GENETICS AND BROODSTOCK MANAGEMENT OF COHO SALMON
James M. Myers1, Per O. Heggelund2, Greg
Hudson3, and Robert N. Iwamoto1
1Northwest Fisheries Science Center
2AquaSeed Corporation
4530 Union Bay Place NE
Seattle, Washington 98105
3Domsea Broodstock, Inc.
10420-A 173rd Avenue SW
.
ABSTRACT
The success of finfish broodstock
operations for both aquaculture and captive restoration purposes will depend on
accurate prediction and management of the influences of a myriad of genetic
effects. Broodstock development
programs for aquacultural species have historically relied on terrestrial
models. As with terrestrial species,
much of the early genetic research on finfish focused on estimates of
inbreeding depression (through sib mating), estimates of heritabilities from
covariance analysis, and estimates of genetic effects through interspecific or
interstrain hybridization. Recent innovations in
biotechnology, including transgenic manipulations and isolation of DNA markers,
have shown some promise to assist traditional broodstock improvement
programs. This report will first
provide a general overview of some basic genetic principles that have proven
useful for developing classical broodstock programs and may be equally
important for the production of genetically modified (clonal, polyploid, or
transgenic) broodstocks. The remainder
of the report will focus on two aspects of broodstock development programs,
genotype-environment interactions and inbreeding, and present research data
from a multi-generational coho salmon (Oncorhynchus
kisutch) selection program.
Keywords:
coho salmon, selection, genotype-environment interactions, inbreeding
1. INTRODUCTION
Broodstock management generally involves the control of two
seemingly antagonistic processes:
1)
The accumulation of novel genotypes as a direct outcome of the
modification of certain traits (classical selection) or 2) through genetic
manipulation and the maintenance of genetic variability through avoiding the
loss of alleles or the accumulation of deleterious alleles. The amount and kind of genetic intervention
will depend on whether genetic improvement (e.g. commercial aquaculture crop)
or genetic homeostasis (e.g. a captive broodstock program for maintenance of an
endangered species) is the principal goal of the program. In either case, well-defined program goals
as well as the initial choice of the foundation stock(s) are critical. In addition, any broodstock genetics program
will have a central breeding plan that is comprised of two principal
components: selecting the parents for the next generation (selection) and
determining how the selected parents will be mated (mating systems) (Turner and
Young 1969).
The design of
most broodstock programs is based on several assumptions. First, a number of studies involving the
effects of inbreeding through sib matings predict significant declines in
reproduction and growth traits with inbreeding coefficients (DFs, the
probability of two alleles being
identical by descent) of 10% or more.
Inbreeding can be estimated through pedigree analysis or through direct
experimental measures of changes in genetic variability (allozymes and
DNA). Second, improvements in
phenotypic traits through selection are dependent on selection intensity and
the heritability (h2, the proportion of phenotypic variation due to additive genetic
variation) of the trait. Furthermore,
changes in the phenotypic characteristic of one trait (both desired and unintended)
can occur indirectly through selection on another trait. The magnitude of these changes is dependent
on the genetic correlation between the two traits, the heritability of both
traits, and the selection intensity (Falconer 1989).
In most cases, broodstock programs for fish and shellfish have had
to rely on models derived from terrestrial organisms; however, as our
understanding of the response of aquatic organisms to long-term selection
programs accumulates, it may be necessary to reevaluate many of the underlying
assumptions used in developing commercial broodstock programs. Furthermore, the success of captive
broodstock programs to preserve the genetic characteristics of a natural
population may be limited by the magnitude of inadvertent genetic changes due
to artificial propagation (e.g. domestication) (Hard and Hershberger
1995). Finally, the development of
broodstock programs for genetically manipulated or transgenic aquatic organisms
does not benefit from the same volume of existing scientific literature as is
found for classical selection programs; however, many aspects of the management
of genetically modified organisms will be the same as those for classical
selection programs.
This report will focus on two of the issues involved with
broodstock genetics:
genotype-environment interactions (GxE) and inbreeding. It will draw principally from the growing
quantitative genetics information from a long-term (13 generations) coho salmon
(Oncorhynchus kisutch) breeding
program. This report proposes that GxE
interactions and inbreeding are universal issues of concern in the development
of a broodstock regardless of the program goal or type of genetic manipulation
or management employed.
