Mate choice play a role in genetic diversity and parasite resistance.
Title: MHC-mediated mate choice increases parasite resistance in salmon
Proceedings of the Royal Society B. Volume 275, Number 1641 / June 22, 2008
Natural (parasite-driven) and sexual selection are thought to maintain high polymorphism in the genes of the major histocompatibility complex (MHC), but support for a link between mate choice, MHC variation and increased parasite resistance is circumstantial. We compared MHC diversity and Anisakis loads among anadromous Atlantic salmon (Salmo salar L.) returning to four rivers to spawn, which had originated from natural spawning (parents allowed to mate freely) or artificial crosses (parents deprived from the potential benefits of mate choice). We found that the offspring of artificially bred salmon had higher parasite loads and were almost four times more likely to be infected than free-mating salmon, despite having similar levels of MHC diversity. Moreover, the offspring of wild salmon were more MHC dissimilar than the offspring of artificially crossed salmon, and uninfected fish were more dissimilar for MHC than infected fish. Thus, our results suggest a link between disassortative mating and offspring benefits and indicate that MHC-mediated mate choice and natural (parasite-driven) selection act in combination to maintain MHC diversity, and hence fitness. Therefore, artificial breeding programmes that negate the potential genetic benefits of mate choice may result in inherently inferior offspring, regardless of population size, rearing conditions or genetic diversity.
6 comments:
I'm certainly no advocate for hatcheries, but couldn't there be quite a few alternative hypotheses that remain unaddressed here?
Hi Brook,
Not sure what aspect of hatchery you are concerning here... indeed, I agree hatchery issue is tremendous big and complex, otherwise it wouldn't rise those debating and discussion from different communities such as biology, social economy or politic...
As focused on biology aspect, we have seen and discovered the artificial breeding program do have negative impacts in the wild populations. Especially in the perspective of evolutionary point of view.
What this paper have demonstrated is that in the wild breeding population, "mate choice" is another potential mechanism for maintaining a healthy population and did increase their progeny's parasite resistance ability.
What I think is, this is a fairly important evidence to show people that increase the number of fish in the river (hatchery), not necessary would have increased the number of healthy population, also the elimination of the "mate choice" could potentially interrupt the important selection and cause extinction.
"What this paper have demonstrated is that in the wild breeding population, "mate choice" is another potential mechanism for maintaining a healthy population and did increase their progeny's parasite resistance ability."
Yes, but how did they isolate "mate choice" as the specific variable causing this effect? Fish reared in hatcheries differ from wild fish in an unlimited number of ways. How do they eliminate those alternative hypotheses and prove that assortative mating specifically lowers parasitic loads?
I vaugely remember a much older study with northern pike that also showed higher parasite loads on hatchery-reared fish.
Does this newer paper provide the "smoking gun" that points to mate choice?
Hi Brook,
In this paper, they have pretty solid evidence to show mate choice is important, the MHC genes complex in this paper is the direct evidence of mate choice. AS we all know, MHC is very important in immune functions. They have been intensely studied in human and some other model species (mice, chicken etc)... they play a key role in immune function and disease defense, however, it's very expensive and time consuming to isolate this complex genes. Not many species have this privilege to
been explored into MHC. Atlantic Salmon is special and important in western culture. So it has been intensely invested...
Refer to mate choice function, you brought up a very important question. How could this mate choice not happening in hatchery? Well...(I guess) even there are so many fish in the hatchery, so many effort in "randomization in the pairing process" in hatchery. The results show, female salmon have their special rule in selecting their mate in the wild, and those criteria seems doing quite well in terms of parasite defense. This is a very new study,however, people are not quiet understand how female choice mechanism play a rule in other organism... but for some fish (salmon) and mammals...they have some evidence to show olfactory cue is link to the MHC gene, and female are directly targeting those gene for making mate choice decision.
The specific evidence linking MHC, mate choice and hatcheries is what I don't understand here.
According to the abstract, MHC diversity was higher in wild fish, and MHC diversity was associated with lower parasite loads in general.
That could support the study's conclusions. If the assumption is correct that assortative mating raises MHC diversity (has that been shown?) hatchery spawning techniques that lower MHC diversity could in turn be responsible for high infection rates in the field.
...but...
The study also says that infections were higher in hatchery fish regardless of their levels of MHC diversity. A hatchery fish will be infected regardless of its' MHC diversity. A hatchery that allowed assortative mating would, by this analysis, still produce infected fish.
The cause for parasitic infections at the hatchery is apparently something other than MHC diversity and assortative mating won't help in that setting.
I suppose the larger concern is subsequent generations of low diversity hatchery fish and their effect on MHC diversity in the field.
Hi Brook,
It is really nice to know you are interested and concerning about the hatchery program. Thanks for the response. Sincerely, I would love to sent the original article to you, I have read it once and it should have more complete information in there. You can reach me at liumark@auburn.edu
For the concern about the hatchery fish and they have less dissimilar MHC diversity. here is the quote from the abstract: "Moreover, the offspring of wild salmon were more MHC dissimilar than the offspring of artificially crossed salmon, and uninfected fish were more dissimilar for MHC than infected fish." I think this is the major finding of this reserach. Then there are another part they mentioned : even the wild offspring and hatchery offspring have similar MHC diversity, the hatchery fish still have more parasites. I think in this part they are addressing (for what I read) even they have similar MHC, the hatchery fish still get more parasites. This might due to other factors that other than genes.which is, early rearing environments, artificial semination etc... but this should not use to against the result as a negative evidence of MHC diversity is not differ between those groups or MHC is not important in parasite defense in Atlantic salmon. I think the author just want to address the issure "even in the same level of MHC, hatchery fish are still weaker and have more parasite". Like I said the main finding in this paper is wild offspring have more diverse MHC.
Just let me know if you need the original paper. I will post more relative papers about MHC and mate choice... Take care!
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