The manipulation of chromosomes and chromosome sets is a major component of our shellfish genetics program. By manipulating chromosomes, we intend to create new genetic constructs that exhibit commercially useful traits. Triploids, organisms with three sets of chromosomes instead of the normal two sets, for example, are useful in shellfish aquaculture. Triploid shellfish have several advantages over normal diploids, including sterility, faster growth, improved meat quality and increased disease resistance during the spawning season. Triploid Pacific oysters (Crassostrea gigas Thunberg) have become an important part of the aquaculture industry on the West Coast of the U.S., accounting for one-third to one-half of the total production.

   Traditionally triploid shellfish are produced by blocking the release of the polar body II in newly fertilized eggs with a highly toxic chemical, cytochalasin B. The chemical induction process is complicated, expensive and rarely 100% effective. After several years of research, we have successfully created tetraploid oysters (with four sets of chromosomes). Tetraploids are fertile and produce 100% pure triploids when mated with normal diploids. The development of tetraploid oysters has revolutionized triploid production for the oyster culture industry.





A newly fertilized egg of the Pacific oyster with polar body I, II, maternal and paternal chromosome groups, all of which can be artificially manipulated to produce polyploid and aneuploid oysters.

   Research is also underway on the production and isolation of certain aneuploids. Aneuploids are organisms with incomplete sets of chromosomes. Aneuploids such as trisomics, 2n+1, are useful in chromosomal assignment of genes and quantitative trait loci (QTLs). Five trisomic families have been isolated in 1997, and further research is being conducted to produce more and evaluate the phenotypic expression of trisomics. The work on aneuploid oysters is funded by a USDA/CSREES grant.

Metaphases from polyploid and aneuploid Pacific oysters produced by chromosome manipulation. Chromosome constitutions shown are A, 21; B, 30; C, 31; D, 32; E, 33; F, 38; G, 39; H, 40; and I, 40. The normal diploid number is 20.