1.1 BACKGROUND
1.1.1.
GENOTYPE-ENVIRONMENT INTERACTIONS
Many traits have continuous rather than discrete distributions and
are under the control of many genes with small, independent effects. Ultimate expression of the trait will depend
on the form and nature of that genetic control as well as a composite of
influencing environmental factors.
Estimating the relative contributions of genotype and environment is
largely accomplished through the analysis of variance and subsequent variance
component analyses (Falconer 1989).
Certain combinations of genotype and environment may not yield a
phenotypic value that is equal to the sum of the two factors. In those cases, a third function must be
added, which changes the equation for phenotypic expression to
P = G + E +
f(GE).
GxE interactions may be manifested either as changes in magnitude
of the differences among the genotypes or as changes in ranking of the
different genotypes. It is the latter
type of interactions that are biologically important. Furthermore, significant interactions are more likely to be
detected when environmental differences are large rather than small. Genotype differences, on the other hand, may
be large or small (Hohenboken 1985).
The accumulating evidence from salmonid research indicates that,
because of the difficulty in predicting the presence and magnitude of
interactions a priori, GxE interactions should be considered when developing
selection programs. Iwamoto et al.
(1984) and Heath et al. (1994) detected significant interactions among families
and two rearing temperatures in the incidence of precocial maturation in coho
salmon and chinook salmon (O. tshawytscha),
respectively. Similarly, Wild et al.
(1994) reported significant interactions for early sexual maturity among
Atlantic salmon (Salmo salar)
families and different net-cage sites in Norway. Balfry et al. (1997) found that while resistance of chinook
salmon to vibriosis has a significant genetic component, there were no
significant GxE interactions for families grown in two net-pen
environments. Significant GxE interactions
(but without changes in rank) were reported by Hanke et al. (1989) for growth
of underyearling Atlantic salmon grown under different photoperiod
treatments. Wangila and Dick (1988)
found that specific growth rate of two rainbow trout (O. mykiss) strains and their hybrids were differentially affected
by differences in the rearing temperature.
Beacham (1987) determined that families of chum salmon (O. keta) fry reared in a higher
temperature rearing environment and a lower temperature freshwater rearing
environment showed changes in rank in body weight after 196 days of
rearing. Iwamoto et al. (1986)
determined that GxE interactions, while significant, were not biologically
important for growth performance of three rainbow trout strains and their
hybrids under several density and ration treatment levels. Finally, significant GxE interactions
including rank changes were reported by Iwamoto (1982) for growth and seawater
survival of two strains of coho salmon and their hybrids reared under four different
rearing temperatures and three photoperiod treatments.
In natural salmonid populations, local adaptation is viewed as an
evolutionarily important form of GxE (Taylor 1991). For captively reared populations, the magnitude of GxE
interactions and the resulting potential for selection effects, such as
domestication, may be a major concern.
For the commercial fish culturist, strong GxE effects may determine the
general utility of a broodstock under a variety of culture conditions. Furthermore, the accuracy of heritability
estimates and selection programs derived under certain conditions may be
minimized under different culture conditions if GxE effects are
significant.
Under culture conditions, if the performance of a given genotype
is fairly uniform under a variety of environmental conditions (small GxE
interaction), such a strain could be used as a general-purpose broodstock and
be expected to perform moderately well under a range of conditions. On the other hand, large GxE interactions
suggest the need for special purpose broodstocks for each environmental
condition or anticipated change in the environment. Consequently, measurement of the magnitude of these responses
will be particularly important in defining the range of the utilization of specific
broodstocks in the industry.
Conservation programs that utilize captive broodstocks need to
assess the risk that captive culture may directly select for traits that lower
the fitness of fish under natural conditions or simply relax selection for genotypes
that are adapted to the native ecosystem for that population.
