RESEARCH ARTICLE

Induced dispersal in wildlife management: experimentalevaluation of the risk of hybrid breakdown and the benefitof hybrid vigor in the F1 generation

Clare E. Holleley Æ Richard A. Nichols ÆMichael R. Whitehead Æ Melissa R. Gunn ÆJyoutsna Gupta Æ William B. Sherwin

Received: 6 April 2009 / Accepted: 29 August 2009 / Published online: 16 September 2009

� Springer Science+Business Media B.V. 2009

Abstract Management practices often aim to increase the

level of gene flow by either: introducing animals from

captive breeding programs, translocating animals from

abundant areas, or increasing the chance of animals dis-

persing between populations by creating habitat corridors.

These practices provide opportunity for the hybrid off-

spring of introduced and resident animals to experience

either increased fitness (hybrid vigor) or decreased fitness

(hybrid breakdown). There is very little quantitative data

available to adequately assess whether hybridization is

likely to be beneficial or detrimental to populations

managed in these ways. Using Drosophila melanogaster

populations, we conducted two experiments that simulate

the common management practices of translocation and

wildlife habitat corridors. We monitored the frequency and

magnitude of hybrid vigor and hybrid breakdown in F1

hybrids to assess the relative risks and benefits to popula-

tions and also monitored net productivity (number of adults

produced from controlled crosses) to assess whether the

populations were stable or in decline. In the translocation

experiment, we observed instances of both significant

hybrid vigor and hybrid breakdown, both occurring at a

frequency of 9%. In the habitat corridor experiments,

populations with moderate to high dispersal (1–4% per

generation) did not develop significant hybrid vigor or

hybrid breakdown. However, of the populations experi-

encing low dispersal (0.25% per generation) for 34 gen-

erations, 6% displayed significant hybrid vigor and 6%

displayed significant hybrid breakdown. These results

suggest that in first generation hybrids there may be limited

opportunity to utilize hybrid vigor as a tool to increase the

short-term viability of populations because there is an

equal likelihood of encountering hybrid breakdown that

may drive the population into further decline. However,

our results apply only to populations of moderate size

(N = 50; Ne = 14.3) in the absence of deliberate consan-

guineous mating. Lastly, we observed that net productivity

was positively correlated with dispersal rate, suggesting

that initial F1 declines in fitness may be temporary and that

it is preferable to maintain high levels of selectable vari-

ation via induced dispersal to assist the long-term survival

of vulnerable populations.

Keywords Hybrid vigor � Hybrid breakdown �Inbreeding depression � Drosophila melanogaster �Conservation � Management � Genetic drift

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10592-009-9984-z) contains supplementarymaterial, which is available to authorized users.

C. E. Holleley � M. R. Whitehead � M. R. Gunn � J. Gupta �W. B. Sherwin

Evolution & Ecology Research Centre and School of Biological,

Earth & Environmental Sciences, University of New South

Wales, Sydney, NSW 2052, Australia

R. A. Nichols

School of Biological and Chemical Sciences, Queen Mary

University of London, Mile End Road, London E1 4NS, UK

M. R. Whitehead

Botany and Zoology, School of Biological Sciences, Australian

National University, Canberra, ACT 0200, Australia

M. R. Gunn

Food and Environment Research Agency, Sand Hutton, York

Y041 1LZ, UK

C. E. Holleley (&)

Laboratory of Genomic Diversity, National Cancer Institute,

Frederick, MD 21702, USA

e-mail: holleleyce@mail.nih.gov

123

Conserv Genet (2011) 12:31–40

DOI 10.1007/s10592-009-9984-z

Introduction

Population fragmentation is a major contributing factor to

the risk of population extinction in the wild (Tilman et al.

1994; Henle et al. 2004; Reed 2004). A consequence of

habitat fragmentation is that dispersal and gene flow are

limited between populations. Management practices are

often designed to increase the level of gene flow in remnant

populations by either: introducing animals from captive

breeding programs (Seddon et al. 2007), translocating

animals from abundant areas (Miller et al. 1999) or

increasing the chance of animals dispersing between pop-

ulations by creating habitat corridors (Beier and Noss

1998). These induced dispersal methods ultimately aim to

reduce the probability of population extinction by

increasing local population sizes and alleviating any neg-

ative effects of inbreeding depression (Frankel 1983; We-

stemeier et al. 1998; Vila et al. 2002; Pimm et al. 2006).

However, another consequence of induced dispersal is that

distantly related populations can begin to hybridize, pro-

ducing a spectrum of fitness outcomes in the offspring,

from hybrid vigor to hybrid breakdown.

Our working definitions of hybrid vigor and of hybrid

breakdown are respectively, any increase or decrease in the

fitness of hybrids between lineages that have experienced

periods of separate evolution (Ehiobu and Goddard 1990a,

b). In practical terms we expect lineages that have experi-

enced periods of separate evolution to display significant

genetic population structure and in this study we deal with

periods of separate evolution that would be discernable on a

management timescale, for example tens of generations.

