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Abstracts
Session: The State of Oyster Disease
Current and future roles of genetics in oyster restoration or recovery
Presented By:
Standish K. Allen, Jr., ska@vims.edu
Aquaculture Genetics and Breeding Technology Center
Virginia Institute of Marine Science
Aquaculture Genetics and Breeding Technology Center
Virginia Institute of Marine Science
Speaking specifically to recovery of oysters in Chesapeake Bay, the practical role of genetics has been with assessment of and selection for disease resistance. Stock assessments have found genetic distinctions among populations. Field tests of distinct populations have uncovered important variation in disease resistance that is of practical value in selective breeding. Selective breeding programs originally intended for improvement of oysters for aquaculture have taken on a new life in oyster restoration, raising much broader questions concerning the genetic effects of stock enhancement.
Original programs for selection of disease resistance by Haskin (Rutgers) and Andrews (VIMS) were focused on survival to MSX-disease. The Haskin lines are extant in the form of the Haskin CROSBreed(tm) strains, which have had significant support from ODRP. Andrews original lines are extinct, but another line selected in Virginia was developed by VIMS, named Andrews DEBY(tm). DEBY oysters have been selected in an area with high intensity of both diseases and consequently, seem more Dermo-resistant than CROSBreed lines. CROSBreed and DEBY lines are appropriate for the mid-Atlantic, having been derived from oysters in Delaware Bay. Other lines have been developed from oysters north of Delaware Bay, the so-called NEH lines (NorthEast Haskin), also an offshoot of original lines developed by Haskin. NEH lines have been crossed with commercially derived JOD resistant strains in response to the episodic occurrence of this disease in areas north of Long Island.
Based on genetic differences among stocks along the eastern seaboard, testing programs have addressed variation in resistance to diseases. Stocks from the northeast are basically susceptible to both MSX- and Dermo-disease having seen little or none of it until recently. Northeast stocks also have markedly different reproductive strategies from mid-Atlantic ones. Stocks from Delaware Bay, although not clearly genetically distinct from other mid-Atlantic populations, have built up an innate resistance to MSX-disease probably attributable to the retention of larvae from survivors in the natural population. Circulation patterns are much more complex in the Chesapeake and movement of oysters is rampant. These factors probably contribute to the general lack of natural disease resistance there. Consequently, wild Chesapeake populations remain susceptible to both Dermo- and MSX-disease, which is the most serious impediment to oyster restoration. The genetically distinct stocks from the Gulf coast have developed innate resistance to Dermo-disease but are remarkably susceptible to MSX-disease. While not useful in the mid-Atlantic directly, germ plasm from Gulf stocks could play an important role in selection programs for dual disease resistance. Louisiana lines have been crossed into CROSBreed and DEBY lines.
Progress in selective breeding could be greatly enhanced through use of molecular markers in marker-assisted selection or pedigree analysis. Various classes of markers have been developed and applied to population genetics and pedigree analysis, but not mapping. These include allozymes, RFLP, microsatellites and SNPs. Different classes of markers have different utility. Microsatellites are broadly useful depending on the number of alleles. SNPs are more generally useful for mapping. As useful as mapping would be to obtain QTLs for disease resistance or other traits, mapping requires the generation of pedigreed families, in and of itself not a difficult task. However, there is an underappreciated logistical problem with raising pedigreed (inbred) families. Simply, there is no place to raise them out of the shadow of disease. Genetic studies could benefit from facilities that were designed for long-term isolation of oysters.
A new approach to reef restoration has seen the confluence of selective breeding, stock assessment, and molecular marker activities. Under the moniker genetic rehabilitation, disease resistant stocks increasingly have been incorporated into outplantings of seed for oyster reefs. The notion arose almost by default. Reef building has been important in the Chesapeake for nearly 10 years. Stocking reefs with oysters has been popular since the mid-1990s. The notion to stock reefs with selectively bred, disease resistant seed came as a direct result of the release of CROSBreed oysters, and later DEBYs, in the early 90s. Now it seems that the welfare of reef restoration in general is pinned on the hopes of disease resistant stocks - at least in Virginia.
Genetic rehabilitation is a simple concept with dastardly complex logistics and assessment issues. Conceptually, disease resistant (or at least disease tolerant) oysters are created through selective breeding program(s) and released to hatcheries for production of seed which is nurtured to refuge size. Hatchery derived seed is planted on "incubator" (or "breeder") reefs where it has the opportunity to spawn producing a new crop of recruits on prepared, shelled areas. This reef-derived seed is then harvested and moved to other reefs in a continual transfer of seed hopefully enhanced for disease resistance. In genetic rehabilitation, molecular markers play a key role in assessment of recruitment success of enhanced stocks.
From the perspective of the selective breeding that gives rise to the disease resistant lines, there are some serious genetic questions concerning the most optimum approach for restoration versus aquaculture. For example, if disease resistance is additive, then there is merit to developing unique and specific lines with specific combining ability for both programs. If disease resistance shows pleiotropy or epistasis, a more productive approach might be selection for general combining ability for restoration, but not necessarily for aquaculture. Genetic dissection of these characters will be essential for both improving the rate of approach to disease resistance and understanding the prognosis for success in genetic rehabilitation.
Finally, non-native species are starting to crowd center stage. ODRP supported work with non-native species in the early 90's in studies on hybridization potential among C. virginica, C. gigas, and C. ariakensis. An important offshoot of this work - despite the failure of C. virginica to hybridize with the others - was the importation of stocks of C. ariakensis. These later became the basis of stocks used to assess the value of this species in Chesapeake Bay. The value seems high. Up to now, no field work on C. ariakensis has been funded by ODRP, but as the momentum for introduction grows, so does the need for answers concerning the ecological impact. Only some of these questions are genetic in nature. Later, however, if C. ariakensis is introduced, genetics will be important because of questions concerned with stock enhancement, domestication, creation of sterile stocks, or all the above.
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