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Reproduc)ve Biology II
Breeding Systems
Summary of previous lecture:
Plants Influence Pollinators
Pollina)on Plant Costs Success Progeny
Growth Dispersal Seed banks Establishment Germina)on
BUT:
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Breeding Systems
• In the context of – costs – success
Inbreeding <-‐-‐-‐-‐-‐-‐> Outcrossing
• Obligate inbreeding -‐ rare • Highly Inbred -‐ very common
• Par)ally Inbred -‐ most common condi)on
• Obligate Outcrossing -‐ less common (woody plants mostly)
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Examples of ranges of outcrossing -‐ inbreeding con)nuum.
Outcrossing Advantages
• Gene)c diversity within popula)ons • Heterosis (hybrid vigor) • Epistasis -‐ traits derived from 2 or more gene)c lines give superior performance
• Dominance masks lethals
• Avoidance of frequency dependent problems (like pathogen suscep)bility)
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Inbreeding Advantages • Reproduc)ve efficiency; insures some reproduc)on
• Fixa)on of highly adapted genotypes • Within a popula)on of mixed inbreeders and outcrossers, selfers are dispropor)onately “males” in the popula)on and selfers thus donate more genes to the next genera)on.
• Less gene)c load • Decreased costs in some species (cleistogamy)
• Selfers usually show high phenotypic plas)city
Ecological Advantages and Constraints of Breeding System
• Breeding System Interacts with Habitat Characteris)cs – Probability of Pollina)on (presence/absence of pollinators, or wind or )ming)
– Energy Costs (growth poten)al of habitat) – Survival Poten)al of Adults and Progeny
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Under what condi)ons will outcrossing be favored?
Under what condi)ons will inbreeding be favored?
Habitat Characteris)cs
• Outcrossing favored: – temporally or spa)ally heterogeneous environment (remember the scale of the organism, long-‐lived trees vs annuals)
• Inbreeding favored: – coloniza)on of rare or distant habitats – insuring seed produc)on in marginal sites – protects adap)ve genotypes in stable habitats
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Costs of Inbreeding: Inbreeding Depression?
Inbreeding depression declines with the degree of selfing
Husband & Schemske 1996 Evolution 50:54-70.
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Inbreeding depression can differen)ally impact different life history stages. Species that are primarily selfing in their breeding systems are rela)vely free of inbreeding depression.
This means popula)ons have already purged many of those lethal or growth limi)ng genes.
Note limited impact except on growth and reproduc)on stage.
Husband & Schemske 1996 Evolution 50:54-70.
Savings of Inbreeding: Reduc)on of expense on pollen and reproduc)ve
structures
Less pollen per ovule Less pollen-‐bearing area Smaller pollen Fewer ovules
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Pollen: Ovule Ra)os
Example: Gilia achilleifolia
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Example: Gilia achilleifolia varies in the degree of inbreeding. Different popula)ons are subject to different selec)ve pressures.
Survivorship of an experimental field popula)on of Gilia.
Outcrossed progeny survived beaer in the circumstances tested here. You can imagine how they would be substan)ally selected if this were a natural popula)on.
outcrossed
inbred
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Popula)ons interac)ng with heterogeneous habitats
• Next example involves Impa/ens capensis, an annual from the east coast of North America. This annual can be found in a variety of habitats.
• Most cri)cally, it produces two types of flowers, chasmogamous and cleistogamous flowers.
Chasmogamy vs. Cleistogamy
• Chasmogamy refers to open flowers that are openly pollinated
• Cleistogamy refers to flowers that are selfed while s)ll in the bud, they never open.
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Impa/ens capensis
Here is the floral phenology for Impa/ens capensis in a number of different habitats.
The light areas are cleistogamous flowers, the dark areas are chasmogamous flowers.
Note that only three areas contained the showy chasmogamous flowers. These were the three areas with the highest levels of natural sunlight.
Freshwater Tidal Marsh
Old Field
Lowland Forest
Pine Woods
Red Maple Swamp
Flood plain
Typha Marsh
Phragmites Marsh
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Here, seedlings were transplanted from each habitat to either a floodplain (above) or lowland forest (below).
Where planted, only one of the habitats had sufficient light (energy) for the chasmogamous flowers to be produced.
Note that the phenology ()ming) wasn’t really changed. Gene)cally, each popula)on is adjusted to different habitat condi)ons.
Quan)ta)ve results for Impa/ens by habitat.
Note the large differences in the numbers of flowers/capsules produced.
Plants generate gene)c diversity with chasmogamous flowers, increase seed produc)on with cleistomagamous.
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Mixed breeding systems in the last two examples:
Allows a balance between: reproduc)ve efficiency offspring gene)c composi)on total fecundity habitat temporal and spa)al diversity
Addi)onal topics we won’t consider:
– maternal control via fixed abor)on systems
– maternal control during fer)liza)on via incompa)bility systems (even discrimina)ng among pollen with different gene)c cons)tu)on)
– fruit and seed abor)on rates and influence – gene)c load
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Things to think about:
• Breeding Systems with respect to: – Reproduc)ve efficiency
– Habitat variability – Pollinator variability – Gene)c diversity
• Are there condi)ons under which inbreeding vs outcrossing might be differen)ally selected?