Hybrid vigor, where hybrid individuals experience higher

fitness than observed in the two parental populations, has at

least two explanations. Firstly, hybrid vigor can be a result of

the alleviation of inbreeding depression when deleterious

recessive alleles are masked in the highly heterozygous

hybrid (dominance and overdominance; Crow 1948; Birch-

ler et al. 2003). Secondly, hybrid vigor can occur in the

absence of inbreeding due to the formation of novel genetic

interactions in the hybrid (Whitlock et al. 2000; Birchler

et al. 2003). There have been several examples where hybrid

vigor appears to have been exploited successfully to manage

vulnerable populations, such as the Florida panther (Pimm

et al. 2006) and the Scandinavian grey wolf (Ingvarsson

2002). In contrast to these examples, induced dispersal can

also result in negative conservation outcomes if the hybrid

offspring are less fit than the offspring of the parental lines, a

phenomenon called hybrid breakdown (Wallace 1968).

Hybrid breakdown can be a consequence of chromosomal

rearrangements (Fishman and Willis 2001), disruption of co-

adapted gene complexes (Templeton 1986), underdomi-

nance (Schierup and Christiansen 1995) or interaction

among genomic elements (Rhode and Cruzan 2005). Cases

that unequivocally link hybrid breakdown with increased

susceptibility to extinction are rare and poorly documented

(Turcek 1951) but there is a large body of evidence sug-

gesting that genetic differentiation can result in hybrid

breakdown and that this process can occur rapidly (Kidwell

and Novy 1979; Boussy and Kidwell 1987; Lozovskaya et al.

1990).

To correctly manage the risk of hybrid breakdown and

the opportunity of hybrid vigor, the manager of a wild

population needs to know the probability of each of these

outcomes. Given that the managers usually only have very

few populations, any appreciable chance of adverse out-

comes must be very carefully managed and a precautionary

approach applied. Induced dispersal is sometimes consid-

ered to be a low risk conservation strategy because hybrid

breakdown is assumed to occur less frequently than hybrid

vigor (Edmands 1999), however, there is very little quan-

titative data to support this assumption (Moll et al. 1965;

Ehiobu and Goddard 1990a). Conversely, wrongly assum-

ing that hybrid breakdown is widespread and common may

result in overly restrictive management strategies that

exacerbate the negative effects of fragmentation. In either

case it is imperative to obtain quantitative information that

evaluates the risks versus benefits involved with induced

dispersal. The large body of literature available on hybrid

vigor and breakdown generally focuses upon the recovery

of populations in which there has been deliberate consan-

guineous mating and/or extremely small effective popula-

tion sizes (total population size N = 2–8), conditions

which are appropriate to critically endangered species

(Spielman and Frankham 1992; Backus et al. 1995; Ball

et al. 2000; Marr et al. 2002). However, there is little

information about the inherent frequency of hybrid vigor

and breakdown under conditions experienced by wild

managed populations that have not reached this threshold

of extinction risk, and thus have not experienced enforced

consanguineous mating and have moderate to small

effective population sizes. Additionally the current litera-

ture typically assumes that populations are completely

isolated and ignores the effects of ongoing low-level dis-

persal (Loebel et al. 1992; Bryant et al. 1999; Reed 2005),

a scenario that is often a more realistic representation of

wild populations.

In this study we conducted two replicated, controlled

experiments in moderately sized populations of Dro-

sophila melanogaster that aimed to evaluate the relative

risks and benefits of current management practices in the

absence of deliberate consanguineous mating. The first

experiment simulated translocation practices, where we

investigated the occurrence of hybrid vigor and hybrid

breakdown among fragmented populations that had

experienced an extended period of complete isolation

(zero gene flow). The second experiment simulated

32 Conserv Genet (2011) 12:31–40

123

habitat corridors, where we investigated the occurrence of

hybrid vigor and hybrid breakdown in fragmented popu-

lations that experienced low levels of gene flow. In each

experiment our general aim was to evaluate the risk of

hybrid breakdown impairing conservation efforts, relative

to the benefit of hybrid vigor. For populations that are

large enough to have avoided the fixation of deleterious

alleles we predict that the major mechanism of population

differentiation is the action of genetic drift to change the

frequency alleles at polymorphic loci. Depending on the

fitness effects of the alleles that happen to drift to higher

frequency, both hybrid vigor and hybrid breakdown could

occur. Consequently, we formulated and addressed the

following two hypotheses. Firstly, we predicted that

periods of population isolation (translocation experiment)

or restricted dispersal (corridor experiment) would result

in significant hybrid vigor or hybrid breakdown in the F1

generation. Our second hypothesis predicted that, since

these were not severely inbred lines (in either the trans-

location or the corridor experiment), the frequency and

effects of hybrid breakdown would be of the same order

as hybrid vigor, hence both phenomena should be

detectable in some replicates.