Inbreeding is defined as the probability of two alleles in an
individual being identical by descent, and is normally the result of mating
related individuals. The rate of
inbreeding is a function of the characteristics of the foundation stock as well
as limited population sizes in subsequent generations (Falconer 1989). The deleterious effects of inbreeding have
been documented in aquatic species, but there has been particular emphasis on
salmonids (Aulstad and Kittelsen 1971; Kincaid 1976, 1983, and 1995; Gjerde et
al. 1983; Su et al. 1996). The majority of these studies produced relatively
high inbreeding levels (∆F=10-25%) through sib matings. Hershberger et al. (1990a) analyzed the
growth performance of coho salmon under selection and increasing levels of
inbreeding. Despite accumulated inbreeding levels after four generations
approaching those of full-sib mating, there was no apparent decrease in growth
performance. Whether selection gains
masked deleterious effects or the accumulation of inbreeding levels over
several generations does not result in the same deleterious effects as has been
reported for closely-related (sib) matings was not determined.
2. MATERIALS AND METHODS
2.1
A CASE STUDY: THE DOMSEA COHO SALMON BROODSTOCK SELECTION PROGRAM
A selection and breeding program began in 1977 by the University
of Washington, the Washington Sea Grant Program, and Domsea Farms, Inc. to
develop coho salmon broodstock for the marine net-pen industry in the state of
Washington. The founding population had
been derived from the Washington Department of Fish and Wildlife’s Skykomish
Hatchery, Skykomish, WA in 1971 and 1972, and had been subjected to three
generations of mass selection for growth (by mating several hundred adults)
before the initiation of the Domsea Farms broodstock program (Novotny
1975). A selection scheme, raising 40
families of 600 individuals, was designed to yield maximum response and to be
useful under commercial culture conditions.
This scheme involved several different types of concurrent selection
(e.g., family and individual) and used a selection index that incorporated
estimates for heritability, relative economic values, genetic correlations, and
mean values on all the traits of interest (survival and growth rate) (Iwamoto
et al. 1982, Saxton et al. 1984, Hershberger et al. 1990b). Breeding was conducted by a circular mating
procedure to minimize the possibility of crossing closely related families
while assortatively mating the top performing families. On a theoretical basis, these steps should
limit the change in inbreeding to about 2% per generation (Hershberger and
Iwamoto 1984). Because of the 2-year
life cycle of coho salmon under captive culture, odd- and even-year spawning
broodstock were developed as independent broodlines. Initially, odd- and even-year lines were founded independently;
however, in 1991 a catastrophic accident at the rearing facilities eliminated
the even-year line. A new even-year
line was produced from three-year old odd-year line spawners. As of 1997, the odd- and even-year lines have
been separated for three generations.
As part of the selection program,
a number of reproductive traits (spawner weight and length, fecundity, egg size
(at the eyed stage), survival to the eyed stage, and survival to ponding) and
growth traits (length and weight at 1800 temperature units (T.U.s) post
ponding, and weight after 3.5 and 8 months of saltwater rearing) were
monitored. In 1986, the program was
modified to rear broodstock in freshwater throughout their life cycle. This change in rearing strategy
substantially reduced the mortality normally experienced following transfer to
salt water and during the summer months prior to maturation. The timing of the measurements previously
made in salt water was adjusted to reflect the different temperature regime
experienced by the all-freshwater reared broodstock.
2.1.1 GENOTYPE-ENVIRONMENT
INTERACTIONS
To evaluate the possible consequences of maintaining broodstocks
in freshwater relative to salt water, randomly selected portions of each
full-sib family were reared in each environment after the initial 7-month
freshwater common rearing period. In
1985, the full-sib families were apportioned to a saltwater net-pen and to
land-based circular tanks with a well-water source of approximately 100C
constant temperature. They were reared
under those conditions for the subsequent 18 months to maturation and
spawning. In 1986, in addition to the
two environments mentioned above, the full-sib families from that brood-year
were also reared at an additional freshwater location in 13.50C
spring water. In both cases,
individuals from each of the full-sib families were measured for length at
approximately 8 months post-transfer adjusted for the differences in rearing
temperatures. Weight was estimated
using the length data, and a length-weight regression was generated for each
site by subsampling the population at each location/environment. Because the same regression equation was
applied to all families within a location, genetically influenced differences
for weight among families as well as GxE interactions were biased to some
unknown degree.
