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Family differences in relative growth of diploid and triploid Atlantic salmon (Salmo salar L.)
Aquaculture 192 Ž2001. 23–29 www.elsevier.nlrlocateraqua-online
Family differences in relative growth of diploid and triploid Atlantic salmon žSalmo salar L./ Gerald W. Friars a,1, Ian McMillan b,) , V. Margaret Quinton b, Fiona M. O’Flynn c , S. Anne McGeachy d , Tillmann J. Benfey d a
Atlantic Salmon Federation, P.O. Box 5200, St. Andrews, NB, Canada E5B 3S8 Department of Animal and Poultry Science, UniÕersity of Guelph, Guelph, ON, Canada N1G 2W1 c Department of Chemical and Life Science, School of Science, Institute of Technology College, Tralee, Co. Kerry, Ireland d Department of Biology, UniÕersity of New Brunswick, P.O. Box 4000, Station A, Bag SerÕice 45111, Fredericton, NB, Canada E3B 6E1 b
Received 16 October 1999; received in revised form 19 February 2000; accepted 17 May 2000
Abstract Comparisons of growth for diploid and triploid full sibs from 20 families of Atlantic salmon Ž Salmo salar L.., ranging from 4 to 24 progenyrfamily, involved 628 individuals. Length and weight data, after approximately 75 weeks in seawater, indicated higher within and between family variances in triploids than in diploids. Regression analyses revealed that improvement of triploid populations might require measurements on triploid sibs of the diploid to be involved in reproduction. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Ploidy=family interaction; Sib selection; Salmo salar L.; Atlantic salmon
1. Introduction Research on control of fertility and sexual maturation in Atlantic salmon Ž Salmo salar L.. has been encouraged not only by economic concerns related to market production, but also by environmental concerns. Sterility through triploidy has been 1 )
Present address: 397 Prince of Wales St., St. Andrews, NB, Canada E5B 1R1. Corresponding author. Tel.: q1-519-824-4120 ext. 3506; fax: q1-519-767-0573. E-mail address: [email protected] ŽI. McMillan..
0044-8486r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 Ž 0 0 . 0 0 4 3 8 - 5
24
G.W. Friars et al.r Aquaculture 192 (2001) 23–29
advocated as a means of inhibiting sea cage escapees from disrupting gene pools of surrounding natural populations. Also, the prevention of sexual maturity in aquaculture enhances market quality. The induction of sterility through the use of triploidy has been reviewed by Benfey Ž1999.. A summary of the literature and experimental data from the Salmon Genetics Research Program by O’Flynn et al. Ž1997. indicated that overall yields of triploid Atlantic salmon were generally lower than those of diploid contemporaries in aquacultural environments. If triploidy is to be used as a method of controlling the unwanted spread of genetic material into the surrounding ecosystem, the economic effects of such a procedure must be investigated. The objective of this study was to investigate the differences between families with respect to size of diploid and triploid contemporaries, after approximately 75 weeks in seawater.
