Type Of Gene Action Research Articles

Genetic Analysis of Inbred and Hybrid Grain Yield under Stress and Nonstress Environments in Tropical Maize

Drought and low soil N cause significant yield reductions in maize (Zea mays L.) grown in the tropics. Understanding the genetic basis of hybrid performance under these stresses is crucial to designing appropriate breeding strategies. This study evaluates under optimal, drought and low N stress conditions (i) the performance, combining abilities and stability of a group of tropical white inbred lines; (ii) the genetic control and modes of gene action for grain yield; and (iii) the relationship between line per se and hybrid performance. Seventeen lowland white‐grained tropical maize inbred lines were used in a diallel study. Lines and their hybrids were evaluated separately in trials under drought stress, low N, and optimal conditions in a total of 12 environments. The differences in grain yield between hybrids and inbreds (i.e., heterosis) increased with the intensity of drought stress. Significant interactions were observed for combining abilities under low and high N. The type of gene action appeared to be different under drought than under low N, with additive effects more important under drought and dominance effects more important under low N. The importance of additive effects increased with intensity of drought stress. This suggests the need for drought tolerance in both parental lines to achieve acceptable hybrid performance under severe drought. Inbreds derived from the population ‘La Posta Sequía’ exhibited the highest GCA effects, stability coefficients, and frequency of dominant alleles for grain yield. Good performance across stress levels can be achieved in tropical maize hybrids.

A seven-parental diallel analysis of nutritional composition of common beans

Genetic information on combining ability and the type of gene action that governs the inheritance of economically important quantitative characters can be an immense help to the breeder. Such information can assists the plant breeders in selection of suitable parents and the appropriate breeding procedure. Therefore, a 7 × 7 diallel analysis of combining ability of three nutritional components in beans (Phaseolus vulgaris L.) were conducted. The aim of the study was to obtain information on the inheritance of traits and to identify best parents for hybridization. This study showed that estimates of both the combining ability (general and specific), and reciprocal variances were significant for both protein and tannin content, whereas only reciprocal variance was significant for phytate. The protein and tannin ratios of the general to specific combining ability variances were 1.0 and 2.0, respectively. These values indicated that both additive and non-additive genetic variances are important in the inheritance of protein. However, the magnitude of general combining ability of tannin was higher than the specific combining ability variance, indicating the predominant role of additive genetic variance. The performance of the parents for percent protein content was highly associated with their general combining ability effects. The three parents, BBL 254 (P7), PI 300657 (P4), and BBL 290 (P3) were good general combiners for protein and, thus, could be utilized for breeding high protein genotypes. The best combiners for tannins were BBL 254, BBL 290, N80097 (P5), and Eagle (P1). The best specific combinations for protein were PI 300657 × PI 304833 (P4 × P6), Eagle × PI 304833 (P1 × P6), Eagle × N80097 (P1 × P5), Pinto 111 × PI 300657 P2 × P4), and Eagle × BBL 290 (P1 × P3). Lower tannin content was observed in white-seeded genotypes compared to black or pinto beans. Reciprocal crosses were also significant and desirable for protein, tannin, and phytate values. Therefore, it is suggested that caution should be exercised when selecting male and female parents in hybrid production. Recurrent selection and reciprocal recurrent selection could be suitable breeding procedures for rapid improvement.

The effect of epistasis on the excess of the additive and nonadditive variances after population bottlenecks.

