ANACAT COMMITTEE
~1:27, Ref. F.
ICES C.M. 1994
RElATIONSHIPS BETWEEN GENETle VARIABILITY AND PRECOCIOUS
MATURITY AND SMOlTING IN ATLANTIC SALMON.
Sonchez J.A., VOzQuez,E. Ijnd Blljnco, G.
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Universidad de Oviedo. Departamento de Biologla Funciona1. Area de
Genetica.
33071 Oviedo. ASTURIAS. ESPANA.
ABSTRACTS
In this work, we monitbred in an Asturias hatchery (Northern Spain)
a population of Atlantie salmon during Hs first year of life in fresh water.
We had analyze the existenee of genetie differenees (using the six most
polymorphies protein-eoding-loei deteeted in Atlantie salmon: sAAT:4*,
IDDH-t *, IDDH-2*, IDHP-3*, MDH-3,4* end mMEP-2) among different
physiologieal groups (parr, preeoeious mature male and smolts).
Our results show that the smolt group has the greatest
heterozygosity (30,12~) and mean number of heterozygous locus per
individual (1.56). Parr group (non-mature and non-smolt fish) has
signiflcantly fewer yalues (21.43~ and 1,25) than the other two groups,
but preeocious mature male (26,35% and 1.50) and smolts groups did not
differ significantly (t:0.49; P> 0.05). In other hand, smolts group shows
lower yalues (0,474) of freQueney of mMEP-2*-100 allele than another two
group.
These results suggest 13 positive relationship between the degre~of
heter:ozygosity and physiologieol changes in indiYiduols under hotchery'
conditions. Due to the importanee of this relationship a more intensive
investigation on this sUbject is warranted.
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Introduction
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Atlant1c salmon, So/mo StJlor L., lS an anadromous specie which
spawns in fresh water and migrate to the sea to feed.. During its fresh
water phase, a large phenotypic Yariabi1ity amcing individuals of the same
population was observed. This interindiYidual variation is present in
morphological and in physiological traits. So~ in natural and cultured
populations of Atlantlc s8lmon~ the existence of individuals with different
growtl1 rates results in 8 bi modal distribution of length and weight at the
end of the flrst years of life (Thorpe et a1. 1982, Metcalfe et a1.1989,
Nicieza et 81 1991, Presa et 81 1991). In terms of size, the "large" mode,
(mainly females) is basically composed of individuals which smolt on the
fol1owinQ sprinQ 8nd "smaller" mode contains remains Darr and 8 var j":ltl!?,
fraction of Dr~L:~lt:iOus rllijtur~ rlltJie;.; (RH.:kt:'f 1972, lhorpe and t10ran 1980,.
Rowe and Thorpe, 1990, Presti el 61 1991). It has been demonstrated thlit
environmental conditions (photoperiod j temperature, etc.) playa role in the
formation of this distribution, but the putative genetic bases of this
intrapopulational variation remain unclear (Thorpe; 1986, lundquist et al.
1986, Nicieza et al 1991).
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Many
recent
publlcations
describe
associations
between
heterozygosity at protein-coding-Ioci and morphological variation within
or among populations. In salmonids, as in other species, heterozygosity is
positively associated wHh size, superior.growth rate, superior vlabllHy,
feeding rate or fecundity (Mitton and Grant, 1984; liskauskas and Ferguson,
1990, 1991). Mitton and Koehn (1985) suggest that these associations exist
because enzyme neterozygosity enhances physiological efficiency by
decreasing the energetlc cost of standard metabollsm.
The aim of our work was to monitor a population of Atlantic salmon
during Hs first year of 11fe. At the end of the year the existence of 3
different subclasses of individuals was recognizable. Here, we analyze the
possible genetie differences among groups by estimating their
heterozygosHy level loci using the 6 most polymorphic protein-coding-Ioci
detected in Salmo salar in Spain: mMEP-2*. sIDHP-3*. sIDDH-1 *. sIDDH-2*.
sAAT-3* and sMDH-3.4* (Sanchez et a1., 1991, 1993>' The proposed null
hypothesis is that the average heterozygosity is the same within each
subgroup.
