Plant Reproduction

 Gene transfer, asexual reproduction, sexual reproduction, and apomixis

viachicago.wordpress.com

tinyfarmblog.com

birdsandbloomsblog.com

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 reproductionAdvantages > 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 reproductionAdvantages > 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 generationsThe sporophytic generation may be diploid (2n = 2x) or polyploid (2n = _x)

VAVA VAVAVBVB VAVAVBVBVDVD

2n = 2x = 1430,000 genes

2n = 4x = 2860,000 genes

2n = 6x = 4290,000 genes

1 pair homologous chromosomes0 sets of homoeologous chromosomesAA

2 pairs of homologous chromosomes2 sets of homoeologous chromosomesAABB

3 pairs of homologous chromosomes3 sets of homoeologous chromosomesAABBDD

A AA 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

Megaspore mother cell (MMC) 

• MMC undergoes meiosis

• Of 4 megaspores produced 1 survives (most species)

• Three post-meiotic mitoses  

Angiosperm reproductive organs and gamete formationDevelopment of the female gametophyte

1 2 3

The 8-nucleate embryo sac (1 egg, 2 synergids, 2 primary endosperm nuclei, 3 antipodals) 

Angiosperm reproductive organs and gamete formationDevelopment of the female gametophyte

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

Pollinator attraction: Petals, nectaries, etc.

Angiosperm reproductive organs and gamete formationDevelopment of the female gametophyte

Source: arizonabeetlesbugsbirdsandmore.blogspot.com

Angiosperm reproductive organs and gamete formationDevelopment of the male gametophyte

Reproductive structures: Anthers; pollen within anthers

Angiosperm reproductive organs and gamete formationDevelopment of the male gametophyte

Pollen mother cell (PMC)  

• PMC undergoes meiosis

• Meiosis gives a tetrad of microspores

Meiosis 1 Meiosis 2

Note, this is different than ♀

Angiosperm reproductive organs and gamete formationDevelopment of the male gametophyte

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

mitosismitosis

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

The pollen pathway and double fertilization

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

polarnuclei♀ ♀

♀

♀ ♀

♀

♂

♂

3nendosperm

2n

embryo

The pollen pathway and double fertilization

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…

MMCV v

PMCV v

vvVV

VV

vv

vvVV

VVvv

V V 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 vsporophyticgeneration

A review of sexual reproduction in an F1 plant, heterozygous for the Vv locus

V vV VV Vv

v Vv vv

+ =V

V V

V V VV V

V V V

V V V

V V

+ =v

v v

V V VV V

V V V

V V v

V v

+ =v

v v

v v vv v

v v v

v v v

v v

+ =V

V V

v v vv 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 fertilizedegg 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: 

Seeds without sex: Apomixis

Source: Grossniklaus et al. 2001. The Plant Cell.

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

Seeds without sex: Apomixis

Prevalence400 species ; 40 families; Common in Poaceae, Asteraceae, Rosaceae

• Examples • Tripsacum • Poa pratensis • Pennisetum • Dandelion (Taraxacum spp) • Rubus

Seeds without sex: Apomixis

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)  

Seeds without sex: Apomixis

Genetic basis

• Reported to be anything from one gene to many

• Lots of breeding effort; little success

• Perhaps all genes related to megasporogenesis!

Seeds without sex: Apomixis

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?