The effect of population bottlenecks on the components of the genetic variance generated by two neutral independent epistatic loci has been studied theoretically (VA, additive; VD, dominant; VAA, additive x additive; VAD, additive x dominant; VDD; dominant x dominant components of variance). Nonoverdominance and overdominance models were considered, covering all possible types of marginal gene action at the single locus level. The variance components in an infinitely large panmictic population (ancestral components) were compared with their expected values at equilibrium, after t consecutive bottlenecks of equal size N (derived components). Formulae were obtained in terms of allele frequencies and effects at each locus and the corresponding epistatic value. An excess of VA after bottlenecks can be assigned to two sources: (1) the spatiotemporal changes in the marginal average effects of gene substitution alpha(i), which are equal to zero only for additive gene action within and between loci; and (2) the covariance between alpha2(i) and the heterozygosity at the loci involved, which is generated by dominance, with or without epistasis. Numerical examples were analyzed, indicating that an increase in VA after bottlenecks will only occur if its ancestral value is minimal or very small. For the nonoverdominance model with weak reinforcing epistasis, that increase has been detected only for extreme frequencies of the negative allele at one or both loci. With strong epistasis, however, this result can be extended to a broad range of intermediate frequencies. With no epistasis, the same qualitative results were found, indicating that dominance can be considered as the primary cause of an increase in VA following bottlenecks. In parallel, the derived total nonadditive variance exceeded its ancestral value (V(NA) = V(D) + V(AA) + V(AD) + V(DD)) for a range of combinations of allele frequencies covering those for an excess of VA and for very large frequencies of the negative allele at both loci. For the overdominance model, an increase in V(A) and V(NA) was respectively observed for equilibrium (intermediate) frequencies at one or both loci or for extreme frequencies at both loci. For all models, the magnitude of the change of V(A) and V(NA) was inversely related to N and t. At low levels of inbreeding, the between-line variance was not affected by the type of gene action. For the models considered, the results indicate that it is unlikely that the rate of evolution may be accelerated after population bottlenecks, in spite of occasional increments of the derived V(A) over its ancestral value.

THE EFFECT OF EPISTASIS ON THE EXCESS OF THE ADDITIVE AND NONADDITIVE VARIANCES AFTER POPULATION BOTTLENECKS

The effect of population bottlenecks on the components of the genetic variance generated by two neutral independent epistatic loci has been studied theoretically (VA, additive; VD, dominant; VAA, additive × additive; VAD, additive × dominant; VDD; dominant × dominant components of variance). Nonoverdominance and overdominance models were considered, covering all possible types of marginal gene action at the single locus level. The variance components in an infinitely large panmictic population (ancestral components) were compared with their expected values at equilibrium, after t consecutive bottlenecks of equal size N (derived components). Formulae were obtained in terms of allele frequencies and effects at each locus and the corresponding epistatic value. An excess of VA after bottlenecks can be assigned to two sources: (1) the spatiotemporal changes in the marginal average effects of gene substitution αi, which are equal to zero only for additive gene action within and between loci; and (2) the covariance between αi2 and the heterozygosity at the loci involved, which is generated by dominance, with or without epistasis. Numerical examples were analyzed, indicating that an increase in VA after bottlenecks will only occur if its ancestral value is minimal or very small. For the nonoverdominance model with weak reinforcing epistasis, that increase has been detected only for extreme frequencies of the negative allele at one or both loci. With strong epistasis, however, this result can be extended to a broad range of intermediate frequencies. With no epistasis, the same qualitative results were found, indicating that dominance can be considered as the primary cause of an increase in VA following bottlenecks. In parallel, the derived total nonadditive variance exceeded its ancestral value (VNA = VD VAA VAD VDD) for a range of combinations of allele frequencies covering those for an excess of VA and for very large frequencies of the negative allele at both loci. For the overdominance model, an increase in VA and VNA was respectively observed for equilibrium (intermediate) frequencies at one or both loci or for extreme frequencies at both loci. For all models, the magnitude of the change of VA and VNA was inversely related to N and t. At low levels of inbreeding, the between-line variance was not affected by the type of gene action. For the models considered, the results indicate that it is unlikely that the rate of evolution may be accelerated after population bottlenecks, in spite of occasional increments of the derived VA over its ancestral value.Corresponding Editor: S. Tonsor

Quantitative trait loci (QTLs) mapping for growth traits in the mouse: a review.