Four statistical methods were
used for measuring GxE interactions, each with a basis in analysis of variance
and variance component analysis: 1) statistical significance of the interaction
term; 2) percentage of total variation contributed by each interaction term; 3)
the ratio of the interaction term to the sum of its component parts; and 4) the
genetic correlation approach (Robertson 1959).
The analysis of variance model
used in tests for significance and variance component partitioning was:
Yijk
= u + Gi + Ej +
GEij + 0ijk
where
u is the overall mean;
Gi is the effect of the ith family;
Ej is the effect of the jth environment;
GEij is the effect of the ith family and
the jth environment;
0ijk is the deviation
of individual k from the mean of the family-environment subgroup
Statistical significance (P < .05) was determined for each main
effect and the interaction term (mixed effects model where families were random
and environment was fixed). Analysis of
variance tests were run using Statview 5.1 (SAS Institute, Cary, NC, USA). Once variance components were partitioned,
the percentage of each component part to the total variance was
determined. Two ratio estimates
(1: VGE/VTotal;
2: VGE/VG +VE+VGE)
were examined as arbitrary determinants of the importance of GxE interactions
in each case (VGE/VTotal
> 5%; VGE/(VG +VE+VGE )
> 10%). Variance components were
further used to calculate genetic correlations (rG; where rG
= VG / (VG + VGE). The genetic correlation is a measure of whether genetic control
of performance in one environment is
similar to that in a second environment.
Generally, if rG is < 0.8, GxE interactions are considered
biologically and practically important (Robertson 1959).
The significance of family rank change in different rearing
environments for BY1985(F6BR73) and 1986(F6BR74) was estimated using the
Mann-Whitney Test (Zar 1974).
2.1.2 INBREEDING
Estimates of
inbreeding (Wright’s inbreeding coefficient) for each of the families sampled
were computed from the family pedigree using the CompuPed v4.0 program (RCI
Software, Loveland, CO, USA). The
inbreeding level of the founding generation, F3BR73 (Fig 3), was assumed to be
0.00 for the purposes of the calculation. Estimates of inbreeding levels under
a random mating scenario (excluding sib matings) were derived using the
equation ∆F=1/(2N+4) for each
generation (Falconer 1989).
In 1986, to test
the effects of intensive inbreeding (brother-sister mating) fifth generation
females (F= 0.059) from one family were crossed with full-sib males or
outcrossed with males from unrelated families.
Individuals from inbred and outbred families were weighed after 2,000
degree-days of rearing.
2.1.2.1 Allozyme Analysis
For the current study, 159 and 165 juveniles representing the 40
full-sib families from BY1997 and BY1998, respectively, were examined for
allozyme variation using methods outlined by Aebersold et al. (1987). The following 66 loci were resolved (locus
nomenclature follows Shaklee et al. 1990):
AAT12**, sAAT3**, sAAT4**, ADA1**, ADA2**, mAH1**, mAH2**, mAH3**,
sAH**, mAAT1**, AK**, FBLD3**, FBLD4**, ALAT**, CKA1**, CKA2**, CKC1**, CKC2**,
CKB**, EST1**, EST4**, FH**, bGLUA**, bGALA**, GAPD3**, GAPD2**, GAPD4**,
GAPD5**, PEPA**, PEPC**, GPIB1**, GPIB2**, GPIA**, GR**, HAGH**, mIDH1**,
mIDH2**, sIDH1**, sIDH2**, LDHA1**, LDHA2**, LDHB1**, LDHB2**, LDHC**, PEPB1**,
PEPLT**, aMAN**, MDA12**, MDB12**, mMDH2**, mMDH3**, MPI**, PNP1**, PNP2**,
PGDH**, PGK1**,PGK2**, PGM1**, PGM2**, PEPD2**, PK2**, IDDH1**, sSOD1**,
TPI1**, TPI2**, TPI4**, and TPI3.
