NOAA/Saltonstall-Kennedy Grant Program: Biosecure domestication of native geoduck clams

Purpose or Scope:

The primary emphasis of this research is the development of methods to increase the biosecurity associated with farming a native species in close contact with wild conspecifics. A secondary focus will be the initial development of genetic lines of geoducks with improved production characteristics that will be utilized in 4Nx2N matings to produce 100% triploid seed cohorts for commercial utilization. In essence, we aim to increase the domestication potential of geoduck though breeding advances while reducing genetic contact between farmed and wild populations by using 3N seed generated by 4Nx2N matings.

Approach:

Specific goals of the project are the following:

  1. Ascertain sterility and enable the production of 100% 3N geoduck by mating tetraploid males with diploid females
  2. Initiate line development and testing for geoduck clams

These goals will be met by fulfilling the following three research objectives:

  1. Determine 3N geoduck gamete and 3N x 2N embryo ploidy and viability
  2. Conduct pilot studies to determine the efficacy of tetraploid induction by
    1. 3N female x 2N male, inhibition of first polar body extrusion
    2. 3N female x 2N male, inhibition of second polar body extrusion
    3. 2N female x irradiated sperm from 2N male, inhibition of extrusion of both polar bodies.
  3. Compare growth and survival of 3N and 2N progeny of 24 pair mated lines at three growout sites in Washington.

Background:

Shellfish aquaculture in the United States is based on both native species (e.g. hard clams and eastern oysters) and species that were introduced into culture after the turn of the last century. Most of the bivalves cultured on the US west coast and Canada (e.g. Pacific oysters and Manila clams) fall into the latter category. The culture of native species is frequently recommended to reduce or avoid harmful interactions among cultured exotics and wild species (Naylor et al., 2001). This strategy does not, however, preclude genetic impacts on native populations of conspecifics. Natural genetic structure and variability can be disrupted via interbreeding between wild and cultured individuals, potentially jeopardizing wild populations by decreasing their potential to adapt to environmental change (Allendorf et al., 2001, Lynch, 1991). Shellfish aquaculture is an important component of rural economies on the US west coast; 2007 revenues exceeded $113 million. This industry produces a critical supply of healthful shellfish for humans and both receives and provides sustained ecosystem services through intensive growout of suspension-feeding bivalves. At the same time, it is increasingly clear that the continued growth of shellfish aquaculture (Fig. 1) Figure 1requires access to larger intertidal areas, raising concerns over the cumulative effects of culture operations on nearshore environments. The culture of native species such as geoduck clams creates the additional potential for adverse genetic interactions with wild conspecifics. The shellfish aquaculture sector is in urgent need of research to develop and implement ecologically and environmentally progressive practices. Wild geoduck clams (Panopea abrupta, Conrad 1849) comprise a large proportion of Puget Sound biomass and, like other suspension feeders, provide vital ecosystem services as both primary consumers of phytoplankton and biodepositors (Newell, 2004). A Washington State commercial geoduck fishery, initiated in 1970 and the Southeast Alaska fishery that launched in the 1980s, provide significant economic benefits from the wild resource. Geoducks are currently the target of the most economically important clam fishery in North America (Hoffmann et al., 2000). In addition to a robust demand from Asia, domestic demand for geoduck is increasing, but the capture fishery cannot satisfy demand in an ecologically sustainable fashion.

The potential for geoduck culture was recognized almost 130 years ago (Hemphill, 1881), but was forestalled until development of sufficient economic demand and technical expertise in the early 1990s (Beattie, 1992). Currently, geoduck aquaculture in Washington State is developing in close proximity to wild geoduck aggregations. On a return-per-acre basis geoducks are the western region’s most valuable cultured shellfish species, hence the intense interest in geoduck aquaculture and the predictions for continued rapid growth (Fig. 1). Cultured geoducks now represent approximately 16% of the 5.2 million pounds of Washington State geoduck ($20 million wholesale value). However, given current conditions, cultured geoduck production in Washington is projected to reach approximately 40% of Washington’s wild geoduck harvest by 2010 (Jonathan King, Northern Economics, Anchorage, personal communication). This proportion may increase even further as management of the wild fishery in Washington is expected to reduce spawning biomass per recruit to 40% of the unfished level (Bradbury and Tagart, 2000).

As the geoduck aquaculture industry advances, we have endeavored to take proactive steps, both to understand, and to mitigate, the potential for genetic perturbation of wild geoduck stocks. In previous work, we assessed neutral genetic differentiation among wild Puget Sound geoduck aggregations, which appear low (global FST ~0.002, Vadopalas et al. 2004) suggesting high levels of gene flow. We have also determined that farmed geoducks mature and spawn during the course of the culture cycle, next to and contemporaneous with wild stocks, suggesting that genetic interactions between cultured and wild geoduck are likely. Current and ongoing projects include investigations of neutral genetic differences between cultured and wild geoduck, fecundity at age, and fertilization success at cultured vs. wild densities.

As we gain information on the potential for negative consequences of genetic interactions between cultured and wild geoduck, a proactive component of our research program is to pursue maturation control in cultured animals. We have developed methods for the induction of triploidy (Vadopalas and Davis, 2004) and initiated a pilot-scale investigation of triploidy’s efficacy as a means of maturation control in geoduck. Triploidy can be induced in many marine mollusks because their oocytes are spawned before completion of meiosis. Sperm penetration releases the oocytes from meiotic arrest, and the oocytes progress through meiotic divisions and expulsion of first and second polar bodies. At the end of meiosis, single maternal chromosomes pair with the paternal chromosomes and development continues. By treating the oocytes at the appropriate time during the continuation of meiosis, we can cause retention of first and/or second polar bodies, adding extra maternal chromosome sets to the developing zygote. With an extra maternal chromosome set, triploid development continues normally but at maturation, meioses are inhibited rendering the triploid effectively sterile.

Induced triploidy has been previously contemplated by resource agencies to confer sterility (Beaumont and Fairbrother, 1991). For example, 3N Crassostrea gigas oysters (Thorgaard and Allen, 1988) and C. ariakensis (Allen, 2000) have been contemplated for widespread outplanting into Chesapeake Bay, where C. virginica is the native oyster. It is important to note that while 3Ns may not completely eliminate genetic risks to wild stocks, 3N Crassostrea gigas were observed to have only 0.1075% of the reproductive potential of 2Ns (Gong et al., 2004). If total or partial sterility can be conferred upon hatchery-produced geoduck, using 3N seed would drastically decrease the potential genetic risk to naturally occurring populations.

Figure 3Figure 2. Triploid production in Crassostrea gigas by mating tetraploid male x diploid female. The tetraploid male gamete becomes a diploid, rather than haploid, pronucleus (green chromosomes), releases the oocyte from the meiotic block at first metaphase, allowing the oocyte to resume meiosis through expulsion of the first (blue) and second (red) polar bodies, leaving the zygote to develop as a triploid with one maternal and two paternal chromosome sets. It is important to note that this process in Crassostrea gigas is governed by U.S. Patent number 5824841.

Tetraploid males crossed with normal 2N females reliably produce 100% 3N progeny in Crassostrea gigas (Guo et al. 1996) without manipulating post-fertilization meiosis in the developing oocyte (Fig. 2). We will adapt and develop this technology for geoduck clams, ultimately benefiting the aquaculture industry, the wild fishery, and the environment.

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