Materials and methods

Construction of the metapopulation

Both the translocation experiment and the habitat corridor

experiment involved studying the effects of fragmentation,

thus we first had to create an artificially fragmented

metapopulation that was the starting point for both exper-

iments (Fig. 1). The source population of D. melanogaster

was a large wild population, collected from Tyrell’s Win-

ery, Hunter Valley, New South Wales (Australia) in April

2000. Wild caught individuals were used to establish 12

smaller laboratory populations (referred to as lines),

founded by 100 males and 100 non-virgin females. The 12

laboratory lines were maintained at large population sizes

(C200 individuals), with zero gene flow for 50 months

(*60 generations). All lines were maintained on an instant

potato-sugar artificial insect food medium (Holleley et al.

2008). This manipulation simulated an episode of frag-

mentation where a large population was reduced into

genetically isolated sub-populations for an extended period

of time. To quantify the effect that the imposed fragmen-

tation had on the level of genetic differentiation, we gen-

otyped 20 individuals per line at six autosomal

microsatellites (England et al. 1996; Gunn 2003) and

estimated the mean pair-wise fixation index FST (Weir and

Cockerham 1984).

Experiment 1: Translocation

Using the artificially fragmented metapopulation, we sim-

ulated translocation by allowing reproduction between

previously genetically isolated lines (Fig. 1). We used the

results of controlled crosses to estimate the mean hybrid

performance statistic (Hij). See below for crossing meth-

odology and calculation of Hij. Investigating Hij in all 132

pair-wise combinations of lines within the artificial meta-

population was prohibitive, thus we chose a subset of 11

line pairs for detailed investigation (line pairs: 1_11, 1_4,

5_11, 9_11, 18_20, 5_22, 17_21, 18_22, 1_22, 18_21).

Using the observed magnitude and frequency of Hij in the

11 line pairs, we tested our two hypotheses and evaluated

the risk of hybrid breakdown versus the benefit of hybrid

vigor in translocation practices.

Experiment 2: Habitat corridors

In natural populations habitat corridors are tracts of land or

bodies of water that connect geographically and genetically

isolated populations. Corridors aim to encourage dispersal

Tyrell’s Winery Large, wild, panmictic population

Artificially Fragmented Metapopulation

100 males 100 females collected from wild to establish each Lab Line

Expt.1: Translocation Expt.2: Habitat Corridors

1 Isolated

Lab Lines FST = 0.08 ± 0.01SE 12

3 4 5 9

22 11

17

18

20

21

1 11

1 4

Zero gene flow between lines for ~ 60 generations

3 4

17 21

3 3

17 17

Genetically differentiated line pairs:

FST = 0.2 ± 0.01SE ~60 generations of isolation

Genetically similar line pairs:

FST = 0.01 ± 0.006SE 1 generation of isolation

Conduct crosses as in Fig. S1 and estimate Hij

H1_11

Low levels of dispersal through the corridors

(m = 0.04, 0.01, 0.0025)

H1_4

5 11 H5_11

9 11 H9_11

18 20 H18_20

5 22 H5_22

17 21 H17_21

18 22 H18_22

1 22 H1_22

18 21 H18_21

m

m

m

m

Low levels of dispersal through the corridors

(m = 0.04, 0.01, 0.0025)

Conduct crosses as in Fig. S1 and estimate Hij, at three sampling times:

T0, T1 & T2 (see Table 1)

(X 4)

(X 4)

(X 4)

(X 4) 3 4 H9_11

Fig. 1 Experimental design flow diagram. Illustrates the construction

of the artificial metapopulation which was subsequently used to

investigate the frequency and magnitude of hybrid vigor and hybrid

breakdown under two induced gene flow management practices:

translocation (Experiment 1) and habitat corridors (Experiment 2)

Conserv Genet (2011) 12:31–40 33

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and gene flow between populations. To simulate the effects

of habitat corridors we allowed a low level of dispersal (m)

between previously isolated pairs of populations (Fig. 1).

