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 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?
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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?