65 Aquacu/lure, 111 (1993) 65-74 Elsevier Science Publishers B.V., Amsterdam

AQUA 30042

Genetic improvement of disease resistance in fish: an overview

Kjersti T. Fjalestad, Trygve Gjedrem and Bjame Gjerde AKVAFORSK (InSlilUle 01Aquacu/lure Research), s, Norway

ABSTRACT

Fjalestad, K.T., Gjedrem, T. and Gjerde, B., 1993. Genetic improvement of disease resistance in fish: an overview. Aquacu/lure, 1I J: 65-74.

It is of great importance to increase the natural disease resistance offarmed fish. Response to direct selection for increased survival in commercial environments has been low, basically due to inaccurate records of the trail. However, challenge tests with family groups which were infected with specific pathogens showed significant genetic variation in mortality, indicating that improvement by selection is possible. Several studies have shown a significant positive gene tic correlation between growth rate and survival. Therefore survival would be expected to increase as a correlated response when selecting for groW1h rateo Significant genetic variation has been found in immunological parameters, but more knowledge is needed about the genetic correlations between these parameters and survival befo re indirect selection for increased survival can be applied. Heterosis for survival has not shown large effects. However, it may be stiJI of interest to develop resistant lines to specific diseases and cross them to produce hybrids. Disease resistance genes ha ve not been identified in fish. However, the production of transgenic fish with enhanced resistance to specific diseases remains as a possibility for the future.

lNTRODUCfION

In fish farming, diseases are quite common and their occurrences reduce profitability substantally. To prevent outbreak of diseases, proper management, such as high quality feedstuffs, low fish density and use of effective vaccines is important. Fish already infected are difficult to isolate, and possibilities of preventing other fish, farmed and wild alike, from becoming infected are therefore limited. Antibiotics are frequently used to cure diseases, but there is always a risk

of bacteria developing resistance and of residues in the product. Moreover, these attempts to reduce mortality, although they are important, do not sol ve the problem on a long-term basis. Considering costs and difficulties of the

Correspondence lo: K.T. Fjalestad, AKVAFORSK (Institute of Aquaculture Research), P .o. Box 5010, N-J432 s, Norway.

0044-8486/93/$06.00 1993 Elsevier Science Publishers B. V. A1l rights reserved.

K.T. FJALESTAD ET AL. 66

aboye practices in preventing and curing diseases, the most effective method can only be the development of natural disease resistance in fish. The purpose ofthis paper is to discuss problems in volved in increasirig disease resistance, with emphasis on doing so through selection.

MEASUREMENT OF DISEASE RESIST ANCE

In a breeding program the economically important traits included in the breeding goal must be defined and the measurement of each trait must be done meaningfully. The following three subsections will deal with possible ways ofmeasuring disease resistance.

Survival rate Rate of survival, a desirable breeding goal in dealing with resistance, is a

complex trait. It may be influenced by various pathogens. The level of survival may also depend on many irrelevant factors in disease resistance, such as accidents and management problems. The effects of the various pathogen and non-pathogen factor s differ between farms and between years within fanns. In addition, recording of survival rate requires a marking system which allows identification of dead fish along with individual diagnosis. In commercial farms this is not possible when using freeze-branding because it is impossible to read the brands on dead fish. For an individual fish, survival only can be recorded as dead or alive. For

a group of individuals, such as full- or half-sibs, the mean survival rate in each group can be recorded. For a large number of groups, survival would tend to be normally distributed. When applying family selection, as opposed to individual selection, the intensity of selection thus becomes independent of the rate of survival and the genetic progress will be maximized when the rate of survival is 50% (Table 1). This requirement will be difficult to meet when recording survival rate in different family groups in commercial farms.

TABLE 1

Expected responses to direct selection based 00 family for survival at different mean survival rates , ~, .~

after one generation. A heritability of 0.1 00 the uoderlying liability scale and a family size of LO fullsibs and 20 half-sibs are assumed .

