Angiosperm mating systems and pollination Flashcards
Mating (breeding) system:
the way in which a group of organisms is structured in relation to its reproductive behaviour
Why is cosexuality adaptive?
optimises the benefits of animal pollination
The majority of flowers are
cosexual
Angiosperm mating system definition
Structural and physiological features of the flower
Angiosperm pollination system definition
Structural features of the flower and inflorescence – adaptations for pollination
What is one factor contributing to angiosperm success?
variety of mating systems
Describe the factors influencing mating system evolution
- immobility
- inbreeding depression
- reproductive assurance
- optimal allocation of male and female reproductive effort (maintenance of sexual polymorphisms)
Most angiosperms
outcross
What is reproductive assurance?
if mates or pollinators are scarce, it is better to self or reproduce asexually
apomixis
- asexual reproduction
- vegetative
- agamospermy
Give some angiosperm sexual dimorphisms
dioecy, gynodioecy
Describe angiosperm hermaphroditism
- ~96% flowering plants
- flowers co-sexual or unisexual
- monoecious
Describe the triangle of mating system interfaces
- reproductive assurance at apomixis
- reproductive assurance at inbreeding
- inbreeding depression at outbreeding
Inbreeding
- autogamy
- selfing (can be facultative or obligate)
Obligate autogamy
cleistogamy
Outbreeding
- panmixis
- can be facultative or obligate (self-incompatibility or dioecy)
In co-sexual flowers
autogamy is always possible
Describe some adaptations for autogametic avoidance
- dichogamy
- herkogamy
- dicliny
- SI: plants able to recognize and reject their own pollen
dichogamy
- separation of sexual organs in time
- protandry
- protogyny
protandry
stamens mature before carpels/pistils
protogyny
or carpels/pistils mature before stamens
herkogamy
- separation of sexual organs in space
– style length polymorphisms - bizarre flowers e.g. Coryanthes sp. (Bucket Orchids)
Give some style length polymorphisms
distyly and tristyly
Describe dicliny
- sexual polymorphisms
- plants are dioecious, monoecious, gynodioecious or gynomonoecious
dioecious
male or female
monoecious
unisexual flowers on same plant are male and female
gynodiecious
combinations of unisexual and co-sexual plants
gynomonoecious
unisexual and co-sexual flowers on same plant
Describe SI
- self-incompatibility
- the ability of a plant to recognize and reject self or self-related pollen
- pre-zygotic barrier: incompatible pollen is recognized and rejected before the pollen tube reaches the ovule
- ~65% of hermaphrodite angiosperms
- visualised with fluorescent stain aniline blue which stains callose
Describe genetic control of SI
- usually controlled by a single locus (S) with multiple alleles: S1, S2, S3 etc
- highly polymorphic: populations maintain around 40 S alleles by negative frequency-dependent selection
- Self (or cross with shared S alleles)
Describe the two main genetic types of SI:
- GSI
- SSI
Describe SSI
- sporophytic SI
- incompatibility phenotype of pollen determined by diploid genome of its parental sporophyte
- S alleles show dominance in pistil and ‘in’ pollen
Describe GSI
- gametophytic SI
- incompatibility phenotype of pollen determined by its own haploid genome
- S alleles are co-dominant in pistil
List some GSI lineages
- Solanaceae (Potato family)
- Rosaceae
- Plantaginaceae
- Campanulaceae
- Papaveraceae (Poppy)
- Poaceae (Grasses)
List some SSI lineages
- Brassicaceae
- Asteraceae
- Convolvulaceae (Ipomoea sp.)
- Betulaceae
- Caryophyllaceae
- Malvaceae
Describe distyly in Primula and Primulaceae
- two S alleles: S (dominant) and s (recessive/null)
- 388/426sp. = distylous
- pin = ss
- thrum = Ss
- 50% Ss, 50% ss
Pin
long-styled morph
Thrum
short-styled morph
Describe the heteromorphic S locus
- ‘supergene’
- controls SI and morphological characters (style length, stamen position)
Give a pin and thrum species
Limonium meyeri
Describe autogamy
- selfing within outbreeding groups = common across all angiosperm lineages
- 35-40% co-sexual angiosperms able to self fertilize
- 20% = habitual selfers
List some autogamous angiosperms
- Cardamine hirsuta (Brassicaceae)
- Arabidopsis thaliana (Brassicaceae)
Describe the genetic consequences of selfing
- recessive individuals are candidates for inbreeding depression
- genetic variation declines with time
Inbreeding depression can be studied in
SI obligate outcrossers
Describe inbreeding depression
recessive alleles are ‘purged’ and dominant alleles are fixed in homozygotes
Why don’t habitual selfers suffer from inbreeding depression?
homozygous at most loci
Describe the advantages of autogamy
- Reduces male expenditure and removes the ‘costs of outbreeding’
- Optimizes reproductive assurance when:
a.) Pollinators are scarce or unreliable
b.) Plants are ‘pioneers’ or colonizers (Bakers Law)
c.) Populations pass through ‘bottlenecks’ (threat of extinction)
Describe the evolution of autogamy in Eichhornia paniculata
- Brazil (outcrossing population)
- Nicaragua (selfing population)
- once selfing established, selection favours reduction in: flower size, pollen production, nectar production; disappearance of adaptations for herkogamy, dichogamy, dicliny
Describe the disadvantages of autogamy
- decreases (but also maintains) genetic diversity
- cannot adapt to changing environmental conditions
- ‘Evolutionary dead end’
Describe apomixis (agamospermy)
- production of fertile seeds in the absence of sexual fusion between gametes
Give some agamospermous lineages
- Taraxacum sp. (Asteraceae)
- Alchemilla sp. (Rosaceae)
- Rubus sp. (Rosaceae)
Demonstrating Agamospermy in a dandelion (Taraxacum officinale)
- emasculate bud
- all offspring are clones and reproductively isolated
Describe gametophytic agamospermy
- diploid egg cell forms embryo
- other diploid cells becomes embryo(s)
- pollination may be needed for endosperm formation
Describe sporophytic agamospermy
- embryo sac haploid
- egg cell fertilised and diploid embryo forms
- polar nuclei fertilised: triploid endosperm
- sexual embryo aborts and diploid embryo forms one or more cells of nucellus
Describe the advantages of agamospermy
- reproductive assurance
- avoids costs of meiosis
- transmits 100% mothers genes to offspring
- fixes well-adapted genotypes
- frequently maintains/fixes new hybrids/polyploids
Describe the disadvantages of agamospermy
- unable to escape from disadvantageous mutations (‘Muller’s Ratchet’)
- unable to recombine advantageous mutations
- very narrow niche tolerance
- inability to adapt: evolutionary dead-end
