proportions of tetraploids ranged from 13 to 92 after 8 days for an average of 55, and seven of eight replicates went through metamorphosis and settlement. At settlement, the percentage of
tetraploids ranged from 7 to 96, averaging 45. Average survival in all the replicates at 8 days was 4.4, which is acceptable considering tetraploid progeny are destined for use as brood
stock. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Triploid; Tetraploid; Crassostrea gigas; Cytochalasin B; Meiosis; Flow cytometry; Oyster; Shellfish; Breeding
1. Introduction
In the last 10 years, triploids have become an important part of Pacific oyster, Crassostrea gigas, aquaculture. For example, about one third of the hatchery produced
Ž .
seed on the US West Coast were triploid Chew, 1994 and there is considerable interest in Australia, China and Europe. Triploids are interesting principally due to their reduced
Ž gonadal development which prevents deterioration of meat quality Downing and Allen,
. Ž
. 1986 . Consequently, triploids can be marketed year-round Allen, 1988 . Reduced
gonadal development may also favor more rapid growth rates, although probably only in Ž
. favorable conditions Davis, 1989 . Given their very low fecundity, triploids have also
been used in some cases for population control and biological containment of non-native Ž
. species Allen, 1993; Allen and Wattendorf, 1987; Guo and Allen, 1994a .
Currently, in commercial hatcheries, triploid oysters are produced by inhibiting polar Ž
. Ž
. Ž .
body 2 PB 2 with cytochalasin B CB Allen et al., 1989 , or more recently with
Ž . Ž
. 6-dimethylaminopurine 6-DMAP
Desrosiers et al., 1993 . However, there are major limitations to the use of these chemicals. First, these techniques always produce less
than 100 triploidy because of inherent variation among dividing eggs. This leads to larval populations containing both diploids and triploids. Batches of triploids in lower
proportions than 80 are a problem for hatchery management and a waste of production space and money. Secondly, CB is toxic, and has come under the scrutiny of the United
States Food and Drug Administration because of its potential danger to hatchery personnel. Health and safety concerns currently restrict its use in oyster hatcheries.
Finally, blocking PB 2 may negatively affect the survival and growth of induced
Ž .
triploids, probably due to the treatment itself Downing and Allen, 1987 or to genetic Ž
. considerations, such as inbreeding Chourrout et al., 1986; Guo et al., 1990 . All these
problems are eliminated with the development of tetraploids. After more than a decade Ž
. of attempts, Guo and Allen 1994b finally succeeded in producing tetraploid Pacific
oysters using eggs from triploids in which they blocked the first polar body with CB Ž
. treatment applied from 5 to 20 min post fertilization PF . Since then, they have shown
Ž .
the general utility of tetraploids for making 100-triploid populations Guo et al., 1996 . Ž
. While Guo and Allen 1994b reported a successful spawn for tetraploids from work
in 1993, there were in fact a considerable number of other tetraploid attempts that were less successful, including a major effort in 1996 to produce tetraploids in a number of
Ž .
commercial hatcheries Allen, unpublished data . Treatments were based on the timing Ž
. recommended by Guo and Allen 1994b . Clearly, however, something was missing
from these techniques or they had less general utility than first thought. Based on these
uncertainties, we examined some of the fundamental aspects of tetraploid production, Ž
. including dynamics of development in triploid eggs Eudeline et al., 2000 . Here we
report the results of our investigations into improving production of tetraploids in C. gigas.
2. Materials and methods