2. Materials and methods The stock used in this research was derived from broodstock fish, which had been through one generation of selection for increased growth and related traits, following collection from the Saint John River, New Brunswick. Single-pair matings were used to create individual families. Full-sib diploids and triploids were created within each family by subdividing the eggs immediately after fertilization, with half of the eggs remaining untreated Ždiploids. and the other half being subjected to 5 min at 9500 psi Ž65 = 10 3 kPa. for the induction of triploidy. Pressure treatments began at 3008C-min after fertilization, and were applied using a commercial system ŽTRC Hydraulics, Dieppe, New Brunswick. modified from that described by Benfey et al. Ž1988.. Flow cytometry measurement of erthrocytic DNA content ŽAllen, 1983. was used to confirm ploidy level in 10 randomly selected pressure-treated individualsrfamily at the fry stage; all tested individuals were found to be triploid. Diploid and triploid sibs from each family were reared separately until the early parr stage. Adipose and pectoral fin clips were then used to identify family–ploidy groups for combined rearing to the pre-smolt stage. Selection of presumptive S1 Žyearling. smolts was made in early spring ŽJanuary and February. of the 2nd year of age on the basis of size Žfork length) 13 cm.. Freshwater rearing took place at the Atlantic Salmon Federation ŽSt. Andrews, New Brunswick.. Eggs and yolk-sac fry were incubated at 7.88C. Rearing temperatures for fry and parr ranged from a high of 158C in the summer to a low of 38C during winter. Heat brands were applied to 25 diploid and 25 triploid smolts from each of 20 families prior to the transfer of all 1000 fish to a single sea cage at the Atlantic Salmon Demonstration and Development Farm ŽSt. George, New Brunswick.. These heat brands, in combination with the earlier fin clips, allowed for the subsequent identification of individuals to the family and ploidy level. Another 2500 unmarked S1 smolts were added to the cage to increase stocking density. Temperatures during seawater growth ranged from 108C at smolt transfer in May to a high of 148C in August and a
G.W. Friars et al.r Aquaculture 192 (2001) 23–29
25
low of 18C in late February. Sea cage rearing continued for 75 weeks, at which time lengths and weights of individual fish were recorded and their condition factor calcuweight Ž gm . lated = 100 . Due to mortality and brand loss, data were only collected 3 length Ž cm . from 4 to 24 fishrfamily–ploidy groups, for a total of 628 fish. A mixed model analysis of variance was performed for length, weight and condition factor, considering the two ploidy types Ždiploid and triploid. as fixed effects and families as random effects. This was done using the SAS MIXED procedure ŽSAS.. Estimates of the homogeneity of the variances within diploids and triploids were tested using likelihood ratio tests. The differences between diploid and triploid estimates of each family were linearly regressed on diploid and triploid least square means. As well, the family least squares means for the triploids were linearly regressed on the corresponding diploid means.
ž
/
3. Results The within and between family variances for weight, length and condition factor are given in Table 1 for each ploidy group. Not all fish had both length and weight measures taken. Average family sizes were 17.7 Ž11–21. and 10.1 Ž7–16. for diploid length and weight, respectively. The same figures for triploid families were 15.1 Ž4–24. and 9.2 Ž3–18.. Using a mixed model analysis of variance, the ratio of the variance within triploid families to that within diploid families for both weight and length was significant Ž P - 0.05.. This was not so for condition factor. Fig. 1 suggests distributions of deviations of individual measures of weight and length from the corresponding family means did not differ for the two ploidy groups. For the fixed group effect, the diploids had a higher mean weight Ž P - 0.06. and condition factor Ž P - 0.001. than the triploids ŽTable 2., but there was no significant difference in length Ž P - 0.93.. The differences between diploid and triploid random effects solutions within family for all three variables, regressed over the corresponding performance of either ploidy group, as well as the regressions of triploid on diploid least squares means, are
Table 1 Within and between family variances for weight, length and condition factor in diploid and triploid Atlantic salmon Variable
Ploidy
Between family
Within family
Weight Žkg.
Diploid Triploid Diploid Triploid Diploid Triploid
0.0330 0.1094 2.7534 5.1396 0.0022 0.0019
0.5153 0.8905 22.5419 45.7586 0.0093 0.0095
Length Žcm. Condition factor
26
G.W. Friars et al.r Aquaculture 192 (2001) 23–29
Fig. 1. Frequency distributions of deviations from family–ploidy group means for length and weight in diploid and triploid Atlantic salmon.
summarized in Table 3 for each trait. Using t tests with a Sattherwaite’s correction for degrees of freedom, weight and length had significant slopes Ž P - 0.01. for the regression of difference on either ploidy group. These slopes were positive for the diploids and negative for the triploids. The condition factor had a significant Ž P - 0.01. positive slope for the difference regressed on the diploids, but not on the triploids. For both weight and length, the coefficient of determination Ž R 2 . was highest for the regression having the triploid effect as the independent variable Ž P - 0.005.. In the case of the condition factor, the diploid effect as the independent variable gave the higher, significant Ž P - 0.005. R 2 . Using triploid value as the independent variable, the
G.W. Friars et al.r Aquaculture 192 (2001) 23–29
27
Table 2 Least squares means for weight, length and condition factor in diploid and triploid Atlantic salmon Variable
Ploidy
Mean
Standard error
Weight Žkg.
