Inbreeding

inbreeding coefficient F – probability that given alleles are identical by descent - note: homozygotes may arise in population from unrelated parents but: generally will have less overall homozygosity by random chance than from inbreeding

Inbreeding Result of inbreeding is inbreeding depression: - loss of fitness due to deficient heterozygosity - recessive traits are expressed

Inbreeding inbreeding coefficient F F = 1 – H F is a function of the ratio 2pq of observed over expected H (# heterozygotes) H = observed frequency of heterozygotes in the population p = frequency of one allele in the population q = frequency of alternate allele, or 1-p 2pq = expected frequency of heterozygotes in the population

Inbreeding inbreeding coefficient F F = 1 – H F is a function of the ratio 2pq of observed over expected H example: p = 0.5, therefore q = 0.5 2pq = 0.5

Inbreeding inbreeding coefficient F F = 1 – H F is a function of the ratio 2pq of observed over expected H example: p = 0.5, therefore q = 0.5 2pq = 0.5 if in H-W equilibrium, H = 0.5 so F = 0 = random mating if no heterozygotes, H = 0 F = 1 = complete inbreeding

Inbreeding selfing: F = 0.5 (loss of 50% of total variation per generation)

Inbreeding selfing: F = 0.5 (loss of 50% of total variation per generation) AA Aa aa p q Generation

Inbreeding selfing: F = 0.5 (loss of 50% of total variation per generation) AA Aa aa p q Generation

Inbreeding selfing: F = 0.5 (loss of 50% of total variation per generation) AA Aa aa p q Generation

Inbreeding selfing: F = 0.5 (loss of 50% of total variation per generation) AA Aa aa p q Generation

Inbreeding selfing: F = 0.5 (loss of 50% of total variation per generation) AA Aa aa p q = ½ of het. frequency = 1 x hom. frequency + ¼ of het. frequency Generation

Inbreeding selfing: F = 0.5 (loss of 50% of total variation per generation) AA Aa aa p q Generation = ½ of het. frequency = 1 x hom. frequency + ¼ of het. frequency

Inbreeding selfing: F = 0.5 (loss of 50% of total variation per generation) AA Aa aa p q Generation = ½ of het. frequency = 1 x hom. frequency + ¼ of het. frequency If there is a recessive deleterious allele, the population is in trouble….

Inbreeding part 2 – or, how to predict the changes in genotype frequencies F = 1 – H 2pq H = observed frequency of heterozygotes in the population 2pq = expected frequency of heterozygotes

Inbreeding F = 1 – H 2pq H = observed frequency of heterozygotes in the population 2pq = expected frequency of heterozygotes Define H = # heterozygotes (freq Aa) = 2pq –F2pq D = # homozygotes (freq AA) R = # alternate homozygotes (freq aa)

Inbreeding selfing: F = 0.5 (loss of 50% of total variation per generation) AA Aa aa p q Generation heterozygote freq. decreased by F homozygote freq. increased by ½ F

Inbreeding Thus, the impact of inbreeding on each genotype is: freq (AA) = D = p 2 + Fpq freq (Aa) = H = 2pq – 2Fpq freq (aa) = R = q 2 + Fpq (remember F = 1 is the result of complete inbreeding, no heterozygotes)

Inbreeding Example of the impact of inbreeding: p = 0.6, q = 0.4, 2pq = 0.48 F = 0 D = p 2 + Fpq0.36 H = 2pq – 2Fpq0.48 R = q 2 + Fpq0.16 i.e., no inbreeding, population is in Hardy-Weinberg equilibrium, genotypes are predictable based H-W equation, p 2 + 2pq + q 2

Inbreeding Example of the impact of inbreeding: p = 0.6, q = 0.4, 2pq = 0.48 F = 0 F = 0.5 D = p 2 + Fpq H = 2pq – 2Fpq R = q 2 + Fpq heterozygotes have decreased by ½ of 0.24; homozygotes have increased by ¼ of 0.24

Inbreeding Example of the impact of inbreeding: p = 0.6, q = 0.4, 2pq = 0.48 F = 0 F = 0.5F = 1 D = p 2 + Fpq H = 2pq – 2Fpq R = q 2 + Fpq heterozygotes have decreased by all of 0.24; homozygotes have increased by ½ of 0.24

Inbreeding How much inbreeding is acceptable? Slow increase in inbreeding results in less inbreeding depression than rapid inbreeding –slow purging of deleterious alleles

Inbreeding How much inbreeding is acceptable? Slow increase in inbreeding results in less inbreeding depression than rapid inbreeding –slow purging of deleterious alleles Low genetic variability is much less important than loss of variability

Inbreeding Deliberate use of inbreeding - breed out deleterious alleles - temporary reduction in fitness, then stabilizes - increase fitness by crossing inbred lines

Inbreeding Examples? Beware of file drawer effect/publication bias! - In stable populations, low variation is uninteresting - In small populations, high variation is uninteresting

Small populations do not evolve Forces that change neutral genes among sub-populations founder effect > reduced diversity genetic drift > changes in allelic frequency

“ Smallness and randomness are inseparable.” M. Soulé (1985)

How big does a population need to be to avoid loss of genetic variation? What is a ‘small’ population?

