Lecture 3 Notes

EEB 304 Lecture 3 – Plant Manipulation and Naming

In this class we will consider various ways that people have modified food plants for their needs. We will also discuss how plants are classified and named.

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Plant Manipulation – the Raw Material Flowering Plants

– ca 300,000 species

Plants eaten regularly by people – ca

2,500 species

Plants in World Commerce

– ca

150 species

Major Economic Importance

– ca

20 species

Crops recently domesticated

–

0 species

Only a small proportion of the estimated 300,000 species of flowering plants are regularly consumed by people. Of these, fewer than 20 species provide the bulk of food that humans obtain from plants. Our ancestors were judicious in their choice of plants to domesticate – no major additions to our crop fields have been added in historical times.

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Crops of Major Economic Importance Wheat, Rice, Maize (Corn), and Potatoes – the Big 4

This graph emphasizes the fact that the bulk of our plant foods come from a very few species. In fact, the top four account for well over half of the plant foods consumed by people.

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Traditional Methods of Plant Manipulation - Selection - Polyploidy - Asexual Reproduction (=cloning) - Inbreeding

The textbook discusses 4 major methods of plant manipulation that have been utilized in manipulating plants for human uses.

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Variation and Selection Keystone of Evolutionary Theory – “Selection of the Fittest” Natural Selection – wild populations Artificial Selection – when done by people Note: For selection to work, there must be variation, and it must be heritable - mutations (natural and induced) - geographic variation

A key facet of naturally occurring organisms is that they vary. Detailed studies have shown that the underlying factor in producing variation is mutation, and that within a species there is often a large geographical component in where variants occur. Variation provides the raw material for selection to operate and produce changes over time. People have been the selective agent for crop plants, and have produced striking modifications in plants, particularly in the structures that are used.

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Artificial Selection in Cole Crops

The cole crops provide a striking example of the ability of humans to change the appearance of plants in a relatively short time. Each of the crops shown in this diagram belong to the same species, Brassica oleracea (we know this, in part, because each can be intercrossed to produce fertile hybrids if they are allowed to flower). These vegetable crops all originated in the Mediterranean region from a naturally occurring wild species. The changes in each have involved the part of the plant that is utilized: the leaves in kale; the stem in kohrabi; the terminal bud in cabbage; the lateral buds in Brussels sprouts; and the flowers and inflorescences in broccoli and cabbage. These various crops have been produced within the ca. 10,000 year period within which people have been utilizing agriculture.

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Polyploidy Eukaryotic organisms, typically 2 sets of chromosomes/nucleus = diploid Gametes – have one set of chromosomes/nucleus (result of meiosis) = haploid Some plants – cells have >2 sets of chromosomes = polyploid triploid

= 3 sets

tetraploid = 4 sets pentaploid = 5 sets hexaploid = 6 sets etc.

A distinctive feature of plant evolution is the widespread occurrence of polyploidy, and this genetic phenomenon is often encountered in crops. Polyploidy occurs when a sporophyte plant has more than the standard two sets of chromosomes in each cell. Various levels of polyploidy are observed, although multiples higher than about 8 sets (octoploid) are rare (probably because of physical constraints that limit the volume of the nucleus).

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Polyploidy continued “Odd” polyploids (3x, 5x, 7x) – usually sterile - advantage for seedless fruit “Even” polyploids (4x, 6x, 8x) – often fertile - organs can be larger, including fruits, seeds - heterosis fixed Many crop plants are polyploid – see text, Table 1.1 Coffee, Cotton, Potato, Strawberry, Sugar cane, Tobacco, Wheat Even some crops that appear to be diploid are ancient polyploids: Corn, sunflower

Because of the need for each chromosome to pair with its counterpart (homolog) at meiosis, polyploids with an odd number of chromosome sets are usually sterile. This can be advantageous when it is desirable to produce fruits that lack seeds (such as in bananas and grapes) but generally limits further development of the crop because reproduction must be vegetative. Even polyploids, in contrast, may often be fertile and in fact occur commonly in nature (estimates are that 50-80% of naturally occurring plant species are polyploid). Many crop plants are polyploid, and induced polyploidy has been significant in breeding, particularly for ornamental plants. Even some crops that appear to be diploid have been shown to be genetically polyploid, based discovery of widespread duplication of genetic material within their genomes.

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Hybrid sterility

The production of new species through hybridization was once thought to be rare, but recent new evidence suggests that it is common and highly important. Many pairs of closely related plant species can be crossed to produce hybrids, but the hybrid plants are often sterile because differences between their chromosomes.

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Polyploidy can overcome hybrid sterility

A feature of polyploidy that has contributed to its major role as a element in plant speciation is that the simple step of doubling the chromosome sets can restore fertility to hybrids that may be sterile because of incompatibility between the chromosome sets of the original parents. This occurs because, after doubling of the chromosome sets, each individual chromosome will have a duplicate (homolog) with which it can pair at meiosis.

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Inbreeding Most Plants are Outcrossing – gametes from different individuals Some plants, particularly weeds and crop plants, are inbreeding - self-fertilization - self-compatibility Forced Inbreeding: - increased homozygosity - inbreeding depression Crossing between homozygous lines Æ Heterosis (hybrid vigor) - uniformity - need to produce new seed each year

Inbreeding is another mechanism that has been employed to improve crop plants. Most plants are outcrossing (and the various ways that can be observed in plants to facilitate or ensure outcrossing were one of the things that caught Darwin’s attention). Crops and weeds, in contrast, are often inbreeding – this has advantages in producing uniform progeny and ensuring seed set even under adverse conditions. Forced inbreeding has been used as a breeding tool in some crops – when a normally outcrossing crop such as corn is forced to inbreed, initially the resulting plants are actually smaller and stunted because of inbreeding depression. However, crosses between different inbred lines produce offspring that are both uniform (because the basic genetic makeup is the same in each individual) and also often larger and more vigorous than either parental line. This is called hybrid vigor, or heterosis, and is the basis for the highly successful industry of hybrid corn.