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Motedol ond methods:
Semoles:
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In December 1989, 15 female and 10 rnale Atlantlc s8lmon, of twci
sea vVinters, were' caught in Sella river (Asturies, Northern Spein) to be
used .as broodstocks to repopulate this river. Eggs from erosses between, ät
least seven females and flve males, were collected to use in this
experiment and ralsed In a hatchery. This minimum number (7 females and 5
males) represents an effective population size of 11.66 and, aceording to
Nei et 81. (1975); they would be expected to cantoin around of 90% the total
genette variabllHy of the natural populatIon. In September 1990, progenies
from these eggs. were fendomly sampled and 200 individuals, previously
anesthetized with Ms 222 (3-aminobenzoic acid ethyl ester), were
1t"ldividually tagged wHh a bar code using electrie dermograph type V.L.G.
(electrical tattooing pan) and used for this stUdy. From September to
Decernber, fork:-length, weight precocios maturatlon end smolting v'Ias
monthly monitored. Afterwards, the fish were sacrHied for
electrophoreticel analysis end to correlate morphological ond Physiological
parameters wHh their enzymatic genotype.
"p'
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El ectrophoresi s:
Tissue samples of liver and muscle we~e analyzed by horizontal
starch gel electrophoresis and seored for the ,6, loei whieh. sho~v
polymorphism ,in Asturias salmon populations: sMDH-3.4*. sIDHP-3*,
mt"IEP-2*. sAAT-3*; sIDDH-I* and sIDDH":2*. The proeedures fo11owed those
deseribed by Sänehez et al., 1991 and Blaneo et a1., 1992.
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Statlsdeal analysis.
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Student-t-test was used to test differenees in length and weight
between groups, as well as differenees in mean heterozygosity.
The assoeiations between number 01. heterozygous loei per flsh and
fish length, ffsh weight and growth rate in length and welght 'ivas
determined using linear regression analysis.
,
All these tests are deseri bed in Sokal and Rho 1f, 1981.
Resul ts and Discussion.
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The September distribution of lengths approximated anormal
distt-ibudon (fig. 1a).Fork hmgths vary among 2.37 to 11.2 cm. wlth mean.of
7.45 .!.. 0.11.
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By Deeember the distribution was distinetly bi modal (fig. 1b). If at
this moment we also eonsider phys~ological criteria (precocious maturatlon
and smoltifleation) we can divide the flsh into 3 groups (flg. 1c):
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1) Pre-smolts (33%): individuals from, in term' of size, the larger
modal group. These fish ere mainly femel es end will smolt e little after
their first year of life (in spring 1991).
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2) Preeoeious mature individuals (5.6%): These individuals are
sHuClted between the two modCll elClsses (see flg. 1b, e). All of them are
males whieh have maturated at the end of theh- flrst yeer of life.
3) PClrr (61.4%): individuals. whieh were a11 immature and would not
smolt.
Fig. 1c shows the populatlon thus elasslfied and Table 1 shows'
the mean values in length and weight in eaeh group. The greatest
differenees in weightand length were found between pre-smolts group end
the other one. WHhin groups from lhe small modal class (parr and
precocious mature individuals) there are significant statistieal differenees
in weight, bilt not in length (Table 1).
Teble 2 shows different velues of genetie diversity in each of the
three different physiological classes of individuals. Significant
heterogeneHy of allele freQuencies was found between group at 3 of the six
enzymaUc loei asseyed (reble 2). Since same technical difficulties arisen
to properly distinguish IDDH-1 * and IDDH-2* genotlpes in small individuals,
results present for these loei should be reletlvized. FrsQueneies at mMEP2* locus show great differenees between groups. Pre-smolt individual has
the lower values of mt1EP-2* 100 allele (rable 2). Reeent studies provide
evidences of an association between mMEP-2* yariation and growth and
spawning in Atlantie salmon -(Jordan; et a1. 1990). For this reason
addi ti ona1 researches on thi s loeus are needed.
