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{{Short description|Reduced biological fitness}}
{{Short description|Reduced biological fitness}}
In [[biology]], '''outbreeding depression''' happens when crosses between two genetically distant groups or populations result in a reduction of fitness.<ref name=":0">{{Cite journal |last=Frankham |first=Richard |last2=Ballou |first2=Jonathan D. |last3=Eldridge |first3=Mark D. B. |last4=Lacy |first4=Robert C. |last5=Ralls |first5=Katherine |last6=Dudash |first6=Michele R. |last7=Fenster |first7=Charles B. |date=June 2011 |title=Predicting the probability of outbreeding depression |url=https://pubmed.ncbi.nlm.nih.gov/21486369/ |journal=Conservation Biology: The Journal of the Society for Conservation Biology |volume=25 |issue=3 |pages=465–475 |doi=10.1111/j.1523-1739.2011.01662.x |issn=1523-1739 |pmid=21486369}}</ref> This is particularly likely if the subspecies have different habitats or if no genetic exchange has occurred within 500 years.<ref>{{Cite journal |last=Banes |first=Graham L. |last2=Galdikas |first2=Biruté M. F. |last3=Vigilant |first3=Linda |date=2016-02-25 |title=Reintroduction of confiscated and displaced mammals risks outbreeding and introgression in natural populations, as evidenced by orang-utans of divergent subspecies |url=https://www.nature.com/articles/srep22026 |journal=Scientific Reports |language=en |volume=6 |issue=1 |pages=22026 |doi=10.1038/srep22026 |issn=2045-2322}}</ref> The concept is in contrast to [[inbreeding depression]], although the two effects can occur simultaneously.<ref name=rf1>{{Cite book|title=Introduction to Conservation Genetics|url=https://archive.org/details/introductiontoco00fran_084|url-access=limited|last1=Frankham|last2=Ballou|last3=Briscoe|first1=R.|first2=J.D.|first3=D.A.|publisher=Cambridge|year=2002|isbn=0521702712|pages=[https://archive.org/details/introductiontoco00fran_084/page/n404 382]}}</ref> [[Outbreeding]] depression is a risk that sometimes limits the potential for [[genetic rescue]] or augmentations.<ref name=":0" /> Indeed, studies that report hybridization in mammals find resulting negative consequences about 4 times more likely than positive consequences.<ref>{{Cite journal |last=Adavoudi |first=Roya |last2=Pilot |first2=Małgorzata |date=January 2022 |title=Consequences of Hybridization in Mammals: A Systematic Review |url=https://www.mdpi.com/2073-4425/13/1/50 |journal=Genes |language=en |volume=13 |issue=1 |pages=50 |doi=10.3390/genes13010050 |issn=2073-4425 |pmc=8774782 |pmid=35052393}}</ref> Outbreeding depression can occur between an invasive population and a native populations; hybridization can result in extinction of the native species<ref>{{Cite journal |last=Rhymer |first=Judith M. |last2=Simberloff |first2=Daniel |date=1996 |title=Extinction by Hybridization and Introgression |url=https://www.jstor.org/stable/2097230 |journal=Annual Review of Ecology and Systematics |volume=27 |pages=83–109 |issn=0066-4162}}</ref> or the loss of native adaptations.<ref>{{Cite journal |last=Randi |first=Ettore |date=January 2008 |title=Detecting hybridization between wild species and their domesticated relatives |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1365-294X.2007.03417.x |journal=Molecular Ecology |language=en |volume=17 |issue=1 |pages=285–293 |doi=10.1111/j.1365-294X.2007.03417.x}}</ref> Outbreeding depression considered post-zygotic response because outbreeding depression is noted usually in the performance of the progeny.<ref name=":1">{{Cite journal|last=Waser|first=Nickolas M.|last2=Price|first2=Mary V.|last3=Shaw|first3=Ruth G.|date=2000|journal=Evolution|volume=54|issue=2|pages=485–91|doi=10.1554/0014-3820(2000)054[0485:odvaco]2.0.co;2|pmid=10937225|issn=0014-3820|title=Outbreeding Depression Varies Among Cohorts of Ipomopsis Aggregata Planted in Nature}}</ref>
In [[biology]], '''outbreeding depression''' happens when crosses between two genetically distant groups or populations result in a reduction of fitness.<ref name=":0">{{Cite journal|last1=Frankham|first1=Richard|last2=Ballou|first2=Jonathan D.|last3=Eldridge|first3=Mark D. B.|last4=Lacy|first4=Robert C.|last5=Ralls|first5=Katherine|last6=Dudash|first6=Michele R.|last7=Fenster|first7=Charles B.|date=2011-04-12|title=Predicting the Probability of Outbreeding Depression|journal=Conservation Biology|volume=25|issue=3|pages=465–475|doi=10.1111/j.1523-1739.2011.01662.x|pmid=21486369|s2cid=14824257 |issn=0888-8892|doi-access=free|bibcode=2011ConBi..25..465F }}</ref> The concept is in contrast to [[inbreeding depression]], although the two effects can occur simultaneously on different traits.<ref>{{Cite book|title=Introduction to Conservation Genetics|url=https://archive.org/details/introductiontoco00fran_084|url-access=limited|last1=Frankham|first1=R.|last2=Ballou|first2=J.D.|last3=Briscoe|first3=D.A.|publisher=Cambridge|year=2002|isbn=0521702712|pages=[https://archive.org/details/introductiontoco00fran_084/page/n404 382]}}</ref> [[Outbreeding]] depression is a risk that sometimes limits the potential for [[genetic rescue]] or augmentations.<ref name=":0" /> It is considered postzygotic response because outbreeding depression is noted usually in the performance of the progeny.<ref name=":1">{{Cite journal|last1=Waser|first1=Nickolas M.|last2=Price|first2=Mary V.|last3=Shaw|first3=Ruth G.|date=2000|journal=Evolution|volume=54|issue=2|pages=485–91|doi=10.1554/0014-3820(2000)054[0485:odvaco]2.0.co;2|pmid=10937225|issn=0014-3820|title=Outbreeding Depression Varies Among Cohorts of Ipomopsis Aggregata Planted in Nature}}</ref>


