INTRODUCTION TO PLANT BREEDING

AGRONOMY 815 / COURSE NOTES

P. STEPHEN BAENZIGER, 338 Keim Hall, 472-1538

DEPARTMENT OF AGRONOMY / UNIVERSITY OF NEBRASKA

REPRODUCTION IN PLANTS


B and K, Chapt. 4
Allard, Chapt. 4
Poehlman, Chapt. 2
Hybridization of Crop Plants. 1980.
American Scientist 75:44-50. The Effects of Pollen Competition.


THIS IS PART OF UNDERSTANDING THE BOTANY OF YOUR CROP AS WELL AS THE MOST IMPORTANT ASPECT OF DETERMINING THE TYPE OF BREEDING METHOD THAT YOU WILL USE.

Terminology:

Reproduction can be asexual (from vegetative parts — non-gametic/non-fertilized) or by sexual (requiring effective fertilization/hybridization forming seed) methods.

Sporophyte — produces spores (microspores and megaspores), in higher plants is the conspicuous plant.

Gametophyte — produces gametes (sperm and eggs) in higher plants is the pollen grain and oogocyte. The inconspicuous "plant" in the higher plant life cycle.

Flower parts: Sepals, petals, stamen, pistil.

Complete flower has — sepals, petals, stamen, and pistil.

Incomplete flowers — lacking one of the above.

Perfect flowers stamens and pistils are in the same floral structure wheat.

Imperfect flower stamen and pistil not in the same floral structure corn.

Monoecious ("one house") — stamens and pistils on the same plant.

Dioecious ("two houses") — stamens and pistils on different plants, ex., hemp, hops, buffalo grass.

GAMETE FORMATION:

MALE

  1. Microspore (pollen) mother cells, within immature anthers, undergo meiosis tetrad of four microspores.

  2. Microspores mature to form pollen grains.


    1. Spore wall thickens.


Tube cell nucleus

Microspore nucleus divides

Generative nucleus
  1. Anther matures.


    1. Pollen sacs open.

    2. Pollen grains dispersed.


FEMALE

  1. Megaspore mother cell, within each ovule, undergoes meiosis tetrad of four megaspores.

  2. One megaspore, usually the furthest from the micropyle, undergoes additional divisions an ovoid, 8 nucleate embryo sac. The other 3 megaspores disintegrate.

  3. The female gamete(egg) + 2 synergids(nuclei) lie near the micropyle, 3 antipodals(nuclei) are located at the opposite end of the embryo sac, and 2 polar nuclei are positioned in the middle.


    4.

ANTHESIS:

Maturation of the anther accompanied by the extension of the filament.

POLLINATION:

Transfer of pollen grains from anther to stigma.

Method of transfer varies with crop.

Pollen germinates on the stigma and the pollen tube enters the ovule via the micropyle.

The generative nucleus divides 2 male germ cells(gametes). These male gametes enter the embryo sac.

FERTILIZATION:

1 male gamete(sperm) fuses with the egg zygote.

The other male gamete unites with the 2 polar nuclei.

This triple fusion the primary endosperm nucleus.


SEED DEVELOPMENT:

ZYGOTE+PRIMARY ENDOSPERM NUCLEUS+INTEGUMENTS

surrounding
provides foodthe ovule.
during germination

EMBRYO ENDOSPERM*SEEDCOAT
and early seedling(storing starch,
growth.protein)

*In soybeans and other
legumes, the endosperm is
absorbed by the developing
embryo and food materials
are stored in the cotyledons.

Comments:

  1. Many more microspores are produced than megaspores.

  2. Microspores are normally in abundance in higher plants relative to the number of megaspores that need to be pollinated.

  3. Approximately 60% of the genes expressed in the plant are expressed in the microspore large overlap of gene expression and function.

  4. Emphasize the process of double fertilization — endosperm is triploid, the embryo is diploid. Note differences between legumes (have cotyledons) and grasses.


Poehlman, 1979. "Orderly (effective) and efficient breeding procedures can only be developed with a clear understanding of the details of pollination, fertilization and seed development for the crop of interest."

Evolution of breeding systems.

There is substantial evidence indicating that many of the self-pollinated domestic crops evolved from naturally cross pollinated species despite the prevalence of self-incompatibility mechanisms. It has been speculated that inbreeders (self-pollinated species) arise frequently from outbreeders (cross-pollinated species) because uniformity associated with homozygosity (alike alleles at a locus e.g., AA ) may give immediate fitness to a specific environment such as cultivation.

Characteristic of self-pollinated population . . . mixture of homozygous lines. If a cross should occur, heterozygosity (unlike alleles at a locus e.g., Aa) disappears rapidly.