The attainment of a specific mature body size is one of the most fundamental differences among species of mammals. Moreover, body size seems to be the central factor underlying differences in traits such as growth rate, energy metabolism and body composition. An important proportion of this variability is of genetic origin. The goal of the genetic analysis of animal growth is to understand its "genetic architecture", that is the number and position of loci affecting the trait, the magnitude of their effects, allele frequencies and types of gene action. In this review, the different strategies developed to identify and characterize genes involved in the regulation of growth in the mouse are described, with emphasis on the methods developed to map loci contributing to the regulation of quantitative traits (QTLs).

Quantitative Trait Loci (QTLs) mapping for growth traits in the mouse: A review

The attainment of a specific mature body size is one of the most fundamental differences among species of mammals. Moreover, body size seems to be the central factor underlying differences in traits such as growth rate, energy metabolism and body composition. An important proportion of this variability is of genetic origin. The goal of the genetic analysis of animal growth is to understand its "genetic architecture", that is the number and position of loci affecting the trait, the magnitude of their effects, allele frequencies and types of gene action. In this review, the different strategies developed to identify and characterize genes involved in the regulation of growth in the mouse are described, with emphasis on the methods developed to map loci contributing to the regulation of quantitative traits (QTLs).

Inheritance of resistance to ear damage caused by Sesamia nonagrioides (Lepidoptera: Noctuidae) in maize.

Sesamia nonagrioides Lefebvre is a major insect pest of maize (Zea mays L.) in northwestern Spain. The inheritance of ear resistance in field corn to this pest has not been studied. This work aims to determine the importance of genetic (additive, dominance, and epistatic effects) and environmental effects in the inheritance of ear resistance to S. nonagrioides. Three field corn inbreds (CM109, EP31, and EP42) were used as parents and two crosses (CM109 x EP31 and CM109 x EP42) were made. These inbreds show different ear resistance levels to S. nonagrioides, with CM109 more resistant than EP31 and EP42. For each cross, parents (P1, P2), F1, F2, and backcrosses to each parent (BC1 and BC2) were evaluated. Correlations among ear damage traits showed that general appearance of the ear should be useful indicator of ear resistance. Ear resistance was dominant to susceptibility and was transmitted from inbreds to their hybrids. The additive-dominance model fit the generation mean analysis for both crosses and the degree of genetic control varied depending on the cross and trait. For both crosses, additive and dominance effects were significant for most ear damage traits. Epistatic gene effects were significant for husk and shank damage, and gene effects for number and length of tunnels were not significant. Because ear resistance involved additive and dominance effects for this set of inbreds, breeding procedures based on both types of gene action should be effective.

Testing drug response in the presence of genetic information: sampling issues for clinical trials.

Progress towards construction of a dense map of di-allelic markers across the human genome has generated considerable enthusiasm for pharmacogenomic applications. To date, however, nearly all of the effort on single nucleotide polymorphism (SNP) projects has been focused on marker identification and screening, not on how the SNP genotype data actually can be used in clinical trials to advance medical practice. Here, we explore how different properties of SNPs impact the size, scope and design of clinical trials using a simple trial design. We evaluate the clinical trial sampling requirements under different allele frequencies, gene action, gene effect size and number of markers in a genome screen. Power and sample size calculations suggest that allele frequency and type of gene action can have a dramatic impact on trial sample sizes, in that under some conditions the required sample sizes are too large to be applicable in a costly clinical trial setting. In other situations, however, pharmacogenomic clinical trials can yield significant sampling/cost savings over traditional trials. These properties are discussed with regard to the general usage of genetic information in clinical trial settings.

