Selection In Aquaculture

Sapto AndriyonoDepartemen Kelautan

Fakultas Perikanan dan KelautanUniversitas Airlangga

Introduction

Definition• Selection entails choosing some individuals

from the population to produce more offspring than others.

• These individuals are selected from the animals, which reach sexual maturation, as only those, which can reproduce can affect the future population.

Aims:• The aim of selection is to identify and select as

parents for the next generation the individuals whose progeny, as a group, have the highest possible additive genetic merit for the trait or traits in question

• Selection does not create new genes, but rather changes gene frequencies.

• The frequencies of alleles with favorable effects on the phenotype under selection are increased, and the frequency of less favorable genes decreased.

• If the purpose of selection is to improve a production trait, the first step is to measure or record this trait on all animals in the population, and then estimate the average and standard deviation.

• Selection is then practiced by selecting those animals, which have highest breeding values.

Natural Selection

• The contribution of offspring to the next generation is called the fitness of an individual, adaptive value, or selective value (Falconer and Mackay, 1996)

• and fitness is the "character" that natural selection selects for.

• Animals, which have the highest fitness in the current environment, will reproduce at a higher level and have a higher survival rate than less fit animals.

• The effect of natural selection over generations is to establish a population adapted to the new environmental condition.

• Thus adaptability is a response of a population rather than of individuals

• Natural selection is rather inefficient over short time periods.

• One of the reasons for its inefficiency is that it only utilizes individual or mass selection.

Articificial Selection• Man creates artificial selection in order to

change a population in the wanted direction.• However, at the same time the population will

continue to be affected by natural selection, which may act in the same or in the opposite direction as artificial selection.

• Artificial selection may be performed in several ways.

a: Animals with average performance are selected. b: Extreme animals are selected and mated interse. c: Animals with good performance in one direction

are selected.

• practiced by selecting phenotypes around the mean and discarding extremes.

• The aim of stabilizing selection is to standardize the population around an average.

• It results in a fairly constant mean with somewhat reduced variance for the trait in question.

Stabilising selection

• some traits there may be an intermediate optimum which is dependent on the biology and life history of the organism.

• in fishes an optimum fat content may be wanted in the filet, while high and low fat percentage are less desirable.

Diversifying selection extremes

• selected parents in each extreme group were mated, offspring variance would be increased and eventually separate and distinct subpopulations could emerge.

• This method of selection is rarely used in animal breeding.

• The most common form of selection applied in agriculture and aquaculture breeding programs is directional selection

• The aim is to improve traits of economic importance. The effect of directional selection for heritable traits is a change in gene frequency at the loci affecting the trait in the next generation.

• Assuming no change in environmental conditions the average phenotypic value of progenies of selected parents is increased.

Directional selection

Multiple trait selection

• A selection program will usually focus on several traits of economic importance.

• There are basically three methods of selection when several traits n are involved (Hazel and Lush, 1942):

1. tandem selection2. cull simultaneously3. selection index or total score

tandem selection

• In tandem selection the individual traits are improved in succession.

• Selection is carried out first for one trait, until a desired genetic level is reached, then selection for the second, third and following traits are performed.

cull simultaneously

• The second method is to cull simultaneously but independently for each trait, a selection strategy called independent culling levels.

• Then a level is set for each trait which represent the culling or selection level for each trait.

selection index or total score.

• The third method is to apply selection simultaneously to all traits giving appropriate economic weight, heritability, and phenotypic and genetic correlations between the traits.

• This method is called selection index or total score.

The tandem method is the least efficient of the three,

The selection index method is the most efficient in terms of selection response in the direction of the multiple trait breeding goal

Comparison of efficiencies of total-score method and tandem method relative to that of the method of independent culling levels, where the n traits are uncorrelated and are equally

important. Reproduced from Hazel and Lush (1942) by permission of Oxford University Press

Indirect Selection

• This method may be exploited in breeding programs.

• If a trait is difficult or very expensive to measure or record, a correlated trait may be used instead.

• The correlated trait may be of importance or may have no economic importance at all.

Marker Traits

1. Disease Resistance Selection Growt Rate2. Feed Convertion Efficiency3. Absorption of Nutrient 4. Reduced Fat Deposition

Disease Resistance Selection

• Growth is also correlated with disease resistance, or at least with survival

• Accordingly, selection for growth should lead to a positive correlated response in survival when selecting for increased growth rate.

immunological and physiological parameters such as

1. lysozyme (Lund et al., 1995), 2. haemolytic activity (Røed et al., 1990;

Røed et al., 1992),3. cortisol (Refstie, 1982), 4. IGM (Lund et al., 1995;

Strømsheim, et al., 1994a), 5. Antibody titre (Lund et al., 1995;

Strømsheim, et al., 1994b), 6. Plasma α2 - antiplasmin activity (Salte et al., 1993)• which has shown genetic variation and genetic correlations with survival. • However, none of these genetic correlations exceeded ±0.37, so the

correlated response would not be expected to be particularly large.

Feed Convertion Efficiency

• feed cost might constitute more than 60% of production cost in Intensive farming

• The feed efficiency in fish farming has been improved both by optimising the feed formulations and improving the feed management.

• “Feed conversion ratio” (FCR, kg feed consumed per kg weight gain)

• “Feed efficiency ratio” (FER, kg weight gain per kg feed consumed)

• Selection for improved feed efficiency may change the requirement of nutrients.

• The optimal ratio between dietary protein and energy will increase in lean fish compared to fat fish

• Therefore, feed efficiency should preferably be tested using diets formulated for a leaner fish

Absorption of nutrients

• This Selection in fish might improve feed efficiency in a similar way as increasing the nutritional density of the diet, as less feed would be necessary to supply sufficient amounts of nutrients and energy for growth, maintenance and activity.

