Genetic erosion and persistence of biodiversity

Transcription

1 Genetic erosion and persistence of biodiversity Kuke Bijlsma Population & Conservation Genetics Evolutionary Genetics Wageningen Biodiversity crisis: human impact Habitat deterioration, habitat destruction and fragmentation fragmentation of heather habitat in the province of Drenthe see for illustration: Flora-kartering Drenthe

2 the consequence of fragmentation: small, mostly isolated populations that become subject to stochastic forces: demographic risks environmental risks genetic risks Genetic risks the focus of this talk Genetic processes in small isolated populaties Genetic drift loss of genetic variability increase in homozygosity increased relatedness (inbreeding) deleterious alleles homozygous loss adaptability to future changes decreased survival and reproductive output Genetic Erosion 2

3 Increase in relatedness (inbreeding): Breeding programs in zoos Plant & animal breeding In small populations (even under random mating) complex vertebral malformation (CVM) in cattle as an example: Denmark 31% and Japan 32% of the top bulls carrier of this recessive deleterious allele Random mutations are the ultimate source of biodiversity, but. most mutations that affect fitness are deleterious in E. coli 226 mutations: 80% negative effect on fitness Non significantly positive Elena et al 1998 (Genetica 102/103: 349) 3

4 Most mutations that affect fitness are negative Haldane-Muller Principle: the equilibrium frequency for a deleterious allele is depends only on the mutation frequency n n mean fitness of a population: W = (1-µ) or W = (1-2µ) (n= number of loci) Mutational load in humans: genes of which carry deleterious alleles; mutation rate: µ= 10-5 or µ= n n W = (1-µ) or W = (1-2µ) Mean fitness reduction 14-53% or 53% 78% genomes of most normally outcrossing species harbour recessive deleterious alleles that become expressed when homozygosity increases. Drosophila: 2-4 lethal equivalents Mammals : 2-6 lethal equivalents Domestic mammals : lethal equivalents (NB: lethal equivalents per zygote) 4

5 The mutational load is the negative side effect of the need to evolve and when it becomes expressed during inbreeding we call it inbreeding depression! The effect of severe inbreeding (F 0.785) on survival during different life stages of Drosophila melanogaster egg-to pupa survival (%) egg + larval survival outbreds inbreds pupal survival (%) pupal survival mean lifespan (days) lifespan Bijlsma, unpublished 0 Does genetic erosion increase the extinction risk? 5

6 Does genetic drift/inbreeding increase the extinction risk? non-inbred significantly lower extinction rate the more inbred the higher the extinction moreover. % population extinction F=0 F=0.10 F=0 F=0.35 F=0.57 F= Generations control moreover. extinction much higher under stress conditions strong interaction between the inbreeding level and and the environment! % population extinction % population extinction F=0 F=0.10 F=0 F=0.35 F=0.57 F=0.79 high temp. Bijlsma et al control Generations 6

7 how bad is it? Survival overlevingskans probability kleine of populations populaties degree mate of van inbreeding inteelt Moderate effect when conditions are favourable but when environmental conditions deteriorate extinction probability increases strongly hamster in Blij-dorp hamster in grim nature Effects of genetic erosion are long-lasting! Survival overlevingskans probability kleine of populations populaties degree mate of van inbreeding inteelt 50 generations later we observe still the same negative impact of inbreeding on the persistence of small populations The negative effects of genetic erosion can stay hidden for a long period but can suddenly strike hard under very specific environmental conditions! 7

8 Does loss of variation decrease adaptive potential? Joke Bakker: metapopulations maintained for 40 generations fragmented populations vial non-fragmented populations 6 replicate metapopulations for each model, each comprising 6 subpopulations (vials) Migration between subpopulations was allowed, after most offspring had hatched, during a short period Adaptive potential To what extent can fragmented and non-fragmented populations adapt when exposed for 6 generations to different stresses? Relative increase in stress resistance High temperature NaCl ( Van Rijswijk et al, manuscipt) Ethanol Fragmented Non-Fragmented The loss of genetic variation in individual subpopulations due to fragmentation decreases evolutionary potential! 8

9 Contradictions from nature Felis concolor coryi, top, photo courtesy US Fish and Wildlife Services Florida panther: signs of inbreeding abnormal sperm cryptorchidism Northern elephant seal: bottleneck around 1900 s now: individuals, no signs of inbreeding greatly reduced variation but very healthy Purging inbreeding depression? Not all organisms seem to suffer from genetic erosion in the same way: selfing plants Arabidopsis agricultural animals beaver northern elephant seal 9

