Transcript Document

Inbreeding
if population is finite, and mating is random, there
is some probability of mating with a relative
effects of small population size, mating with
related individuals are similar
drift, inbreeding, population subdivision all
reduce within population genetic variance
more likely if population size is small
consequence is assortative mating over entire genome
--deviations from expected heterozygosity
(vs. HWE expectations) over all genes
look at one locus, consider an individual who is A1A1
1) random combination from unrelated parents
2) identical by descent (both A1 alleles from a
common ancestor)
A1A2
F = inbreeding coefficient
= probability that an individual that is
homozygous carries two alleles that
are identical by descent, i.e., from
a common ancestor
A1A2
A1A2
A1A2
when a population is totally outbred, F = 0
when a population is totally inbred, F = 1
A1A1
extreme cases:
single fertilized female--->sib-mating
single hermaphrodite--->selfing
example—one generation of selfing
start with a single heterozygous hermaphrodite
A1A2
1
4
A1A1
H1 = ( 12 )H0
1
AA
2 1 2
Hobs = 1.0
1
A A
4 2 2
Ht = ( 12 )tH0
Hobs = 0.5
limHt = 0
t 64
Calculating Inbreeding Coefficients from Genealogies
What is the chance of a individual
Becoming homozygous due to alleles
From the same source?
A 3A4
A1 A 2
p = 1/2
p = 1/2
A1
A1
p = 1/2
p = 1/2
A1A1
Chance of all events occurring = (1/2) 4
However, there are four possible alleles that could be
Made homozygous due to inbreeding, therefore the
Probability of homozygosity due to inbreeding is 4(1/2)
Inbreeding coefficient
4
= 1/4
A1 A 2
A1
A1
A1A1
The chance of events occurring is again (1/2)
However, only two possible pathways
Inbreeding coefficient = 1/8
F = 1/8
4
A 3A4
A1 A 2
A1
A1
A1
A1
A1A1
extreme cases:
single fertilized female--->sib-mating
single hermaphrodite--->selfing
example—one generation of selfing
start with a single heterozygous hermaphrodite
A1A2
1
4
A1A1
H1 = ( 12 )H0
1
AA
2 1 2
Hobs = 1.0
1
A A
4 2 2
Ht = ( 12 )tH0
Hobs = 0.5
limHt = 0
t 64
Inbreeding Reduces Heterozygosity:
A1A1
A1A2
A2A2
outbred
inbred
genotype fr.
p2(1-F)
2pq(1-F)
q2(1-F)
+
pF
+
qF
=
=
=
P
H
Q
if F=0,
HWE
Measuring inbreeding:
Observed Heterozygosity = 2pq(1-F)
or, Hobs / 2pq = 1-F
or,
F = 1 - [Hobs/Hexp]; Hexp = 2pq
How Does F Change Over Time in a Population Undergoing Inbreeding?
Ft = (1/2Ne) (1)
+
identical
by descent
Ft
(1 - (1/2Ne)) (Ft-1)
indentical
by chance
= 1 - ( 1 - (1/2Ne)t
in small popns, as t --> 4, [1 – (1/2Ne) --> 0, Ft --> 1
but, if Ne -->
4,
[1 – (1/2Ne) --> 1, Ft stays near 0
in popns known to inbreed:
Ht = Ho(1-F)t
Drift and Inbreeding May Occur in a Subdivided Population:
A1A1
A1A2
A2A2
i
j
0.16
0.64
0.48
0.32
0.36
0.04
pi = 0.4, qi = 0.6
pj = 0.8, qj = 0.2
X
exp
0.40
0.36
0.40
0.48
0.20
0.16
p = 0.6, q =0.4
heterozygote
deficiency
Estimates of Wahlund’s fst For Bougainville Islanders
fst
ABO
Rh
Gm
Inv
Hp
PHs
MNSs
0.0522
0.0113
0.0767
0.0777
0.0563
0.0490
0.0430
Mean
0.0477
Predicted Effects of Inbreeding
1) inbred populations become genetically uniform;
no longer respond to selection
2) inbred populations may become phenotypically
more uniform due to loss of genetic variance
3) inbreeding depression—fixation of deleterious
recessives and loss of selectively favored
heterozygotes leads to decreased fertility,
viability, etc.
(Lerner 1954)
lab studies have expected effects of inbreeding
but most field studies suggest ecological rather than
genetic factors cause extinction in
small populations
Saccheri et al. 1998 Nature 392:491
Inbreeding depression in the
Glanville Fritillary,
Melitea cinxia
Aland Islands in southwest
Finland
many small, isolated populations
~1600 suitable sites
~350-500 occupied sites
Model 1: Extinction Throughout Aland Islands (1993-94)
risk of extinction increases with:
decreasing population size
decreasing density of butterflies in the
neighborhood of the focal population
decreasing regional trend in butterfly density
modelling extinction risk 1995-96:
data on heterozygosity ( 7 allozyme loci) for 42 popns
336 additional populations with only ecological data
does genetic data improve model’s ability to predict
extinction??
extinct
alive
Effects of inbreeding on M. cinxia
probability of extinction is affected by:
global model (n=336 populations; 185 extinct 1995-96)
decreasing regional trend in butterfly density
decreasing habitat patch size
decreasing heterozygosity (increased inbreeding)
sample model (n=42 populations; 7 extinct 1995-96)
small size in 1995
decreasing density of butterflies in the area
surrounding the focal population
decreasing abundance of flowers
decreasing heterozygosity (increased inbreeding)
Consequences of Inbreeding in M. cinxia
reduced rate of egg hatching
reduced rate of larval survival
longer pupal period--->increased risk of
being parasitized
shortened female lifespan (lower female
fecundity)
Inbreeding Results in the Loss of Heterozygosity
more likley to occur in small populations (inbreeding
and drift may both contribute to loss of genetic
variation)
in previously outbred populations, habitat fragmentation
(and smaller population size) may lead to inbreeding
and subsequent extinction
in species that routinely inbreed (e.g., parasitic wasps)
inbreeding is not deleterious