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Plant Reproduction

Gene transfer, asexual reproduction, sexual reproduction, and apomixis

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Asexual reproduction The clone is immortal

Example: Allium sativum “As far as we know, garlic in cultivation throughout history has only been propagated asexually by way of vegetative cloves, bulbs, and bulbils (or topsets), not from seed. These asexually propagated, genetically distinct selections of garlic we cultivate are more generally called "clones". Yet this asexual lifestyle of cultivated garlic forgoes the possibility of combining traits proffered by interpollinating diverse parental stocks.” Source: http://www.ars.usda.gov/Research/docs.htm?docid=5232

Asexual reproduction The clone is immortal

Example: Populus tremuloides

• The world's heaviest living thing

• 1 clone in the Wasatch Mountains of Utah

• 47,000 stems of genetically identical aspen trees

• Total weight: 6 million kilograms

• Aspen is dioecious species - this clone is one big male

source: http://waynesword.palomar.edu/ww0601.htm#aspen

Sexual reproduction Advantages > disadvantages

• Advantages:

• Genetic variation:

• Allele exchange via cross-pollination

• New combinations of alleles via meiosis

• Purge deleterious mutations

• Stay ahead in the host-pathogen “arms race”

• Potential adaptation to a changing climate

Sexual reproduction Advantages > disadvantages

• Disadvantages:

• In a dioecious species, half the reproductive effort is wasted in

producing males

• Meiosis produces some "unfit" combinations of alleles

• Cross-pollinated plants may be subject to environmental conditions

unfavorable to pollination

• Finding a mate

Alternation of generations

In plants there is an alternation of the gametophytic (n) and

sporophytic (2n) generations

(Gametophytic = haploid)

Alternation of generations

The sporophytic generation may be diploid (2n = 2x) or polyploid (2n = _x)

VAVA VAVAVBVB

VAVAVBVBVDVD

2n = 2x = 14

30,000 genes 2n = 4x = 28

60,000 genes

2n = 6x = 42

90,000 genes

1 pair homologous chromosomes

0 sets of homoeologous chromosomes

AA

2 pairs of homologous chromosomes


AABB

3 pairs of homologous chromosomes


AABBDD

A A A A B B

A A B B D D

Angiosperm reproductive organs and gamete

formation Development of the female gametophyte

• Reproductive structure: Ovule(s), style, stigma

http://www.extension.org/pages/32204/stigma

http://www.extension.org/pages/32204/stigma

Megaspore mother cell (MMC)

• MMC undergoes meiosis

• Of 4 megaspores produced 1 survives (most species)

• Three post-meiotic mitoses



1 2 3

The 8-nucleate embryo sac (1 egg, 2 synergids, 2 primary

endosperm nuclei, 3 antipodals)



Source: yougems.reflectionsinfos.com lima.ohio-state.edu

Pollinator attraction: Petals, nectaries, etc.



Source: arizonabeetlesbugsbirdsandmore.blogspot.com


formation Development of the male gametophyte

Reproductive structures: Anthers; pollen within anthers



Pollen mother cell (PMC)

• PMC undergoes meiosis

• Meiosis gives a tetrad of microspores

Meiosis 1 Meiosis 2

Note, this is different than ♀



The first mitosis gives vegetative and generative

nuclei; at the second mitotic division, the generative

nucleus gives 2 sperms.

mitosis

mitosis

mitosis

mitosis

mitosis

mitosis

mitosis mitosis

The pollen pathway and double fertilization

• The stigma is the site of pollen recognition

• Pollen germinates and the vegetative (tube nucleus)

grows through the style to the ovule

• The two sperm use the tube as conduit


Double fertilization: One sperm fertilizes the egg to give

the 2n embryo, the other fertilizes the polar nuclei to give

the 3n endosperm

antipodals

egg

synergids

polar nuclei ♀ ♀

♀

♀ ♀

♀

♂

♂

3n endosperm

2n

embryo


After double fertilization, there are at least four

independent and genetically distinct generations

coexisting in the seed:

• maternal sporophyte diploid tissue

• maternal gametophyte haploid tissue

• offspring sporophyte diploid tissue

• fusion of male (1) and female (2) gametophyte to

form triploid tissue

A review…

MMC

V v

PMC

V v

v

v

V

V

V

V

v

v

v

v

V

V

V V

v

v

V V V

V V

V V V

v v v

v v

v v v

v

v v

v

v v

v

v

V

V

V

V V

V

V V

V v sporophytic generation

A review of sexual reproduction in an F1 plant,

heterozygous for the Vv locus

V v

V VV Vv

v Vv vv

+ = V

V V

V V V V V

V V V

V V V

V V

+ = v

v v

V V V V V

V V V

V V v

V v

+ = v

v v

v v v

v v

v v v

v v v

v v

+ = V

V V

v v v

v v

v v v

v v V

v V

Note: At this point in the figure, the antipodals and synergids are deleted and only the fertilized endosperm nuclei (now 3n) and fertilized egg (now 2n) are shown. Only the fertilized egg is carried to the Punnett square.

Seeds without sex: Apomixis

Apomixis involves parthenogenesis – development of an egg

without fertilization, as opposed to parthenocarpy –

development of fruits (seedless) without fertilization

Implication - Apomixis allows for seed propagation of a

heterozygote – genetically identical from generation to

generation.

1. Obligate:

2. Facultative:


Hand and Koltunow. 2014. Genetics 197: 441-450.

Economic implications of apomixis

• Why would apomixis be, or not be, of commercial

interest to purveyors of hybrid seeds?

• If transgenic apomicts are developed and released,

and if apomixis is dominant, it will be necessary to

control pollen flow – if not, there will be frozen gene

pools

Evolutionary implications of apomixis

• Obligate vs. facultative


Prevalence

400 species ; 40 families; Common in Poaceae,

Asteraceae, Rosaceae

• Examples

• Tripsacum

• Poa pratensis

• Pennisetum

• Dandelion (Taraxacum spp)

• Rubus


How it works

• no (or altered) meiosis to produce unreduced female

gametophyte (embryo sac)

• no fertilization – but do get autonomous embryo

formation

• may have autonomous endosperm development, or

endosperm development may be triggered by

fertilization. Most cases it is triggered by fertilization

(pseudogamy = fertilization of central cell)


Genetic basis

• Apomixis loci and candidate genes

• Lots of breeding effort; little success

• Epigenetically regulated?


The Rubus armeniacus case study

Himalayan (Armenian) blackberry

Is the Himalayan blackberry the perfect weed?

promo.idahopotato.com

Designing the perfect invasive

Attribute Description

Flower Hermaphroditic

Pollination biology Self and outcross

Apomixis Facultative

Seeds Small and numerous

Vegetative propagation Yes

Ploidy level Polyploid

Protection Thorns

Attraction Tasty fruit

Was Luther Burbank the “father” of this perfect weed?

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