Gene Regulatory Networks Generating the Phenomena of Additivity, Dominance and Epistasis

We show how the phenomena of genetic dominance, overdominance, additivity, and epistasis are generic features of simple diploid gene regulatory networks. These regulatory network models are together sufficiently complex to catch most of the suggested molecular mechanisms responsible for generating dominant mutations. These include reduced gene dosage, expression or protein activity (haploinsufficiency), increased gene dosage, ectopic or temporarily altered mRNA expression, increased or constitutive protein activity, and dominant negative effects. As classical genetics regards the phenomenon of dominance to be generated by intralocus interactions, we have studied two one-locus models, one with a negative autoregulatory feedback loop, and one with a positive autoregulatory feedback loop. To include the phenomena of epistasis and downstream regulatory effects, a model of a three-locus signal transduction network is also analyzed. It is found that genetic dominance as well as overdominance may be an intra- as well as interlocus interaction phenomenon. In the latter case the dominance phenomenon is intimately connected to either feedback-mediated epistasis or downstream-mediated epistasis. It appears that in the intra- as well as the interlocus case there is considerable room for additive gene action, which may explain to some degree the predictive power of quantitative genetic theory, with its emphasis on this type of gene action. Furthermore, the results illuminate and reconcile the prevailing explanations of heterosis, and they support the old conjecture that the phenomenon of dominance may have an evolutionary explanation related to life history strategy.

Resistance to powdery mildew in selections from Tunisian landraces of barley

AbstractTwo‐hundred and thirty‐two accessions of barley landraces collected from Tunisia were screened for resistance to powdery mildew. A number of race‐specific genes were detected using the detached leaf technique. Among the 232 accessions tested, 169 were susceptible to powdery mildew, 20 were resistant and 43 showed differential reactions to the three isolates of powdery mildew used. An attempt was made to determine the number of genes, the types of gene, the types of gene action and the gene loci in 20 resistant accessions. Three types of cross were made: (1) the accessions were crossed to the susceptible variety ‘Pallas’, (2) the accessions were crossed with ‘Pallas’ isolines, and (2) accessions with identical powdery mildew reaction patterns were intercrossed. Three isolates of Erysiphe graminis f. sp. hordei were used: Bzm‐1, KM 18‐75, R13C. A number of different resistance genes were detected among the 19 resistant accessions. Surprisingly, segregation indicating single genes only were detected with the isolates used. Some of these genes could be associated with loci already known. In 19 cases a dominant and in one a recessive mode of inheritance was detected. The recessive gene was not located at the mlo locus. This investigation represents the first systematic study of race‐specific genes for powdery mildew resistance in Tunisian landraces. The newly identified sources of resistance may be used in many strategies of breeding for disease resistance.

Componentes epidemiológicos y herencia del desarrollo lento de la Roya de la Hoja (Puccinia recondita f. sp. Tritici) en trigo.

Se realizó este estudio para determinar tres componentes epidemiológicos de la roya de la hoja: período de latencia, tasa de desarrollo de la enfermedad y área bajo la curva de progreso de la roya de la hoja, también, la clase de acción génica y heredabilidad implicados en el tipo de resistencia denominada "desarrollo lento" de la roya de la hoja en tres variedades de trigo: Pavón 76, Nacozarí 76 y Hermosillo 77 ;trabajo que se llevó a cabo en el Centro Internacional de Mejoramiento de Maíz y Trigo (CIMMYT),localizado en Texcoco, Estado de México, México. Para comenzar, se obtuvieron seis cruzas simples posibles de éstas tres variedades con una variedad susceptible carente de genes de resistencia, denominada Jupateco 73S. Los progenitores y 74 familias F5, de cada una de las seis cruzas, fueron sembradas en un diseño de bloques completos al azar con un arreglo de parcelas divididas con tres repeticiones. Los resultados indicaron que en las 3 variedades el período de latencia fue largo y la tasa de desarrollo de la enfermedad baja. El análisis de aptitud combinatoria indicó que los efectos de tipo aditivo fueron los más importantes. La heredabilidad en sentido restringido en promedio de todas las cruzas y en cada una de ellas fue alta. No se encontró asociación entre el área bajo la curva de progreso de la roya de la hoja con madurez fisiológica y la altura de la Planta.Inheritance and Components of slow rusting to leaf rust (Puccinia recondita f.sp.tritici) in wheat.In order to determine the latent period and rate of development of the disease, the area under the leaf rust progress curve, the type of gene action, and heritability implicated in the type of resistance called "slow rusting" of the leaf rust in the Pavon 76, Nacozari 76 and Hermosillo 77,was carried out this work at the International Maize and Wheat Improvement Center (CIMMYT),located in Texcoco, State of Mexico, Mexico. All possible single crosses were entered into these three varieties with a susceptible lacking genes resistance, called Jupateco 73S. Parents and 74 F5, Families of each cross were planted in a plot randomized block design with three replications. The results indicate that in the 3 varieties the latent period was long and the rate of disease development was low. Combining ability analysis indicates that the additive variance was the most important. The narrowsense heritability as an average of all crosses, as well as in each cross was high. There was no relationship between the area under the leaf rust progress curve and Plant maturity or height.