Diploid Triploid Diploid Triploid Diploid Triploid
3.95 3.70 68.98 69.05 1.16 1.08
0.06 0.10 0.45 0.65 0.01 0.01
Length Žcm. Condition factor
Table 3 Linear regressions of average family ploidy differences against average family ploidy effects, and regressions of triploid on diploid least squares means Trait Weight
Length
Condition factor
Dependent variable Difference Difference Triploid Difference Difference Triploid Difference Difference Triploid
Independent variable Diploid Triploid Diploid Diploid Triploid Diploid Diploid Triploid Diploid
Slopea
R 2b ))
1.55 y1.13 ) ) y0.55 1.23 ) ) y1.13 ) ) y0.23 0.55 ) ) y0.40 0.44 )
0.38 0.85 0.07 0.48 0.69 0.03 0.36 0.14 0.27
a
Slope of linear regression. Coefficient of determination. ) P - 0.05 )) P - 0.01 b
regression for the condition factor was not significant Ž P ) 0.05.. Significance was present for the regressions of triploid on diploid condition factor least squares means Ž P - 0.05.. 4. Discussion The question of whether the improvement of weight and length in diploid populations carries over to triploid contemporaries is of consequence. The work of Guo et al. Ž1990., Myers and Hershberger Ž1991. and Solar et al. Ž1984. with rainbow trout is pertinent in this connection, where few benefits of triploids over diploids were demonstrated until after sexual maturation. The results in this study were obtained before sexual maturation and revealed generally poorer performance in triploids compared to diploids. The question of competition is pertinent as raised in the study of O’Flynn et al. Ž1997. which recommends separate rearing of the two ploidy types. However, such practice is difficult experimentally unless large numbers of cages for each type can be employed because of the large variances usually observed between cages. Triploids are sterile and therefore the reproduction of a selected population must be performed with diploid parents. The performance of triploid sibs of the diploids is of
28
G.W. Friars et al.r Aquaculture 192 (2001) 23–29
ultimate importance if improved populations must be triploids. The least squares linear regression of triploid on diploid family means was negative for both length and weight, although not significant Ž P ) 0.05; Table 3.. The positive and significant Ž P - 0.05. regression for condition factor is opposite that for the two component traits, namely length and weight, where a ratio of these traits is involved. The reverse trends in the regression of the differences between diploids and triploids, with respect to triploids or, alternatively, diploids as the independent variable suggest that some families are intolerant to triploidy. The difference used is the diploid mean minus the triploid mean and it increased with increasing diploid values and conversely decreased with increasing triploid values. Thus, it is apparent that as the overall population mean increased, the diploid values increased at a higher rate than the triploid values. This perhaps suggests a differential fitness in favour of the diploid genotypes, and selection for generally improved weight or growth in diploids would not result in improved triploid performance. The possibility of improving triploids through selection in diploid contemporaries is not promising, even though the linear regression of triploid on diploid least squares means is not significant Ž P ) 0.05; Table 3.. The low, negative correlations between diploid and triploid means ŽTable 3. are affected by numbers of progeny, as low as four, in family–ploidy groups. However, the trend suggests a problem in the simultaneous improvement of diploid and triploid production characteristics. The higher weights in diploids compared to triploids, where the lengths are very similar ŽTable 2., is reflected in the condition factors. The lower values for the triploids indicate that they are not as plump as diploids, an observation also made by O’Flynn et al. Ž1997.. The consistently higher variance in triploids compared to diploids in this study, both between and within families, with the exception of the between-family condition factor, is interesting. Inspection of the length and weight histograms of diploids and triploids ŽFig. 1. reveals that the increased variance in triploids is not due to runts, as might be conjectured by outliers at the lower ends of the distribution, in light of the similar distributions of diploids and triploids from their family–ploidy means. The occurrence of some high performing triploids outliers, however, raises the possibility of specific genetic combinations for improved performance. The evidence of genetic effect involving triploids is noted in the between-family variance in this study and supported by the maternal strain effects in the study of Guo et al. Ž1990.. The involvement of three sets of chromosomes in triploids appears to disrupt uniformity. Also, a lower variance in triploid performance within families may be of importance in multiple objective selection of contemporary sibling-diploid parents used for the creation of populations to be subjected to triploidy. The relative contribution of sires, with one set of chromosomes, and dams, with two sets of chromosomes, to the within family variance in triploids cannot be partitioned in this study where single pair matings were used. Such detail should be pursued in future research using appropriate mating designs. The cumulative results indicate that the development of populations, which perform under triploid conditions, may need to involve the testing of triploid-sibling contemporaries in the selection of diploid parents. The involvement of family selection will
G.W. Friars et al.r Aquaculture 192 (2001) 23–29
29
require the use of a large number of families to minimize inbreeding effects. A selection experiment testing this concept is probably in order, particularly in light of the gains made with selection in diploid Atlantic salmon ŽFriars, 1998. and in other species such as chickens ŽChambers et al., 1981..
