Reproduction

1
Reproduction Chromosome numbers are unique and
constant within species with the exception of
vegetatively reproduced crops (e.g. grasses or
sugarcane where one can produce interspecific
F1s. Definitions (some we have already
covered) Somatic cell non-reproductive cell
most of the cells in a plant diploid complement
of chromosomes Sex cell specialized cells found
in the anthers (male) and ovaries (female) of
plants very small percentage of the total plant
cells produce haploid cells called pollen and
ovules Diploid each somatic cell of most plants
have 2 copies of each unique chromosome and thus
is dipoid or 2n
2

Reproduction Definitions (contd) Gamete cell of
meiotic origin, specialized for fertilization
each chromosome complement is produced by the
fusion of two gametes, one from the male and one
from the female parent. Haploid each gamete
contains only 1 set o fchromosomes or half the
number found in somatic cells, written n, e.g.
n7 (barley) Basic set the haploid number from
each genome in plants that are polyploid, i.e.
containing genes from 2 or more species (allo) or
containing duplicated sets of itself
(autopolyploid), written x e.g. x13, 2n52
(upland cotton)
3
Reproduction Definitions (contd) Genome the
complete haploid complement of genetic
information. Zygote results from the fusion of 2
gametes, later called the embryo Homologous
Chromosomes in diploid cells, each member of a
chromosome pair each individual member is
referred to as a homolog chromosomes from
different pairs are called non-homologous
chromosomes.
4
Cellular Reproduction Types of cellular
reproduction Mitosis simple cell
division Meiosis reduction-division All cells
undergo mitosis but only sex cells undergo
meiosis. Both processes are technically nuclear
division followed by cell division or
cytokinesis. Nuclear division and cytokinesis
combined are referred to as the Cell Cycle NOTE
the student is to know the phases (not
sub-phases) and events within each for mitosis
and meiosis!!!see last slides in this set!!!!!!
5
Reproduction Crossing Over Reciprocal exchange
of chromosome segments occur between homologous
chromosomes in Meiosis I when they
synapsis. produces new combinations of genes in
a chromatid if the homologues are genetically
different (recall that 1 came from each
parent no loss or addition of genetic material
to either chromatid point of cross-over is
called a chiasma (plchiasmata) all 4
chromatids can be involved in crossing over,
ratios do not change if 2, 3 or 4 chromatids
involved. Recombinant Chromosome a chromosome
that emerges from meiosis with a combination
of genes (alleles) that differs from the
combination with which it started.
6

• Reproduction
• Importance of Meiosis
• Maintains chromosome number in sexually
  reproduced crops
• Produces variability through independent
  assortment
• Creates variability through crossing over

7
Reproduction Importance of Meiosis 2. Produces
variability through independent
assortment Consider a species with 4 pairs of
chromosomes AA BB CC DD with ABCD from
one parent vice versa Independent assortment
will produce at Metaphase I the following
possibilities ( the gametic possibilities) 2n
where n chromos. A A A A A A A
A A A A A A A A A B B B
B B B B B B B B B B
B B B C C C C C C C C
C C C C C C C C D D D
D D D D D D D D D D D
D D Parental types all others are
potentially new combinations
8
Reproduction Importance of Meiosis 2. Produces
variability through independent assortment Since
each homologous pair can produce 2 possibilities,
the total number of possible combinations is 2n
where nthe number of chromosome
pairs. Therefore the total number of possible
combinations is Corn (2n 20, n10) (2)10
1024 Humans (23 pairs) (2)23
gt8,000,000 Cotton (26 pairs) gt 67,000,000
9

• Reproduction
• Importance of Meiosis
• 3. Creates variability through crossing over
• Recombination through crossing over creates
  combinations of alleles that may not have
  occurred before.
• linkage will impact genotypic and phenotypic
  ratios
• epistasis will impact phenotypic ratios
• gene action will impact phenotypic ratios
• ploidy level will impact phenotypic ratios
• genetic abnormalities, e.g., aneuploidy, will
  impact genotypic and phenotypic ratios
• some of these we will discuss