Genetic analysis of heading date and other agronomic characters in barley (Hordeum vulgare L.)

Genetic analyses of heading date, tiller number, plant height, grain yield, kernel weight, and plump and thin kernels were made in three six-rowed barley crosses (Hordeum vulgare L.) involving four cultivars. Six populations, P1 , P2 , F1 , F2 , BC1 , and BC2 , from each cross were grown and evaluated at Fargo and Prosper, North Dakota, 1982. Parental means within crosses generally were different except for tiller number. Comparison of generation means suggested that late heading was dominant to early, high kernel weight was dominant to low, and kernel plumpness was influenced by additive gene action. The relationship between yield and heading date was not consistent among crosses and positive r values were quite low. It should be possible to select early maturing, high yielding segregates with plump kernels. Heterosis over the mid-parent was quite similar among crosses for heading date, but there was no heterosis over the high parent. Inbreeding depression was fairly constant for heading date, but was less consistent for yield. The lack of uniformity for estimates of inbreeding depression can be related to environmental variation and to its influence on type of gene action. The ratio of additive to dominance variance was inconsistent among crosses for heading date and yield. These data suggest selection for these characters should be delayed past the F 2 generation. Broad sense heritabilities for heading date ranged from 42 to 86%. Values obtained for grain yield were more consistent among broad sense than narrow sense estimates. Genetic advance estimates were low due to lack of additive variance.

GENETIC COMPONENTS OF VARIATION IN UPLAND COTTON (G. hirsutum L)

A ten parent diallel cross analysis involving bollworm tolerant donor lines of upland cotton was studied for component of variance analysis for seven traits. Both additive and non-additive gene action were important for all the traits except seed index. However the role of dominance was major for all the traits studied. Dominance was ambidirectional for all the traits studied. Epistasis influenced the performance of number of bolls and seed cotton yield. Narrow sense heritability was high for days to 50 per cent flowering, boll number and seed cotton yield/plant while moderate for boll weight, seed index, lint index and ginning percentage. The number of genes controlling inheritance of the traits were established. Any form of recurrent selection may be followed for exploiting all the three types of gene actions.

Mapping QTLs in breeding for drought tolerance in maize (Zea mays L.)

Grain yield in the maize (Zea mays L) plant is sensitive to drought in the period three weeks either side of flowering. Maize is well-adapted to the use of restriction fragment length polymorphisms (RFLPs) to identify a tight linkage between gene(s) controlling the quantitative trait and a molecular marker. We have determined the chromosomal locations of quantitative trait loci (QTLs) affecting grain yield under drought, anthesis-silking interval, and number of ears per plant. The F3 families derived from the cross SD34(tolerant) × SD35 (intolerant) were evaluated for these traits in a two replicated experiment. RFLP analysis of the maize genome included non-radioactive DNA-DNA hybridization detection using chemiluminescence. To identify QTLs underlying tolerance to drought, the mean phenotypic performances of F3 families were compared based on genotypic classification at each of 70 RFLP marker loci. The genetic linkage map assembled from these markers was in good agreement with previously published maps. The phenotypic correlations between yield and other traits were highly significant. In the combined analyses, genomic regions significantly affecting tolerance to drought were found on chromosomes 1,3,5,6, and 8. For yield, a total of 50% of the phenotypic variance could be explained by five putative QTLs. Different types of gene action were found for the putative QTLs for the three traits.