Since no baseline exists for the genetic content of the founding
population, the results of this analysis were compared to existing data from
five coho salmon populations (Little Pilchuck (N=120), Harris Creek (N=120),
Grizzly Creek BY91 (N=100), Grizzly Creek BY93 (N=100), and Lewis Creek (N=67))
which are found in the same watershed (Snohomish and Skykomish River Basins) as
the founding population for the Domsea Farms broodstock. Allozyme data from the populations was
analyzed with the BIOSYS computer program (Swofford and Selander 1981).
2.1.2.2 Transferrin
Electrophoretic analyses were conducted on serum samples
from 100-120 adult fish in each of four years (1977, 1978, 1985, and
1986). The electrophoretic procedures
employed were those reported in Utter et al. (1970) for analysis of serum
transferrins in coho salmon. For the BY1997 and
BY1998, 159 and 165 juveniles representing the 40 full-sib families were
analyzed according to Van Doornik et al. (1995).
3. RESULTS
3.1 HERITABILITY ESTIMATES
AND RESPONSE TO SELECTION
Hershberger et al. (1990b) summarized the genetic estimates and
performance of the broodstock after four generations of selection. They reported significant gains in body
weight for both the odd-year and even-year broodstocks for the 8-month
saltwater monitoring period (239.0g to 432.5g for the odd-year and 296.2 to
666.7g for the even-year line) reflecting an average gain of 10.1% per
generation (Figure 1). Heritability
estimates derived from sib analysis for length and weight ranged between 0.18
to 0.33 and showed minor change over the four generations of intensive
selection (Table 1). An internal
control line also showed increases in length and weight gain over the time
period suggesting that domestication selection (Gjedrem 1979, Doyle 1983) had
occurred.
3.2 GENOTYPE-ENVIRONMENT
INTERACTIONS
Family (G) and Environment (E) effects were significant for length
in BY1985 (Table 2). The GxE interaction was statistically significant
(P<0.01), indicating that the relative growth of families in saltwater and
freshwater rearing affected growth of each family differently. All variance component ratio estimates (VGE/Vtotal
= 5.5% and VGE/(VG + VE + VGE =
25.6%) as well as the genetic correlation coefficient (rg = .532)
indicated that GxE interactions were important.
For the 1986 broodstock, the splitting and subsequent rearing of
full-sib families in the freshwater and saltwater environments mentioned above
plus an additional freshwater environment permitted several additional
comparisons of the data (Table 2). When
length data from all three environments were combined, the two main effects
(families and environment) and the interaction term were all statistically
significant and comprised significant portions of the total variation (.097,
.136, and .051, respectively).
Moreover, the variance component ratio estimates as well as the genetic
correlation coefficient all exceeded the criteria indicating the biological and
practical importance of GxE interactions.
The average growth performance at each of the
sites varied between years (Table 2).
In 1985 fish reared at the freshwater site were heavier, while during
1986, fish at the saltwater site were larger . For the 1985 weight data, the
environment main effect was not significant and comprised just 0.1% of the
total variation compared with 9.8% of the total variation for the corresponding
length data. All variance component
ratio estimates exceeded the arbitrary significance criteria. In 1986, the same two-environment comparison
indicated a significant environment effect, comprising 15.3% of the total
variation. The three-environment
analysis (comparing one saltwater and two freshwater sites) yielded an even
stronger environmental effect , accounting for 58.2% of the total variation.
Comparisons
of the relative ranking of families at each of the rearing sites during each of
the broodyears also revealed significant GxE interactions (Table 3). There were considerable changes in rank
among families reared at different sites.
On average, only about half of the top 15 families from any one site
were among the top 15 families at the other sites. Furthermore, the correlation in family ranks between the two
freshwater sites used in rearing the BY1986 families was substantially greater
(r = 0.714) than that between either of the freshwater sites and the saltwater
site (r = 0.187 and 0.070). This would
suggest (although only one year of data is available) that differences in freshwater
and saltwater rearing had a substantial effect on relative family performance.
Finally,
genotype x environment interactions were also observed when heritabilities were
computed separately for each of the environments in each of the
broodyears. In BY 1985, under normal
rearing conditions, similar estimates of heritability, around 0.20, for length
and weight were derived for both fresh and salt water. However, in BY 1986, heritability estimates
for length and weight ranged from 0.35 to 0.55 for both freshwater rearing
sites, but only from 0.05 to 0.06 for length and weight at the saltwater site.
