Synthetic Maize Population Research Articles

Divergent Mass Selection for Leaf Chlorophyll Content Measured Using Chlorophyll Meter Readings in a Maize Composite Population

ABSTRACTLeaf chlorophyll content is correlated with photosynthetic activity indicators and leaf nitrogen concentration. Three cycles of divergent mass selection for chlorophyll content were performed to estimate heritability, response to selection, and correlated changes in grain yield for these traits in a synthetic maize (Zea mays L.) population. Selection for high and low chlorophyll content was performed in the field using the SPAD chlorophyll meter, and response to selection was measured in field evaluations of the C0, C1, C2, and C3 populations. The divergent selection responses cycle−1 were 1.2 SPAD unit increase versus 0.2 decrease at anthesis and 0.9 increase versus 0.9 decrease at grain‐filling period relative to the base average of 55.9 and 53.5, respectively. The C3 high and low selections showed significant differences in chlorophyll content at anthesis and grain filling stages, but no significant selection indirect response was detected for grain yield. Thus, our data indicate that leaf chlorophyll content is heritable and that it can be manipulated through conventional breeding methodology. Additional cycles of chlorophyll content selection are needed to determine if chlorophyll content will be useful as an indirect selection criterion for yield improvement.

Analysis of Per Se and Genetic Diversity of Maize Synthetic Populations for Grain Quality: Implications for Breeding

The objective of this research was to determine whether phenotypic and genetic diversity could be identified for maize grain quality traits (percentage of starch, protein, oil, fiber and ash) and agronomic traits. 59 maize synthetic populations which differed in earliness and for geographic origins were evaluated in the field at the Agricultural Research Station Turda - Romania, for their per se value. Each synthetic population was tested only one year and was characterized for their ear characteristics and grain quality attributes. The grain oil and ash content showed high variability among genotypes. 
 The experiment was conducted over 2 years (2006-2007) and 2 locations (Turda and Targu-Mures). Analysis of variance showed significant differences among the genotypes for all traits studied. General combining ability (GCA) effects were more important for grain content than specific combining ability (SCA). The occurrence of low interactions between synthetic populations and testers for all characters suggested the occurrence of high statistical additive effects expressed whatever the system of population evaluation. These results support breeding effort towards the genetic improvement of grain quality traits in “Turda” maize germplasm.

Selection for Forage Yield and Composition on the Wisconsin Quality Synthetic Maize Population

ABSTRACTWisconsin Quality Synthetic (WQS) is a maize population that has undergone four cycles of S2–topcross selection for increased silage yield and feed quality. This study evaluated forage yield and quality for each cycle of WQS per se as well as testcrosses to two Stiff Stalk‐type testers. Linear improvement was seen in whole‐plant yield, stover yield, and whole‐plant quality both in the population per se and in testcrosses. Stover quality has not improved through selection. Starch content has increased while crude protein has decreased. Milk yields on a Mg dry matter and hectare basis have increased with selection. Changes in silage dry matter yield have been greater on a percent basis than changes in silage quality, suggesting that the current selection protocol of selecting S2–topcrosses first for yield then for quality may be more efficient at improving yield than quality. Selection directly focused on stover quality may be necessary if more rapid improvement in stover composition is desired.

Effects of selection for resistance to Sesamia nonagrioides on maize yield, performance and stability under infestation with Sesamia nonagrioides and Ostrinia nubilalis in Spain

A maize synthetic population was improved for resistance to the Mediterranean corn borer (MCB, Sesamia nonagrioides) while maintaining yield. The objectives of this research were to investigate whether yield and yield stability of the maize synthetic population named EPS12 were affected by selection for MCB resistance; also to determine which genotypic and environmental covariates could explain the genotype (G), environment (E) and genotype × environment (GE) effects for yield under corn borer infestation. Plants from three cycles of selection and their testcrosses to three inbred testers (A639, B93 and EP42) were evaluated at two locations in 2 years, under MCB and European corn borer infestations. After selection EPS12 was a more stable genotype. Hybrids derived from crosses between B93 and inbreds obtained from the initial cycles of selection could be recommended for cultivation in northern Spain. The yield of crosses between cycles of selection and testers increased when there were fewer days with temperatures >25°C and higher mean maximum temperatures. Differences in yield among these genotypes were mostly explained by resistance to corn borer attack. In general, among EPS12‐derived materials, genetic characteristics that contribute to increased grain yield were also responsible for increased abiotic stress tolerance.