We commenced the habitat corridor experiment using four

lines from the artificially fragmented metapopulation

(lines: 3, 4, 17, and 21). These lines were chosen for use in

Experiment 2 because they showed a mean Hij closest to

zero in a preliminary low replication screening of line

pairs. This screening step was imperative to avoid selecting

lines for Experiment 2 that already showed a significant

Hij, which would then bias the subsequent experiment

towards either positive or negative Hij (low replicate

screening data not shown, refer to Fig. 2 for high replicate

data). The dispersal schemes were designed to simulate two

scenarios: firstly induced gene flow between pairs of

genetically differentiated populations (line pairs: 3_4 and

17_21) and secondly induced gene flow between pairs of

recently isolated genetically similar populations (line pairs:

3_3 and 17_17). The effect of restricted dispersal on

genetic differentiation was quantified by genotyping 24

individuals per population at seven autosomal microsatel-

lites (Holleley 2007; Holleley and Sherwin 2007) and

estimating FST (Weir and Cockerham 1984). The geneti-

cally differentiated line pairs were selected such that

FST = 0.2 ± 0.01 SE (P = 0.001). Genetically similar

line pairs (3_3 and 17_17) were constructed by splitting

lines 3 and 17 after the 50-month period of isolation and

thus these line pairs did not significantly differ from an FST

of zero (FST = 0.01 ± 0.006 SE; P = 0.268). All lines

were maintained with a constant population size of 25

males and 25 females, which corresponds to an eigenvalue

effective population size (Ne) of 14.3 (Gunn 2003; Gilligan

et al. 2005; Holleley 2009). We allowed founding indi-

viduals to mate randomly for 7 days, after which we

removed the adults for the proliferation of the next discrete

generation. We randomly chose 25 males and 25 non-vir-

gin females from the progeny in the proliferation step to be

the mating individuals for the next generation. Induced

dispersal was conducted between the line pairs at this stage

by exchanging the required number of non-virgin individ-

uals between lines according to the experimental dispersal

regime.

There were four replicates of each line pair at three

independent dispersal regimes: high dispersal, where two

individuals were transferred per generation (m = 0.04);

moderate dispersal, where one individual was transferred

every second generation (m = 0.01); and low dispersal

where one individual was transferred every eight genera-

tions (m = 0.0025). We maintained an equal sex ratio of

dispersers either by transferring a 1:1 sex ratio (high m) or

alternating the sex of the dispersing individuals at each

dispersal event (moderate and low m). We continued the

cultures by repeating this procedure until we had sampled

at three time points (Table 1). The first sample was taken at

the beginning of the experiment before the three dispersal

regimes had been implemented (T0) and then at two sub-

sequent points after the dispersal regimes had been

implemented (T1 and T2). The T1 and T2 sampling points

were calculated using the expected time for line pairs to

reach the half drift-dispersal equilibrium FST value,

because our definition of hybrid vigor assumes that the

lines have experienced independent evolution, which we

practically defined as significant genetic structure (FST).

Thus, sampling points T1 and T2 for different dispersal

rates are not comparable on a temporal scale, but are at the

same point of the drift-dispersal equilibrium trajectory.

Specifically, T1 was calculated as the expected time for

line pairs to reach the half drift-dispersal equilibrium FST

value, rounded up to the nearest whole number (Whitlock

1992; Eq. 1).

Fig. 2 Translocation experiment. Rank ordered mean Hij values of

the fitness component, net productivity, measured in recently isolated

Drosophila melanogaster lines. The line pairs axis designates which

two genetically differentiated lines are being assayed (Mean

FST = 0.0805 ± 0.01 SE; P = 0.001). Bars indicate bootstrap 95%

confidence interval. Asterisks (*) indicate significant deviation (P \0.05) from Hij = 0 after FDR correction for multiple tests

34 Conserv Genet (2011) 12:31–40

123

T1 ¼ lnð0:5Þln ð1� mÞ2 1� 1

2Ne

� �h i ð1Þ

where m is the dispersal rate and Ne is the effective pop-

ulation size (Table 1). T2 was calculated as 2 9 T1. We

confirmed that equilibrium had been attained by T2

through convergence of FST in line pairs starting from high

and low initial differentiation.

At each time point, we assessed hybrid performance (Hij

defined below) and we also compared the gross effects of

the dispersal regimes on net productivity by regressing the

number of adults produced from all the controlled crosses

in the habitat corridor experiment against the number of

generations that the line pairs had been exposed to a dis-

persal regime. Our hypotheses assumed that extensive

consanguineous mating was not occurring in our popula-

tions, thus we tested the veracity of this assumption at T0,

T1, T2 by conducting a Fisher’s Exact test for global

heterozygote deficiency in the seven autosomal microsat-

ellite loci, corrected for multiple tests using the FDR cor-

rection (Devlin et al. 2003).

