Sexual and Asexual Reproduction
Long term survival requires reproduction. Even the longest-lived
organisms are less than 10,000 years old.
– Cellular machinery wears out, or gets clogged with waste products.
– Environmental conditions change
Plants often reproduce asexually, through cuttings or runners or buds
(e.g. potatoes). The resulting plants are clones: they are genetically
identical to the parent.
– Used to preserve good combinations of traits.
Sexual reproduction is also found in plants, and in all animals. Sexual
reproduction means combining genes from two different parents,
resulting in new combinations of genes. Each parent contributes a
randomly-chosen half of their genes to the offspring.
– This can be a good thing, because some new combinations will survive
better than the old ones.
– It can also be bad: lack of uniformity in the offspring.
Plant meristems are usually capable of generating
all the tissue types found in a plant.
Some plants naturally propagate vegetatively,
usually through modified stems: flower bulbs,
corms, and rhizomes; suckers; stolons (runners).
Artificial vegetative propagation can be down by
taking a cutting from the stem (containing at least
one meristem) and encouraging it to grow roots.
More advanced methods include grafting and
Grafting is common among fruit trees: the stem
tissue of one plant is fused to a stem from
Commonly done: a hardy rootstock is grafted to
the stem of a better fruit variety.
Another method: attaching a bud form one plant
to the stem of another. You can produce an
apple tree bearing several different types of apple
The cambium layers of the two plants being
grafted need to be brought into contact. This
allows the xylem and phloem to connect to each
It is possible to graft a potato rootstock to a
tomato top, so both underground potato tubers
and aboveground tomato fruits are formed on
the same plant.
A more modern way of propagating plants
vegetatively is through tissue culture. This is often
– Useful for genetically engineered plants, for plants
that don’t set viable seeds, and for rare and valuable
Unlike animals, many plant cells, especially in the
meristems, are totipotent: they can generate an
entire plant under the proper conditions.
Pieces of the plant are cut out and placed on an
agar medium under sterile conditions.
Manipulating plant hormones is the key: an excess
of auxin produces roots, and excess of cytokinin
produces shoots, and a balanced mixture allows the
cells to multiply as an undifferentiated mass of cells
called a callus.
Pieces of the callus can be cut out and propagated
Diploid: having 2 copies of each chromosome, one set from each
– Humans have 46 chromosomes, 23 from each parent.
– Almost any organisms you can see: plant, animal, fungus, is diploid.
Haploid: having only 1 copy of each chromosome.
– Sperm and eggs (=gametes) are haploid
– moss, a primitive plant, is haploid for most of its life
Plants, animals, and other eukaryotes alternate between haploid and
diploid phases. This is called alternation of generations.
• Diploid organism generates haploid
gametes using the process of meiosis.
The gametes combine during the process
of fertilization to form a new diploid
• In animals, the haploid phase is just one
cell generation, the gametes, which
immediately do fertilization to produce a
diploid zygote, the first cell of the new
• In plants, the haploid phase is several cell
generations at least.
– Lower plants are mostly haploid
– Higher plants are haploid for only a few
• The diploid plant is called the sporophyte,
and the haploid plant is called the
The science of genetics is devoted to understanding the patterns of how
traits are inherited during sexual reproduction. It was founded by Gregor
Mendel in the 1850's, using pea plants. Despite the obvious differences,
humans and peas have very similar inheritance patterns.
The fundamental observation of genetics: within a species, there are a fixed
number of genes, and each gene has a fixed location on one of the
– This allows genes to be mapped: a gene's neighbors are always the same.
– Most species of higher organism have about 25,000 different genes distributed
onto 10-30 different chromosomes.
Alleles. Many genes have several variant forms, which are called
– For example, a gene the produces color in the flower might have a purple
allele and a white allele. These alleles are designated P and p.
– Differences in alleles are what makes each human different from all
True-breeding lines. If you cross close relatives with each other for
many generations, eventually all the offspring look alike.
– Mendel started with several true-breeding lines, which differed from
each other in 7 distinctive characteristics
In many plants, you can self-pollinate: cross the
male parts of a plant with the female parts of the
– In this case, both copies of any given gene are
identical. This is called homozygous. The plants are
homozygotes, either PP (purple) or pp (white).
– The closest cross you can do in animals is brother x
Hybrids. If you cross two true-breeding lines with
each other and examine some trait where the
parents had different alleles, you produce a
heterozygote: the two copies of the gene are
– Surprisingly, you often find that the heterozygote
looks just like one of the parents. The Pp
heterozygote is purple, just like its PP parent.
– This is the F1 generation in the diagram.
Dominant and recessive. If a heterozygote is
identical to one parent, the allele from that
parent is dominant. The allele from the other
parent is recessive. That is, the heterozygote
looks like the dominant parent.
– This is why we say purple is dominant to white,
and give purple the capital letter P.