10 The If effect it we would of severe select allow inbreeding the natural blue selection circled (F 0.785) inbred to operate on lines survival viability in the during would different be inbred nearly life population the stages same of as what Drosophila the would non-inbred melanogaster happen? lines egg-to pupa survival (%) egg + larval survival outbreds inbreds pupal survival (%) pupal survival 50 lifespan mean lifespan (days) Bijlsma, unpublished 0 If expressed natural selection can purge the inbreeding load: Large effect mutations faster removed than those with small effect slightly deleterious lethals (Hedrick, 1994) Cost of purging too high for endangered populations? 10

11 domestication genetic purging amount of inbeeding depression in full-sib offspring 1.0 inbreeding depression Chicken Turkey Quail Chukar Abplanalp (1974) domesticated species semi-wild species both the rate of inbreeding and the strength of selection determine the success of purging However, natural selection cannot purge what it cannot see at the phenotypic level and not Conditionally expressed genes: 1. alleles that are deleterious under different environmental conditions than they were selected at. for ectotherms, temperature may play an important ro 2. alleles that are beneficial under specific (restrictive) environmental conditions, but (slightly) deleterious under other conditions resistance genes 11

12 Can we prevent the bad effects of genetic erosion? Could introduction of new genetic variation help to overcome the problems? Migration??? Genetic rescue of the viper! A small population of vipers of which the numbers were severely declining since 1981 was rescued genetically in 1992 by introducing a few unralated males from another population; since then the numbers of individuals increased again and were doubled in for figures see: Madsen et al (1999) Nature 402: migration can possibly alleviate the problems of genetic erosion 12

13 conclusion: genetic rescue might help but. (i) we have to be aware of local adaptation: outbreeding depression (ii) if the population stays small we have to repeat this measure from time to time (iii) bad genes from the rescuer may attain high frequencies in the rescues population worsening the situation on the long term source: Vila et al (2003), Proc. Royal Soc. B 270:

14 General conclusions: Genetic erosion increases extinction risk effects are long lasting effects increase under stress Genetic erosion decreases adaptive potential while most endangered species face changing and deteriorating environmental Purging of inbreeding load possible can be effective only partially depends on the rate of inbreeding dependent on the environment General conclusions: Conditional expressed variation important cryptic inbreeding load cannot be purged; danger if environment changes greatly delayed problems Genetic rescue can alleviate problems in isolated populations only temporary effect possible problems with outbreeding depression 14

15 Questions still to be resolved: what kind of genes cause inbreeding depression? major vs minor genes mainly recessive deleterious genes? architecture and epistasis? Questions still to be resolved: what kind of genes cause inbreeding depression? major vs minor genes mainly recessive deleterious genes? architecture and epistasis? how do these genes relate to stress? genotype-by-environment interactions conditional expression major versus minor genes 15

16 Questions still to be resolved: what kind of genes cause inbreeding depression? major vs minor genes mainly recessive deleterious genes? architecture and epistasis? how do these genes relate to stress? genotype-by-environment interactions conditional expression major versus minor genes how does genetic erosion affect adaptation? what genes involved depletion of standing genetic variation? constrained by genetic architecture? Questions still to be resolved: what kind of genes cause inbreeding depression? major vs minor genes mainly recessive deleterious genes? architecture and epistasis? how do these genes relate to stress? genotype-by-environment interactions conditional expression major versus minor genes how does genetic erosion affect adaptation? what genes involved depletion of standing genetic variation? constrained by genetic architecture? what is the importance of local adaptation? does outbreeding depression occur frequently? is it a short- or long-term problem? 16

17 Results of the past are no guarantee for the future! Literature Bijlsma R., Loeschcke V Environmental stress, adaptation and evolution: an overview. J. EVOL. BIOL. 18: Vermeulen C.J., Bijlsma R Changes in mortality patterns and temperature dependence of lifespan in Drosophila melanogaster caused by inbreeding HEREDITY 92: Bijlsma R., Bundgaard J., Boerema A.C Does inbreeding affect the extinction risk of small populations? predictions from Drosophila J. EVOL. BIOL. 13: Bijlsma R., Bundgaard J., Van Putten W.F Environmental dependence of inbreeding depression and purging in Drosophila melanogaster J. EVOL. BIOL. 12: Hedrick P.W Conservation genetics: where are we now? TRENDS ECOL. EVOL. 16: Vila C, Sundqvist AK, Flagstad O, et al Rescue of a severely bottlenecked wolf (Canis lupus) population by a single immigrant PROC ROYAL SOC LONDON B 270: take home message: results from the past are no guarantee for the future! 17