Acknowledgements The data used in this study were procured in the Salmon Genetics Research Program, conducted by the Atlantic Salmon Federation, with funding from the Department of Fisheries and Oceans ŽCanada., the New Brunswick Salmon Growers’ Association, the Industrial Research Assistance Program of the National Research Council ŽCanada., the Atlantic Canada Opportunities Agency, and the Atlantic Salmon Federation.
References Allen, S.K. Jr., 1983. Flow cytometry: assaying experimental polyploid fish and shellfish. Aquaculture 33, 317–328. Benfey, T.J., 1999. The physiology and behaviour of triploid fishes. Rev. Fish. Sci. 7, 39–67. Benfey, T.J., Bosa, P.G., Richardson, N.L., Donaldson, E.M., 1988. Effectiveness of a commercial-scale shocking device for producing triploid salmonids. Aquacult. Eng. 7, 147–154. Chambers, J.R., Gavora, J.S., Fortin, A., 1981. Genetic changes in meat-type chickens in the last twenty years. Can. J. Anim. Sci. 61, 555–563. Friars, G.W., 1998. Transfer of breeding technology from terrestrial agriculture to aquaculture. Aquacult. Assoc. Can. Bull. 98 Ž3., 5–8. Guo, X., Hershberger, W.K., Myers, J.M., 1990. Growth and survival of intrastrain and interstrain rainbow trout Ž Oncorhynchus mykiss . triploids. J. World Aquacult. Soc. 21 Ž4., 250–256. Myers, J.M., Hershberger, W.K., 1991. Early growth and survival of heat-shocked and tetraploid-derived triploid rainbow trout Ž Oncorrynchus mykiss .. Aquaculture 96 Ž2., 97–108. O’Flynn, F.M., McGeachy, S.A., Friars, G.W., Benfey, T.J., Bailey, J.K., 1997. Comparisons of cultured triploid and diploid Atlantic salmon Ž Salmo salar L... ICES J. Mar. Sci. 54, 1160–1165. Solar, I.I., Donaldson, E.M., Hunter, G.A., 1984. Induction of triploidy in rainbow trout Ž Salmo gairdner: Richardson. by heat shock, and investigation of early growth. Aquaculture 42 Ž1., 57–67.
Family differences in relative growth of diploid and triploid Atlantic salmon žSalmo salar L./ Gerald W. Friars a,1, Ian McMillan b,) , V. Margaret Quinton b, Fiona M. O’Flynn c , S. Anne McGeachy d , Tillmann J. Benfey d a
Atlantic Salmon Federation, P.O. Box 5200, St. Andrews, NB, Canada E5B 3S8 Department of Animal and Poultry Science, UniÕersity of Guelph, Guelph, ON, Canada N1G 2W1 c Department of Chemical and Life Science, School of Science, Institute of Technology College, Tralee, Co. Kerry, Ireland d Department of Biology, UniÕersity of New Brunswick, P.O. Box 4000, Station A, Bag SerÕice 45111, Fredericton, NB, Canada E3B 6E1 b
Received 16 October 1999; received in revised form 19 February 2000; accepted 17 May 2000
Abstract Comparisons of growth for diploid and triploid full sibs from 20 families of Atlantic salmon Ž Salmo salar L.., ranging from 4 to 24 progenyrfamily, involved 628 individuals. Length and weight data, after approximately 75 weeks in seawater, indicated higher within and between family variances in triploids than in diploids. Regression analyses revealed that improvement of triploid populations might require measurements on triploid sibs of the diploid to be involved in reproduction. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Ploidy=family interaction; Sib selection; Salmo salar L.; Atlantic salmon
1. Introduction Research on control of fertility and sexual maturation in Atlantic salmon Ž Salmo salar L.. has been encouraged not only by economic concerns related to market production, but also by environmental concerns. Sterility through triploidy has been 1 )
Present address: 397 Prince of Wales St., St. Andrews, NB, Canada E5B 1R1. Corresponding author. Tel.: q1-519-824-4120 ext. 3506; fax: q1-519-767-0573. E-mail address: [email protected] ŽI. McMillan..