Outbreeding depression manifests in two ways:
Outbreeding depression manifests in two ways:
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* Breakdown of biochemical or physiological compatibility. Within isolated breeding populations, [[allele]]s are selected in the context of the local [[epistasis|genetic background]]. Because the same alleles may have rather different effects in different genetic backgrounds, this can result in different locally [[coadaptation|coadapted]] [[gene complex]]es. Outcrossing between individuals with differently adapted gene complexes can result in disruption of this selective advantage, resulting in a loss of fitness.
* Breakdown of biochemical or physiological compatibility. Within isolated breeding populations, [[allele]]s are selected in the context of the local [[epistasis|genetic background]]. Because the same alleles may have rather different effects in different genetic backgrounds, this can result in different locally [[coadaptation|coadapted]] [[gene complex]]es. Outcrossing between individuals with differently adapted gene complexes can result in disruption of this selective advantage, resulting in a loss of fitness.


== Mechanisms for generating outbreeding depression ==
== Mechanisms ==
The different mechanisms of outbreeding depression can operate at the same time. However, determining which mechanism is likely to occur in a particular population can be very difficult.
The different mechanisms of outbreeding depression can operate at the same time. However, determining which mechanism is likely to occur in a particular population can be very difficult.


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# [[Population bottleneck]]s and [[genetic drift]]
# [[Population bottleneck]]s and [[genetic drift]]


The first mechanism for generating outbreeding depression has the greatest effects on fitness for [[Polyploidy|polyploids]], an intermediate effect on [[Chromosomal translocation|translocations]], and a modest effect on centric fusions and inversions.<ref name=":0" /> Generally this mechanism will be more prevalent in the first generation (F<sub>1</sub>) after the initial outcrossing when most individuals are made up of the intermediate [[phenotype]]. Some mechanisms may not appear until two or more generations later (F<sub>2</sub> or greater),<ref>{{Cite journal|last=Fenster|first=Charles|date=2000|title=Inbreeding and Outbreeding Depression in Natural Populations of ''Chamaecrista fasciculata'' (Fabaceae)|url=https://semanticscholar.org/paper/fa36616cd061d836d7b09b9d6b2ac6a26a80e527|journal=Conservation Biology|volume=14|issue=5|pages=1406–1412|doi=10.1046/j.1523-1739.2000.99234.x}}</ref> when [[Genetic recombination|recombination]] has undermined vitality of positive [[epistasis]]. [[Heterosis|Hybrid vigor]] in the first generation can, in some circumstances, be strong enough to mask the effects of outbreeding depression. An example of this is that plant breeders will make [[F1 hybrid|F<sub>1</sub> hybrid]]s from purebred strains, which will improve the uniformity and vigor of the offspring, however the F<sub>2</sub> generation are not used for further breeding because of unpredictable phenotypes in their offspring. Unless there is strong [[selective pressure]], outbreeding depression can increase in further generations as coadapted gene complexes are broken apart without the forging of new coadapted gene complexes to take their place. If the outcrossing is limited and populations are large enough, selective pressure acting on each generation can restore fitness. Unless the F<sub>1</sub> hybrid generation is sterile or very low fitness, selection will act in each generation using the increased diversity to adapt to the environment.