We pursue a number of research projects that inform and thus enhance our breeding program. Many of these are accomplished through our family breeding program, whose purpose is to provide detailed estimates of genetic and environmental contributions to economically important and desirable traits.
Quantitative genetic studies of growth rate
In the absence of disease, fast growth rate is emerging as the primary breeding objective in Virginia, particularly in low-salinity areas where mortality from the diseases MSX and Dermo is low. Commercial operations aim to harvest their crop in 18 months. Can we improve this by selection? If this is the case how early in the grow-out cycle is it possible to select oysters for size? Early selection can reduce the handling of oysters in the field and perhaps shorten the generation time from 2 years to 1. Are there environmental conditions that will retard or realize this genetic potential? To answer these questions, we are using our family breeding program to estimate the heritability and genetic correlations between early and late growth. We are also growing our families under various environmental conditions (salinity, temperature, and depth) to understand and quantify non-genetic contributions.
Genetic correlations among old and new traits
For a number of generations, our principal selection trait has been resistance (tolerance) to the diseases MSX and Dermo. However, as oyster aquaculture develops, new traits of importance are coming to the fore. It is no longer sufficient for our animals just to survive until harvest. We are now incorporating other production traits such as fast growth rate and high meat yield to add more value at harvest. This begs the questions, are these new traits positively or negatively correlated with disease tolerance, and each other? Furthermore, all our emphasis has been placed on grow-out performance but there is another side to oyster aquaculture, the hatchery. Starting in 2010 we will begin a series of long-term breeding trials to define hatchery traits and determine the possibility of improving these traits by selection and the consequences these could have on previously selected field traits.
Variation in disease resistance among natural populations
Certain natural populations have been shown to harbor resistance to the diseases MSX and Dermo. For example, Louisiana oyster populations are tolerant to Dermo disease. We are systematically examining local Chesapeake Bay populations from 4 different rivers, which are believed to have developed tolerance to these diseases. Survivors from a two-year natural disease challenge will be used to develop a new line—the WTS (Wild ThingS).
Examining variation among different sets of triploid progeny
Triploids are fast becoming the animal of choice throughout Virginia. This is because of their proven superior growth and high survival, even under intense disease pressure. Can we further improve their performance by breeding? This sounds like an oxymoron because these animals are sterile, but it isn't. This is because in addition to the standard tools of selective breeding, ABC also has technology to increase chromosome numbers of oysters, so-called polyploid production and we produce triploid (3n) oysters—containing an extra set of chromosomes—by using tetraploid (4n) and diploid (2n) parents. Triploids are made by mating 4n males with 2n females. We have begun testing to see if breeding better triploids is possible. The first step in this process is to see if and how much variation exists among triploid spawns produced from same tetraploid males and different females. This will tell us if the genetic advantage from a diploid female is carried into the triploid cross and thus if selection for an improved triploid is possible. We are also making several new pure-bred tetraploid lines for an upcoming crossbreeding experiment. Managing tetraploid broodstock populations represents an extensive program, which can be considered distinct from our family and line breeding programs. To what degree will tetraploids have to be managed to optimize their use, or are triploid produced from them more or less equivalent?
Stability of polyploidy oysters over time
We have been systematically examining triploid and tetraploid populations of oysters for chromosome stability. We know that both tend to lose chromosomes over time. For triploids, this process is slower than tetraploids. Does this chromosome loss affect sterility of triploids or fertility of tetraploids? Are there implications for evolutionary processes?
Research by ABC graduate students
My goal is to determine the fecundity of triploid Crassostrea virginica females in relation to the generation of their tetraploid parent. Commercial oyster growers in Virginia reported significant mortality events of triploid oysters during recent spring and summer months. The summer of 2014 was the worst, as growers across the state reported summer mortality, most severe on the Eastern Shore, in some cases as high as 85% of the crop. Surviving oysters from these mortality events were brought to VIMS and 38% of the triploid oysters examined were found to be females, 31% with significant gonadal development. This was an unusual finding because, historically, triploid C. virginica are effectively sterile. Genetics may be an explanation for the unusually high reproductive effort and mortality rate. A plausible theory is that the generation of the tetraploid parent is correlated to fecundity and survival. By back-crossing tetraploid lines year after year, we at ABC may have inadvertently selected for increased tetraploid fecundity which is translating to subsequent terminal triploid outcrosses used by the industry. The specific objectives of my project are to determine variability in fecundity of triploids owing to crosses from four different generations of tetraploid parents and to establish the relationship between this variation and mortality. Results will guide breeding decisions for future tetraploid broodstock production at ABC.