Breeding Terms and Theory
|R. A. Smith - Severn, MD|
following eight-part discussion focuses on plant breeding topics that
some applicability to one’s hosta hybridizing program. It is meant to
standard plant improvement theories often employed with crop plants,
common terms of reference and documentation annotation styles, and
present some concepts related to handling gene variants (alleles) that
affect the outward appearances (phenotype) and approaching a
hosta breeding program[i].
This set of plant breeding topics was originally drafted in response to
discussions on the Hosta Seed Growers forum and has subsequently been
for the Online Journal. My hope is that some will find the terms
reference and the theoretical ideas presented both informative and thought
when designing hosta breeding goals and specific protocols to achieve
Plant Mating Strategies
to one of the leading distinguished professors in the field of plant
George Acquaah (Principles of Plant Genetics and Breeding, 2007), there
primary gene actions to consider when working with plants: 1)
additive, 3) overdominance, and 4) epistatic. Additionally, Acquaah
in his text books how some genes are major genes for a trait while
more of a minor role (e.g., polygenes)[iv]. For an excellent coverage
of these topics,
I’d also recommend the “Fundamentals of Plant Genetics and Breeding”
Welsh, 1990 reprint[v].
Of the text books on this topic I’ve read (12), the two referenced
provide excellent plant breeding general topic coverage of these
briefly look at these types of gene actions a bit more.
Dominance: Mendel's work highlighted traits that exhibited complete dominance --the all on or off type approach. This gene action equates to what is often called dominant or recessive genes --the theory being, that in the right combination, one or the other trait shows full dominance. Building on Gregor Mendel's work on dominance theories, plant breeders over the past 100 years discovered that sometimes genes may partially mask a trait. This is referred to as incomplete dominance or partial dominance. The classic example is a red and a white flowering annual; crossing with each other results in some red, some pink (blended), and some white progeny with 1/2 being pink (a 1:2:1 ratio versus the expected 3:1 ratio (red to white) with complete dominate expressions). Another concept to mention here is one of codominance -- two traits can be expressed completely in the progeny (to make a new look or phenotype altogether)[vi]. In simple terms, incomplete dominance likely produces a blended phenotype while codominance produces unique and separate phenotypes (Acquaah, 2007) with both traits being fully expressed (not blended). In hostas, to identify and then cross plants that have different looks (phenotypic disassortative) with the hope for both leaf traits to be fully expressed would be a worthwhile goal.
Additive: This deals with the concept that each gene enhances the overall expression of a trait (up and beyond what is expressed in either parent), then it is said to be additive gene actions. This concept’s basis is that various gene variants (alleles) involved in trait expression each add one unit of the trait, assuming that the plants that have good General Combining Ability (GCA). For instance, if 4 different units are combined (2 from each parent), then the progeny may have up to 4 total units of the trait expressed (and thus, greater trait expression than either parent that only had the 2 units each). For example, in wheat, the red kernel colors can range from dark red to white based on 3 loci (the location of a gene on a chromosome) that can be added together to make a continuous range between these two colors (ratios being 1:6:15:20:15:6:1 from white to medium red to dark red -- Nilsson-Ehle 1911[vii]). If I had to take a guess with hostas, it appears that the puckered leaf trait may behave in this way. (Please let me know if your experience holds true as this initial observation is based on only very limited examples.) I’ve also made the assumption that the amount of red/purple pigments expressed in hosta petioles is an additive trait -- this will take some dedicated hybrid crossing experiments and a few years to fully explore.
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