<ref>{{Cite journal|last=Erickson and Fenster|date=2006|title=Intraspecific hybridization and the recovery of fitness in the native legume ''Chamaecrista fasciculata''|journal=Evolution|volume=60|issue=2|pages=225–33|doi=10.1554/05-020.1|jstor=4095211|pmid=16610315}}</ref> This can lead to recovery in fitness to baseline, and sometimes even greater fitness than original parental types in that environment.<ref>{{Cite journal|last=Lewontin & Birch|first=R.C. & L.C.|date=February 3, 1966|title=Hybridization as a source of variation for adaptation to new environments|journal=Evolution|volume=20|issue=3|pages=315–336|doi=10.2307/2406633|jstor=2406633|pmid=28562982}}</ref> However, as the hybrid population will likely to go through a decline in fitness for a few generations, they will need to persist long enough to allow selection to act before they can rebound.<ref name=rf1/>
Some mechanisms may not appear until two or more generations later (F<sub>2</sub> or greater),<ref>{{Cite journal|last=Fenster|first=Charles|date=2000|title=Inbreeding and Outbreeding Depression in Natural Populations of ''Chamaecrista fasciculata'' (Fabaceae)|journal=Conservation Biology|volume=14|issue=5|pages=1406–1412|doi=10.1046/j.1523-1739.2000.99234.x|bibcode=2000ConBi..14.1406F |s2cid=16051555 }}</ref> when [[Genetic recombination|recombination]] has undermined vitality of positive [[epistasis]]. [[Heterosis|Hybrid vigor]] in the first generation can, in some circumstances, be strong enough to mask the effects of outbreeding depression. An example of this is that plant breeders will make [[F1 hybrid|F<sub>1</sub> hybrid]]s from purebred strains, which will improve the uniformity and vigor of the offspring; however, the F<sub>2</sub> generation are not used for further breeding because of unpredictable phenotypes in their offspring. Unless there is strong [[selective pressure]], outbreeding depression can increase in further generations as coadapted gene complexes are broken apart without the forging of new coadapted gene complexes to take their place. If the outcrossing is limited and populations are large enough, selective pressure acting on each generation can restore fitness. Unless the F<sub>1</sub> hybrid generation is sterile or very low fitness, selection will act in each generation using the increased diversity to adapt to the environment.<ref>{{Cite journal|last=Erickson and Fenster|date=2006|title=Intraspecific hybridization and the recovery of fitness in the native legume ''Chamaecrista fasciculata''|journal=Evolution|volume=60|issue=2|pages=225–33|doi=10.1554/05-020.1|jstor=4095211|pmid=16610315|s2cid=6822055 }}</ref> This can lead to recovery in fitness to baseline, and sometimes even greater fitness than original parental types in that environment.<ref>{{Cite journal|last=Lewontin & Birch|first=R.C. & L.C.|date=February 3, 1966|title=Hybridization as a source of variation for adaptation to new environments|journal=Evolution|volume=20|issue=3|pages=315–336|doi=10.2307/2406633|jstor=2406633|pmid=28562982}}</ref> However, as the hybrid population will likely to go through a decline in fitness for a few generations, they will need to persist long enough to allow selection to act before they can rebound.<ref>Frankham, Ballou, & Briscoe, R., J.D. & D.A. (2002). Introduction to Conservation Genetics. Cambridge. p. 388 {{ISBN|0521702712}}</ref>


