Plant Reproduction

Genes can be passed from one plant to another via asexual reproduction, sexual reproduction, and apomixis

I. Asexual reproduction:

• The clone is immortal _____________________

• The world's heaviest living thing _________________

• the garlic example _______________________

II. Sexual reproduction:

A. Advantages and disadvantages of sexual reproduction (More Advantages)

• Advantages
  • Environments are highly heterogeneous and unstable - sexual reproduction generates variability and variability is the basis for selection (natural and artificial).

 

• Disadvantages
  • In a dioecious species, half the reproductive effort is wasted in producing males
• Meiosis will produce some "unfit" combinations of genes
• Cross-pollinated plants may be subject to environmental conditions unfavorable to pollination

B. Review of sexual reproduction in higher plants

Alternation of generations: In plants there is an alternation of the gametophytic (n) and sporophytic (2n) generations. The sporophytic generation may be diploid (2n = 2x) or polyploid (2n = _x).

 

 

Angiosperm reproductive organs and gamete formation

The development of the female gametophyte

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

Megaspore mother cell (MMC): __________________

MMC undergoes meiosis.

 

Of 4 megaspores produced 1 survives.

 

Three post-meiotic mitoses yield the 8-nucleate embryo sac (1 egg, 2 synergids, 2 primary endosperm nuclei, 3 antipodals).

 

Petals, nectaries, etc.: Animal attraction

The development of the male gametophyte

Reproductive structures: Anthers; pollen

Pollen mother cell (PMC) _________________

 

PMC undergoes meiosis.

 

Meiosis gives a tetrad of microspores.

 

There are two subsequent mitotic events:

The first mitosis gives vegetative and generative nuclei.

At the second mitotic division, the generative nucleus gives 2 sperms.

 

Summary of megasporogenesis and microsporogeneis:

The pollen pathway and double fertilization

The stigma is the site of pollen recognition.

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

Genetics of embryo and endosperm in reciprocal crosses ________________________

III. Seeds without sex: Apomixis

A. Definition (Illustrative figure)

• Apomixis involves parthenogenesis – development of an egg without fertilization, as opposed to parthenocarpy – development of fruits (seedless) without fertilization

 

• Allows for seed propagation of a heterozygote – genetically identical from generation to generation: ___________________

 

• Obligate: ___________

 

• Facultative: _______________

 

B. Evolutionary and economic importance

C. Prevalence

• 400 species ; 40 families; Common in Poaceae, Asteraceae, Rosaceae
• Examples
  • Tripsacum
  • Poa pratensis
  • Pennisetum
  • Dandelion (Taraxacum spp)
  • Rubus
    

C. 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).  (More on endosperm balance numbers)

D. More details:

Apomixis can be classified by the origin and location of the cell that initiates apomixis

Sporophytic apomixis: Also known as adventitious embryony. An embryo forms directly from an unreduced sporophytic cell.

Gametophytic apomixis - Formation of an unreduced embryo sac

• Diplospory – MMC undergoes mitosis only (or altered meiosis) to give unreduced cells including an egg cell with genetic constitution of maternal plant
  Altered meiosis means first division restitution – no pairing at Meiosis I and normal meiosis II. No pairing = no recombination
• Apospory – a cell within the ovule other than MMC produces unreduced embryo sac

E. Genetic basis

• Reported to be anything from one gene to many
• Lots of breeding effort; little success
  
• Perhaps all genes related to megasporogenesis!

F. Considerations:

• If transgenic apomicts are developed and released, would need to control pollen flow from a dominant apomict – or else get frozen gene pools.

G. The Rubus (blackberry) example

Valley Invasive_1 Invasive_2

• A perfect weed? For more on the genetics of invasiveness see Amsellem et al. (2000)

Was the AFLP a good choice of molecular marker for this work?

• AFLPs (More about Keygene)
• What AFLPs revealed about genetic diversity in Rubus alceiifolius

FYI: Blackberry breeding and genetics (Clark et al. 2007)

Useful Links:

Reproductive characters (Botany 301, TAMU)
Vascular Plant Image Gallery
Garlic and Carrot home page
Apomixis reviews in Plant Cell. 2001.