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
Ž Although the Pacific oyster has a diploid chromosome number of 20 Ahmed and
. Sparks, 1967 , hypodiploid cells of 2 N s 19, 18 or 17 were observed in all of the four
Ž .
recent progenies studied Table 2 . The level of aneuploidy was significantly different Ž
. between populations F s 12.454; P - 0.001 , the oysters of Scottish origin being less
aneuploid than those of the other three populations. The size class was tested as a Ž
. within-population effect and found to be highly significant F s 8.703; P - 0.001 .
Individual aneuploidy levels were scored for 36 20-month-old oysters from the APort-des-BarquesB progeny. To give a concise view of the extent of aneuploidy in this
Ž progeny, oysters were grouped into Aslow-B, Amedium-B and Afast-growingB groups 12
. oysters in each . The number of aneuploid cells with 2 N s 19, 18 or 17 was calculated
Ž .
for each group Table 3 . Highly significant linear negative correlations were observed Ž
2
between aneuploidy and total weight in 12-, 15- and 20-month-old oysters R s 0.40, .
Ž .
0.53 and 0.58; P - 0.001 , respectively Fig. 1 . Ž
Looking at the 13 populations studied since 1988, including the present ones, Tables .
1 and 2, Fig. 2 , slow-growing oysters always showed higher levels of aneuploidy than fast-growing oysters. The difference in aneuploidy between fast- and slow-growing
oysters ranged from 5 to 22. Statistical analyses of the unpublished data obtained in Ž
. Ž
1988 four populations
revealed significant between-population F s 10.553; P -
. Ž
. 0.001 and within-population size effects F s 21.536; P - 0.001 .
When comparing aneuploidy between fast- and slow-growing oysters in the 13 populations studied, two different groups can be clearly distinguished: populations 8, 9,
Ž . Ž . Ž
. 10 and 11 in one group 1 and the other populations in a second group 2
Fig. 3 . In populations of group 1, fast growers are 0.72–0.76 times less aneuploid than slow
Table 3 Observed aneuploidy of individually labelled oysters from the APort-des-BarquesB pogeny
Growth Mean
Total number Number of cells observed with
Total number Aneuploidy
Ž . category
weight of mitoses
of aneuploid 2 N s 20
2 N s19 2 N s18
2 N s17 Ž
. Ž .
N s12 g
studied cells
Slow 31.6
360 281
44 25
10 79
22 Medium
53.7 360
300 38
17 5
60 17
Fast 83.1
360 337
17 4
2 23
6
Fig. 1. Correlation between individual aneuploidy level and total weight in 20-month-old oysters of the APort-des-BarquesB population.
Ž .
growers, while in group 2, this ratio is much lower 0.26–0.43 . In other words, the difference in aneuploidy between fast and slow growers was twice as much in group 2
as in group 1. An analysis of covariance revealed no significant difference in the slopes of linear regression between aneuploidy of fast- and slow-growing oysters between these
Ž .
Ž .
Fig. 2. Comparison of aneuploidy between fast- black and slow-growing white oysters in the 13 Ž
. populations studied 1988–1999 . See Tables 1 and 2 for the numbering of the populations.
Fig. 3. Relationship between mean aneuploidy of slow-growing and fast-growing oysters in the 13 populations studied.
Ž .
two groups of populations F s 2.47; P s 0.15 . However, this analysis showed a
Ž .
significant group effect F s 199.7; P - 0.001 .
4. Discussion