0044-8486r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 Ž 0 0 . 0 0 4 3 8 - 5
24
G.W. Friars et al.r Aquaculture 192 (2001) 23–29
advocated as a means of inhibiting sea cage escapees from disrupting gene pools of surrounding natural populations. Also, the prevention of sexual maturity in aquaculture enhances market quality. The induction of sterility through the use of triploidy has been reviewed by Benfey Ž1999.. A summary of the literature and experimental data from the Salmon Genetics Research Program by O’Flynn et al. Ž1997. indicated that overall yields of triploid Atlantic salmon were generally lower than those of diploid contemporaries in aquacultural environments. If triploidy is to be used as a method of controlling the unwanted spread of genetic material into the surrounding ecosystem, the economic effects of such a procedure must be investigated. The objective of this study was to investigate the differences between families with respect to size of diploid and triploid contemporaries, after approximately 75 weeks in seawater.
2. Materials and methods The stock used in this research was derived from broodstock fish, which had been through one generation of selection for increased growth and related traits, following collection from the Saint John River, New Brunswick. Single-pair matings were used to create individual families. Full-sib diploids and triploids were created within each family by subdividing the eggs immediately after fertilization, with half of the eggs remaining untreated Ždiploids. and the other half being subjected to 5 min at 9500 psi Ž65 = 10 3 kPa. for the induction of triploidy. Pressure treatments began at 3008C-min after fertilization, and were applied using a commercial system ŽTRC Hydraulics, Dieppe, New Brunswick. modified from that described by Benfey et al. Ž1988.. Flow cytometry measurement of erthrocytic DNA content ŽAllen, 1983. was used to confirm ploidy level in 10 randomly selected pressure-treated individualsrfamily at the fry stage; all tested individuals were found to be triploid. Diploid and triploid sibs from each family were reared separately until the early parr stage. Adipose and pectoral fin clips were then used to identify family–ploidy groups for combined rearing to the pre-smolt stage. Selection of presumptive S1 Žyearling. smolts was made in early spring ŽJanuary and February. of the 2nd year of age on the basis of size Žfork length) 13 cm.. Freshwater rearing took place at the Atlantic Salmon Federation ŽSt. Andrews, New Brunswick.. Eggs and yolk-sac fry were incubated at 7.88C. Rearing temperatures for fry and parr ranged from a high of 158C in the summer to a low of 38C during winter. Heat brands were applied to 25 diploid and 25 triploid smolts from each of 20 families prior to the transfer of all 1000 fish to a single sea cage at the Atlantic Salmon Demonstration and Development Farm ŽSt. George, New Brunswick.. These heat brands, in combination with the earlier fin clips, allowed for the subsequent identification of individuals to the family and ploidy level. Another 2500 unmarked S1 smolts were added to the cage to increase stocking density. Temperatures during seawater growth ranged from 108C at smolt transfer in May to a high of 148C in August and a
G.W. Friars et al.r Aquaculture 192 (2001) 23–29
25
low of 18C in late February. Sea cage rearing continued for 75 weeks, at which time lengths and weights of individual fish were recorded and their condition factor calcuweight Ž gm . lated = 100 . Due to mortality and brand loss, data were only collected 3 length Ž cm . from 4 to 24 fishrfamily–ploidy groups, for a total of 628 fish. A mixed model analysis of variance was performed for length, weight and condition factor, considering the two ploidy types Ždiploid and triploid. as fixed effects and families as random effects. This was done using the SAS MIXED procedure ŽSAS.. Estimates of the homogeneity of the variances within diploids and triploids were tested using likelihood ratio tests. The differences between diploid and triploid estimates of each family were linearly regressed on diploid and triploid least square means. As well, the family least squares means for the triploids were linearly regressed on the corresponding diploid means.