3.3
INBREEDING
Genetic
improvement has been rapid and significant due in part to the intense selection
imposed upon the population every generation.
Program design created significant genetic bottlenecks in every
generation, with six males and six females from each of 10 full-sib families
serving as the nucleus for the succeeding generation. Because of the limited numbers of spawners per generation, the mating
of close relatives became inevitable.
By the fourth generation of the selection program (BY1983), all
individuals were related to some degree.
Analysis of the
pedigree records for the even- and odd-year broodlines (Figs. 2 and 3)
determined that inbreeding coefficients of 16.30% and 14.95% had accumulated in
the odd- and even-year broodlines, respectively after 9 and 10 generations of
selection (Table 4). Based solely on
population size and excluding self- and sib-mating, the inbreeding coefficients
after 9 (even-year) and 10 (odd-year) generations were estimated to be 21.88%
and 22.80%, respectively. Although the
mating design was successful in reducing the rate of inbreeding accumulation,
both estimates are well within the levels expected to result in inbreeding
depression (Gall 1987).
Furthermore,
allozyme analysis indicated that although overall heterozygosity levels were
equivalent to natural populations from the watershed where the Domsea stock
originated, there was a loss in variability at more than 70% of the loci found
to be variable in the natural stocks (Table 5). At 6 of the 30 variable loci, the frequency of rare alleles was
actually higher in the two Domsea lines than in the natural populations. Given that the two Domsea lines diverged
from a common stock in 1991 (with 3 generations of subsequent isolation) the
similarity in allozyme variability between the two Domsea lines may indicate
that gene frequencies have stabilized in recent years and that much of the
divergence from the founding natural populations occurred earlier in the
selection program.
Transferrin
samples from Domsea coho salmon analyzed in 1977 and 1985 Hershberger et al.
1990a) provide a baseline for analyzing genetic changes. Samples taken in 1998 and 1999 indicate that
there had been a substantial shift in transferrin allelic frequencies, with the
complete loss of the “A” allele in both odd- and even-year lines (Table
6). Furthermore, the decrease in the
frequency of the “A” allele was similar in the period from 1977-1985 and from
1985 to the present. It is unknown
whether the loss of the “A” allele occurred before or after the divergence of
the two lines in 1991.
While
the general population failed to show any significant inbreeding effects after
five generations of selection, the results of experimental intensive inbreeding
in 1986, on the other hand, indicated significant growth depression in the
resultant progeny. Progeny from the
highly inbred full-sib matings (F = 0.265) weighed a significant 25% less than
their out-bred half-sibs (Table 7).
4. DISCUSSION
Historically,
strain selection has followed the principle that strains have presumably
adapted to local environmental conditions and will demonstrate optimal fitness
within those same environmental parameters (Namkoong 1979). However, as Levins (1968) has suggested, the
overall fitness of a population may be enhanced, not by specific adaptation to
a restricted range of environmental circumstances, but by general adaptability
to a wider array of conditions. Under
this argument, then, the long-term survival of a population may well depend on
average performance under many environments.
This point may be especially relevant where environmental conditions are
variable, and changes are largely unpredictable.
The homing fidelity exhibited by salmonids
provides an important mechanism, the creation of discrete localized breeding
populations, for GxE interactions to occur.
Differences in the size and degree of isolation (geographic separation
of spawning populations, ecological distinctiveness, and homing fidelity) of
each population will determine the potential for local adaptation. A further consequence of the
establishment of discrete populations is the potential for the accumulation of
inbreeding. However, the success of the
salmonid life history strategy would suggest that the deleterious effects of
inbreeding are not significant under natural conditions. There may be sufficient gene flow between
populations, the effective size of most populations may be sufficiently large
to minimize inbreeding effects, or salmonids may be able to tolerate inbreeding
levels if accumulated at a slow rate (perhaps due to their tetraploid
genome).