10
Reproduction NOTE the student is to know the
phases (not sub-phases) and events within each
for mitosis and meiosis!!!see next 6 slides in
this set!!!!!! MITOSIS 5 phases 1 long and 4
relative short Interphase longest DNA of
each chromosome is replicated (but cant
see) the product is 2 exact copies now called
sister chromotids that are held together at
the centromere Note a Chromotid is one of the
two visibly distinct, longitudinal subunits of
all replicated chromosomes, which become visible
during mitosis. After mitotic anaphase, they
become sister chromosomes.
11
Reproduction Mitotic Interphase Sister A T A
T A T A T A chromotid T A T A T A T A T
Original Sister A T A T A T A T A
chromomsome chromotid T A T A T A T A T
centromere Replication is
similar to transcription except that both strands
are duplicated and the result is two chromotids
(chromosomes after anaphase) exact copies of the
original and attached at centromere
12
Reproduction Mitosis (contd) Interphase Prophase
doubled chromosomes (chromatids, attached at the
centromere) shorten and become visible. Nuclear
membrane disappears. Metaphase align along the
center of the cell (or equator) Anaphase spindle
fibers form and sister chromotids separate and
move toward opposite ends of the
cell. Teleophase formation of cell plate and new
nuclear membranes resulting in two cells with
identical genetic information
13
Reproduction MEIOSIS two successive divisions of
a diploid nucleus following a single replication
event in plants, the process results in the
formation of haploid gametes, a process called
gametogenesis Divided into 2 successive
phases Meiosis I (5 phases I, P, M, A, T)
(e.g. interphase I, etc). In Meiosis I,
chromosome replication occurs and homologues
pair, exchange chromatin, and
separate. Meiosis II (5 phases I, P, M, A, T)
(e.g. interphase II, etc.) In Meiosis II,
chromotids separate resulting in 4 cells each
with a haploid complement.
14
Reproduction Meiosis I Interphase I
replication of chromosomes to produce sister
chromotids joined at the centromere Prophase I
homologous chromosomes pair, or synapsis.
Homologous chromosomes have genes at
corresponding loci controlling a common
hereditary trait. There are also genes that
regulate synapses. Chromosomes become visible.
Each synapsed set of homologous chromosomes
consists of 4 chromotids and is referred to as a
bivalent or tetrad. Exchange of alleles, i.e.
crossing over occurs. Nuclear membrane
disappears. Metaphase I homologous chromosomes,
still showing synapses points, line up at the
equatorial plate. Note that in mitosis it was
chromosomes with sister chromotids that aligned,
here it is homologues
15
Reproduction Meiosis I (contd) Anaphase I
spindle fibers appear attached to the aligned
homologous chromosomes homologues separate and
move along the spindle fiber to opposite ends of
the cell. NOTE that homologous chromosomes
separate randomly. This is called Reductional
Division because we have reduced the number of
chromosomes although the absolute amount of DNA
is the same because each chromosome is composed
of sister chromatids. Teleophase I spindle
fibers disappear nuclear membranes appear and a
cell plate appears to form 2 separate cells in
most Megaspore Mother Cells but not necessarily
in Microsporogenesis.
16
Reproduction Meiosis II Essentially a repeat of
mitosis but remember that we have only one of
each homologous pair of chromosomes, i.e. we have
n number or the haploid compliment. Interphase
II may not exist in all plant species, brief
period in others. Nothing happens, no crossing
over, no duplication Prophase II shortening of
chromatids Metaphase II alignment of
chromatids Anaphase II separation of
chromatids Telophase II development of
membranes in 4 haploid cells
17
Reproduction Asexual
18

• Reproduction in Plants Asexual
• New plants develop from cells in the absence of
  sexual process
• Vegetative reproduction or propagation
• roots, tubers, stolons, rhizomes, stem/leaf
  cuttings, tissue culture
• Clone plants propagated vegetatively from a
  single plant
• genetically identically, barring mutations, to
  the parent plt.
• used to develop breeding lines when seed prod.
  is inadequate
• used for genetic studies when genetic uniformity
  is required
• seed production fields of forage crops may be
  established vegetatively to insure genetic
  purity
• some grasses/crops do not produce enough or any
  seed

19
Reproduction in Plants Asexual Other vegetative
propagation includes root crops such as cassava
and sweet potato tubers as with potatoes and
most yams
20

