Modes of Reproduction

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Modes of Reproduction
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• Sexual reproduction involves union of gametes
• Sperm and egg produced via sporogenesis
• Asexual reproduction bypasses fertilization
• Some plants combine both strategies
• Differences between animals and plants
• Movement vs. stasis..pollen can move
• Developmental fate vs. meristematic plasticity
• Totipotency
• Alternation of generations and endosperm
  development
• Embryo to adult vs. embryo-dormant-adult

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Terms and Concepts

• Megasporogenesis
• Formation of 4 haploid (n) megaspores from
  meiotic division of the 2n megaspore mother cell
• Microsporogenesis
• Formation of 4 haploid (n) microspores from
  meiotic division of the 2n microspore mother cell
  
• Meiotic Stages-Reductional Division
• Meiosis I is first division of MMC, each of the
  two cells divides again in Meiosis II to form 4
  cells
• Mitotic Stages-Equational Division
• No pairing or crossing over of chromosomes

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Overview of Gamete Formation and Fertilization

• Male
• Each of 4 microspores develops into a pollen
  grain
• Each microspore undergoes a mitotic division and
  forms a generative and vegetative nucleus
• The generative nucleus divides mitotically to
  form 2 sperm
• Female
• 3 of 4 microspores disintegrate
• 4th megaspore undergoes 3 mitotic divisions to
  form the 8 nucleate female gametophyte, or embryo
  sac
• Fertilization
• Egg cell unites with sperm from pollen to form a
  2n zygote
• Mitotic division of zygote results in embryo
  (seed)
• Two polar nuclei and second sperm form 3n
  endosperm

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Meiosis
Mitosis of Nucleus (2x)
2 sperm and tube nucleus
Microspore Mother Cell 2n
Pollen tube
Pollen Grain Tube and generative nucleus
Stigma Style SEED
Male
Microspores
All functional
Double Fertilization 2 polar nuclei sperm 3n
endosperm Egg sperm zygote
Female
Megaspores
Mitosis of Nucleus (3x)
Antipodals Polar nuclei Egg
Megaspore Mother Cell 2n
3 degenerate
8-nucleate female gametophyte
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Endosperm Genotypes

• Endosperm is triploid tissue
• Of great importance in human diet
• More genotypes possible in endosperm
• AA x aa results in endosperm AAa
• Reciprocal cross would have Aaa endosperm
• Zygote Aa in either case
• Xenia effect immediate effect of pollen parent
• on female seed parent
• Implications for seed crops

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Cross and Self-Pollination

• Individual flowers
• Hermaphrodite bisexual
• Pistillate female
• Staminate male
• Individual plants and plant populations
• Hermaphrodite bisexual
• Monoecious male and female
• Dioecious male or female
• Gynoecious female
• Androecious male
• Gynomonoecious bisexual and female
• Andromonoecious bisexual and male
• Trimonoecious / trioecious bisexual and male
  and female

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Floral Modifications in Hermaphroditesadapted
from Dellaporta and Calderon-Urrea, 1993, Plant
Cell 51241

• Mechanism Description
• Facilitates self-pollination
• Cleistogamy flowers self before opening
• Homogamy synchronous maturation of m female
• Facilitates cross-pollination
• Chasmogamy open flowers can cross-pollinate
• Dichogamy stigma and stamen mature
  differentially
• Protogyny stigma receptive before anthesis
• Protandry anthesis precedes stigma receptivity
• Incompatibility sexual crosses fail with like
  plants
• Sterility nonfunctional sex organs / gametes
• Heterostyly modification of floral parts

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Sex Expression in Cucumber

• The m allele conditions andromonoecy
• The F allele conditions femaleness
• The A locus conditions maleness
• The De gene enhances pistillate flowers
• M-F- plants are gynoecious
• M-ff plants are monoecious
• mmF- plants are hermaphroditic
• Mmff plants are andromonoecious
• Standard cultivars monoecious, gynoecious
• Methionine SAM ACC Ethylene
  Femaleness
• F
  Cobalt Silver Nitrate, GA

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Populations and Reproductive Modeadapted from
R.J. Lambert, Univ. Illinois

• Characteristic Selfer
  Crosser
• Genotypes of sporophyte Homozygous
  Heterozygous
• Genotypes of single plant Same
  Different
• Progeny of single plant Homogeneous
  Heterogeneous
• Progeny of F1 plant Heterogeneous
  Heterogeneous
• Self-incompatibility None
  Frequent
• Inbreeding depression Usually none
  Usual

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Evolution of Self-Pollination

• Inbreeding thought to arise from outbreeding
• Wheat, tomato, pea are derived inbreeders
• High immediate fitness with inbreeding?
• Take advantage of niche environment
• Showy flowers not useful in many legumes
• But this facilitates some cross pollination
• Even low levels of crossing may be very important
• Inbreeders may have some flexibility
• More devices in nature to prevent inbreeding

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Some Examples
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• Selfers
• Barley, oat, rice, sorghum, wheat, pea, bean,
• peanut, soybean, apricot, nectarine,
• peach, citrus, cotton, eggplant, lettuce, pepper,
  tobacco, tomato, parsnip, endive
• Crossers
• Maize, rye, apple, avocado, banana, cherry, fig,
  grape, mango, olive, pear, plum, alfalfa, almond,
  pecan, walnut, beet carrot, artichoke, onion,
  cucumber, pumpkin, spinach, squash

