygous AS-R sires could also be explained by the same model. These results suggested that 2 major genes are responsible for the difference between the AS-R and AS-S lines. Resource populations derived from these lines will facilitate an efficient genomic search for these 2 genes. Once the alleles of these genes are identified and genotyping tests are developed, breeders will easily be able to select against AS susceptibility.

Effects of Selection for the Timing of Vegetative Phase Transition on Corn Borer (Lepidoptera: Noctuidae and Crambidae) Damage

In maize, Zea mays L., the timing of vegetative phase transition from juvenile to adult vegetative phases can be modified through selection. A reduction in the juvenile vegetative phase has been associated with resistance to diseases and pests. The major maize pest in temperate areas is Ostrinia nubilalis (Hübner) and in Europe Sesamia nonagrioides Lefebvre. The objective of our study was to determine the effects of divergent selection for the timing of vegetative phase transition in maize on resistance to corn borers. Three cycles of divergent selection for early and late phase transition in a field corn synthetic and in a sweet corn population were evaluated separately under S. nonagrioides and O. nubilalis artificial infestation. For the field corn experiment, yield and moisture improved with selection for phase transition in both directions, but improvement was due to artifacts of selection, rather than to the change in phase transition. There were no correlated responses for corn borer damage, yield, or grain moisture due to selection for the timing of vegetative phase transition. In the sweet corn experiment, selection for the timing of vegetative phase transition had no significant effects on corn borer damage in sweet corn harvested at the fresh stage. Our results do not support the use of phase transition as an indirect criterion for improving resistance to corn borers in maize. The relationship between phase transition and pest resistance reported by other studies could depend on the genotypes or could be too weak to be detected in a selection program with wild-type maize.

Agricultural Fitness of Smooth Bromegrass Populations Selected for Divergent Fiber Concentration

Selection for reduced neutral detergent fiber (NDF) concentration has been used as a mechanism of improving intake potential of perennial grasses by ruminant livestock. However, reduced NDF concentration is typically associated with reduced forage yield, although the reasons for this genetic correlation are unknown. The objectives of this study were to determine the relative contributions of pleiotropy, linkage, and drift to the genetic correlation of NDF concentration with four agricultural fitness traits: forage yield, survival, seed yield, and lodging. These traits were measured on four smooth bromegrass (Bromus inermis Leyss.) populations that had undergone one cycle of divergent selection for NDF concentration. Selection responses for forage yield were linear and homogeneous, suggesting pleiotropic effects. Growth and expansion of the cell wall appears to be essential for accumulation of forage yield, lending a certain allometry to these two traits. However, natural selection within swards appeared to regulate this response, because forage yield responses disappeared by the third production year. Selection responses for survival and lodging were linear, but nonhomogeneous, suggesting linkage. Selection responses for seed yield appeared to be regulated by all three phenomena, with drift (asymmetry) the most important. Seed yield is highly sensitive to inbreeding depression, which occurs as a result of drift. Short-term divergent selection experiments, analyzed by a factorial ANOVA model, provide a mechanism to identify genetic phenomena responsible for observed genetic correlations.

Selection methods for maize seedlings in greenhouse as related to aluminum tolerance

Genetic improvement is a useful approach to increase aluminum tolerance in maize. This experiment was carried out to compare two screening techniques under greenhouse conditions and estimate the association between the results obtained for both bioassays with grain yield. Nutrient solution (NS) and pots with acid soil (AS) were utilized as screening methodologies to perform one cycle of divergent selection for aluminum tolerance in the tropical maize population SIKALQ. The four sub-populations obtained by both approaches were compared with the original and checks in a greenhouse and in multilocation field trials. Evaluation under the conditions of NS and AS showed that the populations with the best performances were those selected for aluminum tolerance under the same condition of evaluation. The variables measured in greenhouse showed good correlation and the most closely related were fresh root weight (FRW) and dry rot weight (DRW) (0.79). All of these variables showed low correlation with yield in non acid conditions (< 0.30). The correlation was more important (~ 0.50) as soil aluminum saturation increased. The best correlation was observed for visual scoring (VS) (0.68), FRW (0.47) and net root growth (NRG) (0.52).

