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Synopsis

Synopsis. Adaptation to environments? Why is sex good? Evolutionary theory of the maintenance of sex Case studies. Adaptation to physical & biological environments. Physical and biological environments differ radically in PREDICTABILITY

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Synopsis

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

  2. 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

  3. 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.

  4. 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

  5. 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

  6. 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

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

  8. 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?

  9. Gene flow resulting in the homogenization of allele frequencies Barriers to dispersal resulting in differentiation due to mutation & random genetic drift POPULATION STRUCTURING Forces reducing differentiation  Forces increasing differentiation

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

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

  12. 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

  13. 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

  14. Reproductive strategies • partitioning genetic variation bryozoans 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

  15. 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)

  16. 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?

  17. 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

  18. 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

  19. 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

  20. Arionid slugs &the nematodePhasmarhabditis hermaphrodita

  21. Reproductive strategies • partitioning genetic variation slugs bryozoans 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

  22. 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!!

  23. 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?

  24. Selfers vs. Outcrossers • SelfersOutcrossers 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

  25. 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.

  26. 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?

  27. 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

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

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

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