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Asexual Reproduction and Apomixis
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• Vegetative reproduction and apomictic
  reproduction are means of asexual reproduction
• Vegetative stolons, rhizomes, buds, shoots,
  cuttings, etc.
• Apomixis production of seed without fusion of
  gametes
• Facultative apomicts typical
• Procreation without recreation (S. Peloquin)
• Megaspore mother cell divides meiotically, but
  embryo sac aborts
• Somatic cells in ovule divide mitotically to form
  embryo sac
• Diplospory embryo sac from MMC via mitosis
• Apospory embryo sac from nucellus cells
• Adventitious embryony diploid ovule cell forms
  embryo mitotically

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Sexual Reproduction Vegetative Apomixis Allo
gamy Autogamy stolon Protandry cleisto
gamy cutting Protogyny time of pollen
shed rhizome Monoecy perfect flowers bulb Dioecy
tiller Exserted style bulbil Self-incompat
ibility graft Male sterility bud
Recurrent Non-Recurrent Gameto
phytic Adventitious Embryony embryo sac
cell Nucellus or integument leads to 1n
leads to 2n sporophyte
sporophyte Diplospory / Aposproy Parthenogene
sis / Apogamety
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Gametophytic Apomixis Adventitious
Embryony Alternation of Generations No
Alternation of Generations
Diplospory Apospory MMC forms
embryo somatic cells Sac mitotically in ovule
form embryo sac
Development of embryosac Development of
embryo
2n Gametophyte
Parthenogenesis Apogamety Egg cell Non egg
cell
2n Sporophyte
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Floral Biology
Stigma Ovary Nectaries Anthers Petals
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Case Study
Pollen Flow and Gene Dispersal

• Wind pollination, insect pollination,
  self-pollination
• Outcrossing percentages vary in facultative
  selfers
• Selfing rates vary in facultative crossers
• Faba bean is partially allogamous and is
  pollinated by honeybees and several different
  kinds of non-social ground nesting apoids
• Average cross-pollination is 50
• Pollination involves tripping, where
  pollinators cause release of style and rupture
  the stigmatic surface
• Spontaneous disruption of membrane causes selfing
• Pollinator situation very dependent upon
  environment

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Case Study
Pollen Flow and Gene Dispersal
Carre et al., 1998, Crop Science 38322-325

• Position of flower in bee visitation sequence
  studied in Faba bean
• Caged plants with single Bombus terrestris
  individuals used
• 17 bees visited 1261 flowers, forming 2812 seeds
• Seeds analyzed for hybridity
• 21.3 and 17.5 outcrossing for first 5 and 10
  flowers visited
• 78, 13.4, and 8.5 of pods contained 1,2, and 3
  hybrid seeds
• Unaffected by floral node
• After first visits to recipient flowers, crossing
  loses efficiency
• High frequency of alternate foraging needed
• Placement of parental lines is crucial for
  crossing success

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Case Study
Average Pollen Dispersal
Lavigne et al., 1998, TAG 96886-896

• Transgenic canola now a reality on world market
• Herbicide-resistant strains of canola available
• Efforts to limit uncontrolled escape of genes to
  wild/weedy plants
• Or, crop itself could become a weed outside of
  crop production
• Or, transgene could be transferred to another
  nearby field of crop
• Or, a traits like herbicide resistance is
  transferred to weeds
• Pollen dispersal of a single plant measured
• 50 of pollen fell within 3 m probability of
  pollination after that decreased slowly along a
  negative exponential of distance

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Case Study
Case Studies in Reproduction

• Single dominant gene control of apomixis in
  Tripsacum, due to displospory, mapped as cluster
  of genes
• (Grimanelli et al., 1998, Heredity, 8033-39)
• Apomicts identified in Malus spp. when
  single-gene dominant conditioned red pigmentation
  in seedlings is absent
• (Ur-Rahman et al., 1997, TAG, 951080-1083)
• Self-incompatibility in Indian mango, Floridian
  mango is self-fertile. Outcrossing rates in
  self-fertile mango increased dramatically as
  fruit matured- selective abscission of selfed
  progeny
• (Dag et al., 1998, JASHS 123618-622)

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Case Study
Case Studies in Reproduction

• Maintenance of hybrids or other genotypes
  difficult to propagate sexually can be conducted
  via apomixis
• Kentucky bluegrass (Poa pratensis) is a major
  turfgrass species
• It is a facultative aposporic apomict, but can be
  sexual as well
• Apomixis in this species requires fertilization
  of polar nuclei for endosperm development, but
  progenies are of maternal origin only. This
  process is known as Pseudogamy
• Parthenogenesis in Poa pratensis likely
  controlled by a single dominant gene. Sexual x
  apomictic matings support a single dominant gene
  model that is simplex in polyploid Poa
• (Barcaccia et al., 1998, TAG)

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Case Study
Utilizing Apomixis in Breeding

• Current efforts to transfer apomixis genes to
  sexual species
• Major cereal grasses have been targeted
• Fixation of hybridity a major commercial target
• Vielle Calzada et al. (Science 274, 1996)
  Asexual Revolution
• The harnessing of apomixis will lead to large
  increases in agricultural production
• Apomixis seemingly under simple genetic control
• Mutants have arisen which result in apomixis
• However the process may not be simple
• And efforts so far have not resulted in transfer
  of apomixis