The role of allelic and nonallelic interactions of genes in the mechanism of heterosis

Aim. Studying the heterosis effect from the point of view of allelic and nonallelic gene interaction in F1 hybrids from crossing the arabidopsis ecotypes Col-0 and La-0. Methods. Hybridological method of crossing mutant lines and analysis of the inheritance of the signs of the root system. Results. When plants of different races Col-0 and La-0 cross in the F2 generation, polymeric interaction of genes occurs. In that case splitting in F2 goes in the ratio 15: 1. At the same time, hybrids of the first generation exhibit heterosis, which is manifested in a more powerful development of a rosette of leaves in comparison with the initial forms. Conclusions. The emergence of heterosis in hybrids of the first generation can be explained on the basis of the allelic and nonallelic gene interaction, which creates a favorable combination of genes during hybridization, causing the best manifestation of an economically valuable trait. Keywords: Arabidopsis thaliana (L.) Heynh., heterosis, gene, mutation, race.

Inheritance of Resistance to Powdery Mildew Race 2 in Citrullus lanatus var. lanatus

Information on the mode of inheritance of powdery mildew resistance in watermelon is important for designing a breeding strategy for the development of new cultivars. Resistance in the watermelon accession PI 270545 was investigated by generation means analysis by crossing it with susceptible PI 267677. The analyses showed involvement of two genes, a recessive resistance gene, pmr-1, and a dominant gene for moderate resistance, Pmr-2. Resistance to powdery mildew in the leaf had a large dominance effect and a heritability of 71%. The additive-dominance model was inadequate in explaining variation in leaf resistance as revealed by the joint scaling test. However, nonallelic interactions could not be detected by the nonweighted six-parameter scaling test. For stem resistance, the additive-dominance model was adequate, and inheritance was controlled mainly by additive effects. A high narrow-sense heritability of 79% suggested that selection for stem resistance in early generations would be effective.

Correlations, epistasis, and heterosis of plant height and internode length in barley

The correlation between total height and its components, and between these and yield components, and the presence of nonallelic interactions and heterosis were studied in barley by means of a 6 × 6 diallel cross. The estimation of interaction and heterosis parameters were made in P1, P2, F1, F2, B1, and B2 generations according to the Jinks and Jones model. Total height had a positive correlation with each internode length. The values of such correlations seemed to indicate that peduncle length could be a good index of total height. The plants with short basal internodes had less total height and longer intermediate internodes. The positive correlation of total height with yield and its components suggests that the selection for short culm and short basal internodes would be to the detriment of yield. The predominant interaction was of the duplicate type, the total height being the character with greater epistasis. In the crosses with heterosis, this was always positive. 'Albacete' × M-168 and A.1.-2 × M-168 were the crosses where the selection for short culm and lodging resistance would be most effective. Key words: diallel cross, correlations, epistasis, heterosis, barley.

AN ANALYSIS OF HETEROSIS VS. INBREEDING EFFECTS WITH AN AUTOTETRAPLOID CROSS-FERTILIZED PLANT: MEDICAGO SATIVA L

ABSTRACT Self-fertilization and crossing were combined to produce a large number of levels of inbreeding and of degrees of kinship. The inbreeding effect increases with the complexity of the character and with its supposed relationship with fitness. A certain amount of heterozygosity appears to be necessary for the expression of variability. With crossing of unrelated noninbred plants, genetic variance is mainly additive, but with inbreeding its major part is nonadditive. High additivity in crossing, therefore, coexists with strong inbreeding depression. However, even in inbreeding the genetic coefficient of covariation among relatives appears to be strongly and linearly related to the classical coefficient of kinship. This means that deviations from the additive model with inbreeding could be partly due to an effect of inbreeding on variances through an effect on means. An attempt to analyze genetic effects from a theoretical model, based upon the identity by descent relationship at the level of means and of covariances between relatives, tends to show that allelic interactions are more important and nonallelic interactions are less important for a character closely related to fitness. For a complex character, these results lead to the conception of a genome organized in polygenic complementary blocks integrating epistasis and dominance. Some consequences for plant breeding are also discussed.

SIMULATION OF QUANTITATIVE CHARACTERS BY GENES WITH BIOCHEMICALLY DEFINABLE ACTION. VI. MODIFICATIONS OF A SIMPLE MODEL

ABSTRACT Investigations on metric characters of defined genotypes of Matthiola incana, and application of different linear models for the estimation of genetic parameters, indicate that the use of midparental value as a reference point results in parameter estimates that do not correspond to the actual biological situation. Use of the most recessive genotype as a reference point causes all of the contributions of single loci to be unidirectional and positive, and all the allelic and nonallelic interactions to be unidirectional and negative, in accord with our Model 2.2. The results indicate that the phenotypic response to allelic substitutions follows the characteristics of a saturation curve. The possibility is discussed that the saturation character results from regulating processes, whereas deviations of single measurements from the response curve, or response surface, reflect real interactions between allelic and nonallelic genes.

A STUDY OF COMBINING ABILITY FOR SOME DEVELOPMENTAL TRAITS IN A DIALLEL SET OF CROSSES IN PEARL MILLET

A diallel cross involving ten selected inbred parents in pearl millet Pennisetum typhoides (Burm.) Staph and Hubb. was analyzed for the inheritance of number of functioning leaves and leaf size. The study revealed that both additive and dominance genetic variances were important for leaf size whereas additive genetic variance was of major importance for number of functioning leaves. However, nonallelic interactions evidently had considerable impact in the inheritance of both characters. Heritability for number of functioning leaves was higher than that for leaf size. Although specific and general combining abilities were significant for both characters, gca effect appeared to be more important than sca effect for both characters. It was proposed that a breeding procedure which could make an efficient use of both additive and nonadditive genetic variances would be appropriate for the improvement of pearl millet.

A QUANTITATIVE ANALYSIS OF THE GENETICS OF RESTING BLOOD LACTIC ACID LEVELS IN MICE

ABSTRACT Resting blood lactate levels were measured in inbred mouse strains, their F1, and several of their segregating generations to determine whether the level of lactic acid is influenced by genetic factors. The inbred strains in each of the two sets used differed significantly from one another for this character. Only one strain showed a significant sex difference. The data could not be fully analyzed because of the failure to fulfill Mather's first criterion for an adequate scale. Nonallelic interactions, in particular, additive x dominance and dominance x dominance, were found to influence the generation means. Genotype x environment interaction was detected and eliminated by log transformation. Negative heterosis was exhibited by all but one noninbred generation.—The data suggest that genes influencing the character are dispersed between the parental lines and that interactions are predominantly of the duplicate kind. A buffering system by which lactate levels are kept at a minimum is proposed.