By Janet Krenn
Part 3 in a 3-part series: Selecting a Better Oyster.
When Stan Allen found that the oysters he spent his career breeding for faster growth could also resist diseases that were decimating oyster populations, it was a happy coincidence. Now Allen, Director Virginia Institute of Marine Science Aquaculture Genetics and Breeding Technology Center (ABC), is working to develop a strategy for breeding oysters with improved disease resistance and other profitable characteristics for Virginia’s oyster aquaculture industry.
Think about the last baby you saw. Was it identical to its mother or father? Of course, it’s a trick question. Children’s characteristics are a combination of their two parents. The same goes for oysters.
Triploid oysters also have two parents. Triploids come from mating a tetraploid oyster—an oyster with four sets of genes—with a typical oyster with two sets of genes. When it comes time for these animals to reproduce, they each put half of their genes into the sperm or egg. When a tetraploid’s sperm carrying two sets of genes (half of 4 = 2) fertilizes an egg containing one set of genes (half of 2 = 1), you get triploid oyster babies (2+1=3).
ABC produces tetraploid oysters as broodstock and distributes them to commercial hatcheries, where triploids are made. But to breed a better triploid oyster, Allen says, they need to focus on the typical diploid oyster parent. The tetraploid parent’s eggs are more fragile and the adult tetraploid oysters are highly variable, making them difficult to select and breed.
Recently ABC’s research has shown that selecting traits in the parent with two gene sets does effect the triploid offspring, even though they only supply one set of genes to the other parent’s two.
In a Sea Grant-funded study, ABC mated tetraploid oysters with either wild oysters or selected diploid oysters. The selected oysters were from a long line of oysters that ABC exposed to disease, mating the survivors to select for disease resistance. When these oysters are bred with tetraploids, the triploid offspring should have the benefit of three sets of genes plus the added benefit of one of those sets having additional disease resistance.
“If [offspring from] selected parents do better than the wild, then by selective breeding we can make the triploid better,” says Anu Frank-Lawale, aquaculutre geneticist at ABC.
The team found that triploid oysters with a selectively bred parent survived better than those mated with wild oysters. This gave the team confidence that other traits could be passed down from the typical two-gene parent to their triploid offspring. Now it comes down to identifying desirable traits.
Knowing that the diploid parent can greatly affect disease resistance in its triploid offspring, Allen and Frank-Lawale hope that they can breed these oysters to have other traits that would be beneficial to growers.
The researchers began by asking growers what the most important traits were for them. The top three traits were disease resistance (for historical reasons), growth rate (because of the importance of meat weight in the shucked and half-shell markets), and shell shape (presumably for aesthetic appeal in the half-shell market).
Now that they know what traits to shoot for, ABC’s task comes down to mating oysters that will reliably produce the traits that growers want. But it’s different from just breeding disease resistance, says Frank-Lawale. “When you’re breeding for disease resistance, the disease will just come and kill those that are vulnerable, so that everything alive is presumably resistant.” Breeding for shape is different because it is not as clear-cut which family is best, he explains. “You need to look at the length, width and height of each family and say, alright this is the [average] shape. Is it consistent for all brothers and sisters? Does one family always do better than the other?”
Yet selective breeding for one trait could make the animal weaker in another area, says Peter Kube, an oyster geneticist from Australia who will work with ABC on developing a breeding strategy. Kube’s expertise is in calculating the economic value of multiple trait combinations and developing breeding plans to achieve the maximum financial return for growers. He has worked on oysters, salmon, and other species.
“You need to identify traits that are important and breed for those without adversely affecting other quality traits… traits you don’t even think about can get compromised,” he says.
Take chickens, for example. The typical chicken you might find on your plate is bred for fast growth, reaching market size within four weeks, but breeding chickens for fast growth has unintentionally made the bones weaker and easier to break.
The next wave of oysters will need to have carefully balanced traits, enhancing the traits that are important without losing others. Then ABC can implement the breeding plan and start providing better oysters to the industry.
Looking ahead, Frank-Lawale envisions a world where growers are in the drivers seat.
“Right now the grower basically doesn’t have much he can control. He can’t control the salinity or the environment. What he can control is the kind of oyster he grows. And that’s where we come in because they’re growing our oysters,” Frank-Lawale says. “These projects are really important because the industry’s at a cross road, and what’s amazing, is we’re basically helping chart the course.”
This is Part 3 in the series: Selecting a Better Oyster about how Virginia Sea Grant-funded research will get more profitable oysters in the hands of oyster aquaculture growers.
