Cause Of Heterosis Research Articles

Genetic causes of heterosis in juvenile aspen:a quantitative comparison across intra- and inter-specific hybrids

Genetic causes of heterosis in juvenile aspen:a quantitative comparison across intra- and inter-specific hybrids

The genetical basis of hybrid vigour in a highly heterotic cross of Nicotiana tabacum

The genetical control of F1 heterosis, observed in a cross of desirable Nicotiana tabacum varieties, was investigated by analysing the data of the basic generations, triple test cross-families and random samples of doubled haploids (DH) and single-seed descent (SSD) lines. Analyses of the first-degree statistics revealed a complex control underlying the genetic variation, including the presence of epistasis, linkage, maternal effects and their interactions, in addition to the additive and dominance effects of the genes segregating in the cross. These analyses identified gene dispersion, directional dominance, and duplicate epistasis, as the main causes of heterosis. The triple test-cross analysis also confirmed the presence of non-allelic interactions and indicated that the dominance ratio, although inflated by epistasis, is consistently partial for all the traits. The extent of transgression in the recombinant inbred lines finally established unequivocally that, as in numerous other crosses, gene dispersion and unidirectional, but partial, dominance are the true causes of heterosis in this cross too.

Allozyme Heterosis in Radiata Pine is Poorly Explained by Overdominance

Heterosis, the superior vegetative or reproductive vigor associated with heterozygosity, has been observed for allozymes in a wide variety of plant and animal taxa. Its causes, however, are widely debated. To gain insight into the causes of heterosis, we examined the relationship of allozyme heterozygosity and growth rate in a sample from a natural population of radiata pine. Some 244 clones derived from 30 natural stands were grown for 14 yr in an experimental plantation. Height, diameter, and crown growth rates were determined for six to nine ramets per clone; clonal genotypes were scored at 27 polymorphic allozyme loci. Levels of heterozygosity well below average were associated with depressed growth rates. The strength of heterosis within stands was positively associated with the estimated level of natural inbreeding and negatively associated with mean observed heterozygosity. Heterosis varied sporadically among loci and was unrelated to their levels of polymorphism, thus failing to support the hypothesis that the major polymorphisms are maintained by overdominant selection. Rates of growth also declined at high levels of heterozygosity, suggesting that the less frequent allozymes and/or tightly linked loci, which occur predominantly in heterozygous condition, tended to have deleterious effects.

The genetic conditions in heterozygous and homozygous populations ofDrosophila I. The fate of alien chromosomes

In experimental populations ofDrosophila melanogaster lethal chromosomes with dominant markers and inversions were introduced and the frequency changes of the markers studied during a period of several generations. The base populations of the various experiments differed from each other with respect to their degree of heterozygosity. Monochromosomal populations were isogenic for a quasinormal + chromosome, dichromosomal populations contained the genetic material of two different + chromosomes, trichromosomal of three, tetrachromosomal of four, hexachromosomal of six and polychromosomal populations of many normal chromosomes. Marker chromosomes with the dominant genesLCy, Cy, Pm orD respectively were added to the populations with an initial frequency of 16,6 per cent. The fate of the dominant markers was different in different populations. In some the marker chromosome reached equilibrium frequencies, in others they were eliminated with variable speed. As a rule the lethal marker chromosomes were accepted by monochromosomal populations; however, they were eliminated from populations with a higher degree of heterozygosity. Since in all populations one genotype, namely the homozygote for the marker chromosome, was lethal, the adaptive values “c” of the +/LCy, +/Cy, +/Pm or +/D heterozygotes could easily be calculated from the experimental data. This “c” value can be used as a measure for the combining ability of the marker chromosomes. It could be shown that “c” depends on the degree of heterozygosity of a population or in other words that the average degree of heterozygosity of the marker free individuals determines the selection processes. An equation can be arrived at which fits the experimental results very well if superiority of heterozygous +/+ individuals over +/+ homozygotes is assumed. From that it was concluded that heterosis is the determining variable in our experiments. An attempt was undertaken in order to decide if in our case the observed heterosis was due to dominance or to overdominance. It was postulated that in di-, tri-, tetra- or hexachromosomal populations the adaptive values of the marker free normals should progressively increase if recessive detrimental genes are the cause of heterosis but not if heterozygosity on many loci leads to overdominance. The “a” values of the +/+ individuals were ealeulated from the frequency changes of the marker chromosomes for each subsequent two-generation period. Unfortunately only two different dichromosomal populations were available. These showed increasing adaptive values for the normals. The tri-, tetra-, and hexachromosomal populations, however, gave different results, some with increasing, some with fluctuating adaptive values. From that it was concluded that heterosis can be due in one case to dominance and in the other to overdominance. In either case, the recessive genetic load may be rather important as a determinating factor in the dynamics of populations.

