dr. i.j singh lecture

8/7/2019 dr. i.j singh lecture

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I.J. Singh, Professor

Department of Fishery Biology

College of Fisheries, G.B.P.U.A.&T., Pantnagar

GENETIC QUALITY

MANAGEMENT IN FISHSEED PRODUCTION


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FISH SEED PRODUCTION

Several species of commercially important food andsport fishes are bred routinely by hormonetreatment.

Eggs are fertilized by mixing of spermatozoa and

eggs after hand-stripping as in Clarias batrachus.

Hormone treated males and females are released intoenclosures to breed on their own. Practiced with

Indian major carps and Chinese carps in hatcheries.

Conducive environment is provided to fertilized eggsin the hatchery for proper embryonic development toobtain hatchlings.


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NEED FOR GENETIC QUALITYMANAGEMENT

Genetic changes take place inadvertently in cultured

populations.

Farmed fish populations experience different kind of selectionregimes due to lack of competition for food or fear forpredators.

Regular breeding and culture of domesticated species bringsabout changes in the gene pool.

Understanding of the genetic rationale would be helpful in

avoiding inadvertent mistakes taking place.

Selection of largest individuals from Catla and Rohupopulations as broodstock for improving the growth rate hadfailed to give desired results as these tended to be aged slow

growing individuals.


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Number of fishes to be used as broodstock

Age composition of the broodstock

Sex ratio in the broodstock

Family background of the broodstock

Important Aspects for the GeneticManagement of hatchery

populations


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Selection of broodstock is the initial requirementfor any breeding program and often depends onthe objectives of the programs.

1. In case of hatchery production for the purpose

of restocking or stock enhancement, thebroodstock should be acquired from the samesource where juveniles are to be released.

2. When objective of the breeding program is forfood fish production, many factors should betaken into account in broodstock selection.

SELECTION OFBROODSTOCK


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The species of interest must be fully domesticated

e.g., full life cycle is controlled in captivity.

Selective breeding program should be taken onlywhen aquaculture of species under consideration

is sustainable.

There should be genetic variations associated withtraits of commercial interest.

FACTORS FOR SELECTION OFBROODSTOCK


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2. Inbreeding provides opportunity to deleterious recessive

alleles, otherwise hidden in heterozygous individuals, tocombine together in offspring and express.

3. Decreased productivity, reduced fecundity, diseaseresistance and survival of seed stock and increased

incidence of abnormalities are associated with inbreedingdepression.

4. Exchanges between more homozygous hatcherypopulations have produced better results probably due to

increased heterozygosity as a result of cross breeding.


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MANAGING GENETIC VARIANCE INCULTURED STOCKS

Avoiding inbreeding and random genetic drift is critical

for the maintenance of genetic variance in cultured stocks.

This problem is more likely to be countered by highlyfecund species like Indian and Chinese carps.

Generally, there is a tendency to use a fewer number ofbroodstock for seed production.

Consequently, genetic problems associated with small genepools, such as inbreeding have a greater probability tooccur.

In large sized hatcheries using large number of brooders atone time the occurrence of inbreeding to some extent isdiminished.


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INBREEDING

Inbreeding is measured by the inbreeding coefficient F.

The objective is to prevent F from reaching 0.25- the level

where inbreeding is likely to occur in fish.

The simplest method to calculate the accumulation of

inbreeding per generation with random mating is-

F =

1

8 Nem+

1

8 Nef

Where Nem and Nef are numbers of males and femalesthat successfully breed, respectively.

Avoidance of inbreeding often primarily resolve aroundpopulation size.


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INBREEDING IN RAINBOW TROUT(SALMO GAIRDNERI)

Number of Fish Inbreeding pergeneration (inpercent)Female Male

100 100 0.2

50 50 0.550 25 0.25

10 10 2.0

10 05 3.0


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EFFECTIVE POPULATION SIZE(Ne)

Effective population size as an important concept inbroodstock management, as it is inversely related to bothinbreeding and genetic drift.

