Download Plant Breeding and Plant Biotechnology

Variants in the
Reproductive Process
Apomixis




Substitute for sexual reproduction
Development of an embryo w/o sex
Identified in over 300 species across
37 families.
Not rare but relatively uncommon
Reproductive Behavior



Obligate Apomict: a plant which reproduces only by
asexual reproduction.
Facultative Apomict: a plant which has the
potential to reproduce either sexually or asexually.
Both process may occur simultaneously or one may
be predominant.
Within a genus or species, all reproduction
strategies may be possible
Apomixis

Must bypass two different processes:
 Meiosis:
no reduction of chromosome #
 Fertilization: development of embryo w/o
fusion of sperm and egg
Stimulus for Embryo
Development


Autonomous: requires no further stimulation embryo and endosperm may be developed before
flowers open
Pseudogamy (False-fertilization): development of
2n gametophyte requires pollination and/or pollen
tube growth. Sometimes a sperm nucleus fuses
with polar nuclei, but never with egg
Genetics of Apomicts




Apomixis is genetically controlled in most
cases studied
Control is complex (multigenic) and
recessive to sexual reproduction
Polyploidy may affect apomictic
reproduction
Hybridization tends to increase apomixis
Use of Apomixis in
Breeding

Production of exact replicas of diploid
 Fix
hybrid vigor
 Fix heterozygous status
Breeding and
Pollination Systems
Plant Breeding Systems
General Issues to consider:
• A flower is an adapted leaf specialized for sex.
• Its function is to bring the gametes together thus
forming zygotes with new genetic combinations
• A species’ survival is dependent on the production of
offspring adapted to a particular environment. Plants
have evolved a variety of adaptations that either
encourage outbreeding (cross-pollination) or
inbreeding (self-pollination or selfing).
• Each breeding system has genetic ramifications
Basic Terms:
• Outbreeding: Sexual reproduction between individuals
(cross-pollination). (It involves two individual plants)
• Inbreeding: Sexual reproduction within an individual
(self-pollination). (It involves one individual plant)
• Sexual Reproduction: Meiosis and fertilization.
The sexual process is a mechanism to bring
about gene recombination. Recombination is the chief
source of hereditary variation and provides the raw
materials for species to adapt to changing
environmental conditions.
Outbreeder or Inbreeder??
Often one can tell just by looking at a flower whether it
cross-pollinates or self-pollinates
Trait
Incompatibility
Flower #
Flower size
Flower color
Nectories
Scent
Nector guides
Anther position
Pollen #
Style position
Stigma
OUTBREEDER
self-incompatible
many flowers
large flowers
bright colors
nectaries present
scented flowers
nectar guides present
anthers far from stigma
many pollen grains
style exserted from flower
stigmatic area well-defined
INBREEDER
self-compatible
few flowers
small flowers
mono-colored
nectaries absent
unscented flowers
nectar guides absent
anthers close to stigma
fewer pollen grains
style included in flower
stigmatic area poorly-defined
Outbreeding (Cross-pollination)
• Advantages:
Increases genetic variability
Strong evolutionary potential
Adaptation to changing conditions
Successful long-term
• Disadvantages:
Can destroy well-adapted genotypes
Relies on effective cross-pollination, seed
dispersal and establishment
Inbreeding (Self-pollination)
• Advantages:
Preserves well-adapted genotypes
Insures seed set in the absence of pollinators
Single colonizing individual possible
• Disadvantages:
Decreases (or maintains) genetic variability
Evolutionary dead-end
Cannot adapt to changing environmental conditions
Successful short-term