Rapid screening for aluminum tolerance in maize (Zea mays L.)

A significant decrease in maize grain yield due to aluminum toxicity is considered to be one of the most important agricultural problems for tropical regions. Genetic improvement is a useful approach to increase maize yield in acid soils, but this requires a rapid and reliable method to discriminate between genotypes. In our work we investigated the feasibility of using hematoxylin staining (HS) to detect Al-tolerant plants at the seedling stage. The original population along with two populations obtained after one cycle of divergent selection were evaluated by net root growth (NRG) and HS after 7 days in nutrient solution. Results showed a negative correlation between NRG and HS in all populations, in which sensitive plants, characterized by low NRG, exhibited more intense staining than tolerant plants. These results indicate that HS is a useful procedure for selecting Al-tolerant maize seedlings.

Response to divergent selection for ease of particle size reduction of dried leaves of smooth bromegrass (Bromus inermis Leyss) and correlated effects on nutritive value indicators and plant fitness

The rate at which forage can be broken down and cleared from the rumen is a major constraint on voluntary intake by ruminants. Responsiveness to selection has been demonstrated for physical characters possibly related to the ease of breakdown of forage grasses and is a prerequisite for the exploitation of these characters in a breeding program. The objective of this study was to evaluate at two locations the response to one cycle of divergent selection for the ease of breakdown, or particle size reduction index (PSRI), of dried leaf blades in four populations of smooth bromegrass (Bromus inermis Leyss.) grown as spaced plants and the correlated response in neutral detergent fibre (NDF), in vitro dry matter digestibility (IVDMD) and on seedling performance and second year spread as indicators of plant fitness. Divergent selection for PSRI resulted in significant differences between high and low groups in all populations. The contrast for asymmetry of response was not significant, but response tended to be higher in the downwards direction relative to an unselected group. Realised heritabilities ranged from 10–21% when the unselected group was used to represent the parental generation. In the most responsive population, WB19e, seedling growth was poorer at both sites and seedling mortality higher at one site in plants selected for increased PSRI. Selection for PSRI had no effect on IVDMD but there were closely correlated changes in NDF (r = –0.73 across populations, p<0.01). However, comparisons between populations suggested that differences in PSRI independent of NDF also existed. PSRI can be exploited in a breeding program if gains in forage intake by ruminants, independent of changes in NDF, are demonstrated.

Productivity under contrasting cutting regimes of perennial ryegrass selected for short and long leaves

The productivity of short-leaved and long-leaved perennial ryegrass was evaluated in 5 different sites. In each site, half-sib families, derived from the first cycle of divergent selection on leaf length, were managed both under infrequent cutting and under frequent cutting simulating grazing. Despite the importance of the effects of the site and the cutting frequency, the dry matter yield was dependent on the interaction between the morphogenetic type resulting from the selection and the cutting frequency imposed by the management. Thus, under infrequent cutting, the long-leaved families on average out-yielded the short-leaved families. Conversely, under frequent cutting the short-leaved families were on average the most productive. The use of morphogenetic traits as selection criteria in breeding for management-dependent yield is briefly discussed.

Genetic variability for leaf development in perennial ryegrass populations

In perennial ryegrass (Lolium perenne, L.), leaf length is a highly heritable trait likely to respond to both natural and artificial selection. The genetic variability of components of leaf morphogenesis of perennial ryegrass was studied in 2 populations obtained from two cycles of divergent selection for lamina length, in 42 natural populations, and in forage and turf cultivars. The genetic progress in the two branches of the divergent selection was accompanied by a significant change in the leaf elongation rate, whereas the phyllochron and the leaf elongation duration remained relatively unchanged. The forage cultivars were not morphogenetically different from the natural populations, and hence progress for yield under infrequent cutting could be made by selection for long-leaf populations. Turf cultivars, however, were morphogenetically different from natural populations. Turf cultivars had small leaves with short sheaths and laminae. The population resulting from the selection for short leaf had laminae as long as those of turf cultivars but with longer sheaths.