Effects of Improvement by Mass Selection on the Different Maize Synthetic Populations

On the basis of statistical analysis of field test and genetic parameters evaluation by Simple Sequence Repeat (SSR) markers,the experiment was conducted with three maize synthetic populations GP3,GP4,and GP5 with mass selection for 4 cycles.The results showed that there existed genetic differences in most traits for the different populations and cycles,and the significant interaction effects between populations with cycles.The genetic gain was GP3GP5GP4 in kernel depth and GP3 GP5GP4 in rows per ear.After 4 cycles of mass selection,there was an obvious increase in the mean kernel depth and rows per ear,which displayed significant direct responses.Plant height and ear height exhibited an increase with selection,and the increase in GP4 was significantly higher than that in GP3 and GP5.The genetic gain of GP4 in yield per plant and ear diameter was higher than that of GP3 and GP5.Compared the cycle C4 with C0,most yield traits were improved for all the populations,which implied a significant correlated response.Excluding plant height and ear height,the differences in GCA were significant for the other traits.The GCA effects of GP5 in most yield traits were higher than those of GP3 and GP4.The population improvement by mass selection resulted in an increase in the GCA effects of most yield traits.Most test-cross combinations,with more than 8% of yield per plant over the check,were derived from the cycle C4 of all the populations,which displayed significant indirect responses to mass selection.For the coefficients of variation in phenotypic traits,there were no obvious changes.For the genetic similarity coefficients of intrapopulation,there were a gradual increase in the derivatives of GP3 and GP4 and no changes in those of GP5.The genotype number in the derivatives of GP3 and GP4 showed a decreasing tendency while a little increases in those of GP5.The results above indicated that populations might be improved effectively and retained a high genetic diversity by mass selection.However,there existed different effects of genetic improvement on different populations and traits.

Effects of Mass Selection on Maize Synthetic Populations

For evaluating the effects on improvement of different maize ( Zea mays L.) synthetic populations, genetic parameters derived from simple sequence repeat (SSR) markers were statistically analyzed with maize synthetic populations GP3, GP4, and GP5 after 4 cycles of mass selection. Genetic differences were observed in most traits across populations or cycles, and the interaction between population and cycle was significant. The genetic gains ranked as GP3 < GP5 < GP4 for kernel depth and as GP3 > GP5 > GP4 for row number per ear. Means of kernel depth and row number per ear were obviously increased after 4 cycles of mass selection, showing significantly direct responses to selection. Plant height and ear height were enhanced by selection, and the increment in GP4 was significantly higher than that in GP3 and GP5. The genetic gains of yield per plant and ear diameter were higher in GP4 than in GP3 and GP5. In all populations, most yield traits were improved in cycle C4 compared to those in cycle C0, showing a significant correlated response. Except for plant height and ear height, the other traits showed significant variations in GCA, and the GCA effects of GP5 in most yield traits were higher than those of GP3 and GP4. In most yield traits, the GCA effects tended to increase with the progress of population improvement through mass selection. Testcross combinations with yield per plant more than 8% over the controls were derived from the cycle C4, showing a significantly indirect response to mass selection. The variation coefficients of phenotypic traits varied slightly across selection cycles. The genetic similarity based on SSR data showed slight increases in GP3 and GP4, but relative stability in GP5. With the increase of selection cycle, the genotype number in derivatives had small reductions in GP3 and GP4, but an increase tendency in GP5. These results indicate that mass selection is not only effective for population improvement but also maintained the genetic diversity of the population. The effect of genetic improvement varied across populations of traits.