Hybrid performance assay methods

We measured first generation (F1) Hij for net productivity,

estimated as the number of adult offspring observed a

specified number of days after a mating (Fig. S1 of the

supplementary information). Net productivity, which

comprises fecundity and survivorship, is a commonly

quantified fitness trait that can easily be monitored in both

laboratory and wild populations. We investigated Hij in the

F1 generation because both fitness increases and decreases

can be apparent in the F1 generation of crosses between

more inbred lines (Falconer and Mackay 1996; Edmands

1999). Our research did not specifically address fitness in

F2 and subsequent generations. To estimate Hij for net

productivity we performed F1 reciprocal crosses with

respect to sex between isolated lines as well as within-line

crosses (Fig. S1). To obtain mean net productivity esti-

mates all crosses were replicated at least 10 times. One-to-

six day old virgin flies of each sex were pooled and ran-

domly distributed among crosses. All crosses were per-

formed by mating five virgin males with five virgin females

(Ackermann et al. 2001). Mating was conducted for 3 days

after which the mating adults were removed. Cultures were

incubated for 16 days at 25�C to allow time for eclosion of

offspring (Fig. S1). To estimate Hij, the translocation

experiment required 12 replicates of 31 line-pair combi-

nations thus a minimum of 372 successful virgin crosses

(redundant within-line crosses were not conducted to

reduce workload). The hybrid performance assays for the

habitat corridor experiment were more extensive due to

temporal replication and required 10 replicates of 384 line-

pair combinations, thus a minimum of 3,840 successful

virgin crosses. Wherever possible, extra replicates of each

cross were carried out to compensate for occasional cross

failure and microbial contamination. The mean hybrid

performance statistic (Hij) for net productivity between

lines i and j was calculated as follows,

Hij ¼ logð�wij þ �wjiÞð�wii þ �wjjÞ

� �ð2Þ

where ð�wij þ �wjiÞ is the sum of mean net productivity

estimates of the reciprocal crosses between lines i and j,

and ð�wii þ �wjjÞ is the sum of mean net productivity

Table 1 Habitat corridor experiment

Dispersal regime Temporal sample

(generations)

Genetically differentiated line pairs Genetically similar line pairs

3_4 17_21 3_3 17_17

Mean Hij SE Mean Hij SE Mean Hij SE Mean Hij SE

High dispersal m = 0.04 T0 0 0.106 0.09 0.318 0.11 0.044 0.12 0.005 0.09

T1 6 0.302a 0.23 -0.010 0.07 0.201 0.14 0.160 0.06

T2 12 0.006 0.05 -0.046 0.03 -0.113 0.08 -0.054 0.04

Moderate dispersal m = 0.01 T0 0 0.106 0.09 0.318 0.11 0.044 0.12 0.005 0.09

T1 13 0.072 0.07 0.089 0.03 -0.048 0.06 -0.072 0.06

T2 26 -0.055 0.09 -0.003 0.08 0.064 0.05 -0.015 0.04

Low dispersal m = 0.0025 T0 0 0.106 0.09 0.318 0.11 0.044 0.12 0.005 0.09

T1 17 0.058 0.02 0.006 0.03 -0.028 0.06 0.055 0.05

T2 34 -0.119 0.11 0.022 0.15 -0.063 0.11 -0.155 0.30b

Mean Hij values at T0, T1 and T2 for all 24 different dispersal-line pair combinations, where the number of replicates (N) is four. Estimates of Hij

did not vary significantly from zero except for two line pair replicates of 17_17 that experienced low dispersala N = 3 instead of 4 due to an experimental errorb Two of the four replicates had a significant deviation (P \ 0.05) from Hij = 0 after FDR correction for multiple tests

Conserv Genet (2011) 12:31–40 35

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estimates of the within-line crosses. We used this ratio-

based hybrid performance statistic instead of calculating the

difference between F1 fitness and the parent mean (Dick-

erson 1969) because the difference method is dependent on

the magnitude of the fitness measure. However, a secondary

problem of ratios is the compression of hybrid breakdown

values into a range of 0–1, compared to a range of unity to

positive infinity for hybrid vigor. Therefore we log trans-

formed to ensure that hybrid breakdown had the same range

of values as hybrid vigor (zero to minus infinity or plus

infinity, respectively). The 95% confidence limits of Hij

were calculated using a bootstrap procedure, resampling

within each category of cross ( �wij, �wji, �wii and �wjj) using the

boot function within the boot package of R. The confidence

intervals were obtained from the appropriate quantile of the

bootstrap sample. See Supplementary Information for R

code. A line pair was considered to have significant Hij if

the interval did not include zero after correction for multiple

tests using the false discovery rate (FDR) correction (Devlin

et al. 2003).

Results

Experiment 1: Translocation

The experimentally imposed habitat fragmentation resulted

in significant levels of population structure among the iso-

lated laboratory lines (FST = 0.0805 ± 0.01 SE; P =

0.001). Unfortunately due to an irreconcilable labeling error

that occurred during a staff-changeover we are unable to

report pair-wise FST values for Experiment 1. We also note

that FST estimates in Experiment 1 were based on only six

microsatellite loci (England et al. 1996; Gunn 2003). Whilst

this relatively low number of loci may have resulted in low

power to detect genetic differentiation, we still observed

significant but low levels of genetic differentiation.