Sun/ival rate (%) Response (%) Relative response

50 5.04 100 60 4.84 96 70 4.24 84 80 3.22 64 90 1.77 35

al hi m m an l)'l ciJ IS en ga;

re~ ha 1m (S 19: 1

tail SU( pie spo ID to s

SEL

fl IS P haH chal

67 TAL.

hod lose [lce,

the t be ible

is a sur,uch Igen thin lich omit is

For :ach d to ndi'the e of 'hen

rates ) full-

GENETlC lMPROVEMENT OF DISEASE RESISfANCE IN FISH

Challenge test Disease resistance may be recorded also by exposing the fish to specific

disease agents. This must take place in closed facilities without risk of infecting the breeding stock, commercial fish farms or wild populations. To reduce costs, such challenge tests are performed with small fish and over a sbort period of time. Since the environmental conditions under challenge tests differ from those in commercial farms, the efficiency of a challenge test may be reduced. The response to selection in the field will depend on the genetic correlation between survival after challenge test and general resistance in commercial farms. However, this may be compensated for by obtaining around 50% survival in such tests.

Immunological or physiological parameters AH factors that enable fish to resist the pathogens in a specific environment

are entities of a disease resistance complexo The immune system of fish exhibits all the characteristic features of the immune system as known in mammals according to Ellis ( 1982), Mechanical and chemical factors such as skin, mucus (Fletcher, 1978; Ingram, 1980), lysozyme (Hinge et al., 1976; Fletcher and White, 1976), interferon (De Kinkelin and Dorson, 1973) and proteolytic enzymes (Hjelmeland et al., 1983) probably constitute a first non-specific line of defence against pathogens in fish. Another important component is the complement system which nvolves both specific and non-specific defence mechanisms (Harrel et al., 1976; Nonaka et al., 1981; Sakai, 1981). Regarding the antibody dependent immunity, it is believed that fish possess a restricted repertoire because of a simplified gene arrangement and that fish have one IgM-like immunoglobulin class only (Ellis, 1982). Furthermore, the immune response is very sensitive to stress and temperature variations (Snieszko, 1974; Avtalion, 1981; Pickering et al., 1982; Ellsaesser and Clem, 1986; Pickering, 1989). Immunological or physi010gical parameters from healthy fish may be ob

tained from blood samples as indirect measurements of disease resistance. Such parameters could be: cortisol, lysozyme, total-haemolytic acti vity (complement), transferrin, total antibody activity (total IgM), or antibody response against specific agents, or combinations ofsuch parameters. Their value in a breeding program depends on their heritabilities and genetic correlation to survi val.

SELECTION

High fecundity and external fertilization are advantages in fish breeding.1t is possible to produce large full-sib groups as well as maternal and paternal ha1f-sib groups. Each group may be split into subgroups for testing ofdifferent characters under varying environmental conditions. The gametes are easily

68 K.T. FJALESTAD ET AL.

transportable and may be disinfected against most pathogens. Selection schemes could be designed either by direct selection or indirectly by selecting for correlated traits to disease resistance.

DireCI selection Heritability estimates for survival rate indicate a significant but rather low

additive genetic variance as summarized by Gjedrem (1983). This was confirmed by later estimates in Atlantic salmon (Standal and Gjerde, 1987; Rye et al., 1990) and brook trout (Robison and Luempert, 1984). Several reports show significant differences in survival after specific diseases between populations (Plumb et al., 1975; Suzumoto et al., 1977; Zinn et al., 1977; Winter et al., 1980; Bakke et al., 1990) and significant differences between full- and half-sib families have also been found (Gjedrem and Aulstad, 1974; Amend and Nelson, 1977; McIntyre and Amend, 1978; Refstie, 1982; Bailey, 1986). A study by Gjedrem et al. (1991) showed large genetic variability in sur

vival after challenge by furunculosis. The estimated heritability, h2 = 0.48 ::t 0.1 7, was calculated based on observed either-or data and at a mean survival of 68%. Sorne selection experiments to reduce mortality have shown response, e.g.

selection for increased resistance to dropsy in carp (Kirpichnikov et al., 1979, 1987) and to furunculosis in brook trout and brown trout (Ehlinger, 1964, 1977).