Effects of selection for maize resistance to Sesamia nonagrioides on the additive and dominant components of genetic variance

Abstract Sesamia nonagrioides causes high yield losses in maize in northwestern Spain. Therefore, a recurrent selection programme to improve resistance against this pest has been carried out in a maize synthetic population. The main purposes of this investigation were to determine which are the main components of genetic variance for stem tunnelling and yield under infestation, to determine the variance remaining after selection, and, finally, to establish the genetic relationship between traits used as selection criteria and other ear resistance and agronomic traits. A North Carolina Design I mating system was used. Individual analyses by year were carried out and additive variance was identified as the only significant component of the genetic variance for stem tunnel length while a dominant variance was significant for yield. Those variances have decreased due to selection, but not significantly. Selection could continue for more cycles as there is enough variance. Additive genetic correlation coefficients between tunnel length and yield could not be determined. Thereafter, we propose three different methodologies of the selection to determine the relationship between resistance and yield under infestation.

Genetic Diversity of Maize Populations Developed by Two Kinds of Recurrent Selection Methods Investigated with SSR Markers

Two cycles of biparental mass selection (MS) and one cycle of half-sib-S3 family combining selection (HS-S3) for yield were carried out in 2 synthetic maize populations P 4C 0 and P 5C 0 synchronously. The genetic diversity of 8 maize populations, including both the basic populations and their developed populations, were evaluated by 30 SSR primers. On the 30 SSR loci, a total of 184 alleles had been detected in these populations. At each locus, the number of alleles varied from 2 to 14, with an average of 6.13. The number and ratio of polymorphic loci in both the basic populations were higher than those of their developed populations, respectively. There was nearly no difference after MS but decreased after HS-S3 in both the basic populations in the mean gene heterozygosity. The mean genetic distance changed slightly after MS but decreased in a bigger degree after HS-S3 in both the basic populations. Analyses on the distribution of genetic distances showed that the ranges of the genetic distance were wider after MS and most of the genetic distances in populations developed by HS-S3 were smaller than those in both the basic populations. The number of genotypes increased after MS but decreased after HS-S3 in both the basic populations. The genetic diversity of intra-population was much more than genetic diversity of inter-population in both the basic populations. All these indexes demonstrated that the genetic diversity of populations after MS was similar to their basic populations, and the genetic diversity was maintained during MS, whereas the genetic diversity of populations decreased after HS-S3. This result indicated that heterogeneity between some of the individuals in the developed populations increased after MS, whereas the populations become more homozygotic after HS-S3.

Changes in Phenolic Concentrations during Recurrent Selection for Resistance to the Mediterranean Corn Borer (Sesamia nonagrioides Lef.)

Recurrent selection has been reported as successful for improving maize resistance against corn borers. This study was conducted to determine if phenolics concentration in maize changes during recurrent selection to improve stalk resistance to the Mediterranean corn borer. Three cycles of selection [EPS12(S)C0, ESP12(S)C2, and EPS12(S)C3] from the maize synthetic population EPS12 and test crosses to inbred lines A639, B93, and EP42 were field grown and artificially infested with Mediterranean corn borer larvae, and the pith tissues were sampled for biochemical analyses. Two major simple phenolic acids [p-coumaric (p-CA) and trans-ferulic (E-FA) acids] were identified in free and cell-wall fractions, whereas four isomers of diferulic acid (DFA) (8-5'l, 5-5', 8-o-4', and 8-5' benzofuran form) were present in the cell-wall bound fraction. The selection cycles EPS12(S)C0 and EPS12(S)C3 showed less damage and higher cell wall phenolics concentrations than the cycle EPS12(S)C2. In addition, higher concentrations of total DFAs were associated with shorter tunnel length and lower numbers of larvae per stem. The current study shows new and concrete evidence that the cell-wall bound phenolics could have a determinative role in the resistance to the Mediterranean corn borer, although future development of recurrent and divergent selection cycles will clarify this point.