Following fragmentation we estimated mean net pro-

ductivity ( �wij, �wji, �wii and �wjj) of the 11 line pairs (387

crosses) and we observed that the variance in Drosophila

net productivity was inherently large, even when a large

number of replicates per cross type were conducted (10–12

minimum; Fig. 2). Despite low power resulting from the

inherently high variance in net productivity and after cor-

rection for multiple tests, two of the 11 line pairs exhibited

significant values of Hij, one instance was positive and the

other negative (Fig. 2). The other nine line pairs exhibited

a range of both positive and negative Hij values, however,

these did not differ significantly from zero. Thus, consis-

tent with our first hypothesis, significant Hij was detected in

18% of the translocation experiments. Consistent with our

second hypothesis, positive and negative Hij were both

detected among replicates of the source population.

Experiment 2: Habitat corridors

We designed the experiment such that there were two tra-

jectories towards equilibrium values of FST, by selecting two

initial levels of population differentiation. All FST values for

Experiment 2 are presented in Fig. S2 of the supplementary

information, but here we summarize this information. The

initially differentiated line pairs (T0 FST = 0.2 ± 0.01 SE;

P = 0.001) followed a downward trajectory towards equi-

librium values of FST that were lower than the initial T0 FST

(T2 high m FST = 0.03 ± 0.01 SE; T2 moderate m

FST = 0.12 ± 0.02 SE; T2 low m FST = 0.16 ± 0.03 SE).

In contrast, the initially undifferentiated line pairs (T0

FST = 0.01 ± 0.006 SE; P = 0.27) followed an upward

trajectory towards equilibrium values of FST that were higher

than the initial T0 FST (T2 high m FST = 0.06 ± 0.02 SE; T2

moderate m FST = 0.08 ± 0.01 SE; T2 low m FST = 0.16

± 0.03 SE). These FST estimates were based on seven

microsatellites (Holleley and Sherwin 2007) and thus may

have had slightly higher power to detect genetic differenti-

ation than Experiment 1 which utilized six microsatellites.

From a total of 3,930 controlled crosses, we estimated

the mean net productivity ( �wij, �wji, �wii and �wjj ) of the

replicated line pairs (3_4, 17_21, 3_3, 17_17) for each

dispersal regime (high, moderate, low) at three sampling

times (T0, T1, T2). Using the estimates of mean net pro-

ductivity we estimated Hij for replicates 1–4 of each line

pair (Fig. S2) and we present the grand mean Hij for each

line pair in Table 1. Full Hij information for all replicates is

provided in Fig. S2 of the supplementary information. At T0

no line pairs exhibited significant Hij. Over the course of the

habitat corridor experiment, populations with moderate to

high dispersal (1–4% per generation) did not develop sig-

nificant Hij (Table 1). However, two instances of significant

Hij were observed during the low dispersal regime (0.25%

per generation; Table 1; Fig. 3). At the sampling period T2,

line pair 17_17 displayed one instance of significant hybrid

vigor (H17_17 Rep 4 = 0.598) and one instance of significant

hybrid breakdown (H17_17 Rep 1 = - 0.823; Fig. 3). The

remaining two estimates of Hij for line pair 17_17 (repli-

cates 3 and 4) did not differ significantly from zero (Fig. 3).

Interestingly, the upward trajectory of line pair 17_17 at low

dispersal displayed the largest mean FST value of any line

pair at T1 (FST = 0.218 ± 0.08) or T2 (FST = 0.265 ±

0.07; Fig. 3). Line pairs 3_4, 17_21, and 3_3 did not display

any instances of significant Hij throughout the experiment

(Table 1). See Fig. S2 for FST values of all line pairs. Thus,

consistent with our first hypothesis, 12.5% of the line pairs

experiencing low dispersal displayed significant Hij at T2,

and consistent with our second hypothesis, we observed that

both hybrid vigor and breakdown can occur among repli-

cates of the same initial line pair after a period of inde-

pendent evolution (Fig. 3). Both hypotheses were rejected

36 Conserv Genet (2011) 12:31–40

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for all line pairs experiencing moderate and high dispersal,

as no significant hybrid vigor or breakdown was observed.

Overall there was very little evidence of heterozygote

deficiency in the experimental populations. Specifically, at

both T0 and T2 none of the populations showed a signifi-

cant heterozygote deficit. At T1 only two populations (Line

17Rep 1 at m = 0.01 and Line 4Rep 2 at m = 0.0025)

showed a significant heterozygote deficit after correction

for multiple tests, accounting for just 2.8% of populations

at the T1 sampling point and 1.2% over all populations. Net

productivity tended to increase over time and this increase

was positively correlated with dispersal rate (Fig. 4). Net

productivity in the high dispersal regime increased at a

faster rate than the moderate dispersal regime (Fig. 4).

Under the low dispersal regime net productivity did not

significantly increase or decrease over time (Fig. 4).

