Integrative Biology 335:
Systematics of Plants

Plant Breeding Systems

Announcements:

Your third take-home lecture assignment, worth 2% of your final grade, is due in class on Monday, March 9th. This assignment will help you prepare for the upcoming lab exam.

Lab Exam Review! This Sunday (March 8th), time TBA (Clark will inform you). Come prepared with questions! There will also be a slide review in lecture on Monday, March 9th.

Remember, your lab exam consists of two components: 1) a station-to-station component where living and dissected material will be displayed and questions asked of this material; and 2) a slide component where questions will be asked from projected 35mm slides. The first part will take place on Tuesday, March 10th during your regularly scheduled lab; the second part will take place during lecture on Wednesday, March 11th. Both components will be equally-weighted. This exam is worth 10% of your final grade. Because of the large amount of labor in putting together a lab exam, there will be no make-up or conflict exams.

Sample Laboratory Exam (from Spring 2001). Use this page as a guide to the style of questions you will see at next week's exam. Also, see the sample exam in your class notes (pp. 243-244; Spring 1994). The names of families will be provided!


Text and Other Resources:

Information on the topic of breeding systems is scattered throughout the Judd et al. textbook. In the 3rd edition (2008), refer to these pages:

71 – 72, Self-incompatibility and heterostyly
89 – 90, Agamospermy
123 – 132, Variation in plant populations and species, and Speciation
143 – 146, Plant breeding systems and Species concepts.

This is required reading and you will be examined on it. As a guide, only study that information that is emphasized in lecture.

There is also a tutorial on Breeding Systems in Digital Flowers. (It offers basically the same information as presented here.)

General Objectives:

After studying this material you should be able to:

  1. Explain why sexual reproduction is necessary for the survival of a species over time

  2. Explain the long-term and short-term advantages and disadvantages of outbreeding and inbreeding

  3. Define and illustrate the following terms: protogyny, protandry, dioecy, monoecy, polygamodioecy, heterostyly (distyly)

  4. Explain the difference between sporophytic self-incompatibility and gametophytic self-incompatibility

  5. Explain what is meant by cleistogamous flowers

  6. List the various methods of asexual reproduction

  7. Understand the various prezygotic and postzygotic isolating mechanisms used by species to maintain their identity and distinctiveness over time

  8. Define what a species is, especially with reference to the morphological, biological and genetic species concepts

Plant Breeding Systems

Why is a flower necessary?

A flower is the reproductive structure of the angiosperms. It consists of a determinate, modified shoot (the floral axis or receptacle) bearing modified leaves (the perianth parts, stamens and/or carpels).

It is an adaptation for pollination, and the process of pollination is necessary to bring the gametes together to produce offspring. These gametes can come from the same individual, or from different individuals of the same species. Plants have evolved a variety of adaptations that either encourage inbreeding (self-pollination) or outbreeding (cross-pollination).

In order for a species to survive over time, individuals of that species must produce offspring and these offspring must be adapted to their environment.

Some basic terms:

Outbreeding: Sexual reproduction between individuals (cross-pollination or outcrossing). (It involves two plants!)

Inbreeding: Sexual reproduction within an individual (self-pollination or selfing). (It involves one plant!)

Sexual Reproduction: Meiosis and fertilization are involved (formation of gametes and fusion of sperm and egg). The sexual process is a mechanism to bring about gene recombination. Genetic recombination (crossing-over and independent assortment of homologous chromosomes through meiosis) is the chief source of hereditary variation and provides the raw materials for species to adapt to changing environmental conditions. Sexual reproduction is largely responsible for the rich source of variation observed in the natural world.

Asexual Reproduction: No meiosis and no fertilization.

NOTE! Many species of plants utilize a variety of sexual and asexual breeding systems.

Outbreeding (Cross-pollination)

Inbreeding (Self-pollination)

Outbreeder or Inbreeder?

Often one can tell just by looking at a flower whether it cross-pollinates or self-pollinates.

OUTBREEDER INBREEDER
self-incompatible self-compatible
many flowers few flowers
large flowers small flowers
bright colors mono-colored
nectaries present nectaries absent
scented flowers unscented flowers
nectar guides present nectar guides absent
anthers far from stigma anthers close to stigma
many pollen grains fewer pollen grains
style exserted from flower style included in flower
stigmatic area well-defined stigmatic area poorly-defined

Sexual Reproduction

Many plants avoid self-pollination and the harmful effects of inbreeding by separating the gametes temporally and spatially.