• higher absorption may reduce the minimum requirement of essential nutrients (essential amino acids and fatty acids, minerals and vitamins) in the feed

Reduced Fat Deposition

• The magnitude of selection response for reduced fat deposition will depend on the amount of genetic variation for this trait.

• Some of the variation in fat deposition in their study was perhaps due to differences in size and/or growth rate

• Studies with pigs (reviewed by Vangen and Kolstad, 1986) indicate that leaner individuals may have a higher daily maintenance requirement than fatty individuals

• This argues that selection for decreased fat deposition should be combined with selection for increased growth to prevent a reduced feed efficiency due to a higher relative maintenance energy cost.

selection methods to fish breeders

1. Individual selection, 2. Pedigree selection, 3. Family selection, 4. within family selection, 5. progeny testing6. combined family and within family selection, and

• The efficiency of each method can be predicted by calculating the expected genetic response for a given set of parameters.

• The efficiency of selection is partly dependent on how accurately the breeding values of individual animals are evaluated

Individual selection

• Selection based on an individual's own performance or phenotype is called individual or mass selection.

• This is a well-known and widely used method of selection in animal breeding, and for most aquaculture species the only method practiced.

• Individual selection is usually the simplest method to operate and in many circumstances it yields the most rapid response.

• individual selection is of particular interest for growth rate as the heritability for this trait is fairly high (h2=0.20-0.40) in most species

• methods should be developed for recording also other traits than growth rate like for example ultrasound of live fish to assess fat coverage.

• vital importance that the environmental influence is kept the same for all individuals that are to be compared at any stage of the life cycle.

• to avoid mating of close relatives and rapid accumulation of inbreeding, the number of breeding animals should be kept fairly high, at least 50 mating pairs per generation (Bentsen and Olesen, 2002) thereby securing a large effective population size.

Pedigree selection

• breeding animals are selected based on their parents, grandparents or more remote ancestor’s performances or breeding values

• Selection based on performance of ancestors is generally of limited value if other information is available.

• The weakness of pedigree information in improving the accuracy of selection lies in the fact that information is generally available on only a few ancestors and the effects of genetic segregation become large after one or two generations.

• The accuracy of this selection method will therefore not be high. As a result pedigree selection is not much used as the only method of selection in modern animal breeding.

Family selection

• Family selection refers to a selection method in which family groups are ranked according to the mean performance of each family and whole families are saved or discarded

• The efficiency of family selection rests on the fact that the environmental deviations of the individuals tend to cancel each other out in the mean value of the family.

• Accordingly the phenotypic mean of the family comes close to being a measure of its genotypic mean.

Family selection• Advantage: greater when environmental deviations constitute a

large part of the phenotypic variance. • To reduce: the environment for all families should be

standardised as far as possible in the period the families are kept separate.

• Another important factor affecting the efficiency of family selection is the number of individuals in the families; the family size.

• The larger the family, the closer is the correspondence between mean phenotypic value and the mean genotypic value.

• The high reproductive capacity in aquatic animals makes family selection important for these species.

Summarising to this point

• the conditions that favour family selection compared to individual selection are low heritability, little variation due to common environment and large families.

• The accuracy of family selection is dependent on several parameters; the heritability of the trait (h2), the family size (n) and family type (full and/or half-sibs), and the variation due to common environment (c2)

Within family selection

• The criterion of within family selection is the deviation of each individual from the mean value of the family to which it belongs.

• This type of selection is the reverse of family selection, the family means being given zero weight in the selection decision.

• The chief condition under which this method has an advantage over the others is when there is a large component of environmental variance common to members of a family (Figure).

• Selection within families would eliminate this large non-genetic component from the variation operated on by selection

Relative merits of the different methods of selection. Responses relative to that of combined selection plotted against the phenotypic intraclass correlation, t. I = individual selection; F = family selection; W = within family selection.

• An important practical advantage of selection within families, especially in laboratory experiments, is that it economises breeding space, unlike family selecti

• Within family selection has low efficiency compared to most other selection methods

Progeny testing

• Progeny testing offers the most direct assessment of an individual's breeding value because individuals are selected on the basis of the performance of their offspring

• This method of selection is widely applied in breeding programmes of less prolific species like for example dairy and beef cattle, sheep and goats. In prolific species like fish it is of much less advantageous since family selection can be applied.

• Progeny testing suffers from the serious drawback of a much lengthened generation interval, because the selection of the parents cannot be carried out until the offspring have been measured

• Frequently the generation interval will be doubled. This limitation makes progeny testing impossible or difficult in species, which either spawn only once or suffer high mortality during or after spawning.

• Interestingly, progeny testing is the only selection method for which the accuracy can be 100% (Gjerde, 1991) with large progeny groups and irrespective of the size of the heritability.

Combined selection

• combined selection is used simply to denote that more than one method of selection is used in a breeding plan

• This method, ideally, combines in an optimal way all available sources of information that can add to our knowledge about the breeding value of an animal, information recorded on the animal itself, information about fullsibs and/or half-sibs and progenies as well as pedigree information.

• Combined selection is therefore in principle always the best method

• Selection indexes are the most efficient method to combine information from an individual and its relatives as well as information from several traits.

• All information is combined into an index of merit where the traits are weighted according to their relative economic value.

“Selection” is easier talked about than done~

ARE THERE ANY PROBLEMS? YES, POTENTIAL RISKS…

Reference

• Trygve Gjedrem (ED). 2005. Selection And Breeding Programs In Aquaculture. Published by Springer, The Netherlands.

• Chapter 7 and 11

TERIMA KASIH