Multicellular living organisms and unmodified parts thereof and – Method of using a plant or plant part in a breeding process... – Method of breeding maize
Reexamination Certificate
1999-07-13
2002-09-24
Fox, David T. (Department: 1638)
Multicellular living organisms and unmodified parts thereof and
Method of using a plant or plant part in a breeding process...
Method of breeding maize
C800S267000, C800S266000, C800S320100
Reexamination Certificate
active
06455758
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to a process of predicting the value of a phenotypic numerically representable trait in a plant. A process of the present invention uses a quantitative assessment of the distribution of the numerically representable trait and a genotypic database in a first population to define an association between genotype and phenotype and predict the value of the phenotype in the same or a different plant.
BACKGROUND OF THE INVENTION
Phenotypic traits of agronomic interest tend to be quantitative and continuously distributed. For most such traits, the quantitative distributions, resulting when values of individual plants are graphed against their relative frequency, fit that expected from segregation of alleles at a large number of loci, with each locus (a position on a chromosome) contributing a relatively small amount to the value of the phenotype. This is the polygenic model of inheritance. Another assumption of the basic model is that heritable gene action is additive; that is, each allele contributes some predictably inheritable amount to the total quantitative value of the phenotype. Environmental and unpredictable heritable factors are then superimposed as variation on the genotypic sources of variation to yield the phenotypic variance component of the continuous distribution of the trait. Distributions may be described by moments, notably the mean and variance (or the standard deviation which is the square root of the variance).
Before the polygenic model was developed, the observation of continuous distributions of traits initially posed an obstacle to the universal application of Mendelian theory. This apparent conflict was resolved by proposing the polygenic model for the inheritance of traits that could be described quantitatively (e.g., corn ear length, number of kernels, plant height, yield in bushels). The model proposed an underlying segregation of single genetic entities, thereby being consistent with Mendelian theory. However, because the effects of these individual genes were aggregated in the expression of the quantitative phenotypes, their individual effects could not be teased out. (Johannsen, 1909; Nilsson-Ehle, 1909; East, 1916; Fisher, 1918). Environmental variation further smoothed the distribution, masking boundaries between distinct genotypic classes.
The polygenic model has been used in attempts to enhance selection efficacy in plant breeding programs. By observation and careful measurements of results of various parental-offspring distributions, both in plants and animals, and by expressing the genetic relationships in mathematical correlations, a complex mathematical theory emerged (Fisher; 1918; Falconer, 1960; Wright, 1968, 1977).
A basic tenant of this theory is the expression of phenotypic distribution in terms of its variance and to dissect that variance into its causative components. By studying the variance in offspring distributions where the offspring result from various types of crosses, and by determining the correlation between phenotypic distributions of different pedigree relationships (parent-offspring, offspring of the same cross, subsequent generations, e.g., F
2
-F
3
) it was determined that the phenotypic variance (V
P
) had as basic components genotypic variance (V
G
) and environmental variance (V
E
). In a simple case, the variance of plants of the same genotype grown in different environments provides an estimate of the effects of environment. Factors contributing to the environmental variance include year of growth and differences in the soil composition of plots of land.
In turn, each of these components could be further subdivided, for example, by separating V
G
into additive (V
A
), dominance (V
D
) and epistatic (V
I
) components. The components of the variance could be estimated by breeding experiments. These values were then used to predict results of other breeding crosses. Response to selection was found to be a function of the heritability of the trait, the selection differential and the intensity of selection.
The heritability (h
2
) of a trait is broadly defined as
h
2
=
V
G
V
P
,
or more narrowly,
h
2
=
V
A
V
P
and is a predictor of the degree to which values of traits may be transmitted from parents to offspring.
The intensity of selection is defined as the percent of the distribution from which the parents of the next generation are derived. The selection differential is defined as the difference between the trait in the parental population versus that of the selected parents. The cost effectiveness of selection is determined by the amount of time (in generations) required to achieve a significant change in the distribution of the trait under selection, the number of parents selected for breeding, and the response to selection. The response to selection is the difference between parental and offspring means after selection, e.g., after a generation of selective breeding. The basic mathematical formula to predict gain from selection is as follows:
expected gain=(selection differential)×(heritability)
where selection differential is the mean value of the phenotypic trait in the selected individuals minus the overall parental population mean, and heritability is the proportion of the phenotypic variance that is due to additive genetic variance.
A change of the population mean brought about by selection, i.e., the response to selection, depended on the heritability of a trait and on the intensity of selection (selection differential). This variable depends on the proportion of the population selected, and the standard deviation of the phenotypic trait. The shaded area of the distributions of
FIG. 2A
, FIG.
2
B and
FIG. 2C
are the proportion selected, and S is the selection differential.
Despite concerted attempts to improve commercially important phenotypic traits in plants, the rate of improvement of those traits has been only a few to several percent of the mean per year for the past several decades. In many previously described breeding programs, plants are selected as parents of the next generation on the basis of one or more phenotypic traits (e.g., yield in bushels per acre, number of rows per kernel of corn, percentage of grain oil).
A problem associated with selection based on phenotype is the affect of the environment on that phenotype. For various crop plants, it has been established that roughly half of improvement is due to improved husbandry practices, i.e., environmental effects rather than genetic changes effected by selection. (Lande and Thompson, 1990). For example, over the past 60 years, increases in yield due to genetic improvement have averaged only about one bushel/acre/year (Hallauer, et al., 1988, p. 466). Only a small population of hybrid plants produced commercially ever show enough improvement to be worth marketing. Environmental variables which need to be taken into account include soil type and the amount and distribution of rainfall. One of the important and influential environmental conditions is the temperature range of the climate in which the plants are grown. The time period needed by the plants to reach maturity (growth period) is under genotypic control. For optimum growth, the genotypically based growth period of the plant must fit within the environmental range. For example, if the plant does not fulfill its reproductive potential before the temperature drops below a threshold, the plant will not produce seed or offspring in that environment. Comparison of plants for various traits is typically made among plants of similar or identical maturity.
Another problem of phenotype selection is polygenic control. Most phenotypic waits of commercial interest are under polygenic, rather than single locus, control. This means that expression of alleles at many loci contribute to the phenotype of interest. Polygenically controlled traits, therefore, are not solely determined by any particular locus. Consequently, selecting on the basis of phenotype is a superficial and inefficient strategy. Complex genetic phenomenon lurk
Dekalb Genetics Corporation
Fox David T.
Fulbright & Jaworski LLP
Kruse David H
LandOfFree
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