1. Introduction
High growth rate variability has been often observed in bivalves. Even among individuals with the same age and grown under common conditions, a great variation in
Ž shell size and total weight can be observed e.g. Singh and Zouros, 1978; Mallet and
. Haley, 1983 . Several factors have been found to be associated with bivalve growth rate,
Ž .
including physical and nutritional environment e.g. Askew, 1972; Utting, 1986 ,
Ž .
Ž broodstock conditioning e.g. Gallager and Mann, 1986 , physiological Bayne et al.,
. Ž
1999 and genetic parameters e.g. Newkirk, 1980; Gaffney, 1988; Hedgecock et al., .
1996 . Positive correlations have been observed between heterozygosity and growth, as well as between heterozygosity and other fitness related traits such as viability, fecundity
Ž .
and developmental stability e.g. Mitton and Grant, 1984; Zouros and Foltz, 1987 . The Ž
general significance of these correlations has been extensively debated e.g. Pogson and .
Zouros, 1994; David et al., 1995 . Cytogenetic abnormalities, such as aneuploidy, are known to be common in bivalve
Ž populations Longwell et al., 1968; Ahmed and Sparks, 1967; Dixon, 1982; Thiriot-
Quievreux, 1986; Martinez-Exposito et al., 1992; Cornet, 1993; Li and Havenhand,
´
. Ž
1997 . Aneuploidy has been also observed in triploid and tetraploid oysters Guo and .
Allen, 1994; Wang et al., 1999 . This phenomenon which mainly originates from the non-disjunction of chromosomes
Ž .
during mitosis or meiosis e.g. Bond and Chandley, 1983; Martin and Rademaker, 1990 is often lethal in higher animals, such as mammals, or is associated with growth
Ž .
retardation Vig and Sandberg, 1987 . But the effects of aneuploidy seem less deleteri- Ž
. ous in plants and lower animals Verma, 1990 . A negative correlation between somatic
aneuploidy and growth rate has been described in offspring of cultivated Crassostrea Ž
. gigas oysters Thiriot-Quievreux et al., 1988, 1992 and in natural populations of the
´
Ž .
same species Zouros et al., 1996 , i.e. in a single cohort, fast-growing animals show Ž
. fewer aneuploid cells than slow-growing animals Table 1 . No relationship was seen
Ž between allozyme heterozygosity and either shell length or chromosome loss Thiriot-
. Quievreux et al., 1992; Zouros et al., 1996 .
´
In this paper, we present recent data from populations of hatchery-produced C. gigas, and a global survey of all populations studied since 1988, to assess the consistency of
the growth-aneuploidy relationship.
2. Materials and methods
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