Phenotype and genotype. Phenotype is the
physical appearance, and genotype is the
– The heterozygote in the previous paragraph
has the same phenotype as the homozygous
dominant parent (i.e. purple flowers), but a
different genotype (the heterozygote is Pp and
the parent is PP).
Now we want to move to the next
generation, by self-pollinating the
When a heterozygote undergoes
meiosis to produce the haploid
gametes, half are P and half are p.
– These gametes combine randomly,
producing 1/4 PP, 1/2 Pp, and 1/4
Since PP and Pp have the same
phenotype, 3/4 of the offspring are
purple and 1/4 are white.
Much of Mendel’s work involved pairs of genes:
how do they affect each other when forming
the gametes and combining the gametes to
form the next generation?
Simple answer: in most cases pairs of genes act
completely independently of each other. Each
gamete gets 1 copy of each gene, chosen
1. seed shape. Dominant allele S is smooth;
recessive allele s is wrinkled.
2. seed color. Dominant allele Y is yellow;
recessive allele y is green.
Heterozygous for both has genotype Ss Yy,
which is smooth and yellow. Gametes are
formed by taking 1 copy of each gene randomly,
giving ¼ SY, ¼ Sy, ¼ sY, and ¼ sy.
These gametes can be put into a Punnett square
to show the types of offspring that arise.
– Comes out to 9/16 smooth yellow, 3/16 smooth
green, 3/16 wrinkled yellow, and 1/16 wrinkled
– 3/4 are yellow, 1/4 are green, and 3/4 are round,
1/4 are wrinkled
Many traits don’t seem to fall
into discrete categories: height,
for example. Tall parents usually
have tall children. Short parents
have short children, and tall x
short often gives intermediate
height. In all cases, wide
Simple interactions between
several genes can give rise to
continuous variation. Also:
variations caused by
environment, and our inability to
distinguish fine distinctions lead
us to see continuous variation
where there actually are discrete
Most pairs of genes assort
However, if two genes are close
together on the same
chromosome, they are said to be
linked, which means the genes
don’t do into the gametes
independently of each other.
The closer two genes are, the
more the parental combination
of alleles stays together. This
relationship can be used to make
maps of genes on chromosomes.
Methods of Crop Improvement
The idea that we can improve the inherited characteristics of crop
species is fundamental. Very few of the plants we use are unmodified
wild plants: most of them have been modified to make them easier to
grow and harvest, and to increase the quality and quantity of the
We will see many examples of crop improvement this semester. Here
are some of the basic methods used.
Single Gene Traits and Mutation
Single gene traits. Many useful traits are controlled
by a single gene. Spontaneous mutations can lead to
important, abrupt changes
– A good example: sweet corn. The recessive mutation su
(sugary) produces kernels that are 5-10% sugar. But,
only when homozygous: the non-sugary allele (Su) is
Single gene mutations occur rarely, but often enough
so that observant people notice and propagate them.
– Sweet corn was recognized and propagated by several
Native American tribes. The Iroquois introduced it to
– Mutation rate: 1 in 10,000 to 1 in 1,000,000 plants.
– Artificially-induced mutation occasionally works, but
most are spontaneous.
Single gene traits are inherited in a Mendelian
each individual carries one copy of the gene from each
– the relationship between phenotype (sweet vs. starchy
corn) and genotype (homozygous or heterozygous) is
determined by dominance vs. recessiveness.
Polygenic Traits and
Polygenic traits. Many traits are controlled
by many genes, each of which contributes a
small amount to the phenotype. Grain yield
is a good example: lots of genes contribute
Such traits respond well to selection. In the
simplest sense, selection means using the
best seeds to start the next generation. If
this is done consistently, the crop slowly
improves over many generations.
Genetic research has led to an
understanding of what happens during
selection. This allows much faster and
more effective selection than just saving the
This is often called “conventional breeding”
or “traditional plant breeding”. It has been
the main way crops have been improved for
a long time.
Normal diploids have 2 copies of every
chromosome. Sometimes it is possible to double
this number, making a tetraploid, 4 copies of
– The drug colchicine does this by causing meiosis to
produce diploid gametes instead of the normal
haploids. Then, diploid sperm + diploid egg =
Tetraploids are often bigger, healthier, more
nourishing than their diploid parents.
– Examples: cotton, durum wheat, potato, daylily
Tetraploid is a form of polyploid, which means
having more than 2 sets of chromosomes (2 sets =
There are triploid (e.g. banana and watermelon),
hexaploid (bread wheat, chrysanthemum), and
octaploid (strawberry, sugar cane) crops
Triploids are sterile
Plants are not as rigid in maintaining species boundaries as
animals are. It is often possible to produce hybrids between two
different, but closely related species.