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Asexual Reproduction Asexual Reproduction Æ new plants identical to parent (clones) Applications of Asexual Reproduction: - vegetative propagation (cuttings, rhizome pieces etc.) - grafting

Another distinctive feature of plants is their propensity to produce exact copies of themselves through various mechanisms. This is based in part on the relatively simple development that plants exhibit because of their modular organization. The new plants are clones that are indentical to their parents. There are many applications of asexual reproduction to horticulture.

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Naming of Plants Scientific Hierarchy of Classification (See Table 1.5, page 35) Kingdom Division Class Order Family Genus Species

Phyta

“plants”

Anthophyta

“flowering plants”

Magnoliopsida“dicots” Fabales

“bean order”

Fabaceae

“bean family”

Phaseolus*

“beans”

P. vulgaris*

“common bean”

*Name written in Latin

This slide shows the group at each level of the scientific hierarchy of classification that the common bean would be placed within. Note that names at the family level and above are generally written in the language used within the country of the specialist, although English is so widely used that it can be considered now as the universal scientific language. In practice, taxonomists make the greatest use of the scientific name (genus + species) and the family of the plant.

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Species Names – Binomial Nomenclature Prior to Linnaeus – use of Phrase Names Linnaeus – each species called by genus name + species epithet = binomial Species – only category that is thought to be discrete, objective Species name – consists of genus + species epithet, written in Latin

The only category that scientists consider to be objectively defined is the species. Since the adoption of the work of the Swedish botanist Carolus Linnaeus (in Swedish, Karl Linne) in 1753, a species name is written as a binomial, that is 2 names: the genus name and the species epithet. The species name is written in Latin, and must agree with the grammar of that language (which has number, case, and gender). There is only one correct name for a given species (although there can be and is disagreement between different species as to how to circumscribe a given species). The correct name to use when more than one name is available is governed by the principle of priority, which states that the oldest name is the correct one.

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Principles of Botanical Names 1. Publication – name must be properly published according to rules of International Code of Botanical Nomenclature 2. Type method – each name is associated with a physical plant specimen (= type specimen) 3. Priority – Oldest properly published name is correct one

There is an International Code of Botanical Nomenclature that states explicit rules for the naming of plants. The Code is updated at each International Botanical Conference, and these are held every 6 years (the last one was in St. Louis in 1999). The rules include dictates for how names must be published, for determining the exact application of a name through the type specimen approach, and for deciding which name is to be used if more than one is available for a given species.

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Example – Rule of Priority leads to Change in Plant Name Wyethia trilobata Æ Complaya trilobata Æ Thelechitonia trilobata Æ Sphagneticola trilobata Complaya – published in 1991 Thelechitonia – published in 1954 Sphagneticola – published in 1900

The plant “Creeping Ox Eye” provides an example of how there can be changes in what is considered to be the correct scientific name. The plant is used as a ground cover in subtropical regions such as Florida, and is found throughout the world. It was originally placed in Wyethia, but when scientific studies showed that Wyethia included unrelated species, its name underwent several changes in rapid succession. John Strother coined a new name for it, but other scientists noted that previous names had been published for it although in rather obscure places. It is now placed in the genus Sphagneticola.

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Example – Change in Circumscription of Genus leads to changes in plant names

Chrysanthemum s.s. (3 species)

Leucanthemum

Dendranthemum

Horticulturists can become driven to distraction when scientists change the names of widely grown plants, especially when the scientific name is the same as the common name. Chrysanthemum is a sterling example – until recently, this was considered to be a large genus of wide north temperate distribution. Recent studies, however, showed that it was polyphyletic – a mixture of different lineages. The most recent studies have concluded that the genus name should be used for only 3 species of the Mediterranean region. Thus, the widely grown garden “Mums” now are called Dendranthemum, and our own ox-eye daisy (from which the cultivated Shasta daisy is derived) rejoices under the name Leucanthemum.

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A Rose by Any Other Name …? Cannabis sativa and the law Most Botanists: Cannabis has 1 species, C. sativa Some botanists recognize 3 species: C. sativa (hemp, cultivated for rope) C. ruderalis (wild form, weed) C. indica (high THC-form) Laws: originally proscribe marijuana (C. sativa) Æ argument that defendant not literally breaking law Eventual resolution: looked past botanical “semantics” – illegal regardless of what it is called by scientists

An example where plant names had potential legal implications is the case of marijuana. Although many botanists consider marijuana to be a single, variable species, others have proposed that the weedy, wild form and the most potent drug form are separate from the widely cultivated fiber plant (which is almost or completely devoid of the psychoactive compound, tetrahydrocannibinol, or THC). A legal defence was mounted during the middle and latter parts of the 20th century based on the technicality that some local or state laws mentioned only one species name in making illegal possession or use of this plant. This defense produced limited successes at the trial level, but eventually was overruled and has now been bypassed by more careful writing of the laws.

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Tuesday Lecture – Origins of Agriculture Read: Chapter 2

In the next lecture we will consider issues related to where and when people began to farm.

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