Pollination and fertilisation

define pollination

- transfer of pollen grains from the anther to the mature stigma of a plant of the same species

types of pollination

- self-pollination

- cross-pollination

define self pollination

- transfer of pollen from the anther of a flower to the mature stigma of the same flower or a different flower of the same plant

define cross-pollination

- transfer of pollen from the anther of a flower to a mature stigma of another flower on a different plant of the same species

- most angiosperms use this pollination type

what is the main implications of the different types of pollination

- genetic

outline the genetic implications of self-pollination

- leads to self fertilisation = inbreeding

- self-fertilised species depend only on independent assortment and crossing over during meiosis and on mutation to bring about genetic variation in the genomes of their gametes. So less genetic variation than cross fertilised species which also combine gametes from 2 different individuals

- there is a greater chance of 2 potentially harmful recessive alleles being brought together at fertilisation

- an advantage of inbreeding: it can preserve those successful genomes that are suitable to a relatively STABLE environment

outline genetic implications of cross-pollination

- leads to cross- fertilisation = outbreeding

- outbreeding combines gamete from 2 different individuals, in addition to events in meiosis and mutation so it generates more genetic variation

- outbreeding reduces chance of producing harmful allele combinations

- and has great evolutionary significance: in a particular environment, some genomes are more successful than others. This may allow a species to survive in a CHANGING environment, as there are always likely to be some members of the population with a suitable combination of alleles

state methods that flowering plants use to ensure cross-pollination takes place

- dichogamy

- genetic incompatibility

- structure: anther below the stigma + pin and thrum

- separate male and female flowers on same plant

- separate male and female plants

outline dichogamy + examples

- the stamen and stigma ripening at different times

- in protandry eg: daisy, the stamens ripen first

- in protogyny, eg: bluebell, which is rarer, stigma ripens first

Explain the pin and thrum structures of primrose

- they are pollinated by long tongued bees and the structures are such that pollen from a pin primrose is more likely to be deposited on the stigma of a thrum and vice versa when the bee visits flowers with the different structures

- in a pin-eyed primrose, the anther is below the stigma so pollen cannot fall on it

outline impact of anther being below stigma + example

- so pollen can't fall onto it

- pin-eyed primrose

explain genetic/chemical incompatibility + example

- red clover

- pollen can't germinate on the stigma of the flower that produced

- so self pollination can occur but the pollen tube cannot grow on the stigma of the same flower that produced the pollen

explain separate male and female flowers on the same plant + example

- maize

give example for a plant that has separate male and female plants

- holly

What is fertilisation

- fertilisation in the fusion of the male gamete with the female gamete to form a zygote

- the male gamete is the nucleus contained in the pollen grain

- the female gamete is in the ovule protected by the ovary

- the male gamete is delivered to the female gamete by the pollen tube

Outline germination of the pollen grain

- when a compatible pollen gran lands on the stigma it absorbs water and germinates in the sucrose solution secreted by the stigma and produces a pollen tube

- the pollen tube nucleus is at the tip of the tube with the two generative nuclei behind it

- the pollen tube grows out through the pit, a gap in the exine and down the style along a chemoattractant gradient

Outline the growth of the pollen tube

- pollen tube growth is controlled by the tube nucleus

- hydrolytic enzymes, secreted by the pollen tube, digest a path through style tissues and the products of digestion are used for growth of the pollen tube

- the direction of growth may be in response to chemicals secreted by the synergids in the ovule eg: GABA

- gamete release

Give an example of a chemoattractant

- GABA secreted from the ovule

Outline the gamete release

- the pollen tube grown through the micropyle (gap in the integuments) and passes into the embryo sac

- the tube nucleus degenerates and the tip of the tube breaks open releasing the 2 male gametes into the embryo sac

- double fertilisation

Outline double fertilisation

- One of the male gametes (n) fuses with the oosphere (n) to form the zygote (2n)

- the other male gamete (n) fuses with the diploid polar nucleus (2n) to form the triploid endosperm nucleus (3n) - this generates endosperm tissue (after mitosis)

- = double fertilisation

Function of endosperm

- takes over from nucellus in providing nutrition for developing embryo