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|October 21, 2014|
Daniel Gruenberg Reports on Broodstock
Shrimp News: In this long submission to The Shrimp List, Daniel Gruenberg refers to articles by Dr. Roger Doyle and Dr. Thomas Gitterle on the dangers of inbreeding and the differences in Latin American and Asian breeding programs.
Here is the link to Dr. Doyle’s article: http://www.thefishsite.com/articles/1947/big-picture-connects-shrimp-disease-and-inbreeding.
Here is the link to Dr. Gitterle’s article: http://www.thefishsite.com/articles/contents/The%20Advocate.pdf. If this link does not work, copy and paste it into your web browser.
Daniel Gruenberg (firstname.lastname@example.org): Thailand is currently working hard to approve multiple sources of broodstock from biosecure programs to maintain the genetic diversity of its broodstock. This is a brave policy decision by the Thai authorities, and I have to say the correct one because during an epidemic most countries just respond with a blanket ban on broodstock imports, which promotes inbreeding and therefore decreases the fitness of your animals to handle the disease at time when diversity is needed most. The inextricable fact here is that diversity can get you out of a disease crisis. As I have said before, you can’t make St. Bernards if your founding stock is only chihuahuas.
I believe you will see a slow and steady improvement in Thai production now that Thailand is making these policy moves!
In his article, Dr. Gitterle, explains that simple inbreeding is not the only genetic risk related to the current epidemic. Putting too much emphasis on selection for high growth is another risk factor. When you do that, mortalities increase. In modern breeding programs, these two characteristics are mutually exclusive; when you emphasize one, you produce the opposite response in the other. Asian farmers need high growth animals, but they also need strong animals.
Many well known and professional scientists have strongly disagreed with me on the relationship between genetics and EMS, but I think as time goes by you will find that although genetics didn’t cause the disease, it is a key confounding factor that made the animals more susceptible to the disease, and if that is true the converse is also true—good genetics can reduce the incidence and impact of the disease.
The key argument against my theory is that tiger shrimp (Penaeus monodon) broodstock, which usually come from the wild, not from breeding programs, are also susceptible to EMS. This is the logical equivalent of saying that sober drivers don’t have accidents. Of course they do. In addition, if you question the assumption that P. monodon is diverse because it is not domesticated, and search the literature, you will find that by various methods of testing for genetic diversity, that most populations of monodon lack diversity and contain a high level of inbreeding. This is thought to be due to excessive fishing pressure. So it is a classic case of using the wrong assumptions. I have posted references to this list previously that document the lack of diversity in wild-caught monodon broodstock and monodon PLs.
So good genetics is not going to cure EMS, just as driving sober is not going prevent accidents; however, if you stock good quality PLs with good genetics, you will certainly get better production.
Responses to Gruenberg’s Report
Dallas Weaver (email@example.com): We also have to keep in mind that a lot of the SPF stocks came from a very narrow base of imports to the USA.
When dealing with some of these very high-fecundity, broadcast-spawning animals in the wild, you get patchy inbred areas where one lucky pair produce almost all the local population. Oysters are an example. In one collection area, you get mainly one family or a few families that are genetically distinct from other areas, but these distinctions change over time and are not distinct subspecies.
Greg Lutz (firstname.lastname@example.org): Exactly—and nature often encourages inbreeding, sometimes in the short-term, sometimes in the long term, which is ultimately associated with speciation, for example, Darwin’s finches and colonization of new habitats by Penaeus monodon populations in Gulf of Mexico. It depends on the species’ life-history, of course. White-tail deer in North America can be very inbred in local populations, since dominant males tend to mate with their own daughters when they reach breeding age, while timber wolves inhabiting the same terrain exhibit quite different patterns of genetic variation, since females vigorously reject the advances of their sires even though they may stay in the same packs for a number of years.
There will always be a trade-off between a) adaptation to specific conditions and b) maintenance of sufficient genetic variation to respond when those conditions eventually change. It’s not complicated and there is no right or wrong answer. There is little variation left in commercial lines of poultry, but they don’t live out in the open where there is (comparatively) no biosecurity. I agree completely when Daniel states that when a new challenge eventually shows up, if there is no diversity available your options are limited.
But once you select for pathogen resistance in a line of shrimp, you are automatically reducing the total genetic variation available in favor of those alleles that infer some degree of resistance. If anyone says they are selecting for genetic resistance to EMS (I heard someone in Thailand was doing that?), by default they are rejecting other alleles that do not infer resistance to that specific pathogen (but might ultimately be of great utility against some other disease, environmental or production challenge in the future). A lot can depend on how many alleles are involved. The WSSV resistant animals in Central America are an example where probably only two alleles are involved.
High-fecundity, broadcast-spawning animals, as you put it, Dallas, are not so different from many plant species in terms of population genetics. One strategy that the shrimp industry should consider is that which has served so well in maize production—identifying inbred lines that may have specific limitations on their own, but combine well. This turns the tendency toward inbreeding accumulation into a powerful tool. Of course, the hatcheries would still have a lock—crossbred x crossbred yields all sorts of offspring. You can’t argue with the results this strategy has produced for corn, and similar strategies work well with cattle and other livestock in terms of hardiness and fitness related traits.
I also agree with Daniel that you cannot make St. Bernards if your founding stock is only Chihuahuas. At least, not unless you have about 10,000 years to get the breeding program completed. But, St. Bernards were bred for a specific purpose, and neither Chihuahuas nor St. Bernard x Chihuahua crossbreds, or mongrels off the street can do what St. Bernards have been selected to do. That is the trade-off. The reality may well be that the industry needs mongrel shrimp to better ward off diseases and adapt to stressful conditions, but that is an extreme position.
One last St. Bernard analogy. Let’s say we have an environment that calls for a dog like a St. Bernard, but all the St. Bernards got sick and died. But we need a dog with those capabilities. I’d pick a cross between a bull mastiff and a great Pyrenees over a pack of mongrels from back alleys any day. No need to throw out the baby with the bath water, and no need to dilute all the gains made to date selecting for animals that perform under the conditions specific to shrimp farming.
Source: The Shrimp List (a mailing list for shrimp farmers). Subject: EMS and Genetics. Daniel Gruenberg, Dallas Weaver and Greg Lutz. October 20–21, 2014. 2. Bob Rosenberry, Shrimp News International, October 21, 2014
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