Inheritance of Lint Quality Characteristics in Interspecific Crosses of Cotton1

In interspecific crosses between varieties of upland (Gossypium hirsutum L.) and Egyptian (G. barbadense L.) cotton, tested in Israel from 1959 through 1965, F1 performance for upper‐half‐mean and mean lint length exceeded that of either parent by 10 to 16% and 6 to 13%, respectively. The uniformity ratio of F1 plants was slightly lower than the parental mean. F2 performance for lint length was slightly above parental mean, but a significant F2 deviation was detected for this trait. There were indications that additive, dominance, and additive ✕ additive epistatic effects were operating in the inheritance of lint length.The tensile strength (1/8‐in. gauge, i.e. T1 of the F1's lint was almost as high as that of the stronger G. barbadense parental varieties. The magnitude of heterosis was 9 to 15% for this trait, with no F2 or backcross deviations. This indicated that only additive and dominance effects were operating in the inheritance of lint strength.Micronaire measurement of lint fineness was much lower in F1 hybrids than in either parent (a negative heterosis of 12 to 19%). Only the dominance effects were detected as operating significantly in the inheritance of this trait, but there were considerable deviations from the model with no epistasis.Significant effects of general combining ability were found for lint length and tensile strength. Additive gene‐action was predominant for these traits. Dominance effects were the main cause of heterosis for all the traits of lint quality examined.

HETEROSIS IN SORGHUM RESULTING FROM THE HETEROZYGOUS CONDITION OF A SINGLE GENE THAT AFFECTS DURATION OF GROWTH

THE LITERATURE bearing on theories of heterosis has been reviewed by Whaley (1944) with the exception of the early literature that had previously been thoroughly covered by Jones (1918). It is apparent from this review that the term heterosis has been applied variously in different cases and that no one theory has been demonstrated to cover all the facts. Complementary gene action, masking of deleterious recessives, combination of favorable dominants, and an effect peculiar to heterozygosity have all been shown to produce the effect. The theory of dominant interaction is apparently the theory most generally accepted by geneticists. Hull (1945) and Richey (1945) have recently based recommendations for isolating better foundation inbreds for use in corn hybrids on the assumption that hybrid vigor resides in the interaction of allelic genes and that numerous genes are involved. Singleton (1943) has reported cases in Zea Mays L. where two parents differing in a single gene crossed to a third strain produce plants differing in vigor. Jones (1944, 1945) has recently reported cases in corn where plants heterozygous for a single allele are larger and higher yielding plants than the homozygous dominant, and the stimulus appears in these cases to be heterozygosity as originally postulated by East (1909), East and Hayes (1912), and Shull (1914). Randolph (1942). after observing the effects on growth fromn doubling the chromosome number of both iinbred lines and hvbrid strains of corn, came to the conclusion that heterozygosity in itself may be responsible for much of the vigor or heterosis exhibited by hybrids. -Jones (1945) has pointed out that only certain genes in the heterozygous condition produce heterosis and that heterogosity in itself does not produce the effect. In view of the complexity of the problem and of the practical importance of knowing what the causes of heterosis are, any information that has a bearing upon any of the presently held theories would be welcome. The inlieritance of three genes that influence duration of growth in milo, a group of Sorghum vulgare Pers. has been reported by Quinbv and Karper (1945). Lateness is dominarnt to earliness except that recessive ma is epistatic to dominant Ma2 and Ma3, and Ma2 is epistatic to Ma3. This state of affairs results in eight homozygous genotypes but onlv four phenotypes for maturity that have been called Early (ma Ma2 Man, ma ma2 Ma ,, ma Ma2 ma3, ma ma2 ma3), Intermediate