Effective population size (Ne) Ne reflects the probability that genetic variation will not be lost by random chance

Effective population size (Ne) Ne reflects the probability that genetic variation will not be lost by random chance Ideal: 1:1 sex ratio all individuals live to maturity and breed all adults have equal chances of mating with each other all individuals or pairs contribute equal numbers of offspring all of the offspring live Effective population size (Ne) = N only if all of these are true

Effective population size - evidence Not all individuals can mate with each other: 1,000 grizzly bears left in US. (1980s) less than 1% of range still occupied species now present in 6 isolated subpopulations estimated effective population size ~ 25% of census size (Allendorf et al. 1991)

Effective population size - evidence Not all parents contribute equally to next generation: Social structure with mate competition harem-polygynous species, e.g., lions, some bats 5.4% of spawning male smallmouth bass produce 54.7% of progeny ( Gross & Kapuscinski 1997 ) Sperm competition mass spawning of 2,000 rainbow trout genetically equiv. to 88.5 spawners mass spawning of 10,000 Chinook salmon equiv. to spawners (K. Scribner and others)

Effect of sex ratio on Ne Ne – taking sex ratio (only) into account Ne = 4N m N f N m + N f

Effect of sex ratio on Ne Ne = 4N m N f for 25 of each sex, 4*25*25 = 2,500 = 50 N m + N f

Effect of sex ratio on Ne Ne = 4N m N f for 25 of each sex, 4*25*25 = 2,500 = 50 N m + N f BUT for 40 males, 10 females 4*40*10 = 1600 =

Effect of sex ratio on Ne 25:25 Ne = 50 40:10 Ne = 32 49:1 Ne = 3.9

Effect of fluctuating populations on Ne Calculate Ne as harmonic mean* over several generations Ne = t (1/N t ) t = generation (population sample) N t = number of individuals in generation t * gives greater weight to small numbers 

Effect of fluctuating populations on Ne Example: Gen. (t) Pop. Size (Nt) 1/Nt Arithmetic Ne Mean

Effect of fluctuating populations on Ne Example: Gen. (t) Pop. Size (Nt) 1/Nt Arithmetic Ne Mean

Effect of family size on Ne Ne ur = k(N k – 1) V k + k(k -1) ur = unequal reproductive output k = mean number of surviving progeny V k = variance in family size N = total progeny

Effect of family size on Ne Ne ur = k(N k – 1) V k + k(k -1) ur = unequal reproductive output k = mean number of surviving progeny V k = variance in family size N = total progeny Av N Ne

Inbreeding and Ne rate of inbreeding  F = rate at which heterozygosity is lost (or fixation occurs) 1  F = 2Ne

Inbreeding and Ne effect of changes in sex ratio: 25:25 Ne=50  F = 1/100 = 1% 40:10 Ne=32  F = 1/64 = 1.6% 49:1 Ne=3.9  F = 1/7.8 = 12.8% 1:1Ne=2  F = 1/4 = 25%

Retention of genetic variation in a small population # generations Ne << << < N = 50, M=25, F=25 Assume a population of N = 50 As sex ratio changes, equivalent Ne changes

Retention of genetic variation in a small population # generations Ne << << < N = 50, M=5, F=45

Retention of genetic variation in a small population # generations Ne << << < N = 50, M=3, F=47

Inbreeding How much inbreeding is acceptable? 1-3% per generation – 1% preferred recommended Ne = 50, to maintain inbreeding at <1%

Inbreeding How much inbreeding is acceptable? 1-3% per generation – 1% preferred recommended Ne = 50, to maintain inbreeding at <1% BUT generally only 1/3 to ¼ of popn contributes to next generation - so N should be

Small populations: founder effect/bottlenecks, drift, inbreeding Minimal founder population for captive breeding: 50 (M. Soulé 1980) For long-term breeding, minimal population: 500 (J. Franklin 1980 ) the “50/500 rule” Franklin, I.R Evolutionary change in small populations, in Conservation Biology: An evolutionary-ecological perspective. M.E. Soulé and B.A. Wilcox (eds). Sinauer, MA Soulé, M.E Thresholds for survival: maintaining fitness and evolutionary potential, in Conservation Biology: An evolutionary-ecological perspective. M.E. Soulé and B.A. Wilcox (eds). Sinauer, MA

Small populations: founder effect/bottlenecks, drift, inbreeding Minimal founder population for captive breeding: 50 (M. Soulé 1980) For long-term breeding, minimal population: 500 (J. Franklin 1980 ) To balance mutation and drift: 5,000 (R. Lande 1995) Franklin, I.R Evolutionary change in small populations, in Conservation Biology: An evolutionary-ecological perspective. M.E. Soulé and B.A. Wilcox (eds). Sinauer, MA Soulé, M.E Thresholds for survival: maintaining fitness and evolutionary potential, in Conservation Biology: An evolutionary-ecological perspective. M.E. Soulé and B.A. Wilcox (eds). Sinauer, MA Lande, R Mutation and conservation. Conservation Biology 9:

Inbreeding When is population size too small (hopeless)? Przewalski’s horse13 Guam rail10 black-footed ferret 6 European bison 6 Speke’s gazelle 4 dusky seaside sparrow 2…1..…0 note: these are all captive (regulated) populations….