Asexual Reproduction, Sexual Reproduction, and Apomixis

Gene transfer, asexual reproduction, sexual reproduction, and apomixis

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tinyfarmblog.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 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)

1 pair homologous chromosomes 2 pairs of homologous chromosomes 3 pairs of homologous chromosomes 0 sets of homoeologous chromosomes 2 sets of homoeologous chromosomes 3 sets of homoeologous chromosomes AA AABB AABBDD

A A B B A A A A B B D D

VAVA VAVAVBVB VAVAVBVBVDVD

2n = 2x = 14 2n = 4x = 28 2n = 6x = 42 30,000 genes 60,000 genes 90,000 genes Angiosperm reproductive organs and gamete formation Development of the female gametophyte

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

http://www.extension.org/pages/32204/stigma Angiosperm reproductive organs and gamete formation Development of the female gametophyte Megaspore mother cell (MMC)

• MMC undergoes meiosis

• Of 4 megaspores produced 1 survives (most species)

• Three post-meiotic mitoses

1 2 3 Angiosperm reproductive organs and gamete formation Development of the female gametophyte

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

Source: yougems.reflectionsinfos.com lima.ohio-state.edu Angiosperm reproductive organs and gamete formation Development of the female gametophyte

Pollinator attraction: Petals, nectaries, etc.

Source: arizonabeetlesbugsbirdsandmore.blogspot.com Angiosperm reproductive organs and gamete formation Development of the male gametophyte

Reproductive structures: Anthers; pollen within anthers

Angiosperm reproductive organs and gamete formation Development of the male gametophyte

Pollen mother cell (PMC)

• PMC undergoes meiosis

Note, this is different than ♀

Meiosis 1 Meiosis 2

• Meiosis gives a tetrad of microspores

Angiosperm reproductive organs and gamete formation Development 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 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 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

♀ ♀ 3n polar endosperm ♀ ♀ nuclei ♂

♀ ♀ ♂ egg 2n

embryo synergids 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…

v v V V V v v V V V V V V V V V

MMC V V v v v V v V V v v v v v v v v v

PMC v V V v V v sporophytic v v V V generation v V

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

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 VV Vv

V v v v v v V v v 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 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: Seeds without sex: Apomixis

Hand and Koltunow. 2014. Genetics 197: 441-450. Seeds without sex: Apomixis

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

Prevalence 400 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

• 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?

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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