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On the other hand, pre-smolt group has, the greatest heterozygosity
and number of heterozygous IOCl per .fish. Parr group has signifieantly
fewer'values than the other two groups (t: 2.41; t:2.55; P< 0.05), but
mature and pre-smolt groups did not differ signifieantly (t= 0.49; P>O.05).
(Table 2).
These resu1ts suggest Cl positive relationship between the degree of
heterozygosity and physiologieal ehanges in individuals under halehery
condit j ons.
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As we pointed Clt in Introduetion, a positivs association between.
heterozygosity at enzyme loei . and growth rate. has been reported in recent
years in many animals and plant populations (see Mitton and Grant 1984 for
Cl review). Mitton and Koehn (1985) suggested that this eorrelation is due to
the fact that heterozygous individuals have inereased metabolie effieieney
eomp~red to more homozygous individuals.
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However, wheil we analyzed the growth rate for length (G1) and
weight (Gw) from September to Deeember, the precoeious mature male
showed signifieanl lower values (Gw=0.022 ärid G1=O.O 15) lhan pre-smolts
group (Gw=O.328 and Gl=O.112). However, both classes of individuals did not
show signifieant differences in their level of heterozygosity (26.35% and
30.12% respeetively, t:0.49).
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These results do not invalidate the existence of a positive
IJssociation between heterozygosity and the growth rate. As Saunders
(1982) end Ferguson (1990) suggests, the precocious meture males may
have a greater orsimilar growth rate than other individuals in the spring.
By autumn, these individuals will initiate maturaUon deflecUng their
metabolic pathwey and investlng more energy towards gonads development
emd gametlc productlon and, therefore, less energy to somatic tissue
elaboration end so they wlll reduce thelr growth rate. In the period from .
September to December (when we analyzed Dur sampIes) they will show
lower values of growth rate
than other indlvlduels with· simller
heterozygosity level. In this wey, they will not be able to ettaln minimum
size to smolt (Thorpe, 1977).
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In summary our results suggest a positive relationshlp between the
,jegree of heterozygosity end "performance" of the individuels under
tlatcllery conditions. Oue to the economic importance ot" these relationships
we think that more intenslve lnvestigation ot" thls sUbject is warnmted.
Acknowledgements
We thank to ASOc18cion Asturlana de Pesca lts technlcal support and
assistances during the develop of our experiment in its hatchery. ThlS
research was supported by funds from E. E.C; (AQ-2.492) and Spanish
gouvernement (DIGICVT, PB-90-0992).
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REFERENCES
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enzyme loei in Salmo 8alar. Aguaculture. 84, 199-209.
Ferguson t1Jt, 1990. Enzyme heterozygosity 8nd growth of r8inbow traut
reared 8t two' ratlos. 6io1. J. Linn. Soc.. 40, 215-227.
Jordan W.C. et al. J 990. Genetlc variation at the ma1ic enzime-2 locus and
~ge at maturity in sea-run Atlantie salmon Sdllna stJ/orL. J. Fish Aguat. Sei.
47: 1672-1677.
Liskauskas A.P., tlM. Ferguson, 1990. Enzyme heterozygosity and fecundity
in a natut-alized population of brook trout. C8n.J.Fish. Aguat.Sci.. 47, 20102015.
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in a noturalized population of brook traut (Stllvellinus fontintllis).
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Lundqvist H.,et 81. 1986. Seawater adaptability in three different river
stocks of Baltic salrnon during smolting. Aguaculture, 52, 219-229.
t1etc81fe N.S. et 81. 1989. Early social status and the development of 11fehistory str8te9ies in Atlantic salmon. Proc.R.Soc.Lond.. 236, 7-19.