• Reproduction in Plants Asexual
• Apomixis (w/o mixing)
• an asexual process resulting in the formation
  of plantlets or seeds.
• with apomitic seed the process occurs in plant
  parts normally associated with the sexual process
• Types of Apomixis (2)
• Vivipory formation of plantlets or bulblets
• Agamospermy (without gametes)
• formation of seeds w/o union of eggs and sperm
• pollination fertilization of polar nuclear
  may occur

21

• Reproduction in Plants AsexualApomixis (contd)
• Vivipory
• Agamospermy (3 types)
• 2.1. Apospory (w/o spores)
• an unreduced embryo mother cell results from
  a somatic cell in the ovule
• MMC ?meiosis?Megaspore (aborts)
• nucellar cell (2n)?2 mitotic events w/o
  cytokinesis ?embryo sac
• cytologically looks like a normal embryo sac
  except it contains only 4 nuclei and not 8.

22
Reproduction in Plants Asexual Apomixis
(contd) Agamospermy (3 types) 2.1 Apospory (w/o
spores) 2.2 Diplospory (with spores) MMC ?no
meiosis?megaspore (2n) 2n megaspore?3 mitotic
divisions w/o cytok.?embryo sac (looks
cytologically normal (but all nuclei are 2n))
23
Reproduction in Plants Asexual Apomixis
(contd) Agamospermy (3 types) 2.1 Apospory (w/o
spores) 2.2 Diplospory (with spores) 2.3
Adventitious Embryony is a third type of Apomitic
seed production. Differs from Apospory and
Diplospory in that no embryo sac is
formed an embryo arises from a somatic cell
may be a sexual embryo and an adventitious
embryony embryo and in these species both may
survive
24

Reproduction in Plants Asexual Apomixis
(contd) Agamospermy (3 types) 2.1 Apospory (w/o
spores) 2.2 Diplospory (with spores) 2.3
Adventitious Embryony Pollination and
fertilization Parthenogenesis fertilization is
not required, i.e. autonomous development of
endosperm Pseudogamy fertilization of the polar
nuclei is required for endosperm development
25
Reproduction in Plants Asexual Apomixis
(contd) General information Occurs in gt 300
species and 35 families Prominent in Poaceae
and Rosaceae Grasses (Poa, Panicum,
Pennisetum, Paspalum spp.) Fragaria
(strawberry) Rubus (blackberry) Malus
(apple) Citrus (orange, etc.) Cucumis
(cucumber)
26
Reproduction in Plants Asexual Apomixis
(contd) General information (contd) Obligate
only produces apomictic progeny NO
VARIABILITY Dallasgrass may be the best
example Facultative some sexual seed are
produced providing for the opportunity for
selection of new and superior types sexuality
may be 1 to 99 GRT a species that produces 80
or more of its seed asexually is bred through
techniques applicable to apomictic
species Apomixis is a heritable trait Breeders
and geneticists are interested in this trait for
other crops such as corn. WHY?
27
Reproduction in Plants Asexual Apomixis
(contd) Indicators of Apomixis seed set in
isolated female (in spp. Having ? and ?
plants) uniform progeny that look very similar
(or identical) to parent high seed set in
absence of good pollen failure of
hybridization twin seedlings multiple embryo
sacs within an ovule Remember Apomixis results
in uniform progeny Sexuality produces variable
progeny
28
Reproduction in Plants Asexual Apomixis
(contd) Requirements for breeding apomicts 1.
Cross-compatible sexual germplasm 2. Apomicts
that produce pollen 3. Effective crossing
techniques 4. Recoverability of obligate or
highly apomictic progeny Efficiency of apomictic
breeding 1. Releases a wide range of
variability in F1 hybrids 2. Permanent hybrid
vigor (in obligates) 3. Rapid development of
cultivars 4. Escape from sterility problems in
wide species crosses 5. Simplified seed
production
29

