11.3: Sexual Reproduction - Plants, Fungi and Animals

sexual reproduction

requires production of gametes which occurs via meiosis. Diploid parent cells divide to form haploid daughter cells. Fertilisation occurs when 2 daughter cells combine to form diploid zygote

plant meiosis

produce spores that undergo mitosis forming multicellular haploid gametophyte. Then produces gametes. gametes from 2 different gametophytes combine to produce diploid zygote. Then grows into diploid sporophyte

sexual reproduction in fungi

1. plasmogamy - fusion of cytoplasm from 2 compatible hyphae without fusing nuclei forming dikaryotic individual

2. karyogamy - fusion of nucleic to form diploid zygote

3. meiosis - produces haploid spores that develop into mycelium beginning cycle

how is sexual reproduction thought to of evolved

from a monoecious ancestor where meiosis produced haploid gametes of both sexes

simultaneous hermaphrodites

possess both male and female reproductive organs and produces both eggs and sperms at same time. e.g. earthworms exchange sperm when they mate = eggs of each one is fertilised

sequential hermaphrodite

individuals that can function as either male or female at different life stages. Changes between sexes dictated by enviro conditions, social structure or age. beneficial when likelihood of finding a mate is very low or reproductive season is vert short

evolution of dioecious

species from monoecious ancestors, thought to occurred by gradual increase in investment in one or other of the sex roles

isogamy

dioecious - each parent has an equal investment in production of gametes = gametes are same size

anisogamy

dioecious - when parental investment is different. one parent invests more energy into production of large gametes while other invests less and produces small gametes

• females often produce fewer but larger gametes than males

evolution of anisogamy

due to different selective pressures on different sexes = reflected in their investment in gamete production, care for offspring and mating behaviour

four types of floral organs

1. sepals - encase and protect flower buds

2. petals - protect other flower structures and often brightly coloured which helps attract pollinators

3. and 4 stamens and carpels - fertile flower organs. produce spores which eventually develop into sperms and eggs

stamen

consists of a filament and anther

• cells in anther create spores that develop into pollen gran-ins which give rise to sperm

carpel

consists of an ovary style and stigma

• ovary contains one or more ovules. In ovule an embyro sac including an egg is formed

• style extends from ovary to stigma

• stigma is sticky and traps pollen allowing mobile sperm to reach stationary egg

• fertilised ovules develop into seeds

pollination

transfer of pollen (sperm source) to flower part containing ovules. A pollen tube is firmed, sperm released into ovule and fertilisation occurs

internal vs external fertilisation

Internal - sperm fertilizes an egg inside female's body = protect developing embryo.

External - egg and sperm combining outside of the female = increases number of gametes released but exposing them to environmental hazards.

few vs many offspring

Few = high parental investment leads to improved survival,

many = low parental investment with increasing vulnerability.

Explain why there are variations in the length of the haploid period of the life cycle and what is the advantage of alternation of generations

vary bc of environmental factors and reproductive strategies, so organisms can adapt to changing conditions.

Alternation of generations increases genetic diversity through sexual reproduction in haploid stage, different stages exploit different niches = adaptability and spreads risk between stages