== Effects on species ==
== Examples ==
The first mechanism has the greatest effects on fitness for [[Polyploidy|polyploids]], an intermediate effect on [[Chromosomal translocation|translocations]], and a modest effect on centric fusions and inversions.<ref name=":0" /> Generally this mechanism will be more prevalent in the first generation (F<sub>1</sub>) after the initial outcrossing when most individuals are made up of the intermediate [[phenotype]].
Outbreeding depression has a large effect on conservation efforts in addition to human-generated or other forms of wildlife disruption in which locally adapted organisms move into "new habitats where they can interbreed with resident organisms."<ref>{{Cite journal |last=Goldberg |first=Tony L. |last2=Grant |first2=Emily C. |last3=Inendino |first3=Kate R. |last4=Kassler |first4=Todd W. |last5=Claussen |first5=Julie E. |last6=Philipp |first6=David P. |date=2005-04 |title=Increased Infectious Disease Susceptibility Resulting from Outbreeding Depression |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1523-1739.2005.00091.x |journal=Conservation Biology |language=en |volume=19 |issue=2 |pages=455–462 |doi=10.1111/j.1523-1739.2005.00091.x |issn=0888-8892}}</ref> This should be considered a risk for conservation "even when the degree of genetic divergence" between populations "is small."<ref>{{Cite journal |last=Goldberg |first=Tony L. |last2=Grant |first2=Emily C. |last3=Inendino |first3=Kate R. |last4=Kassler |first4=Todd W. |last5=Claussen |first5=Julie E. |last6=Philipp |first6=David P. |date=2005-04 |title=Increased Infectious Disease Susceptibility Resulting from Outbreeding Depression |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1523-1739.2005.00091.x |journal=Conservation Biology |language=en |volume=19 |issue=2 |pages=455–462 |doi=10.1111/j.1523-1739.2005.00091.x |issn=0888-8892}}</ref> The native genotypes of parental unmixed populations have complex local adaptations across many loci that can disappear through within-species hybridization, so increasing genetic exchange between these populations could be harmful.<ref name=":2" />


⚫
Examples of the second mechanism include [[Stickleback|stickleback fish]], which developed [[Benthic zone|benthic]] and [[Limnetic zone|limnetic]] forms when separated. When crosses occurred between the two forms, there were low spawning rates. However, when the same forms mated with each other and no crossing occurred between lakes, the spawning rates were normal. This pattern has also been studied in ''[[Drosophila]]'' and [[leaf beetle]]s, where the F<sub>1</sub> progeny and later progeny resulted in intermediate fitness between the two parents. This circumstance is more likely to happen and occurs more quickly with selection than genetic drift.<ref name=":0" />
Mixing stocks of bass (largemouth bass and its closest relative, Florida bass) reduced fitness by 50%,<ref>{{Cite book |url=https://www.worldcat.org/oclc/51651181 |title=Black bass : ecology, conservation, and management : proceedings of the symposium black bass 2000--ecology, conservation, and management of black bass in North America, held at St. Louis, Missouri, USA, 21-24 August 2000 |date=2002 |publisher=American Fisheries Society |others=David P. Philipp, Mark Stephen Ridgway |isbn=1-888569-38-7 |location=Bethesda, Md. |oclc=51651181}}</ref> and the author posits that the mixing of Florida and largemouth bass is contributing to an ongoing viral bass infection epidemic.<ref>{{Cite journal |last=Goldberg |first=Tony L. |last2=Grant |first2=Emily C. |last3=Inendino |first3=Kate R. |last4=Kassler |first4=Todd W. |last5=Claussen |first5=Julie E. |last6=Philipp |first6=David P. |date=2005-04 |title=Increased Infectious Disease Susceptibility Resulting from Outbreeding Depression |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1523-1739.2005.00091.x |journal=Conservation Biology |language=en |volume=19 |issue=2 |pages=455–462 |doi=10.1111/j.1523-1739.2005.00091.x |issn=0888-8892}}</ref>