Short communication. Divergent mass selection for different flowering times in a Spanish synthetic maize population

Eight cycles of divergent mass selection for flowering time were performed in the synthetic maize (Zea mays L.) population Lazcano (EZS9) from Guipuzcoa, Spain. A trial with a randomised block design plus three replicates was then performed at two locations in northeastern Spain, and over two years, to compare flowering time in the original population and the material produced in the eight selection cycles. Linear regression coefficients were determined to estimate the average rate of selection response per cycle. Genetic gains with respect to the vegetative cycle and other major agronomic traits were obtained. Direct selection responses were significant and grain yield maintained. No negative correlated responses were obtained for any agronomic trait. Divergent mass selection for early flowering was therefore found to be efficient in this synthetic population.

Field screening maize germplasm for resistance and tolerance to western corn rootworms (Col.: Chrysomelidae)†

Abstract: In the hopes of lessening the current reliance on soil insecticides, developing a viable alternative for transgenic maize hybrids, and providing sustainable options for Europe, researchers recently have been developing novel maize lines that exhibit resistance and/or tolerance to corn rootworm larvae. Here we report the results of a 2‐year field experiment in a northern growing region assessing the resistance and tolerance of 10 experimental synthetic maize populations selected for varying levels of damage from western corn rootworm larvae, Diabrotica virgifera virgifera LeConte (Col.: Chrysomelidae) and four maize hybrids. Maize non‐preference, antibiosis and tolerance to rootworms was evaluated using previously established methods, including: the Iowa 1–6 root damage rating scale, root fresh weight, compensatory root growth ratings and adult rootworm emergence. Among the experimental synthetic maize populations, BS29‐11‐01 was the most susceptible, and had a mean root damage rating that was greater than the highly susceptible maize hybrid B37 × H84. This line also had the lowest mean root fresh weight and one of the lowest mean compensatory root growth ratings. In contrast, CRW8‐3 appeared to be tolerant to western corn rootworms, and had the lowest mean root damage rating, which was comparable with that of the non‐transgenic hybrid DeKalb® 46‐26.

Six cycles of S1 recurrent selection in two Spanish maize synthetics

European flint x US dent is the most common heterotic pattern used in early maize (Zea mays L.) breeding programs in Spain. Two synthetic maize populations, EZS1 (formed from four Spanish flint populations), and EZS2 (formed from four American dent populations), were developed in Zaragoza, Spain. These synthetics, which showed heterotic behaviour, were subjected to six cycles of S1 recurrent selection for increased grain yield and reduced lodging. The aim of this study was to investigate the effectiveness of selection on these and other agronomic traits in both synthetics. The original populations, EZS1C0 and EZS2C0, and the six cycles of selection of both, were evaluated at two locations over two years. Over the six selection cycles, the average yield of EZS1 and EZS2 increased by 0.82 Mg/ha and 0.93 Mg/ha per cycle respectively, while lodging decreased by 2.76% and 2.44% per cycle respectively. Changes due to selection were significant for all test traits in both synthetics. It was concluded that S1 selection is effective in improving the yield of, and reducing lodging in, both synthetics.

Originality of M3S maize population and changes in allele frequencies revealed by SSR markers after two cycles of selfed progeny recurrent selection

Maksimir 3 Synthetic (M3S) maize population was developed at the Faculty of Agriculture University of Zagreb by intercrossing inbred lines, whose origins trace back to several open-pollinated varieties and local populations from different regions of the former Yugoslavia. The population was subjected to two cycles of selfed progeny recurrent selection for grain yield. The objectives of this study were: (i) to determine genetic distances among the parental inbred lines of the M3S population (M3S progenitors), the M3S population before and after two cycles of recurrent selection, and elite inbred lines representing the BSSS and Lancaster heterotic group; and (ii) to examine the effect of two cycles of recurrent selection on allele frequency changes in the population. Nine M3S progenitors, three BSSS lines, and three Lancaster lines were genotyped at 24 SSR loci, out of which nine randomly chosen loci were used for genotyping 96 individuals from both C0 (the M3S population before selection) and from C2 (M3S population after two cycles of selection). A total of 101 alleles were detected across 24 loci in the 15 lines, whereas 83 alleles were found in the nine M3S progenitors. Among the latter 83 alleles 31 were unique, i.e. found only in one of the progenitors. Mean genetic distance among nine M3S progenitors was 0.61 indicating a broad genetic base of the M3S population. High mean genetic distance was found between M3S progenitors and BSSS lines (0.69) and M3S progenitors and Lancaster lines (0.71). This indicates that the M3S population represents a germplasm source unrelated to both the BSSS and Lancaster germplasm. Mean genetic distance between the M3S population and BSSS as well as Lancaster lines decreased slightly after two cycles of recurrent selection suggesting the need to introduce testers from both groups in future selection in the M3S population in order to maintain heterotic complementarity of the M3S population to these groups. A test of selective neutrality identified several non-neutral loci in the population whose allele frequency changes from the C0 to the C2 cannot be explained by genetic drift. The majority of non-neutral alleles, whose frequency increased after two cycles of selection, were present in at least one line from the BSSS or Lancaster heterotic group.