Although local adaptation as a form of GxE appears to be a
significant factor in natural populations, it is less well understood how
captive culture (as one type of environment) influences the selection or
expression of commercially important traits or traits that are important in the
overall fitness of the organism under natural conditions. Domestication is widely recognized in
terrestrial organisms; however, fish being poikilothermic may be much more
sensitive to the modifications in their environment created by captive culture
(water temperature, flow, and chemical composition). Indirect evidence of domestication effects has been observed in
the relatively poorer survival of hatchery fish relative to their naturally
produced counterparts (Reisenbichler and McIntyre 1977, Fleming and Gross 1992,
Hard and Hershberger 1995), although there are a number of non-genetic effects
which can lower the fitness of hatchery fish (behavior, disease, rearing
conditions). Furthermore, the effect of
selection for commercially-important traits on fitness-related traits under
natural conditions is not well understood.
Kincaid (1993) cautioned against the use of domestic broodstocks for
supplementary stocking of natural fisheries.
For the
commercial aquaculturist, the detection of significant GxE effects as in this
study suggests that consideration be given to the environment used to rear
broodstock (Gjedrem 1992). Ideally,
broodstock-rearing conditions should resemble those used for normal production
stock. For a variety of reasons
(disease transmission, high prespawning mortality, slow growth and delayed
maturation, etc.) it may not be desirable to subject broodstock to normal
rearing conditions. An alternate
strategy would be to rear a portion of each family under production conditions
and select broodstock families on the basis of the production performance of
their sibs (sib selection). It should
be emphasized that the above mentioned actions are designed to optimize
selection gains. In those years where
controls were present, the Domsea select lines consistently outperformed the
control lines, regardless of rearing environment. The existence of significant GxE effects in this study does not
necessarily imply that selection gains derived in one environment will not also
be expressed, in part or in whole, in a different environment (this may be more
true for the range of captive culture conditions than for the range of natural
rearing conditions). Gjerde and
Gjedrem (1984) did not observe
significant GxE effects among strains of Atlantic salmon at different net-pen
rearing sites; however, the variability of the environments studied may not
have been substantial enough to elicit a strain specific response. Similarly, in rainbow trout (O. mykiss) ,G x E interactions ( as
expressed by changes in rank) occurred at the extremes of rearing conditions
(very slow or very fast growth), while rank changes were relatively rare under
normal commercial culture conditions (Iwamoto et al. 1986). In this study, growth conditions during BY
1986, especially at the marine net-pen site where growth was exceptional, may
have resulted in large GxE effects that would not be expressed under the normal
range of rearing conditions.
Differences in
the heritabilities that were calculated for the Domsea broodstock reared in
different environments further illustrates the importance of considering
environmental and GxE effects when interpreting genetic parameters. A lack of understanding of the magnitude of
environmental effects can lead to erroneous or improper genetic interpretations
(Gall 1985).
If GxE effects are significant, then it is possible that there has
been additional selection pressure on the broodstock. Alternatively, the culture conditions experienced by the Domsea
broodstock (especially during freshwater rearing) may result in the removal of
selective forces that operate in natural populations. Under natural rearing conditions and, even
more so, during marine net-pen rearing, most stocks experience high mortality
levels at saltwater entry (smoltification).
This potentially provides the opportunity for considerable selection
pressure on a number of life history traits (osmoregulation transition, growth,
disease resistance, predator avoidance, etc).
Alternatively, there is no specific life history stage during freshwater
captive culture that provides a similar opportunity for selection pressure to
be exerted by the environment (as exhibited by marked mortalities during any
one life history phase). It still
remains to be seen how the changes observed have affected the fitnesses of
these fish under their native environmental conditions.
Analysis of theoretical (based on pedigree data) and actual
genetic changes (based on allozyme data and changes in selected traits) would
suggest that there has been a considerable alteration in the genetic
composition of the Domsea coho salmon broodstock. At roughly three-quarters of the variable loci, rare alleles were
completely lost; however, at the remaining variable loci the frequency of rate
alleles increased. Average heterozygosity levels were similar in the Domsea broodstocks relative to the naturally
spawning populations that contributed to the Domsea founding population. Similarly, Winkler et al. (1999) in their
electrophoretic examination of two Chilean coho salmon broodstocks noted
founder effects as the reason for a reduction in number of polymorphic loci but
not in overall heterozygosity relative to other coho salmon stocks.