Outbreeding

1. Androecium and gynoecium mature at different times (temporal separation). The androecium and gynoecium mature a few days apart. Therefore, the flower cannot self-pollinate. It is rare in perfect flowers to have both A and G mature simultaneously.

Protandry - anthers shed pollen before the stigma is receptive

Protandry is more common that protogyny, and is also more common in insect-pollinated plants

Protandry in geranium showing 1st and 2nd day flowers

Another example of a protandrous flower

And another example of a protandrous flower

2. Separation of androecium and gynoecium (spatial separation)

Monoecy - flowers imperfect; staminate and carpellate flowers on same plant

Monoecy is common in wind-pollinated plants, such as oaks and birches

Example of a monoecious plant (Betulaceae)

Another example of a monoecious plant (Liquidamber, or sweetgum)

Polygamodioecious - a few perfect flowers on a plant with imperfect flowers. This ensures pollination in the absence of cross-pollination.

Heterostyly - different arrangements of stamens and styles.

Heterostyly in Primula

Tristyly in Lythrum

3. Self incompatibility

This means that a plant cannot produce a zygote with its own pollen. In many species, this process is controlled by multiple alleles of a single gene. Basically, if the pollen grain has the same allele as that of the stigma, then mating will not be successful. Self-fertilization will not occur.

Even though a diploid individual has only 2 of these alleles, there may be hundreds of other alleles in other individuals in the population

Gametophytic self-incompatibility - interaction between the pollen tube and stigma/style tissues. Here the genotype of the pollen determines what matings will be successful.

This is more widespread among angiosperms

Inbreeding

1. Self-pollination
Genetic variation is eliminated and well-adapted genotypes are preserved.

Remember, selfing involves one plant and maintains or decreases genetic variability

Other examples:
Protandry followed by back-up self-pollination in Campanula

Self-pollination in a rhizotomous plant

2. Cleistogamous flowers
Inconspicuous, bud-like apetalous flowers concealed beneath leaves. They occur after normal flowering period but never open. In these flowers, the anthers are appressed to the stigma. The pollen germinates in anther and the pollen tube grows through anther into stigma.

Polygala polygama, family Polygalaceae

Asexual Reproduction

REMEMBER: Here we have no meiosis and, therefore, no genetic recombination!

Vegetative Propagation: new plants can be formed from stolons, rhizomes, tubers, offset buds on corms and bulbs, suckers, bulbils [bulb-like propagules in inflorescence], and vivipary [tiny plantlets growing on the parent plant].

Onion bulbils in inflorescence

Clones are large numbers of genetically identical individuals. Some are long-lived, such as the coastal redwood which can live for 1000 years. Any variation is due to the environment, not genetic recombination.

Apomixis (Agamospermy): the production of seeds without meiosis and fertilization. The embryo is genetically identical to the single parent. The egg cell is not reduced (in other words, it has the full complement of chromosomes; it is not haploid).

Seeds are important for transportation to new areas (dispersal) and dormancy

Dandelions, an example of an apomictic plant.

Crataegus fruit. Another example.

Many species utilize a variety of breeding methods to ensure offspring production

Isolating Mechanisms

For species to maintain its identity and distinctiveness over time, it is necessary for each species to have ways of making sure that it is isolated in some manner from other closely related species.

These are mechanisms that prevent hybridization (gene flow) among different species.

Prezygotic (mechanisms functioning prior to fertilization)

Postzygotic (mechanisms functioning after fertilization has taken place)

Hybridization can occur when these isolating mechanisms break down. Both interspecific and intergeneric hybrids are known. Unlike animals, hybridization is common in plants. Sterile hybrids can survive by vegetative growth or apomixis.

What is a species?

Species are the most poorly understood unit of biological organization, and many definitions of species have been proposed. Unfortunately, we do not have the time to cover this subject adequately this semester.

Here are three different species concepts that we have already addressed this semester:

Morphological: Plants that look the same are treated as the same species.

Biological: Plants that are reproductively isolated from each other are treated as different species.

Genetic: Species are defined by the amount of genetic distance among populations.

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