measuring inbreeding connection to population...

Alter, S. E., Rynes, E. and Palumbi, S. R. (2007) DNA evidence for historic population size and past ecosystem impacts of gray whales. PNAS

104: 15162–15167

Causes of inbreeding depression

Reduced heterozygosity

Increased exposure of recessive deleterious alleles in homozygotes

Recessive deleterious alleles are common in large populations. These alleles are at low frequencies and typically occur mainly in heterozygotes and are therefore not purged from the populations because there is no associated penalty for the heterozygotes.

In small populations, just by chance, they might get fixed

What is inbreeding?

What is inbreeding depression?

Why are small populations a problem?

What is Mutation - Drift balance?

Read Chapter 11 (pp 376-399)

Measuring inbreeding

F =

HExpected ! Hobserved

HExpected

for population we use the heterozygosity as a proxy

HExpected = 2pq using Hardy-Weinberg proportions and two alleles

F measures the deviation from a random mating population

Measuring inbreeding

F =

He ! Ho

He

if Hois zero, F is maximal and 1

if Ho

is equal to He, F is 0

Connection to population size

F = 1 ! (1 !

1

2Ne

)t

t = number of generations

Probability that variability is maintainedover t generations

Homozygosity

Inbreeding coefficient


F = 1 ! (1 !

1

2Ne

)t

NeF

tF

F

Time t

Ne = 100

Ne = 1000

Ne = 10, 000

Ne = 100, 000


F = 1 ! (1 !

1

2Ne

)t

NeF

tF

Extinction vortexThe frequency of matings between

close relative risesINBREEDING

Heterozygosity is reduced in offspring, reducing the ability to

respond to environmental change

Semilethal recessive alleles are expressed in homozygous

conditions

As a result of this expression, fecundity is reduced and

mortality is increased(inbreeding depression)

Population becomes smaller

“the worse it gets,the worse it gets.”

Caughley 1994

EXTINCTION

Predicting inbreeding depression

ScenarioWhat is the chance of suffering “Inbreeding depression” when

mating with close relativesGenetic load

Large populationHigh

(large number of rare semilethal alleles)

Low

Recently small population

Intermediate(purged some detrimentals of

large effect)

Intermediate(fixed some detrimentals with

small effect)

Long-term small population

Low (purged detrimentals of medium or large effect)

High(fixed many detrimentals with

small effect)

Genetic load = Reduction of the mean fitness resulting from detrimental variation for a population compared to a population without lowered fitness.

Hedrick, P. W. 2001. Conservation Genetics: where are we now? Trends in Ecology and Evolution 16:629-636

Clarkia – a naturally self-pollinating plant that does

not show inbreeding depression

Heredity (1996) 76, 83–91; doi:10.1038/hdy.1996.11

Variation in inbreeding depression among families and populations of Clarkia tembloriensis (Onagraceae)

Timothy P Holtsford1

1Division of Biological Sciences, University of Missouri, Columbia, MO 65203, USA

Received 9 June 1995.

Topof page

AbstractThe relationship between the self-fertilization rate of a population and the severity of inbreeding

depression is difficult to predict because the underlying genetics may be complex. However, it is

important to understand this relationship, and the degree to which inbreeding depression varies

among families within populations, if we are to understand the evolution of plant mating systems.

Inbreeding depression was studied in glasshouse trials using seed derived from two populations of

Clarkia tembloriensis, Cantua Creek and Idria. These had very different rates of self-fertilization (s)

and inbreeding coefficients (F) (s = 0.74, F = 0.77 in the CC-1 population; 5 = 0.16, F = 0.10 in the

I-1 population). Outcrossing between these populations was equivalent to outcrossing within them;

there was no evidence for outbreeding depression or increased heterosis from interpopulation

crosses. The more self-fertilizing CC-1 population had fewer recessive lethal genes than the

outbreeding I-1 population. However, cumulative inbreeding depression at the end of flowering was

not significantly different between these populations. Inbreeding has seemingly purged the lethal

genes from the CC-1 population but overall mutational load, especially in characters that are

manifest late in the life history, is still substantial despite a history of inbreeding. Variation in

inbreeding depression among families within these two populations was surprisingly large. The range

of variation among families was five to seven times larger than the difference in inbreeding

depression between populations. Variation in inbreeding depression among families should make

these populations more susceptible to the invasion of genes which increase the rate of self-

fertilization.

Keywords: Clarkia, inbreeding depression, mating system, self-fertilization

Colonial naked mole rats do not show much inbreeding depression

VIRAL EPIZOOTIC REVEALS INBREEDING DEPRESSION IN A HABITUALLY INBREEDING

MAMMALAdin Ross-Gillespie 1,2,3 , M. Justin O'Riain 1 , and Lukas F. Keller 41 Department of Zoology, University of Cape Town, Rondebosch 7700, South Africa 2 E-mail: [email protected] 4 Zoologisches Museum, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, SwitzerlandCorrespondence to 3 Present address: Institute of Evolutionary Biology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, United Kingdom.Associate Editor: H. KokkoCorrespondence to 3 Present address: Institute of Evolutionary Biology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, United Kingdom.