– Members of the same genus will often hybridize
The resulting plants often have characteristics different
from both parents
– Often sterile, but many plants can be propagated vegetatively
• The grapefruit is a naturally-occurring hybrid between a
pomelo (native to Indonesia) and a sweet orange (native to
Asia).. It was discovered in Barbados in 1750, then brought
to Florida and propagated.
Hybrids have an “x” in their species name: Citrus x paradisi
Sometimes, a hybrid will spontaneously double its chromosomes,
so you end up with a tetraploid . These interspecies tetraploids
are usually fertile, and they benefit from the general effect of
tetraploidy: bigger, healthier plants.
In the last 30 years it has become possible to take a gene out of one
organism and put it into the DNA of another organism. This process
is called genetic engineering. The resulting organisms are genetically
modified organisms (GMOs) and the gene that has been
transplanted is a transgene.
There are no real interspecies barriers here: all organisms use the
same genetic code, so genes from bacteria (for example) will
produce the correct protein in a corn plant.
– However, some modifications must be made to the signals that control
gene expression, since these are more species-specific.
A few examples:
– Bt corn. Bacillus thuringiensis, a soil bacterium, produces a protein that
kills many insect pests, especially the corn earworm. The gene for this
protein has been transplanted into much of the US corn crop.
– Roundup Ready soybeans (plus other crops). Roundup is the Monsanto
brand name for the herbicide glyphosate. A bacterial gene that confers
resistance to this herbicide has been transplanted to many crops. The
farmer can then spray the fields with glyphosate and kill virtually all the
weeds without harming the crop. About 87% of the US soybean crop is
now Roundup Ready transgenic plants.
Some cultural issues here: are GMOs safe to eat?
The first step in genetic engineering is
Molecular cloning means taking a gene, a piece
of DNA, out of the genome and growing it in
bacteria. The bacteria (usually E. coli) produce
large amounts of this particular gene.
The cloned gene can then be used for further
research, or to produce large amounts of
protein, or to be inserted into cells of another
species (to confer a useful trait).
The basic tools:
1. plasmid vector: small circle of DNA that
grows inside the bacteria. It carries the gene
2. Restriction enzymes: cut the DNA at
specific spots, allowing the isolation of specific
3. DNA ligase, an enzyme that attached
pieces of DNA together.
4. transformation. Putting the DNA back
into living cells and having it function.
The Cloning Process
• 1. Cut genomic DNA with a
• 2. Cut plasmid vector with
the same restriction
• 3. Mix the two DNAs
together and join them with
• 4. Put the recombinant DNA
back into E. coli by
• 5. Grow lots of the E. coli
containing your gene.
The real trick, however, is to find
the gene that confers your
Once a gene of interest has been identified and cloned, it
must be put into the plant.
Usually done with plant tissue culture. Small pieces of a
plant can be grown as an undifferentiated mass of cells
on an artificial growth medium.
– Then, when treated with the proper plant hormones, these
cells develop roots and shoots. They can then be
transferred to soil and grown as regular plants.
To make transgenic plants, DNA gets put into the tissue
culture cells, by one of several methods:
One method is the gene gun: tiny gold particles are
coated with the DNA, and then shot at high speed into
the cells. The gold particles penetrate the cell wall and
membrane. Some end up in the nucleus, where the DNA
gets incorporated into the chromosomes.
An important issue: the proteins produced by transgenes
are identical to those produced in the original species,
because the genetic code is universal.
However, the signals needed to express these genes are
plant-specific, not universal.
Centers of Domestication
Primary theory came from Nikolay Vavilov,
– Vavilov was a Russian who came to a bad end in one of Stalin’s prison camps
in Siberia. He believed in Mendelian genetics, which was considered
“bourgeois” and thus evil by the Communist Party. (Lysenko)
“Centers of domestication”. The idea is that a plant was probably
first domesticated where there are many wild relatives living and
where there is a lot of variation in the domesticated plant. Lots
of diversity near a domestication center.
Eight major centers:
– Southern Mexico and Central America: maize, beans, cotton, pepper, sweet
– South America (mostly Peru): potato, common bean, tomato, cocoa, tobacco
– Mediterranean: pea, mustard, flax, cabbage, asparagus, clover, olive
– Middle East (Turkey and eastward): wheat, alfalfa, rye, lentil, melon, fig
– Ethiopia: barley, millet, coffee, indigo, sorghum
– Central Asia: onion, apple, carrot, almond, grape
– India: sugar cane, yam, cucumber, chickpea, orange, coconut, banana,
– China: soybean, buckwheat, peach, opium poppy, tea
• More recently, Jack Harlan (from U of Illinois)
examined genetic data and found that many
crops were domesticated multiple times in
multiple locations. Also, some were
domesticated over very wide areas that don’t
seem much like “centers”.
• Nevertheless, our current crops come from many
different areas of the world. We will look at the
origins of specific crops as we study them.