• Additional Comments Apomixis and Apomitic
  Breeding
• Apomixis affects megasporogenesis and
  megagametogenesis but NOT micorsporogenesis,
  i.e., apomitic plants produce normal pollen (n).
  This occurs in both aposporous and diplosporous
  plants and may be the key to breeding apomitic
  species for utilization of apomixis
• Facultativeness some progeny results from
  normal meiosis and normal fertilization,
  therefore the types of progeny from crossing
  sexual and apomitic types are
• 1. apomeiotic egg (2n) normal pollen (n)
  2nn BIII hybrid
• 2. apomeiotic egg (2n) (noneparthenogenesis)
  2n0 maternal type
• 3. meiotic egg (n) normal pollen (n) nn
  BII hybrid (normal)
• 4. meiotic egg (n) (noneparthenogenesis)
  n0 haploid
• NOTE Extremely rare to find 2n pollen

30

• Additional Comments Apomixis and Apomitic
  Breeding
• Probably no such thing as 100 obligate apomixis
  (Savidan, V. 2000. Apomixis Genetics and
  Breeding. Plant Breeding Reviews. John Wiley
  Sons.)
• Such reports in the literature probably the
  result of poor detection techniques most likely
  relying on phenotype and not cytological
  analyses.
• The most success in apomictic forage grasses to
  date (2000) has been achieved with facultative
  apomictics in Brachiaria and Panicum in Latin
  America, e.g. by CIAT in Colombia.

31

• Additional Comments Apomixis and Apomitic
  Breeding
• Savidan, V. 2000. Apomixis Genetics and
  Breeding. Plant Breeding Reviews. John Wiley
  Sons.
• Simplistic view polyploidy necessary for
  apomixis and it impossible to achieve apomixis
  in diploids. This was thought to be the result
  of either 1 the dominant A allele responsible
  for apomeiosis could not be transmitted through
  haploid gametes or 2 a dosage effect of the
  dominate A apomitic allele for expression, i.e.
  more than 2 copies of A had to be present for
  expression.
• However, dihaploid apomitics have been described
  experimentally in the 1990s. Appears that
  apomixis can only be transmitted through the
  production of dihaploids from apomictic
  tetraploids BUT the dihaploids did not produce
  any apomictic progeny again suggesting that the A
  allele could not be transmitted through haploid
  pollen

32

• Additional Comments Apomixis and Apomitic
  Breeding
• Savidan, V. 2000. Apomixis Genetics and
  Breeding. Plant Breeding Reviews. John Wiley
  Sons.

33
(No Transcript)
34
(No Transcript)
35
(No Transcript)
36
(No Transcript)
37
(No Transcript)
38
(No Transcript)
39
(No Transcript)
40
(No Transcript)
41
(No Transcript)
42
Reproduction in Plants Sexual Self pollination
transfer of pollen from the anther to the stigma
of the same flower or to another flower of the
same plant or clone Cross pollination transfer
of pollen from the anther of a flower of one
plant to the floral stigma of another plant that
is not a clone Self fertilization the union of
the egg and sperm (gametes) from the same plant
or clone Autogamy (self) Allogamy (cross)
43
Reproduction in Plants Autogamy and Allogamy are
not absolute categories e.g. barley and soybean
are selfed but average 0.5 outcrossing near
completely closed flower at pollination cotton
completely open flower, but is self-pollinated in
the absence of bees corn will be mostly cross
pollinated (monoecious flowers) but will be
about 5 selfed under field conditions See
Poehlman Sleeper pages 30 and 32 for a listing
of self and cross pollinated crops. (student
should know the more common)
44
Reproduction in Plants Terminology Protandry
pollen sheds before stigma is receptive
(outcrossing) Protogyny stigma matures and
ceases to be receptive before pollen is shed
(outcrossing) Chasmogamy stigma receptive and
pollen shed after flower is open (outcrossing,
usually) Cleistogamy stigma is receptive and
pollen sheds in closed flower (selfing) Staminate
male flower Carpellate or Pistillate female
flower
45
Reproduction in Plants Terminology on Flower
Distribution Monoecious ? and ? flowers on same
plant e.g. corn Dioecious ? and ? flowers on
separate plants, e.g. asparagus Mixed or
Polygamous ? and ? and perfect flowers on same
plant Note that there are what appears to be
barriers to pollination in Pin flower long
styles and short stamens (outcrossing) Thrum
flower short style and long stamens
(selfing?) HOWEVER, these types express self
incompatibility, usually sporophytic
incompatibility, which we will discuss later!!