Inheritance of resistance to diclofop-methyl and fenoxaprop-p-ethyl in two Avena sativa × A. fatua populations

Since the late 1980s, several reports of herbicide-resistant wild oat have raised concern about potential weed control problems in western Canada. This study was conducted to determine the mode of inheritance, number of genes and type of gene action governing herbicide resistance in two Avena sativa × A. fatua crosses. The herbicide-susceptible A. sativa cultivars, Random and Derby, were crossed with a resistant A. fatua genotype, GP-HR-01. Parents, F2s and F2:3 families were tested for their reactions to two post-emergent wild oat herbicides, diclofop-methyl and fenoxaprop-p-ethyl, in the greenhouse. Inheritance of resistance to diclofop-methyl and fenoxaprop-p-ethyl was dominant and monogenic in the Derby/GP-HR-01 cross, but was controlled by two dominant complementary genes in the Random/GP-HR-01 cross. Resistance to both herbicides appeared to be controlled by the same genes or groups of tightly linked genes. Key words:Avena sativa, Avena fatua, herbicide resistance, weed (genetics), diclofop-methyl, fenoxaprop-p-ethyl

Inheritance and linkage relationships of genes conditioning hullessness, multiflorous spikelet, and giantism in oat (Avena sativa L.)

A study was conducted to determine the number of genes, type of gene action, and linkage relationships of the factors controlling the expression of hullessness, multiflorous spikelet and giantism in oat (Avena sativa L. var. PI 546363). The results showed that giantism was governed by a recessive gene, which in a homozygous (gi-3/gi-3) condition may have an epistatic effect on several other loci including those controlling the expression of plant height; length, width, and thickness of leaves; culm diameter; number of internodes; panicle size; pedicle length; and number of days to anthesis and to maturity. The results also indicated monogenic inheritance for hull and spikelet characteristics. Hullessness (N-1/_) was dominant over hulledness (n-1/n-1), and multiflorous spikelet (Mf-1/-) was partially dominant over normal spikelet (mf-1/mf-1). Linkage was detected between the gi-3 locus controlling giantism, and the n-1 locus controlling hullessness; the recombination value was calculated to be 36.5 ± 0.8%. Linkage was also detected between the gi-3 gene and the Mf-1 locus conditioning multiflorous spikelet; the recombination value was 23.5 ± 0.4%. The N-1 gene appears to have an epistatic effect on the Mf-1 locus, and the recombination frequency between the two loci was 10.5 ± 0.6%. The gene order was gi-3, Mf-1, N-1. Key words: Oat, Avena sativa, genetics, giantism, hullessness, multiflorous spikelet

Salt tolerance in Lycopersicon species. II. Genetic effects and a search for associated traits

Eleven quantitative traits, mostly related to tomato plant growth and fruit set, and their association with salt tolerance in terms of fruit yield under a 171.1 mM NaCl treatment have been investigated in 206 progeny derived from an interspecific hybrid, L. esculentum x L. pimpinellifolium, by self-pollination. None of the traits were highly correlated phenotypically to salt tolerance; however, the immunologically-detected presence of peptide 2' was significantly associated with high total fruit weight (TW) and number (FN) under saline treatment. Broad-sense heritability was estimated for these two salt-tolerance components as 53.44 and 72.59 %, respectively. Non-additive gene effects, which have to be considered in a breeding program for salt tolerance, have been detected in TW, FN and in average fruit weight (FW). Given that different types of gene action have been found depending on the presence or absence of a high NaCl concentration in the nutrient solution, a different set of genes, or genes, differently regulated, must be involved in the expression of TW, FN and other fruit-related characters depending on this environmental condition.