ž
/
3. Results The within and between family variances for weight, length and condition factor are given in Table 1 for each ploidy group. Not all fish had both length and weight measures taken. Average family sizes were 17.7 Ž11–21. and 10.1 Ž7–16. for diploid length and weight, respectively. The same figures for triploid families were 15.1 Ž4–24. and 9.2 Ž3–18.. Using a mixed model analysis of variance, the ratio of the variance within triploid families to that within diploid families for both weight and length was significant Ž P - 0.05.. This was not so for condition factor. Fig. 1 suggests distributions of deviations of individual measures of weight and length from the corresponding family means did not differ for the two ploidy groups. For the fixed group effect, the diploids had a higher mean weight Ž P - 0.06. and condition factor Ž P - 0.001. than the triploids ŽTable 2., but there was no significant difference in length Ž P - 0.93.. The differences between diploid and triploid random effects solutions within family for all three variables, regressed over the corresponding performance of either ploidy group, as well as the regressions of triploid on diploid least squares means, are
Table 1 Within and between family variances for weight, length and condition factor in diploid and triploid Atlantic salmon Variable
Ploidy
Between family
Within family
Weight Žkg.
Diploid Triploid Diploid Triploid Diploid Triploid
0.0330 0.1094 2.7534 5.1396 0.0022 0.0019
0.5153 0.8905 22.5419 45.7586 0.0093 0.0095
Length Žcm. Condition factor
26
G.W. Friars et al.r Aquaculture 192 (2001) 23–29
Fig. 1. Frequency distributions of deviations from family–ploidy group means for length and weight in diploid and triploid Atlantic salmon.
summarized in Table 3 for each trait. Using t tests with a Sattherwaite’s correction for degrees of freedom, weight and length had significant slopes Ž P - 0.01. for the regression of difference on either ploidy group. These slopes were positive for the diploids and negative for the triploids. The condition factor had a significant Ž P - 0.01. positive slope for the difference regressed on the diploids, but not on the triploids. For both weight and length, the coefficient of determination Ž R 2 . was highest for the regression having the triploid effect as the independent variable Ž P - 0.005.. In the case of the condition factor, the diploid effect as the independent variable gave the higher, significant Ž P - 0.005. R 2 . Using triploid value as the independent variable, the
G.W. Friars et al.r Aquaculture 192 (2001) 23–29
27
Table 2 Least squares means for weight, length and condition factor in diploid and triploid Atlantic salmon Variable
Ploidy
Mean
Standard error
Weight Žkg.