In the absence of continuous monitoring it is not known whether
the changes in allozyme variation noted in the current study are directional
(under direct or indirect selection), or if they represent random changes. Theoretically, allozyme variation is
selectively neutral. Furthermore, the
method and physical location of culture has changed considerably over the
course of the broodstock program: 1) marine net-pen culture (1973-1985), 2)
freshwater culture with warmed groundwater (1986-1990), 3) freshwater culture
with unheated groundwater (1991-1999).
In addition, several investigators (Suzumoto et al. 1977, Pratschner
1978, Winter et al. 1980) have suggested that specific transferrin
genotypes may provide improved resistance to specific bacterial pathogens
including those associated with bacterial kidney disease, vibriosis,
coldwater disease, and furunculosis.
The observed shifts in transferrin frequencies may be a product of selection
and inbreeding rather than inbreeding alone (Hershberger 1990a).
Phenotypically,
the Domsea coho salmon broodstock have demonstrated considerable improvements
in the commercially important traits for which they were selected, even in the
presence of moderately high inbreeding.
Although there is substantial evidence for the
loss of, or changes in, genetic variability, there has been little phenotypic
expression of inbreeding depression (Myers et al. 1999). Measurements of reproductive traits, female
spawner weight, egg weight, and survival to ponding, which have been shown to
be the most sensitive to increases in the inbreeding coefficients (Gall 1987),
have not demonstrated any reduction since the inception of the broodstock. Female weight has varied considerably, with
year-to-year variations most probably due to rearing condition
differences. Survival to ponding, with
the one exception of BY1987, has not indicated any significant decrease. The traits under direct selection, pre-smolt
and post-smolt growth and survival continue to show positive response to
selection (data not shown). It is
possible that indirect selection on reproductive traits may have more than
compensated for any inbreeding depression effects.
The apparent lack of a deleterious response, relative to other
intensive inbreeding studies, may indicate that the accumulation of inbreeding
levels (as calculated by pedigree analysis) over a number of generations does
not result in the same expression of deleterious traits as was observed in sib
crosses (Pirchener 1969). Given the
high fecundity of salmonids, there is considerable opportunity for deleterious
alleles to be culled from the population without a major reduction in
fecundity. It would be possible to test
this hypothesis by creating a number of full-sib crosses, as was done in 1986,
from the existing Domsea broodstock and monitoring for changes in traits that
normally exhibit inbreeding depression.
If deleterious recessive alleles have continued to be removed from the
broodstock, the relative growth rate of the full-sib crosses should show less
of an inbreeding depression than was observed in 1986. Alternatively, naturally occurring coho
salmon breeding populations are relatively small in number and coho may have
developed physiological mechanisms to deal with low levels of inbreeding. Relative to other salmon species, coho
salmon have relatively low levels of genetic variation (as detected by allozyme
analysis). Furthermore, salmonids have
evolved through a least one tetraploid event, and the duplication of genes
provides a further buffer against inbreeding effects.
The cumulative
past research with the Domsea stock, and more recently, the results in this
report, have provided a growing understanding of several fundamental genetic
processes that influence growth and survival performance of coho salmon under
captive culture. Similar programs such
as with Atlantic salmon, rainbow trout , catfish, tilapia, marine shrimp, and
oysters, have expanded the knowledge base for other aquatic species. Despite species‑related differences,
selecting a founding population(s), developing mating schemes, and testing
performance are activities that are universal to broodstock programs. It is
likely that this generalist approach will suffice even with the growing
application of molecular technology innovations. Biotechnology approaches such as hypervariable genetic markers or
mitochondrial DNA for pedigree determination have considerable potential as tools
to assist with developing such programs; however, associating specific DNA
markers with phenotypic traits (marker-assisted selection), especially with
traits under polygenic control, may be considerably more difficult (Ferguson
and Danzmann 1998). While the specific
aspects of molecular biotechnology applications to investigate broodstock
development and characterization are relatively new, the underlying effects of
related genetic manipulation and of assessment methods are fundamentally the
same as those found in classical selection programs.
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