For the third mechanism, examples include [[poison dart frog]]s, [[anole lizard]]s, and [[Cichlid|cichlid fish]]. Selection over genetic drift seems to be the dominant mechanism for outbreeding depression.<ref name=":0" />
The [[Mallard|mallard duck]] interbreeding is associated with population decline in the [[New Zealand Grey Duck|New Zealand gray duck]], the Florida mottled duck (threatening the species' existence), Australian black duck, and the endangered [[Hawaiian duck]], hampering conservation efforts. Introgression also threatens the existence other types of duck species, dove species, owl species, multiple other bird species, cat species, wolf species, ferret species, deer, mink, horse, frog species, various fish species, and many plant species. <ref>{{Cite journal |last=Rhymer |first=Judith M. |last2=Simberloff |first2=Daniel |date=1996 |title=Extinction by Hybridization and Introgression |url=https://www.jstor.org/stable/2097230 |journal=Annual Review of Ecology and Systematics |volume=27 |pages=83–109 |issn=0066-4162}}</ref><ref name=":2" />


[[Ligers]] are also an example of outbreeding depression. Although [[tigers]] and [[lions]] share the same amount of chromosomes, their hybrid offspring have genetic abnormalities and the males are often sterile.
== In animals ==

==== Vertebrates ====
Outbreeding depression may be more frequent in vertebrates than was previously thought.<ref>{{Cite journal |last=Marshall |first=T. C. |last2=Spalton |first2=J. A. |date=August 2000 |title=Simultaneous inbreeding and outbreeding depression in reintroduced Arabian oryx |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1469-1795.2000.tb00109.x |journal=Animal Conservation |language=en |volume=3 |issue=3 |pages=241–248 |doi=10.1111/j.1469-1795.2000.tb00109.x |issn=1367-9430}}</ref><ref name=":2">{{Cite journal |last=Allendorf |first=Fred W. |last2=Leary |first2=Robb F. |last3=Spruell |first3=Paul |last4=Wenburg |first4=John K. |date=2001-11-01 |title=The problems with hybrids: setting conservation guidelines |url=https://www.cell.com/trends/ecology-evolution/abstract/S0169-5347(01)02290-X |journal=Trends in Ecology & Evolution |language=English |volume=16 |issue=11 |pages=613–622 |doi=10.1016/S0169-5347(01)02290-X |issn=0169-5347}}</ref> Two populations of pink salmon ([[Pink salmon|''Oncorhynchus gorbuscha'']]) that were separated by about 600 miles, and the offspring of the two populations exhibited decreased survival in the F2 generation compared to either parental generation.