IndireCI selection In a selection program it may be desirable to incIude correlated traits with

no economic value in order to in crease genetic gain in traits of importance (Gjedrem, 1967). Immunological and physiological parameters such as lysozyme (R0ed et al., 1989), haemolytic activity (Roed et al., 1990, 1992) and cortisol (Refstie, 1982) have shown genetic variation and are examples of possible traits for selection. They can be all recorded on the breeding candidates and their full- and half-sibs. Since blood samples can be taken prior to selection of brood stock, the generation interval will not be increased. Selection experiments have been carried out with both Atlantic salmon and rainbow trout for high and low stress response as measured by blood cortisol

,,;: )evels (Fevolden et al., 1991). In Atlantic salmon, the mortality was signifilcantly increased in the line selected for high cortisol stress response (Fevol

I

den et al., 1991). High and low stress response lines of rainbow trout challenged by furunculosis showed a higher survival rate in thelow stress response line than in the hi~ stress response lineo The opposite was found for fish challenged by vibriosis (Fevolden et al., 1992). Cipriano and Heartwell (1986) selected brown trout one generation for a

high level of mucus precipitin activity against Aeromonas salmonicida and produced progeny that were more resistant to furunculosis. Mortality due to

.1

69

ET AL.

tion :ting

low :.:onRye orts >pUoter and end l6). surlity, :ean

e.g. )79, 164,

vith nce lyn) )les an.lor Semd sol ifi'01lalose ish

'r a md : to

GENETlC IMPROVEMENT OF DISEASE RESISTANCE IN FlSH

furunculosis 6 months after hatching was 2% and 48% among progeny of selected and non-selected parents, respectively.

Correlaled response Natural selection wiIl enable fish to adjust to their environmental con di

tions whether they are in the wild or under crowded fanning conditions. Natural selection is therefore important for domestication. Artificial selection for economically important traits such as growth rate and survival wiIl fucilitate the rate of domestication. The result will probably be fish that adapt oetter to life in captivity with lower levels ofstress. Fanned Atlantic salmon in Norway have undergone four generations of selection for increased growth rate and reduced frequency of early sexual maturation. The fish in the later generations seem to be less sensitive to environmental stress than geneticaIly wild fish. These changes are difficult to measure, but are easily seen.

Growth rate is usually focused on in a fish breeding programo Several investigators have studied the genetic correlation between survival and growth rate. Standal and Gjerde ( 1987) estimated a positive genetic correlation (0.18, mean of several estima tes ) between survival after an outbreak of cold water vibriosis and growth rate. Rye et al. ( 1990) estimated a genetic correlation of 0.37 and 0.23 between survival and growth rate in the fresh water period for Atlantic salmon and rainbow trout, respectively. In brook trout, Robison and Luempert (1984) estimated low positive genetic correlations of 144-day weight with survival and negative correlations between 243-day weight and survival. Gjedrem et al. (1991) studied survival after a challenge test with furunculosis and estimated the genetic correlation between survival and growth rate to be 0.3. The positive correlations would lead to a positive correlated response in survival when selecting for increased growth rate.

ExpeCled response lo selection Expected responses to direct and indirect selection for increased survval

have been studied in the context of selection index theories (Cunningham, 1968). Table 1 shows predicted response to family selection at different overall survival rates in the population. The response depends upon the survival rate, and maximal response is achieved when the survival rate is 50%. The expected response to direct family selection when survival is 90% is only 35% ofthe expected response when survival is 50%.

The prediction of relative response to direct and indirect selection, when assuming different heritabilities (0.1,0.3 or 0.6), genetic correlations (0.3, 0.5 or 0.7) and family and/or individual selection, is compared in Table 2 when overall survival rate is 50% and 90% respectively.

Indirect selection for survival based on a correlated trait with no economic value results in a much lower response in survival compared with direct selection (Table 2). This is the expected correlated response in survival when in

70 K.T. FJALESTAD ET AL.

TABLE2

Relative expected responses to direct and indirect selection for survival at an overall survival of 50% and 90% after one generation. A family size of 10 full-sibs and 20 half-sibs and an environmental correlation of zero are assumed

Survival = 50% Survival=900f0

h2* rG Family selection

Individual+ family selection

Family selection

Individual + family selection

Direct selection Survival 0.1 100 100 Indirect selection Correlated trait I 0.3 0.3 44 52 55 66 Correlated trait 11 0.3 0.5 73 87 92 110 Correlated trait 111 0.3 0.7 103 122 129 154 Correlated trait IV 0.6 0.7 113 145 142 183 Combining direct and indirect selection Survival + correla ted 0.3 0.3 105 108 109 115 trait I

Survival + correlated 0.3 0.7 125 140 146 167 trait Il

*Estimate on the assumed underlying liability scale.

