Reproduction and Genetic
Selection
• one of the major criteria in selecting a species for culture is
the existence of either:- Suitable techniques for controlled breeding or
- Easy availability of spawn, larvae or juveniles.
• In the case of “wild seed”, it is essential to achieve
controlled reproduction as early as possible, to ensure
timely availability of young ones in adequate numbers for
large-scale rearing.
• Domestication of the animal is also a basic need, and for
taking advantage of the benefits of genetic selection and
hybridization.
• Controlled breeding will be possible only if there is
adequate knowledge of the factors governing reproduction
of the animal and its breeding behavior.
• Unlike terrestrial livestock, the potential benefits of
selective breeding in aquaculture were not realized until
recently.
• This is because high mortality led to the selection of only a
few broodstock, causing inbreeding depression, which then
forced the use of wild broodstock.
Quality traits in aquaculture
• Aquaculture species are reared for particular traits such as
1. Growth rate
2. Survival rate
3. Meat quality
4. Fecundity
5. Age at sexual maturation
Aquatic species versus terrestrial livestock
• Selective breeding programs for aquatic species provide better outcomes
compared to terrestrial livestock. This higher response to selection of aquatic
farmed species can be attributed to the following:
1. High fecundity in both sexes fish and shellfish enabling higher selection
intensity.
2. Large phenotypic and genetic variation in the selected traits.
• Selective breeding in aquaculture provide remarkable economic benefits to the
industry, the primary one being that it reduces production costs due to faster
turnover rates. This is because of faster growth rates, decreased maintenance
rates, increased energy and protein retention, and better feed efficiency.
Applying such genetic improvement program to aquaculture species will
increase productivity to meet the increasing demands of growing populations.
Control of reproduction
The two major types of control that are possible, consist of
1. manipulation of the reproductive cycle and
2. induction of gonadal gamete release (ovulation and
spermiation).
• The reproductive cycle is manipulated so as to have
gametes available when needed.
• Altered gonadal gamete release can be achieved by
hormonal supplementation, manipulation of environmental
factors or the use of special selected strains.
• In oviparous animals, embryos are dependent on the egg
yolk for their nutritional requirements.
• The interaction between the brain, pituitary gland, testis and
ovary largely mediates the influence of environmental
factors on the reproductive development of finfish.
• The two major environmental factors that affect maturation
and spawning are:
The photoperiodic regime and
Temperature.
Induced reproduction
• The hypothalamus regulates the reproductive functions of
the pituitary gland.
• The correct combination of environmental factors required
for maturation, ovulation and spawning, brings about an
accelerated release of gonadotropin from the pituitary into
the bloodstream.
• Environmental conditions required for the initiation of
oocyte maturation, ovulation and spawning are much more
complex than those for gametogenesis.
• Under culture conditions, the required environmental
conditions may not be available, or may not persist for a
sufficient length of time for spontaneous maturation to
occur. This has led to the development of induced
reproduction or hypophysation techniques.
• By the injection of pituitary homogenates the natural
gonadotropin surge is simulated.
• Besides the advantage of regulating the time of spawning,
this enables the adoption of other methods of artificial
propagation, including hand-stripping fertilization,
incubation, hatching and larval rearing.
Injection of pituitary
homogenates to induce
spawning.
Hand-stripping of a mature female fish.
Hypophysation
• It is a technique of breeding fish by administering pituitary
gland extract injection. (also induced breeding/spawning).
• The mammalian gonadotropic hormones, LH and human
chorionic gonadotropin (HCG), are effective in inducing
maturation and ovulation in fishes.
• Homogenates and extracts of whole pituitary glands and
partially purified fish gonadotropins are more potent in
inducing maturation and ovulation in fishes than
mammalian gonadotropins, and can be used extensively in
commercial fish culture.
Gametes and fertilization
• Injection of pituitary homogenate or extract is usually given into the
dorsal muscles above the lateral line and below the anterior part of
the dorsal fin, or the dorsal part of the caudal peduncle.
• The required quantity of the gland is generally administered in two to
four doses.
• Ovum develops from oocyte in a process called oogenesis.
• Sperm develops from a spermatocyte in a process called
spermatogenesis.
• Spermiation and ovulation are the production of sperm and ovum
respectively.
Preservation of gametes
• In many species, the maturation of gonads in the two sexes
is not synchronous.
• Under such circumstances, it will be most advantageous to
have a suitable means of preserving the gametes for
artificial fertilization.
• Cryopreservation, which consists of cooling and storing at
subzero temperatures of liquid nitrogen is widely used.
Use of sex steroids for sex reversal
• Among the techniques that can be employed for restricting
fertility is the application of hormones to produce mono-sex
populations.
• Androgenic and estrogenic steroids are used for
masculinization of genotypic females and feminization of
genotypic males.
• Since the presence of even a small percentage of the
opposite sex in a population is sufficient to initiate
uncontrolled breeding.
• Production of all-female eggs is now a common practice in
a number of rainbow trout hatcheries.
Genetic selection and
hybridization
• The use of genetically selected strains and hybrids has
contributed very substantially to agriculture and animal
husbandry.
• But aquaculture is yet to gain much from efficient breeding
and selection programs.
• Reasons for this are
– the delays in the development of suitable techniques for
controlled reproduction of many farmed species and
– The paucity of genetic expertise among aquaculturists.
• The main aims of fish selection given Kirpichnikov (1966) are as
follows:
– To increase the growth rate by better utilization of food
– To increase the growth rate by fuller utilization of natural food in ponds and
higher consumption of feed mixtures;
– To increase resistance to oxygen deficiency, to high or low temperature, to
higher salinity or to other deviations from the normal environmental
conditions;
– To improve resistance to infectious diseases and to infestation of parasites.
– To improve the nutritive properties of fish
• Other aims of fish selection may include:
– speeding up of sexual maturation,
– the ability to reproduce at relatively low
temperatures and
– the slowing down of maturation to prevent early
switching over of metabolism to develop sex
products, affecting growth and resulting in prolific
reproduction.
Transgenics in Aquaculture
• Transgenic fishes could be as beneficial as transgenic plants and
land animals and could be more effective than those traditionally
bred in producing improved fish strains
• The transgenic technology can be helpful not only in producing
faster growing fish, but also in regulating maturation/reproduction,
enhancing resistance to diseases
• Improving nutritional and other qualities of meat, and modification
of metabolic pathways to enhance food conversion efficiency.
• The first form of gene transfer has been successfully accomplished
in China.
• Due to lack of fish gene sequences, initial transgenic research
employed mammalian growth hormone (GH).
• Hew and Fletcher (2001) list 13 instances involving eight fish
species (common carp, crucian carp, catfish, loach, tilapia, pike,
Atlantic salmon and Pacific salmon), where growth increases have
been obtained using GH genes, as evident from published reports
between 1986 and 1996.
• When a gene is transferred with the objective of improving a
specific trait, it may affect another trait, causing positive or negative
‘pleiotropic’ effects.
• Transfer of growth hormone genes have been observed to
affect:
–
–
–
–
–
–
–
Body shape and composition,
Feed conversion efficiency,
Disease resistance,
Reproduction
Tolerance of low oxygen concentration,
Carcass yield/weight.
Swimming ability and predator avoidance.
Molecular techniques
– Intraspecific crossbreeding
– Interspecific hybridization
– Genetic selection
– Polyploidy
– Sex manipulation and breeding
– Gynogenesis, androgenesis and cloning
– Linkage mapping
For more details on these techniques, kindly refer to the textbook
(pages 191-194).
References
• T.V.R Pillay, Aquaculture principles and practices (2000)
Chapter 8