Discussion

We have demonstrated in two independent experiments that

it is possible for significant hybrid vigor and hybrid

breakdown to develop between lines of closely related

organisms, after short-term reproductive isolation (translo-

cation) or periods of restricted dispersal (corridors). Our

result challenges the commonly held notion that

populations maintained at a moderate size in isolation will

not exhibit hybrid vigor or hybrid breakdown when

recrossed. This effect is significant after correction for

multiple tests and despite low power resulting from inher-

ently high variation in Drosophila net productivity. Our

timescale is relatively short (60 generations in Experiment 1

and 12–34 generations in Experiment 2) thus relating to

times that might concern a manger of wild populations. We

encourage managers to consider, as we did, whether or not

the populations are likely to be in drift dispersal equilib-

rium, by using any available data on population size, dis-

persal rate and the time since alteration to these parameters.

We suggest that the occurrence of significant hybrid vigor

is not explained by a simple decrease in performance of the

partially isolated populations due to inbreeding depression

because the mean number of offspring in the corridor

experiment either increased or remained stable over the

course of the experiment (Fig. 4) and there was no excess of

consanguineous mating since no persistent heterozygote

deficiency was observed. Genetic causes of hybrid vigor are

a function of both dominance and epistatic effects but

without data from F2 or backcrosses, as is the case here, we

cannot separate these effects. The F1 hybrid breakdown

observed in our experiments is unlikely to be caused by the

disruption of co-adapted gene complexes, because divergent

local adaptation is unlikely to evolve in a mere 32–60 gen-

erations when there are no differences in environmental

conditions or population densities. Additionally, the dis-

ruption of co-adapted gene complexes is only expected to be

Fig. 3 Line pair 17_17 of the habitat corridor experiment. Mean Hij

values of the fitness component, net productivity, measured in line pair17_17 under low dispersal (m = 0.0025). Each line pair replicate

(N = 4) was maintained independently for the duration of the exper-

iment. The x-axis is the temporal sampling regime that was dependent

upon dispersal rate (Table 1). For the low dispersal rate (m = 0.0025),

T0 = generation zero, T1 = generation 17 and T2 = generation 34.

Bars indicate bootstrap 95% confidence interval. Asterisks (*) indicate

significant deviation (P\0.05) from Hij = 0 after FDR correction for

multiple tests. Estimates of genetic differentiation among line-pairs at

each sampling time are presented as mean FST values on the x-axis. See

supplementary Fig. S2 for graphs of complete Hij and FST dataset

Fig. 4 Habitat corridor experiment. Net productivity estimates result-

ing from controlled hybrid performance assays (N = 9.6 ± 0.25 SE)

in each of the imposed experimental dispersal regimes: a high

dispersal (4% per generation), b moderate dispersal (1% per gener-

ation) and c low dispersal (0.25% per generation). See Fig. S1 for

details of cross methodology

Conserv Genet (2011) 12:31–40 37

123

observed in the F2 or backcross generations, which were not

investigated in this study. Thus we suggest that the occur-

rence of both significant hybrid vigor and hybrid breakdown

in our experiments is most likely a result of population dif-

ferentiation in isolation due to genetic drift. Upon hybrid-

ization novel genetic interactions resulted in hybrid vigor

and hybrid breakdown. This explanation is corroborated

firstly by the development of significant population structure

in the translocation experiment and secondly in the habitat

corridor experiment by the observation that the line pair with

significant Hij (17_17; m = 0.0025) had the largest FST

value of any line pair in that experiment (Fig. 3 and Fig. S2).

Therefore we propose that genetic drift is the primary

mechanism affecting hybrid performance among moderately

sized populations.

Our second major observation was that in both of our

experiments, significant hybrid vigor occurred at the same

frequency as significant hybrid breakdown in the F1 gen-

eration. Interestingly this is quite different from results

typically found in animal and plant breeding, where the

crossbreeding of two lines usually has a positive effect on

the fitness of F1 offspring (Birchler et al. 2003; Oettler

et al. 2003; Ferreira and Amos 2006). This discrepancy is

most likely because the primary mechanism driving pop-

ulation differentiation in our experiment was genetic drift

affecting allele frequencies at polymorphic loci, whereas

most previous experiments have involved crosses between

more strongly inbred and/or selected lines which are likely

to have produced fixed or near fixed differences. A risk

averse manager, managing one or a few wild populations

should be aware that the risk of an undesirable outcome

during induced dispersal is not negligible.

Our third major observation drawn from the corridor

experiment, is the trend for populations with higher dis-

persal rates to experience greater net productivity (Fig. 4).

