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

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Presentation on theme: "Synopsis Adaptation to environments? Why is sex good? Evolutionary theory of the maintenance of sex Case studies."— Presentation transcript:

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 parasite Tetracapsula bryozoides J.R. Freeland, L.R. Noble & B. Okamura (2000) J. Evol. Biol. 13:

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 POPULATION STRUCTURING Gene flow resulting in the homogenization of allele frequencies Barriers to dispersal resulting in differentiation due to mutation & random genetic drift 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 partitioning genetic variationsexual/asexual budding, self/outcross budding, self/outcross good dispersal good dispersal few widespread clones few widespread clones fugitive lifestyle fugitive lifestyle novel myxozoan novel myxozoan host-parasite - Red Queen, host escapes by evolution of resistance Molecular Ecology - bryozoan systems bryozoans

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

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19 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

20 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

21 Arionid slugs & the nematode Phasmarhabditis hermaphrodita

22 Reproductive strategies partitioning genetic variation partitioning genetic variationsexual/asexualslugs facultative selfers facultative selfers poor dispersal poor dispersal species complexes species complexes genesis of taxa genesis of taxa nematodes nematodes budding, self/outcross budding, self/outcross good dispersal good dispersal few widespread clones few widespread clones fugitive lifestyle fugitive lifestyle novel myxozoan novel myxozoan host-parasite - Red Queen, host escapes by evolution of resistance Molecular Ecology - slug & bryozoan systems. bryozoans

23 The Large Arions - are they really difficult to identify? Arion ater ater - black? Arion ater ater - black? –Yes - but also red, yellow, white A. ater rufus - orange? A. ater rufus - orange? Yes - but also black and yellow Yes - but also black and yellow A. lusitanicus - Lusitanian distribution? A. lusitanicus - Lusitanian distribution? No! - anything but, prefers drier eastern sites No! - anything but, prefers drier eastern sites A. flagellus - a distinct flagellum? A. flagellus - a distinct flagellum? No! - a matter of taxonomic precedence No! - a matter of taxonomic precedence Yes!! } } A. lusitanicus A. ater

24 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?

25 Selfers vs. Outcrossers SelfersOutcrossers 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

26 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.

27 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?

28 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

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

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


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