Genetic analysis of stay-green trait in broad-based maize populations

One of the major challenges for maize breeders is to develop hybrids that have an advantage in water-limited environments. A considerable number of grain production studies in maize include correlations between agronomic and physiological characteristics and grain yield. The main objective of this study was to identify traits that may relate to stay-green and grain yield in two maize synthetic populations. S 1 progenies were evaluated in an incomplete block design in four environments. Correlations and path coefficient analysis showed that the leaf water content had the greatest impact on stay-green. At the same time, stalk water content had a less direct effect on stay-green than it could be expected from its correlations. The highest positive direct effect on grain yield was determined for stay-green in both populations (P 2.1 =0.717** and P 2.1 =0.649**, respectively). Breeding for stay-green seems to be feasible in many applied breeding programs, especially due to its simplicity to score on a large nu...

Identification of chromosome regions determining kernel high oil content in maize (Zea mays L.) Synthetic populations

Chromosome regions determining kernel high oil content were identified by RFLP analysis of individual plants from C0 and C9 selection cycles of two maize synthetic populations -DS7u and YuSSSu. Identification of chromosome regions was done with informative RFLP markers (that were identified with BSA earlier) using a single plant approach - analysis of individual plants. This analysis revealed the number of plants carrying alleles that endured frequency alterations during selection process. Statistical analysis (?2 test) revealed chromosome regions that comprise putative QTLs affecting expression of kernel high oil content of analyzed maize populations. Four regions on chromosomes 1, 6, 7 and 8 were identified in both DS7u and YuSSSu populations. Additional four regions on chromosome 4, 9 and 10 were detected only in DS7u population.

Genetic analysis of drought adaptive traits in maize synthetic populations

Maize breeders consider tolerance to drought as an important criterion in commercial breeding programs. Two traits, stay green and anthesis-silking interval seem to be closely associated with drought tolerance. The main objective of this study was to obtain estimates of means, variability, heritability and estimates of genetic correlations for several traits in two maize synthetic populations. S1 progenies were evaluated in an incomplete block design in four environments. Large genetic variability existed for all traits in both populations but highest variability was found for anthesis-silking interval. Strong, highly significant correlations between drought adaptive traits and grain moisture may cause some undesirable correlative response throughout cycles of selection.

Application of molecular markers in bulk segragant analysis of yield in maize (Zea mays L) synthetic populations

Chromosome regions which carry potential QTLs for high grain yield in two synthetic maize populations - B73xMol7 and LlxMol7, were identified by bulk segregant analysis (BSA). Yield was evaluated on F2 testcross families in field trials using a Nested design. Based on yield data, p3 families with the corresponding highest and lowest testcross yields were selected for BSA. Genome analysis of F3 families was carried out with 58 RFLP markers. Allele frequency differences were detected at four RFLP loci n chromosomes 1, 2, 6 and 10 (B73xMol7), i.e. four RFLP loci on chromosomes 1, 2, 6 and 9 (LlxMo17). Only one region, at chromosome 6, was identified in both populations, but with two different RFLP markers. In B73xMol7 it was umc65 and in LlxMol7 umc2l RFLP marker. Bulk segregant analysis was shown to be a quick and informative method for identification of chromosome regions which determine high yield expression in maize, i.e. for identification of RFLP markers closely linked to potential genes involved in expression of the trait.