KEYWORDSCoronavirus • disease susceptibility • Heterocephalus glaber • inbreeding depression • purging

ABSTRACT

Inbreeding is typically detrimental to fitness. However, some animal populations are reported to inbreed without incurring inbreeding depression, ostensibly due to past "purging" of deleterious alleles. Challenging this is the position that purging can, at best, only adapt a population to a particular environment; novel selective regimes will always uncover additional inbreeding load. We consider this in a prominent test case: the eusocial naked mole-rat (Heterocephalus glaber), one of the most inbred of all free-living mammals. We investigated factors affecting mortality in a population of naked mole-rats struck by a spontaneous, lethal coronavirus outbreak. In a multivariate model, inbreeding coefficient strongly predicted mortality, with closely inbred mole-rats (F! 0.25) over 300% more likely to die than their outbred counterparts. We demonstrate that, contrary to common assertions, strong inbreeding depression is evident in this species. Our results suggest that loss of genetic diversity through inbreeding may render populations vulnerable to local extinction from emerging infectious diseases even when other inbreeding depression symptoms are absent.

Received January 29, 2007Accepted May 8, 2007

DIGITAL OBJECT IDENTIFIER (DOI)10.1111/j.1558-5646.2007.00177.x

Predicting inbreeding depression

Scenario

What is the chance of suffering “Inbreeding depression” when

mating with close relatives Genetic load

Large populationHigh

(large number of rare semilethal alleles)

Low

Recently small population

Intermediate(purged some detrimentals of

large effect)

Intermediate(fixed some detrimentals with

small effect)

Long-term small population

Low (purged detrimentals of medium or large effect)

High(fixed many detrimentals with

small effect)

Genetic load = Reduction of the mean fitness resulting from detrimental variation for a population compared to a population without lowered fitness.


Inbreeding avoidance

Plants

Self-incompatiblity

Male and female parts flower at different times

Heterostyly (females and male parts are far from each other)

Male and females are different plants

Animals

Mate choice (mice smell whether they belong to the same MHC type or not)

Migration behavior (Young lions get driven from their pack)

LarkspurDelphinium nuttallianum

0.08 a30

0.65 b10

0.42 b3

0.30 a1

Overall fitnessDistance between

parents (m)

Outbreeding depression(=lower fitness due to breeding with unrelated individuals)

Local adaptation: differences in alleles frequencies due to

different selection pressures in different places

Coadapted gene complexes: group of traits that have high fitness

when they occur together, but low fitness when occurring with other

traits. Populations that are separated a long time were selected for

different combinations, bringing them together might be

problematic: Largemouth bass

Increased Infectious Disease Susceptibility Resulting from Outbreeding Depression

Authors: GOLDBERG, TONY L.; GRANT, EMILY C.1; INENDINO, KATE R.1; KASSLER, TODD W.2; CLAUSSEN, JULIE E.3; PHILIPP, DAVID P.Source: Conservation Biology, Volume 19, Number 2, April 2005 , pp. 455-462(8)Publisher: Blackwell Publishing

Abstract:

The mechanisms by which outbreeding

depression leads to reduced fitness are poorly

understood. We considered the hypothesis that

outbreeding can depress fitness by increasing the

susceptibility of hybrid individuals and

populations to infectious disease. Competitive

breeding trials in experimental ponds indicated

that outbred largemouth bass (!Micropterus salmoides) crossed from two geographically and

genetically distinct populations suffered a

reduction in fitness of approximately 14% relative

to parental stocks. We measured the comparative

susceptibility of these same outbred stocks to a

novel viral pathogen, largemouth bass virus.

Following experimental inoculation, F2 generation hybrids suffered mortality at a rate 3.6 times higher than either F1

generation hybrids or wild-type parental fish. Analysis of viral loads indicated that viral replication was more rapid in F2 fish

than in F1 hybrids or wild-type parental fish. We attribute these results to the disruption of coadapted gene complexes in the

immune systems of outbred fish in the F2 generation. Increased susceptibility to infectious disease may be an important but

underappreciated mechanism by which outbreeding reduces the fitness of individuals and populations and by which novel

infectious diseases emerge in populations of hybrid organisms.

Keywords: hybrids; largemouth bass virus; reduced fitness; híbridos; reducción de adaptabilidad; virus de la lobina Micropterus salmoides

Hybrids and Admixture

Two separated populationscome together

Individual of two different species have

offspring(sometimes fertile)

Wahlund-effect: Heterozygote deficiency

Hybrid Zone

Bombina variegata

Bombina bombina

Hybrids

Foto from Website of Beate Nürnberger, Edinburgh, now Munich (toads are anesthetized)

Hybridization,Admixture

Florida panther has a cowlick and a kinked tail because of inbreeding


Structured populations

Mainland Island Stepping Stone model

Metapopulation

Subpopulation

Continuous populations

High density of individuals

Some density of individuals

Low density of individuals

Low density of individuals

Isolation by distance: the genetic distance between individuals is correlated with geographic distance

Population (Genetics)

A population is a group of individuals that interbreed and that is somewhat isolated from other populations.

Species

Population

Population

Population

Population versus Deme

A deme is a group of individuals that interbreed and that is somewhat isolated from other demes.

Species

DemeDeme

Deme

Population

measuring inbreeding connection to population...

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