Power studies in the estimation of genetic parameters and the localization of quantitative trait loci for backcross and doubled haploid populations

A statistical model for doubled haploids and backcrosses based on the interval-mapping methodology has been used to carry out power studies to investigate the effects of different experimental designs, heritabilities of the quantitative trait, and types of gene action, using two test statistics, the F of Fisher-Snedecor and the LOD score. The doubled haploid experimental design is more powerful than backcrosses while keeping actual type I errors similar to nominal ones. For the doubled haploid design, individual QTLs, showing heritabilities as low as 5% were detected in about 90% of the cases using only 250 individuals. The power to detect a given QTL is related to its contribution to the heritability of the trait. For a given nominal type I error, tests using F values are more powerful than with LOD scores. It seems that more conservative levels should be used for the LOD score in order to increase the power and obtain type I errors similar to nominal ones.

Toward an integrated linkage map of common bean. III. Mapping genetic factors controlling host-bacteria interactions.

Restriction fragment length polymorphism (RFLP)-based genetic linkage maps allow us to dissect the genetic control of quantitative traits (QT) by locating individual quantitative trait loci (QTLs) on the linkage map and determining their type of gene action and the magnitude of their contribution to the phenotype of the QT. We have performed such an analysis for two traits in common bean, involving interactions between the plant host and bacteria, namely Rhizobium nodule number (NN) and resistance to common bacterial blight (CBB) caused by Xanthomonas campestris pv. phaseoli. Analyses were conducted in the progeny of a cross between BAT93 (fewer nodules; moderately resistant to CBB) and Jalo EEP558 (more nodules; susceptible to CBB). An RFLP-based linkage map for common bean based on 152 markers had previously been derived in the F2 of this cross. Seventy F2-derived F3 families were inoculated in separate greenhouse experiments with Rhizobium tropici strain UMR1899 or X. c. pv. phaseoli isolate isolate W18. Regression and interval mapping analyses were used to identify genomic regions involved in the genetic control of these traits. These two methods identified the same genomic regions for each trait, with a few exceptions. For each trait, at least four putative QTLs were identified, which accounted for approximately 50% and 75% of the phenotypic variation in NN and CBB resistance, respectively. A chromosome region on linkage group D7 carried factor(s) influencing both traits. In all other cases, the putative QTLs affecting NN and CBB were located in different linkage groups or in the same linkage group, but far apart (more than 50 cM). Both BAT93 and Jalo EEP558 contributed alleles associated with higher NN, whereas CBB resistance was always associated with BAT93 alleles. Further investigations are needed to determine whether the QTLs for NN and CBB on linkage group D7 represent linked genes or the same gene with pleiotropic effects. Identification of the QTLs raises the possibility of initiating map-based cloning and marker-assisted selection for these traits.

Inheritance of early blight resistance in diploid potatoes

Early blight (Alternatia solani) is a fungal disease in hot and humid environments, which causes leaf, stem and tuber lesions. Early blight resistance should be incorporated into potato cultivars because the fungicide spraying is an expensive solution for developing countries. The diploid cultivated species Solanum tuberosum group Phureja and group Stenotomum are sources of resistance alleles. The elucidation of the inheritance for early blight resistance must help to decide what could be the best breeding procedure to improve this diploid germplasm and transfer the resistance to the tetraploid level. Three experiments were carried out under controlled and field conditions to determine the heritability of this trait using nested and diallel mating designs with haploid, species and haploid-species hybrids. The narrow-sense heritability estimates were relatively high (0.64–0.78). This means that additivity was the most important type of gene action for determining resistance to early blight at the diploid level. The results suggested that diploid parents showing highest levels of resistance, throughout the cycle of disease development, can be used in 4x×2x crosses to obtain resistant tetraploid progenies to this fungal disease.