Population Genetic Diversity in a Maize Reciprocal Recurrent Selection Program

The genetic structures of the Iowa Corn Borer Synthetic #1 (CB) and Iowa Stiff Stalk Synthetic (SS) maize (Zea mays L.) populations are important because these populations serve as the model for development of modern commercial hybrids. In 1949, CB and SS were used to start a reciprocal recurrent selection (RRS) breeding program at Iowa State University. This study was conducted to analyze more thoroughly the genetic diversity within this RRS program and illustrate how the RRS program has changed over time at the molecular level. The progress of this program was measured by analyzing the variation at 86 SSR loci among 28 progenitor lines and 30 plants sampled from each of seven cycles (Cycle 0, Cycle 1, Cycle 3, Cycle 6, Cycle 9, Cycle 12, and Cycle 15) in each population. The progenitors of these populations show a high amount of variation on the basis of expected heterozygosity (0.557). As the RRS program proceeded, this variation decreased (Cycle 15, 0.245). In total, a larger amount of genetic variation was found among plants within cycles (66%) than among cycles (13%) or between populations (21%). The repartitioning of variation from within populations (96% in progenitors) to between populations (58% in Cycle 15) over time is consistent with theoretical expectations of divergence between the populations. By sampling intermediate time points, we gained a comprehensive genetic view of CB and SS permitting evaluation of the molecular‐level changes occurring as a result of reciprocal recurrent selection.

Use of Isozymes and RFLP Markers to Assess Genetic Diversity in Introgression Procedures Between “Adapted” and “Non Adapted” Maize Populations (Zea Mays L.)*

Genetic diversity and its organization were investigated in a diallel cross involving six six-line maize synthetic populations, consisting of three Corn Belt dents and three exotics (a Cateto flint, and an adapted version each of Mexican dent and Caribbean flint/dent). Sixteen isozyme loci were analysed on the 6 parental synthetic populations and the 15 F1 crosses. RFLP analysis was also conducted using eighteen genomic DNA probes on material from Cornbelt1, Cateto synthetics and their F1 cross progenies. Genetic diversity revealed by isozymes and RFLPs was high for Corn Belt synthetics, whereas the lines involved in the exotic synthetics did not reflect the high level of polymorphism described by other authors. Some Cateto inbreds (from which the Cateto synthetic population was derived) presented a residual polymorphism for one to four isozyme loci. Cateto and Cornbelt2 synthetics showed the lowest genetic variability. Confrontation between “adapted” and “non adapted” genomes revealed segregation distortion for six isozyme loci, mainly for crosses between exotic and Corn belt on one hand and between exotic populations on the other hand. From RFLP analysis of F1 cross (Cornbelt1 × Cateto) no segregation distortion was evidenced. This study supports discussion on prebreeding strategies needed for genetic resources enhancement.

Quantitative Genetics of Inbreeding in a Synthetic Maize Population

The average effects of inbreeding depression have been measured extensively in maize (Zea mays L.), but the influence of inbreeding on genetic variance has not been well studied. Two hundred random S1, S2, S3, and S4 lines were developed from the BS13(S)C0 population by single-seed descent and a set of 200 related half-sib families were developed from the S1 lines. The lines and half-sib families were evaluated in replicated yield trials for six agronomic characters. Under a purely additive model, the expected variance among inbred individuals is exactly twice the variance of noninbred individuals. The observed variance of inbred individuals in our study was 1.18 times the variance of noninbred individuals or less for five of six traits studied. By contrast, variance of dominance deviations of inbred individuals ranged from 1.6 to 3.3 times the variance of dominance deviations of noninbred individuals for five of six traits studied. A negative covariance between dominance deviations and breeding values in inbred individuals was found for all six traits. An estimator of the degree of dominance for arbitrary allele frequencies was developed. The estimated average degree of dominance in BS13(S)C0 ranged from 1.28 to 2.76, corresponding to overdominance or pseudo-overdominance. Our results suggested that some regions of linked genes have large effects on inbreeding depression in this population.