formation of a correlated trait is included in the selection index and survival ) is the only trait included in the breeding goal. The genetic correlation between

survival and the correlated trait must be large if indirect selection is to compete with direct selection. Table 2 also shows that more can be gained by including information on correlated traits when Oyeran survival rates are high, in particular if indirect records on the breeding candidate itself are included. The same would apply to low survival rates. By combining direct and indirect selection for survival (at an Oyeran sur

vival of 50%) the expected response increases by 5% (family selection) oc 8% (com bined family and individual selection) when the correlated trait has a heritability of 0.3 and a genetic correlation of 0.3 to survival (Table 2). These genetic parameters are close to e.g. estimates obtained for growth rateo If the genetic correlation between the traits is high (0.7) the response will

1' :' . I " increase considerably . . I

CROSSBREEDING

Breeding programs frequently inelude crossbreeding between strains or lines to utilize heterosis effects. Crosses between different strains in carp have shown heterosis effects for survival (Hines et al., 1974; Sovenyi et al., 1988). Plumb et al. (1975) reported heterosis in American catfish crosses conceming viral

71 lETAL. GENETlC IMPROVEMENT OF DISEASE RESISTANCE IN F1SH

diseases. Interstrain crosses in salmonids have given variable results (Klupp, 1979; Ayles and Baker, 1983; Gjerde and Refstie, 1984). However, unlike Df50% improvement through additive genetic effects, it is not possible to predict the mental results of crossbreeding. Thus there is no general answer to whether or not crossbreeding should be used to increase survival rate in fish farming. Chevassus and Dorson (1990) indicated that several inter-species crosses

yield progenies resistant to specific diseases in some instances. However, due dual+ to low viability of such hybrids they were less interesting for commercial

on farming. "( It could be of interest to develop specialized lines for crossbreeding pur

poses if negative genetic correlations exist between disease resistance and other traits in the breeding goal. However, the heterotic results of such breeding strategies must be great enough to balance the extra cost of developing and maintaining such lines.

MAJORGENES

Major genes (single genes with large effect) are rarely found for production traits in farm animals since they will be rapidly fixed by selection. However, changes in production systems andjor environmental conditions may in

val crease the importance of major genes that have not been subjected to selec'een tion earlier. Species under domestication, as most of the fish species are, may om therefore express major genes for production traits. Sehested and Mao (1992) 'In illustrated the change in frequency of a major gene due to selection under an igh, infinitesimal model. They concluded that major genes in such a selection ledo scheme would approach fixation in a few generations. Special breeding ar

rangements to exploit major genes wiU result in an earlier fixation. ;ur Wild strains may have disease resistan ce genes that could be of advantagelor in fish farming. Such genes may be transferred into farming populations by has systematic crossing. 2). Isolation of single genes, production of transgenic fish and gene mapping lte. have attracted much attention in recent years. Some of the genes isolated in .vill fish and experiments with transgenic fish have been reviewed by Maclean and

Penman (1990). The transferrin gene in Atlantic salmon has been isolated recently (Kvingedal, personal communication). Interestingstudies are under way to isolate genes regulating important physiological processes, e.g. the gonadotropin releasing hormone (GnRH) in Atlantic salmon (Klungland, per

nes sonal communication). wn Gene mapping and isolation of genes associated with disease resistance nb should be given high priority in future gene technology research. ral

72 K-T FJALESTAD ET AL.

CONCLUSIONS

lt is difficult to create optimal environmental conditions in fish fanning to reduce mortality, but much can be done including developing new and improved vaccines. However, such short-tenn attempts would not solve the disease problems. lt is important to improve our understanding of the different diseases and their causes, in order to take preventive steps. Selection is likely to be the most efficient way to increase survival rate or

to enhance natural disease resistance in fanned fish. However, selection for general rather than specific resistance should be applied. Selection should be based on survival rateo In addition, highly heritable traits with high genetic correlations to survival should be included in the selection index particulady at high (or low) survi val rates. Individual tags, possible to read on dead fish as well as live, should be used. This is essential to estimate more reliable breeding values and fully utilize diagnostic infonnation. Because survival is an either-or trait, individual selection is in general of low efficiency particulady at high survival rates. Family selection should therefore be applied. For specific diseases creating serious problems, challenge tests could be applied. Studies of disease resistance mechanisms should be stimulated in order to increase basic knowledge and to find traits highly correlated with disease resistance. Crossbreeding could be of interest as a means to develop specialized lines

ifnegative genetic correlation exists between disease resistance and other traits in the breeding goaL Gene technology research should focus on gene mapping and isolation ofgenes regulating disease resistance.

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