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Table 1. Results from 2-way Anova with factors LDH-3* and PGI-1*
genotype, and interaction.
Source
LDH-3*
PGI-1*
Interaction term
F-ratio (df1,df2)
7.79 (1,70)
4.26 (1,70)
0.55 (1,70)
P-value
0.0068
0.0428
0.4598
Growth differences between genotypes at cod
LDH-3* and PGI-1*
Jarle Mork
Trondhjem Biological Station, NTNU
Summary
Among 74 full-sib codlings reared in captivity, genotypes at the
tissue enzyme loci LDH-3* and PGI-1* showed significant
differences in body length measured at day 146 after hatching.
Introduction
LDH-3* and PGI-1* have been used in many studies on cod
stock structure. Among the isozymes commonly employed, these
two show the largest geographic variability in allele frequencies
(Mork et al. 1985, Grant & Ståhl 1988; Jørstad & Nævdal
1989). However, some studies in wild populations have
indicated that these two loci are affected by natural selection
which appears strong enough to change allele frequencies within
and between generations (Jamieson 1975, Mork & Sundnes
1985), and thus compromise their value as population characteristics. This study was designed to test genotype performance
at these two loci, in such a way that no other factor than isozyme
genotype could account for any observed differences.
Material and Methods
A full-sib offspring group was obtained from parents with
known genotype for LDH-3* and PGI-1*. (Mother: heterozygote 100/70 for LDH-3* and 100/135 for PGI-1* and Father:
homozygote 70/70 for LDH-3* and 135/135 for PGI-1*. Below,
heterozygotes are called 12 and homozygotes 22). Expected
Mendelian ratio among the offspring was thus 50% heterozygotes 12 and 50% homozygotes 22 at both loci.
Spawning took place under semi-natural conditions in a broodfish tank at BP Nutrition’s plant at Bessaker, mid-Norway, on
March 19, 1993. Procedures for handling of fertilized eggs (here
one batch of approx. 100.000) and larvae are described
elsewhere (Jørgensen 1987; Jørgensen et al. 1989). The
offspring group was kept separate from others at all stages.
Temperature was kept between 9 and 11C, with short peaks
down to 7.3C and up to 13.9C. Water oxygenation data are not
available. After a mass mortality incident primo August, the
survivors (74 codlings) was collected, measured (length, weight)
and autopsied for muscle, heart and liver tissues on August 12,
1993 (the 146th day). Tissue extracts were scored for isozyme
genotypes by IFPAG as described in Mork & Haug (1983).
Results
The expected genotypic composition among the 74 offspring
was 37 of each genotype at each locus. The actually observed
ratios of heterozygotes vs homozygotes were 42:32 at LDH-3*
and 33:41 at PGI-1*. Neither result is significantly different
from expectations. Likewise, a test for gametic phase disequilibrium did not detect significant linkage (P=0.821).
However, mean body length differed very significantly between
genotype groups. At both loci, the heterozygote showed the
highest mean length (Table 1 and Figs. 1 and 2).
Fig. 1. Mean lengths of LDH-3* genotypes.
Fig. 2. Mean lengths of PGI-1* genotypes.
Discussion
To serve as reliable population markers, allele frequencies
should be stable within and between generations. Loci that are
detectably affected by selection are not expected to fullfill this
requirement.
The present genotype study was performed on full-sibs from one
egg batch and with a common environment from birth to death.
With the usual reservation about ‘closely linked genes’, the
observed growth differences must be caused by the individuals’
genotype at LDH-3* and PGI-1*. At the stage of collection,
there were no significant differences in genotypic mortality
among the offspring. This does not mean that such differences
had not occurred at earlier stages. Also, it cannot be stated at
which development stage the growth differences developed, or
which environmental factor actually caused them.
The lack of a significant interaction term in the Anova (Table 1)
suggest that the effects of LDH-3* and PGI-1* genotype are
independent and additive: The group of double heterozygotes
showed significantly higher mean length (5.01cm vs 4.48cm)
than the rest (Anova; F(1,72)=9.91, P=0.0024). This is some
12% better length growth (weight-wise, the advantage of the
double heterozygotes vs others was actually 46%). This may
directly affect fitness components like survival and fecundity.
In conclusion, this study has confirmed that genotypes at cod
LDH-3* and PGI-1* can be strongly affected by environmental
selection. In practice, this means that it is not justified to
interpret allele frequency differences between cod groups at
these two loci as proofs of reproductive isolation.
References
COD - CHOOSE YOUR PARENTS WITH CARE!
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