In our corridor experiment, dispersal is probably correlated

with net productivity because gene flow can counteract the

effects of genetic drift, augment genetic diversity and

maintain or increase the evolutionary potential of popula-

tions (Gandon et al. 1996; Swindell and Bouzat 2006). We

believe that alleviation of the effects of consanguineous

mating is not a likely explanation for this correlation

because we did not observe a higher frequency of signifi-

cant positive estimates of Hij relative to negative values of

Hij in these corridor experiments and saw no genetic evi-

dence for consanguineous mating (no persistent heterozy-

gote deficit). However, we cannot rule out the possibility

that the alleviation of very low levels of inbreeding

depression not detected by either our estimates of net

productivity or our estimates of heterozygosity could have

contributed to this correlation.

Our results are of great importance to conservation

efforts because we have demonstrated that both hybrid

vigor and hybrid breakdown can develop in moderately

sized but fragmented populations over timescales relevant

to conservation management and that these phenomena

appear to be equally likely to enhance or reduce population

viability in the short-term. Our experiment highlights that

simply managing populations to maximise their size is not

sufficient to ensure the success of an induced dispersal

management regime. The level of connectivity and the

period of isolation are critical factors affecting the fitness

of offspring resulting from an induced dispersal manage-

ment regime and thus the viability of the managed popu-

lations. Whilst our results suggest that managers’ ability to

utilize hybrid vigor as a tool to increase the short-term

viability of isolated populations may be limited, we suggest

that it is overcautious to completely prohibit all artificial

enhancement of dispersal between populations. We did not

reveal any evidence to suggest that hybrid vigor is more

likely than hybrid breakdown in the F1, however, a pre-

cautionary manager should definitely not view our results

as demonstrating that on average there was no preponder-

ance of hybrid vigor or breakdown and therefore feel it safe

to ignore both potential outcomes. Whilst both phenomena

occurred at a moderate to low frequency, they were still

detectable despite low power to detect Hij, due to inherent

variability of net productivity. A manager must therefore

be ready for both potential outcomes because the risk of an

undesirable outcome is not negligible. We note that

translocation practices are generally designed such that

there are restricted opportunities for hybridization by

introducing a small number of individuals are into a larger

population. Thus the size of the managed population will

determine what proportion of the population may be

affected by hybrid vigor and/or hybrid breakdown.

On the basis of our results we see the first priority of

population management to be the prevention of fragmen-

tation before it occurs. We acknowledge that preventing

fragmentation is not always possible and that often man-

agement is only initiated post-fragmentation or when the

population is in decline. Thus, our second recommendation

is to minimize the period of population isolation and pro-

mote connectivity between populations at a rate of C1%

dispersal per generation through mechanisms such as

habitat corridors or translocations. However, we note that

populations smaller than those investigated in this study

(\50 individuals) may require higher rates of dispersal to

counteract drift. This strategy employs the precautionary

principle by reducing genetic differentiation among popu-

lations and reducing the possibility of hybrid breakdown

occurring. Although such moderate to high dispersal rates

will also reduce the probability of hybrid vigor, we

observed a clear trend that dispersal rate is correlated with

net productivity, as have other authors for a wide range of

taxa (Spielman and Frankham 1992; Fischer and Matthies

38 Conserv Genet (2011) 12:31–40

123

1997; Sagvik et al. 2005). Further support for our recom-

mendation to maintain connectivity comes from studies

that have demonstrated that whilst hybrid breakdown can

occur initially when genetically differentiated populations

interbreed, this reduction in fitness is often temporary

(Templeton 2001). For the long-term survival of vulnerable

populations it is better to maintain high levels of selectable

variation via induced dispersal, despite the risk of short-

term negative fitness outcomes (Templeton et al. 2007).

Thus encouraging moderate rates of dispersal between

fragmented populations is the most conservative manage-

ment strategy overall from a genetic perspective.

Finally, we emphasize that there are many variables,

other than those discussed here, that need to be considered

before implementing induced dispersal as a management

practice, for example the transmission of disease (Hess

1994). Whilst the experiments carried out here cannot fully

measure all the potential effects of induced dispersal on the

viability of natural populations, our experiment provides

the first replicated base-line data about the frequency and

magnitude of hybrid vigor and hybrid breakdown in benign

laboratory conditions in a model organism. The frequency

and magnitude of hybrid vigor and hybrid breakdown may

be very different in other species, experiencing varied

selective pressures in the wild (Edmands 2006; Ferreira

and Amos 2006). However, our results provide empirical

data that can act as a starting point for further research,

such as replicated investigations of Hij in the F2 and sub-

sequent generations under varying levels of inbreeding,

which will help to better quantify the relative risks and

benefits of induced dispersal for conservation efforts.

Acknowledgments We acknowledge L. Tsai, E. Ho and J. Chao for

fly culture assistance; O. E. Gaggiotti, A. R. Templeton, J. L. Wang

and M. Mariette for comments on the manuscript and the Ramaciotti

Centre for Gene Function Analysis for DNA fragment size analysis.

This research was supported by Australian Research Council Grant

DP0559363 to WBS and RAN.

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