Two populations of largemouth bass (''[[Largemouth bass|Micropterus salmoides]]'') were crossed, and the F2 generation suffered from a breakdown of coadapted gene complexes in their immune systems, which resulted in a 3.6 times higher rate of mortality from infection and an decrease in fitness compared to the native parental populations. Notably, these populations were of the same species and "have undergone only a small degree of genetic differentiation."<ref>{{Cite journal |last=Goldberg |first=Tony L. |last2=Grant |first2=Emily C. |last3=Inendino |first3=Kate R. |last4=Kassler |first4=Todd W. |last5=Claussen |first5=Julie E. |last6=Philipp |first6=David P. |date=2005-04 |title=Increased Infectious Disease Susceptibility Resulting from Outbreeding Depression |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1523-1739.2005.00091.x |journal=Conservation Biology |language=en |volume=19 |issue=2 |pages=455–462 |doi=10.1111/j.1523-1739.2005.00091.x |issn=0888-8892}}</ref>

⚫
Examples of outbreeding depression generated from adaptive differentiation include [[Stickleback|stickleback fish]], which developed [[Benthic zone|benthic]] and [[Limnetic zone|limnetic]] forms when separated. When crosses occurred between the two forms, there were low spawning rates. However, when the same forms mated with each other and no crossing occurred between lakes, the spawning rates were normal. Outbreeding depression caused by genetic drift has affected [[poison dart frog]]s, [[anole lizard]]s, and [[Cichlid|cichlid fish]].<ref name=":0" />

Banes et al. (2016) suggests the possibility that some [[Bornean orangutan|Bornean orangutans]] have experienced outbreeding depression due to a cross with other orangutan subspecies, as the subspecies diverged 176,000 years ago, much greater than the 500-year or 20-generation<ref name=":0" /> isolation benchmark.<ref>{{Cite journal |last=Groves |first=C. P. |date=3 December 2017 |title=The latest thinking about the taxonomy of great apes |url=https://zslpublications.onlinelibrary.wiley.com/doi/abs/10.1111/izy.12173 |journal=International Zoo Yearbook |volume=52 |issue=1}}</ref>

==== Invertebrates ====
[[Tigriopus californicus|''Tigriopus californicus'']] tends to stay in one area because leaving its native habitat is genetically costly due to significant outbreeding depression, while staying in its native pool is not genetically detrimental. Fitness was reduced when an individuals from different areas mated.<ref>{{Cite journal |last=Brown |first=A. F. |date=August 1991 |title=Outbreeding Depression as a Cost of Dispersal in the Harpacticoid Copepod, Tigriopus californicus |url=https://www.journals.uchicago.edu/doi/10.2307/1542494 |journal=The Biological Bulletin |language=en |volume=181 |issue=1 |pages=123–126 |doi=10.2307/1542494 |issn=0006-3185}}</ref>

The fruit-fly [[Drosophila montana|''Drosophila montana'']] exhibited outbreeding depression (lower fitness than either parental population) due to decreased success mating.<ref>{{Cite web |last=Doty |first=Lewis |date=2022-07-19 |title=Outbreeding depression - Genetic Diversity |url=https://www.ecologycenter.us/genetic-diversity/outbreeding-depression.html |access-date=2022-07-22 |website=Ecology Center |language=en}}</ref> Lower spawn rates than either parental population has been observed in ''[[Drosophila]]'' as well as [[leaf beetle]]s, where the F<sub>1</sub> progeny and later progeny resulted in intermediate fitness between the two parents. This circumstance is more likely to happen and occurs more quickly with selection than with genetic drift.<ref name=":0" />


== In plants ==
== In plants ==
For plants, outbreeding depression represents a partial crossing barrier.<ref name=":1" /> Unfortunately, outbreeding depression is not understood well in [[angiosperms]]. After observing ''[[Ipomopsis aggregata]]'' over time by crossing plants that were between 10–100m apart, a pattern was noticed that plants that were farther away spatially had a higher likelihood of outbreeding depression.<ref name=":1" /> Some general takeaways from this were that spatial patterns of selection on plant genotypes will vary in scale and pattern, and outbreeding depression reflects the genetic constitution of "hybrid" progeny and the environments in which the parents and progeny grow.<ref name=":1" /> This means that although outbreeding depression cannot be predicted in angiosperms yet, the environment has a role in it.
For plants, outbreeding depression represents a partial crossing barrier.<ref name=":1" /> Outbreeding depression is not understood well in [[angiosperms]]. After observing ''[[Ipomopsis aggregata]]'' over time by crossing plants that were between 10–100&nbsp;m apart, a pattern was noticed that plants that were farther away spatially had a higher likelihood of outbreeding depression.<ref name=":1" /> Some general takeaways from this were that spatial patterns of selection on plant genotypes will vary in scale and pattern, and outbreeding depression reflects the genetic constitution of "hybrid" progeny and the environments in which the parents and progeny grow.<ref name=":1" /> This means that although outbreeding depression cannot be predicted in angiosperms yet, the environment has a role in it.


