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
2.1. Origin of the studied oysters Ž
. Oysters from two IFREMER hatcheries, La Tremblade Charente Maritime, France
Ž .
Ž and Argenton Finistere, France , were studied. In La Tremblade, parental oysters five
`
. males and five females per site originating from three sites located along the French
Ž .
Atlantic coast Arcachon, Port-des-Barques and Bonne-Anse were crossed to generate Ž
. three progenies, as described by Collet 1998 . In Argenton, parental oysters originating
from a Scottish hatchery were reproduced by mass spawning. Larvae and spat were Ž
. cultured according to Walne’s 1974 protocol. Spat were transferred to the IFREMER
Table 1 Ž
. Previous data 1988–1996 of aneuploidy in fast- and slow-growing juvenile oysters
Origin of populations studied Crossing
Number Age
Shell Aneuploidy
Reference Ž
Ž .
Ž . produced in hatchery;
of animals months
length .
Ž .
sampled in wild scored
mm Ž .
Argenton 1
pair mating Thiriot-
Quievreux
´
Ž .
et al. 1988 Fast-growing juveniles
47 3
15–20 5
Slow-growing juveniles 46
3 3–5
19 Ž .
La Tremblade 2
pair mating Thiriot-
Quievreux
´
Ž .
et al. 1988 Fast-growing juveniles
55 2.5
15–20 10
Slow-growing juveniles 55
2.5 8–10
27 La Tremblade
Thiriot- Quievreux
´
unpublished 1988 data
Ž . 3
pair mating 1 Fast-growing juveniles
10 1.5
18–22 6
Slow-growing juveniles 10
1.5 6–8
14 Ž .
4 pair mating 2
Fast-growing juveniles 10
1.5 18–22
9 Slow-growing juveniles
10 1.5
6–8 25
Ž . 5
pair mating 3 Fast-growing juveniles
10 1.5
16–21 7
Slow-growing juveniles 10
1.5 6–8
21 Ž .
6 mass mating
Fast-growing juveniles 10
1.5 15–21
12 Slow-growing juveniles
10 1.5
7–8 34
Ž . La Tremblade
7 pair mating
Thiriot- Quievreux
´
Ž .
et al. 1992 Fast-growing juveniles
39 6
20–25 11
Slow-growing juveniles 45
6 10–15
27 Ž .
Bonne Anse 8
mass mating Zouros
Ž .
et al. 1996 Fast-growing juveniles
40 6
25–29 16
Slow-growing juveniles 40
6 9–16
21 Ž .:numbering of populations for Fig. 2.
Ž .
facilities at Bouin Vendee, France
where they were grown with Skeletonema
´
costatum-enriched seawater. 2.2. Growth monitoring
Within each progeny, oysters of the same age, reared under common conditions were sampled and classified as Afast-growingB or Aslow-growingB according to their shell
Table 2 Ž
. Recent data 1996–1999 in fast- and slow-growing juvenile oysters
Hatchery location and Crossing
Number of Age
Shell lengthr Aneuploidy
Ž .
Ž . parental origin of the
animals months
weight populations studied
scored La Tremblade hatchery
Ž . Bonne Anse 9
5 males=5 females Fast-growing juveniles
10 6
40–50 mm 19
Slow-growing juveniles 10
6 20–35 mm
25 Ž
. Arcachon 10
5 males=5 females Fast-growing juveniles
10 6
40–50 mm 22
Slow-growing juveniles 10
6 20–35 mm
30 Ž
. Port des Barques 11
5 males=5 females Fast-growing juveniles
10 6
40–50 mm 18
Slow-growing juveniles 10
6 20–35 mm
25 Ž
. Port des Barques 12
5 males=5 females Fast-growing juveniles
12 20
74.7–94.2 gr 6
Medium-growing juveniles 12
20 46.9–59.3 gr
17 Fast-growing juveniles
12 20
22.9–36.8 gr 22
Argenton hatchery Ž
. Scotland 13
mass mating Fast-growing juveniles
15 17
78–96 mm 5
Slow-growing juveniles 15
17 45–66 mm
16 Ž .: numbering of populations for Fig. 2.
Ž .
length or live weight Table 2 . Additionally, 36 1-year-old oysters, representative of the range of growth performance present in the Port-des-Barques progeny, were individually
labelled and their live weight was measured at 12, 15 and 20 months in order to examine the aneuploidy-growth relationship at the individual level.
2.3. Chromosome scoring The animals were incubated for 8–10 h in seawater containing 0.005 colchicine.
Then gills were dissected in seawater, treated for 30 min in 0.9 sodium citrate and Ž
. fixed in a freshly prepared mixture of absolute alcohol–acetic acid 3:1 with three 20
min changes. Slides were made from one individual gill following the air drying Ž
. technique of Thiriot-Quievreux and Ayraud 1982 . The preparation was stained for 10
´
Ž .
min with Giemsa 4, pH 6.8 . Chromosome counts were made directly by microscope Ž
. observation Zeiss III photomicroscope on apparently intact and well-spread metaphases.
2.4. Data analyses The level of aneuploidy was estimated by counting 30 randomly chosen metaphases
per individual showing a similar good chromosome spread. This is the minimal Ž
statistical number usually accepted in cytogenetic studies Stallard et al., 1981; Wenger .
et al., 1984 .
Since the number of aneuploid metaphases per studied individual was the same in all Ž
. the studied material 30 per individual , it was therefore possible to test for size andror
population effects using one- and two-way analyses of variance. Differences in aneu- ploidy between fast- and slow-growing oysters in the populations studied were also
tested by analysis of covariance. Statistical analyses were computed using SYSTAT 9.0 by SPSS.
3. Results
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