Synopsis

1
Synopsis

• Adaptation to environments?
• Why is sex good?
• Evolutionary theory of the maintenance of sex
• Case studies

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Adaptation to physical biological environments

• Physical and biological environments differ
  radically in PREDICTABILITY
• Physical environment - reln. between conditions
  constant between generations
• Biological environment - reln. between conditions
  can vary WITHIN a generation

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The cost of sex

• 2-fold cost of meiosis
• useless half of the popn. males
• female dilutes her gene pool
• mating
• cost of ornamentation
• mating displays etc.

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Why is sex good?

• Muller's ratchet
• accumulation of deleterious alleles
• hitch-hiking
• breaks up disadvantageous combinations and
  preserves best
• Group seln.
• ultimate in altruism - unlikely unless close
  relatives

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Why is sex good (2)

• Balance theory - states that there is an
  advantage to simultaneous sexual and asexual
  reproduction because of environmental demands.
• little support as most plants/animals serially
  sexual/asexual

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The Biological Environment

• Capricious - can change within a generation
• How can long lived organisms cope with such
  challenges?
• Sex! - produces unpredictable genetic
  combinations each generation
• The Red Queen Hypothesis

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Freshwater bryozoan Cristatella mucedo
Myxozoan parasiteTetracapsula bryozoides
J.R. Freeland, L.R. Noble B. Okamura (2000) J.
Evol. Biol. 13 383-395
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Cristatella mucedo - population structure

• Popns. linked by dispersal geneflow
• Repeated localized extinctions recolonizations
• Balance by drift gene flow - levels of popn.
  differentiation enhanced/diminished
• Affect on popn. genetic structure?

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POPULATION STRUCTURING
Forces reducing differentiation ? Forces
increasing differentiation
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Reproduction in C. mucedo

• Inhabits discrete lakes ponds
• sex infrequent
• disperses via asexual propagules (statoblasts)
• Predominatly asexual reproduction, budding,
  colony growth fission, statoblast prodn.

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Reproduction in C. mucedo

• Inhabits discrete lakes ponds
• sex infrequent
• disperses via asexual propagules (statoblasts)
• Predominatly asexual reproduction, budding,
  colony growth fission, statoblast prodn.

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Dispersal/gene flow

• Some sexual repdn. beginning of season
• Larvae give limited within-site dispersal
• Asexual statoblasts highly resistant, survive
  winter - gas-filled cells allow buoyancy and
  within-site dispersal - hooks allow long distance
  dispersal via animals
• Survive desiccation and passage through digestive
  tract

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Genetic strategy?

• Facultatively sexual animals produce
  overwintering propagules via sex
• Asexual propagules unusual - but can be produced
  in abundance
• Gives greater chance of passive dispersal and
  survival max. chance of (re)colonization
• Dispersal potential metapopulation

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Molecular Ecology - bryozoan systems
sexual/asexual
host-parasite - Red Queen, host escapes by
evolution of resistance

• budding, self/outcross
• good dispersal
• few widespread clones
• fugitive lifestyle
• novel myxozoan

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The parasite -Tetracapsula bryozoides

• Myxozoan thought related to cnidarians now
  not sure
• Kills all colonies it infects heavy infections
  wipe out bryozoan popns.
• Agent of Proliferative Kidney Disease in trout
  (PKD)

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Summary

• Persistence of high levels of clonality
• Clones highly related
• Clones varied in abundance
• Commonest not disproportionately infected
• No evidence for Red Queen
• How does C. mucedo survive?

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The Great Escape

• Metapopulation structure
• evidence of sub-division and gene flow
• high diversity of clones dispersal
• Asexual statoblasts
• produced at end of season - vs. sexual
  overwintering propagules

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Favouring Asexuality

• Asexual repd. favoured when
• metapopulation structure
• successful dispersal
• Big fitness benefits for single clone
• e.g. Loriston Loch
• Risk of extinction reduced by broad temporal and
  spatial spread

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Synopsis

• Adaptation to environments?
• Why is sex good?
• Evolutionary theory of the maintenance of sex
• Red Queen running to stay ahead of parasites
• Speciation separating good genes from bad
• Case studies

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Arionid slugs the nematodePhasmarhabditis
hermaphrodita
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Molecular Ecology - slug bryozoan systems.
sexual/asexual
host-parasite - Red Queen, host escapes by
evolution of resistance

• facultative selfers
• poor dispersal
• species complexes
• genesis of taxa
• nematodes

• budding, self/outcross
• good dispersal
• few widespread clones
• fugitive lifestyle
• novel myxozoan

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The Large Arions - are they really difficult to
identify?

• Arion ater ater - black?
• Yes - but also red, yellow, white
• A. ater rufus - orange?
• Yes - but also black and yellow
• A. lusitanicus - Lusitanian distribution?
• No! - anything but, prefers drier eastern sites
• A. flagellus - a distinct flagellum?
• No! - a matter of taxonomic precedence

A. ater

A.
lusitanicus
Yes!!
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How did this diversity arise?

• Clues from distributions of selfing vs
  outcrossing taxa?
• Respective levels of genetic polymorphism?
• Anatomical similarity?
• Legacy of an Ice Age?

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Selfers vs. Outcrossers

• Selfers Outcrossers

Few clutches Eggs few large High quality
offspring Low juvenile mortality Less repd.
investment Often biennial Short protandry Late
maturation High altitudes/latitudes
Many clutches Eggs many small Low quality
offspring Higher juvenile mortality More repd.
investment Annuals Long protandry Early
maturation Low altitudes/latitudes
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Distribution of selfers vs outcrossers

• 93 of parthenogenic and selfing taxa found at
  higher altitudes and latitudes than closely
  related outcrossing taxa.
• Biologically simple vs biologically complex
  environments.

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But.?

• Selfers also common in the tropics!
• Surely a selfing species can easily be overcome
  by parasites/pathogens in the evolutionary game
  of the Red Queen?

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Avoidance via speciation

• Speciation very rapid and many species complexes
• Each species represent markedly different genetic
  entities
• Method of isolating gene complexes
• more difficult for parasites to invade than one
  species

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Parapatric species

• Have adjacent but non-overlapping distributions.
• Reproductive barriers? sub species
• Rapid speciation in the face of environmental
  change?

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Conclusions

• Biological environment a driving force in
  evolution.
• Promotes
• Rapid genetic change
• Speciation
• Facultative self-fertilization
• Fugitives - movers
• Species - shakers