==See also==
==See also==
Line 51: Line 37:


== References ==
== References ==
:{{NPS.Gov|sourceURL=[http://www.nwfsc.noaa.gov/publications/techmemos/tm30/lynch.html Inbreeding depression and outbreeding depression] |author=Michael Lynch}}
:{{NPS|url=http://www.nwfsc.noaa.gov/publications/techmemos/tm30/lynch.html|title=Inbreeding depression and outbreeding depression |author=Michael Lynch}}
<references/>
<references/>


Latest revision as of 07:15, 2 November 2024

In biology, outbreeding depression happens when crosses between two genetically distant groups or populations result in a reduction of fitness.[1] The concept is in contrast to inbreeding depression, although the two effects can occur simultaneously on different traits.[2] Outbreeding depression is a risk that sometimes limits the potential for genetic rescue or augmentations.[1] It is considered postzygotic response because outbreeding depression is noted usually in the performance of the progeny.[3]

Outbreeding depression manifests in two ways:

  • Generating intermediate genotypes that are less fit than either parental form. For example, selection in one population might favor a large body size, whereas in another population small body size might be more advantageous, while individuals with intermediate body sizes are comparatively disadvantaged in both populations. As another example, in the Tatra Mountains, the introduction of ibex from the Middle East resulted in hybrids which produced calves at the coldest time of the year.[4]
  • Breakdown of biochemical or physiological compatibility. Within isolated breeding populations, alleles are selected in the context of the local genetic background. Because the same alleles may have rather different effects in different genetic backgrounds, this can result in different locally coadapted gene complexes. Outcrossing between individuals with differently adapted gene complexes can result in disruption of this selective advantage, resulting in a loss of fitness.

Mechanisms

[edit]

The different mechanisms of outbreeding depression can operate at the same time. However, determining which mechanism is likely to occur in a particular population can be very difficult.

There are three main mechanisms for generating outbreeding depression:

  1. Fixed chromosomal differences resulting in the partial or complete sterility of F1 hybrids.[1]
  2. Adaptive differentiation among populations
  3. Population bottlenecks and genetic drift

Some mechanisms may not appear until two or more generations later (F2 or greater),[5] when recombination has undermined vitality of positive epistasis. Hybrid vigor in the first generation can, in some circumstances, be strong enough to mask the effects of outbreeding depression. An example of this is that plant breeders will make F1 hybrids from purebred strains, which will improve the uniformity and vigor of the offspring; however, the F2 generation are not used for further breeding because of unpredictable phenotypes in their offspring. Unless there is strong selective pressure, outbreeding depression can increase in further generations as coadapted gene complexes are broken apart without the forging of new coadapted gene complexes to take their place. If the outcrossing is limited and populations are large enough, selective pressure acting on each generation can restore fitness. Unless the F1 hybrid generation is sterile or very low fitness, selection will act in each generation using the increased diversity to adapt to the environment.[6] This can lead to recovery in fitness to baseline, and sometimes even greater fitness than original parental types in that environment.[7] However, as the hybrid population will likely to go through a decline in fitness for a few generations, they will need to persist long enough to allow selection to act before they can rebound.[8]

Examples

[edit]

The first mechanism has the greatest effects on fitness for polyploids, an intermediate effect on translocations, and a modest effect on centric fusions and inversions.[1] Generally this mechanism will be more prevalent in the first generation (F1) after the initial outcrossing when most individuals are made up of the intermediate phenotype.