In Vitro and in Vivo Analyses of Hays of Switchgrass Strains Selected for High and Low in Vitro Dry Matter Digestibility1

In a previous report the progress made in altering the in vitro dry matter digestibility (IVDMD) of switchgrass (Panicum virgatum L.) in one cycle of divergent selection was described. In this study, the detergent system of analyses was used in an attempt to determine what plant constituents had been altered by the selection for IVDMD. Hays from five switchgrass strains differing in IVDMD harvested over a 3‐year period were analyzed for protein content, neutral detergent fiber (NDF), acid detergent fiber (ADF), ash, and lignin. Although there were differences (p ≤ 0.05) among strains for IVDMD as determined by two different procedures (with and without additional N), there were no differences among strains for ADF, ash, lignin, and protein. The strains ranked the same with the two IVDMD procedures but the IVDMD‐N (with additional N) test averaged 6.7 percentage points higher in digestibility. There were small differences (p ≤ 0.1) among strains for NDF in 2 of the 3 years. The strain with the highest IVDMD had the lowest NDF, suggesting that, in part, the improved IVDMD was due to a decrease in the cell wall component of the harvested forage. Hays of three of these strains, high‐IVDMD PC, low‐IVDMD PC, and ‘Pathfinder’ from the 1980 harvest were ground and fed to sheep to compare in vivo and in vitro digestibilities. After grinding, the hays had IVDMD‐N values of 49.9,45.5, and 46.7%, respectively. There were no differences among strains in the sheep feeding trial for in vivo digestibility, dry matter intake, and fiber digestibility. The in vivo digestibilities for these strains were: high‐ IVDMD PC 50.4%, low‐IVDMD PC 49.0%, and Pathfinder 50.8%. In contrast to a previous report in the literature, the IVDMD‐N values were generally similar to in vivo digestibilities for switchgrass hays. Parameters of the conventional detergent system of analyses, while useful in identifying factors responsible for changes in digestibility of switchgrass hays, would probably be of little value as selection criteria in a breeding program. Because of the large number of animals required to obtain statistical significance in in vivo digestibility studies, the IVDMD procedure would be preferable in studies in which breeding material is being evaluated and small differences in digestibility are expected.

Selection for Aluminum Tolerance in Soybeans Based on Seedling‐Root Growth1

AbstractLaboratory techniques for screening soybean (Glycine max (L.) Merrill) genotypes for tolerance to Al would facilitate breeding programs for areas where AI toxicity limits soybean production. Genotypes from a broad‐base soybean population were screened For Al tolerance based on root growth rate of 5‐day‐old seedlings grown in solutions containing 0.4 ppm Al and 10 ppm Ca (pH 4.6). The selections were intermated to produce a second cycle. Three cycles of divergent selection were completed. The changes associated with selection were evaluated in laboratory studies and in greenhouse studies using Lynchburg sandy loam soil (Aeric Paleaquult).Tolerance to Al as measured by root growth rates proved to be heritable (0.67) with responses in each cycle of selection; however selections completely tolerant to 0.4 ppm Al in solution were not obtained. When high quality seed was used, genetic differences for tolerance were not affected by the environment used for seed production. Selection based on seedlings from individual plants was feasible. Genotypes selected for Al tolerance had less reduction in growth in soils with high exchangeable Al saturation than non‐tolerant selections; however, the differences were not as distinct as that found for the seedling root‐growth studies. Divergent selections had similar levels of metabolic intermediates in the root tips when seedlings were grown in 0 ppm Al, but pyruvate and ATP levels increased significantly under Al stress for the tolerant but not the non‐tolerant selections. Selection for Al tolerance based on growth of 5‐day‐old soybean seedlings in 0.4 ppm Al apparently identified genetic differences for tolerance in established plants as well as tolerance which appeared unique to the seedling stage.