Diploid Triploid Diploid Triploid Diploid Triploid
3.95 3.70 68.98 69.05 1.16 1.08
0.06 0.10 0.45 0.65 0.01 0.01
Length Žcm. Condition factor
Table 3 Linear regressions of average family ploidy differences against average family ploidy effects, and regressions of triploid on diploid least squares means Trait Weight
Length
Condition factor
Dependent variable Difference Difference Triploid Difference Difference Triploid Difference Difference Triploid
Independent variable Diploid Triploid Diploid Diploid Triploid Diploid Diploid Triploid Diploid
Slopea
R 2b ))
1.55 y1.13 ) ) y0.55 1.23 ) ) y1.13 ) ) y0.23 0.55 ) ) y0.40 0.44 )
0.38 0.85 0.07 0.48 0.69 0.03 0.36 0.14 0.27
a
Slope of linear regression. Coefficient of determination. ) P - 0.05 )) P - 0.01 b
regression for the condition factor was not significant Ž P ) 0.05.. Significance was present for the regressions of triploid on diploid condition factor least squares means Ž P - 0.05.. 4. Discussion The question of whether the improvement of weight and length in diploid populations carries over to triploid contemporaries is of consequence. The work of Guo et al. Ž1990., Myers and Hershberger Ž1991. and Solar et al. Ž1984. with rainbow trout is pertinent in this connection, where few benefits of triploids over diploids were demonstrated until after sexual maturation. The results in this study were obtained before sexual maturation and revealed generally poorer performance in triploids compared to diploids. The question of competition is pertinent as raised in the study of O’Flynn et al. Ž1997. which recommends separate rearing of the two ploidy types. However, such practice is difficult experimentally unless large numbers of cages for each type can be employed because of the large variances usually observed between cages. Triploids are sterile and therefore the reproduction of a selected population must be performed with diploid parents. The performance of triploid sibs of the diploids is of
28
G.W. Friars et al.r Aquaculture 192 (2001) 23–29
ultimate importance if improved populations must be triploids. The least squares linear regression of triploid on diploid family means was negative for both length and weight, although not significant Ž P ) 0.05; Table 3.. The positive and significant Ž P - 0.05. regression for condition factor is opposite that for the two component traits, namely length and weight, where a ratio of these traits is involved. The reverse trends in the regression of the differences between diploids and triploids, with respect to triploids or, alternatively, diploids as the independent variable suggest that some families are intolerant to triploidy. The difference used is the diploid mean minus the triploid mean and it increased with increasing diploid values and conversely decreased with increasing triploid values. Thus, it is apparent that as the overall population mean increased, the diploid values increased at a higher rate than the triploid values. This perhaps suggests a differential fitness in favour of the diploid genotypes, and selection for generally improved weight or growth in diploids would not result in improved triploid performance. The possibility of improving triploids through selection in diploid contemporaries is not promising, even though the linear regression of triploid on diploid least squares means is not significant Ž P ) 0.05; Table 3.. The low, negative correlations between diploid and triploid means ŽTable 3. are affected by numbers of progeny, as low as four, in family–ploidy groups. However, the trend suggests a problem in the simultaneous improvement of diploid and triploid production characteristics. The higher weights in diploids compared to triploids, where the lengths are very similar ŽTable 2., is reflected in the condition factors. The lower values for the triploids indicate that they are not as plump as diploids, an observation also made by O’Flynn et al. Ž1997.. The consistently higher variance in triploids compared to diploids in this study, both between and within families, with the exception of the between-family condition factor, is interesting. Inspection of the length and weight histograms of diploids and triploids ŽFig. 1. reveals that the increased variance in triploids is not due to runts, as might be conjectured by outliers at the lower ends of the distribution, in light of the similar distributions of diploids and triploids from their family–ploidy means. The occurrence of some high performing triploids outliers, however, raises the possibility of specific genetic combinations for improved performance. The evidence of genetic effect involving triploids is noted in the between-family variance in this study and supported by the maternal strain effects in the study of Guo et al. Ž1990.. The involvement of three sets of chromosomes in triploids appears to disrupt uniformity. Also, a lower variance in triploid performance within families may be of importance in multiple objective selection of contemporary sibling-diploid parents used for the creation of populations to be subjected to triploidy. The relative contribution of sires, with one set of chromosomes, and dams, with two sets of chromosomes, to the within family variance in triploids cannot be partitioned in this study where single pair matings were used. Such detail should be pursued in future research using appropriate mating designs. The cumulative results indicate that the development of populations, which perform under triploid conditions, may need to involve the testing of triploid-sibling contemporaries in the selection of diploid parents. The involvement of family selection will
G.W. Friars et al.r Aquaculture 192 (2001) 23–29
29
require the use of a large number of families to minimize inbreeding effects. A selection experiment testing this concept is probably in order, particularly in light of the gains made with selection in diploid Atlantic salmon ŽFriars, 1998. and in other species such as chickens ŽChambers et al., 1981..
Acknowledgements The data used in this study were procured in the Salmon Genetics Research Program, conducted by the Atlantic Salmon Federation, with funding from the Department of Fisheries and Oceans ŽCanada., the New Brunswick Salmon Growers’ Association, the Industrial Research Assistance Program of the National Research Council ŽCanada., the Atlantic Canada Opportunities Agency, and the Atlantic Salmon Federation.
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