Examples of the second mechanism include stickleback fish, which developed benthic and limnetic forms when separated. When crosses occurred between the two forms, there were low spawning rates. However, when the same forms mated with each other and no crossing occurred between lakes, the spawning rates were normal. This pattern has also been studied in Drosophila and leaf beetles, where the F1 progeny and later progeny resulted in intermediate fitness between the two parents. This circumstance is more likely to happen and occurs more quickly with selection than genetic drift.[1]

For the third mechanism, examples include poison dart frogs, anole lizards, and cichlid fish. Selection over genetic drift seems to be the dominant mechanism for outbreeding depression.[1]

Ligers are also an example of outbreeding depression. Although tigers and lions share the same amount of chromosomes, their hybrid offspring have genetic abnormalities and the males are often sterile.

In plants

[edit]

For plants, outbreeding depression represents a partial crossing barrier.[3] Outbreeding depression is not understood well in angiosperms. After observing Ipomopsis aggregata over time by crossing plants that were between 10–100 m apart, a pattern was noticed that plants that were farther away spatially had a higher likelihood of outbreeding depression.[3] Some general takeaways from this were that spatial patterns of selection on plant genotypes will vary in scale and pattern, and outbreeding depression reflects the genetic constitution of "hybrid" progeny and the environments in which the parents and progeny grow.[3] This means that although outbreeding depression cannot be predicted in angiosperms yet, the environment has a role in it.

See also

[edit]

References

[edit]
Public Domain This article incorporates public domain material from Michael Lynch. Inbreeding depression and outbreeding depression. National Park Service.
  1. ^ a b c d e f Frankham, Richard; Ballou, Jonathan D.; Eldridge, Mark D. B.; Lacy, Robert C.; Ralls, Katherine; Dudash, Michele R.; Fenster, Charles B. (2011-04-12). "Predicting the Probability of Outbreeding Depression". Conservation Biology. 25 (3): 465–475. Bibcode:2011ConBi..25..465F. doi:10.1111/j.1523-1739.2011.01662.x. ISSN 0888-8892. PMID 21486369. S2CID 14824257.
  2. ^ Frankham, R.; Ballou, J.D.; Briscoe, D.A. (2002). Introduction to Conservation Genetics. Cambridge. pp. 382. ISBN 0521702712.
  3. ^ a b c d Waser, Nickolas M.; Price, Mary V.; Shaw, Ruth G. (2000). "Outbreeding Depression Varies Among Cohorts of Ipomopsis Aggregata Planted in Nature". Evolution. 54 (2): 485–91. doi:10.1554/0014-3820(2000)054[0485:odvaco]2.0.co;2. ISSN 0014-3820. PMID 10937225.
  4. ^ Turcek, FJ (1951). "Effect of introductions on two game populations in Czechoslovakia". Journal of Wildlife Management. 15 (1): 113–114. doi:10.2307/3796784. JSTOR 3796784.
  5. ^ Fenster, Charles (2000). "Inbreeding and Outbreeding Depression in Natural Populations of Chamaecrista fasciculata (Fabaceae)". Conservation Biology. 14 (5): 1406–1412. Bibcode:2000ConBi..14.1406F. doi:10.1046/j.1523-1739.2000.99234.x. S2CID 16051555.
  6. ^ Erickson and Fenster (2006). "Intraspecific hybridization and the recovery of fitness in the native legume Chamaecrista fasciculata". Evolution. 60 (2): 225–33. doi:10.1554/05-020.1. JSTOR 4095211. PMID 16610315. S2CID 6822055.
  7. ^ Lewontin & Birch, R.C. & L.C. (February 3, 1966). "Hybridization as a source of variation for adaptation to new environments". Evolution. 20 (3): 315–336. doi:10.2307/2406633. JSTOR 2406633. PMID 28562982.
  8. ^ Frankham, Ballou, & Briscoe, R., J.D. & D.A. (2002). Introduction to Conservation Genetics. Cambridge. p. 388 ISBN 0521702712
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