Mitton J.B., M.C. Grant, 1984. Association among protein heterozygosity,
growth rate and developmental homeostasis. Ann.Rev.Eco1.Syst.. 15, 479499.
i'1j tton J.6., R.K. Koehn, 1985. Shell shape variaUon in the blue mussel and
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80.
Nei tt, T. et al 1975. The bottleneck effect and genetic variablllty in
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Nicieza A.G.et a1. 1991. D~lielopment of tength-bimoda1ity and smolting in
wild stocks of Atlanti·~ >Imon under different growth conditlons. J.Fish
6io1.. 38, 509-523.
Preso,P. et 01. 1991.Anol1s1s genetlco eomporodo de dos poblociones de
Se/mo se/er L de distlnto origen geograflco. Proceeding de la Jornadas
Luso-Espanolas de Genetica.
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Ricker VrE., 1972. Hereditary and environmental factors aHecting certain
salmonid populations. !n.: Stocks concept in PacHic salmon.. H.R. MacMil1an.
Bri tlsh Columbia, Vancouver pp 27-160.
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Rowe D.K. and Thorpe J.E. 1990. Suppresion of maturation in male Atlantic
saltnon parr by reduction in feeding growth during spring' months.
Aquaculture 66: 291-313.
.'
Sanchez .JA.et al. 1991. Allozyme variation in natural populations of
Atlantic salmon in Asturias (northern Spain). Aguacllltllre. 93, 291-298.
Sanchez J.A., et al. 1993. Genetic status of Atlantic salmon (Salmo salar) in
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Fishes,p.p. 219-225. J.Cloud & G.Thogart (eds.).Plenum Press.
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479.
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Thorpe JE, 1977. Bimodal distribution of length of Atlantic salmon under
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Thorpe J.E., and R.I.G. Moran. 1980. Growth rate and smolting rate of progeny
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Thorpe J.E., et a1. 1982. Bimodal1ty of growth end smolting in Atlantlc
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TABLE 1.- t'lean values of weight and length for each of four groups in which
t.he December di stri buti on was di vi ded (see f1 g. 1c).
Parr
Number of
individuals
Precocious mature males
121
11
Pre-smolts
65
'w'eiqht (qrJ
3.94.:!,.O.08 a(1)
6.71=..O.72b
16.32.:!,.O.42c
Lengtti (ern.
7.07 + 0.21 b(l)
8.26:t.0.24 b
11.29."t.0.09 c
(1) differences in mean 'v'ölr.F'S ",\'8~'; ~I:·~tr.i~ U=:ir:J
'ti;tJlljirrc:; 1.';-;1. Idl'=-I 01 '=- ~lali~llcoliy liij"i81 t!IIL
i.'l
':"':tw1ent- t· test, values
-------------------------------------------------------------------TABLE 2 .- FreQuencies of the most common 811ele (100) and enzymatic
variabllity in different physiological classes of individuals of Atlantic
salmon.
Locus
Parr
~1DH-3,4*
Precocious
mature males
Pre-smolts
0.954
IDHP-3*
0.987
mt-1EP-2*
0.655
AAt-4*
0.694
0.778
IDDH-l *
IDDH-2* .0.785
0.986
0.917
0.542
0.736
0.639
0.722
0.915
0.926
0.479
0.723
0.840
0.904
HET(2)
locus(3)
26.35%
1.50
30.12%
1.56
21.43%
1.25
Gtest(1)
2.16
1.534
15.65**
1.016
7.24*
9.382**
(1) G-test for homogenelty of allele freQueneles.
Average of heterozygosHy (2) and mean number of heterozygous locus per
individual (3) over six polymorphie loei seored.
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,.
A
. ,-,
::0
10
l",nQn! in o:-m
c
·?-o
F'r~((o(:iOtJ:::
nutiJr,;.
m~l"$
10
5
10
FltJure I: D1St.nbut.lOn of fork-Ienqth
A) September: B) December; C) different. I~roups oi iish when
December distribution were diviljed using physiological criteria ...