Variants in the Reproductive Process

gooseliverΒιοτεχνολογία

22 Οκτ 2013 (πριν από 3 χρόνια και 11 μήνες)

79 εμφανίσεις

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.


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).



Its function is to bring the gametes together thus




forming zygotes with new genetic combinations



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

OUTBREEDER

INBREEDER

Trait

Incompatibility

Flower #

Flower size

Flower color

Nectories

Scent

Nector guides

Anther position

Pollen #

Style position

Stigma

self
-
incompatible


self
-
compatible

many flowers



few flowers

large flowers



small flowers

bright colors



mono
-
colored

nectaries present



nectaries absent

scented flowers



unscented flowers

nectar guides present


nectar guides absent

anthers far from stigma


anthers close to stigma

many pollen grains



fewer pollen grains

style exserted from flower


style included in flower

stigmatic area well
-
defined


stigmatic area poorly
-
defined

Outbreeding (Cross
-
pollination)


Advantages:


Disadvantages:

Increases genetic variability

Strong evolutionary potential

Adaptation to changing conditions

Successful long
-
term

Can destroy well
-
adapted genotypes

Relies on effective cross
-
pollination, seed


dispersal and establishment

Inbreeding (Self
-
pollination)


Advantages:


Disadvantages:

Preserves well
-
adapted genotypes

Insures seed set in the absence of pollinators

Single colonizing individual possible

Decreases (or maintains) genetic variability

Evolutionary dead
-
end

Cannot adapt to changing environmental conditions

Successful short
-
term