Maintaining effective population size together with

avoiding mating among closely related individuals of ahatchery stock are important measures for controllinggenetic erosion in hatchery produced seed.

Genetic variability decreases rapidity if the effective

population size of the broodstock is small.

Contd


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In a random mating population, effective population size is

calculated as follows-

Ne =4 Nem Nef

Nem + Nefor

1

Ne=

1

4Nm+

1

4Nf

Nm and Nf are the numbers of males and females in the

population respectively.

When Ne decreases, inbreeding and variance in changes ofallele frequencies resulting from genetic drift increase.

The relationship between inbreeding coefficient F andeffective population size Ne is described below-

F =1

2 Ne

Contd


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The effective size of a population is rarely equal to the totalnumber of reproductive individuals in that population.

A major factor which influences Ne is the relative numberof males and females in a population.

The value of Ne is strongly influenced by the sex which is

the least frequent.

For example a population consisting of 2 males and 2females has an Ne of 4, while a population with one maleand 100 females has an Ne also of approximately only 4.

Contd


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BREEDING For the success of any selection breeding program the most

important factor is heritability (h2) which is represented as:

VA (additive genetic variance)

Vp (total phenotypic variance)

h2 =

For the success of a selection program for improvementthe heritability level must be of the tune of 0.25 or above.

At h

2

= 0.15 or below it the selection will be ineffective. In a selection program improvement is assessed by the

formula

R = h2 S


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S- the selection pressure / selection differential is measuredas the difference between the mean of the whole populationand the mean of the group of fish chosen to be parents ofthe next generation.

R- is the selection response expressed as the differencebetween the mean of the original population and that ofthe next.

There are two basic types of selective breeding programs.

1. Individual selection (also called as mass selection)

2. Family selection

INDIVIDUAL OR MASS SELECTION

In individual selection, family relationships are ignored,and comparisons are made among individuals. Individualfish are ranked in order and the best become the selectbrood fish.

c ema c agram o n epen en


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c ema c agram o n epen enculling

c ema c agram o mo e n epen en


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c ema c agram o mo e n epen enculling

FAMILY


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In family selection, family relationships are of paramountimportance and following two methods are used:

1. Family means are compared and ranked and whole familiesare either saved or culled.

2. Individual rankings are made within each family andselection occurs independently within each family.

FAMILYSELECTION

c ema c agram o e ween am y


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c ema c agram o e ween- am yselection

Sc emat c agram o w t n am y


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Sc emat c agram o w t n- am yselection

Sc emat c agram o a se ect ve ree ng


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Sc emat c agram o a se ect ve ree ngprogramme that combines between-family

and within-family selection


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Other genetic interventions forimproving quality

Hybridization between different stocks of same species

(stocks from different hatcheries).

Production of monosex population through hormonal

application and selective breeding.

Production of triploids through retaining polar body after

fertilization by heat, cold or pressure shock applicationsfor improvement in growth and carcass quality.


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RECOMMENDATION FOREFFECTIVE POPULATION SIZE

Effective population size is a useful concept of brood stockmanagement to reduce inbreeding and genetic drift in ahatchery.

FAO/UNDP recommends Ne as 50 for short term and 500 onlong term.

US Fish and Wildlife service recommends 1000.

Tave (1988) recommends 68-685 depending on the goal.

A farmer, however, can maintain maximum number of Ne aspossible and also avoid inbreeding through geneticmanagement.

A id f i b di B G ti


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Avoidance of inbreeding By GeneticManagement

Maintenance of largest feasible Ne

Partial replacement of brood stock procuring from nature

Exchange of brood stocks between the local hatcheries

Breeding of different age groups together

Use of cryopreserved spermatozoa, if possible

Maintenance of separate stocks of distinct populations andmaintenance of pedigree records of all brood fishes toavoid mating of relatives

Crossing of different lines to increase heterozygosity andtaking up of selective breeding if possible


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dr. i.j singh lecture