Formula for percent homozygosity:n = generations of selfing (= no. of segregating generations.),
and x = no. of (gene pairs)

Example 1. 1 locus, F4 generation (3rd segr gen.)
Example 2. 5 loci, F3 generation

Remember, for1 locus 2 homozygous genotypes are possible AA,aa
2 loci 4 . . . AABB, AAbb, aaBB, aabb
3 loci 8
5 loci 32 and so on.

Characteristic of cross pollinated species is heterozygosity.
Inbreeding tends to reduce vigor (crossing inbred individuals restores vigor).

Genotypic frequencies can also be predicted in a cross pollinated population if . . . random mating occurs and there is no linkage, selection, mutation or migration of alleles.

Hardy-Weinberg Law genotypic frequencies remain constant from one generation to the next.

1AA: 2Aa: 1aa

Pollination control mechanisms: Self Incompatibility and Male Sterility. Allard, ch. 20 Briggs & Knowles, ch. 15.

Self incompatibility. System favoring cross pollination. Common. Ref. Pandey, K.K. 1977. Theor.& Appl. Genet. 49: 101-109.

Disadvantages . . . restricts inbreeding & limits genetic combinations possible through crossing.

Pollen incompatible with stylar tissue . . . genetic control in both male and female . . . physiological interaction between pollen (gametophytic, 1n) or pollen plant genotype (sporophytic, 2n) and style (2n). Governed by S allele series at a locus (2 loci may be involved in some cases). Number of alleles varies with species.

Classification:Heteromorphic, e.g., Primula sinensis

(see Briggs & Knowles, p 185)

Characterized by differences in floral morphology.

Incompatibility systems
Gametophytic
Homogorphic
Sporophytic

Gametophytic . . . found in many species, e.g., clovers.
Occurs when pollen grain and stigma have allele in common.
Note . . . diploid stylar tissue will have any 2 alleles but never 2 of the same.
Examples involving gametophytic incompatibility system.


          S1S2    x    S1S2 . . .    incompatible because pollen = S1 or S2, both of which are in common with the stylar tissue.
          S1 S3   x    S1S2  . . .   fertile since male gamete carrying S2 compatible.
      S1S2,    S2S3

Reciprocal cross is also fertile

          S1S2    x     S1S3
                                                S3  compatible.
            S1S3 ,     S2S3

Sporophytic  . . .  incompatibility relationship of pollen determined by the plant producing it. e.g., Crepis, Parthenium, broccoli, kale.  Not as common.

Genetic explanation often complex. May have reciprocal differences where there is dominance and plants have one allele in common.  Dominance is expressed . . . S1 dominant over all other alleles, S2 dominant over all but S1 etc.

Note  . . .  no dominance is expressed on the female side.
      S1S2 x  S1S3  . . .  incompatible, S3 pollen having phenotype of S1.

Reciprocal also incompatible.
Homozygosity of S alleles is possible
with sporophytic system.                                                                                 S2S3         x         S1S2
                                                                                                              S1S2,    S1S3,              S2S2,  S2S3

Gametophytic vs. Sporophytic . . . difference in timing of biochemical substance associated with incompatibility.

Gametophytic . . . late during pollen development.
Sporophytic . . .early"

Self incompatibility . . . handicap to the plant breeder.
Manipulation of self incompatibility systems:

  1. Remove stigmatic tissue;
  2. Pollinate in bud stage;
  3. Lower temperatures;
  4. Mutagens;
  5. Increase ploidy level.
    Mutagens . . . X-ray, P32, chemical.


Gametophytic . . . applied to pollen mother cells prior to anthesis. S1Sf (self fertile). Sf dominant to incompatibility alleles. Stable Sf alleles can be transferred .

S1S2         x        S1Sf    S1Sf   ,   S2Sf  all fertile.

S1S2         x        S3Sf    S1S3   ,   S2S3 (self ster.)  &  S1Sf , S2Sf (self fert.)

                    Sporophytic . . .        treat at early stage. No immediate effect on pollen because mutant allele found to be recessive.

Ploidy.          Gametophytic . . .     increased ploidy, pollen grain can carry more than 1 allele . . . >
                                                     interact . . . < self fertility.

                     Sporophytic . . .        no effect.

Male sterility . . . nonfunctional male gametes. Means of emasculating male plants genetically.

Many causes of sterility . . . chromosomal aberrations, gene action, or cytoplasmic effects that influence development of pollen, embryo sac, embryo or endosperm. Primarily interested in male sterility.

Non functional male gametes as a result of mutant nuclear genes and/or cytoplasmic factors.