Introduction
Hybridization is the
process of combining two complementary single-stranded DNA or RNA molecules and
allowing them to form a single double-stranded molecule through base pairing.
In biology a hybrid is an offspring of two animals or plants of different
breeds, varieties, species, or genera. Using genetics terminology, it may be
defined as follows:
v In
general usage, hybrid is synonymous with heterozygous: any offspring resulting
from the breeding of two genetically distinct individuals.
v A
genetic hybrid carries two different alleles of the same gene.
v A
structural hybrid results from the fusion of gametes that have differing
structure in at least one chromosome, as a result of structural abnormalities.
v A
numerical hybrid results from the fusion of gametes having different haploid
numbers of chromosomes.
v A
permanent hybrid is a situation where only the heterozygous genotype occurs,
because all homozygous combinations are lethal.
From a taxonomic
perspective, hybrid refers to:
ü Offspring
resulting from the interbreeding between two animal species or plant species.
ü Hybrids
between different subspecies within a species (such as between the Bengal tiger
and Siberian tiger) are known as intra-specific hybrids.
ü Hybrids
between different species within the same genus (such as between lions and
tigers) are sometimes known as interspecific hybrids or crosses.
ü Hybrids
between different genera (such as between sheep and goats) are known as intergeneric
hybrids.
ü Extremely rare interfamilial hybrids have been
known to occur (such as the guineafowl hybrids).
ü No
interordinal (between different orders) animal hybrids are known.
ü The
hybrid consists of crosses between populations, breeds or cultivars within a
single species. This meaning is often used in plant and animal breeding, where
hybrids are commonly produced and selected because they have desirable
characteristics not found or inconsistently present in the parent individuals
or populations. This flow of genetic material between populations is often
called hybridization.
In recent years, with
the growing importance of pond fish culture, many countries pay greater
attention to the process of fish domestication. This makes possible and at the
same time necessitates the use of selection in fish rearing. Hybridization is
one of the most effective ways of selection with a view to improving productive
properties of fishes. The application of this method in fish culture, as well
as in other branches of animal husbandry, is closely connected with two major aspects.
Firstly, it is a direct use of the results of crossing, i.e. the effect of
heterosis and favourable combination of some valuable features of parental
forms in hybrids of the first generation (so-called commercial or utilizable
crossing). Secondly, and a no less important point, the prolific hybrid forms
can represent a valuable material for further selection (so called synthetic
selection). It is to be noted that both in pisciculture and in breeding of
other domestic animals the use of heterosis is of growing significance.
Fish culture is a very
young branch of science in comparison with animal husbandry and agriculture.
The practice of fish culture shows, however, that skillful application of hybridization
methods makes it possible to obtain hybrid forms of fish with high economic
value. It is promoted by a number of biological peculiarities of fish: high
fecundity, external fertilization (which implies a possibility of artificial
fertilization), relative simplicity of obtaining viable and even fertile
distant hybrids, and strong, early manifestation of heterosis in hybrids of the
first generation. This can be clearly illustrated by some hybrids of sturgeons,
white fish, Tilapia, American catfishes, and various hybrids of cyprinids,
including hybrids of the first generation that resulted from crossing common
carp with eastern carp and the various groups breed during many years of
selective work on these hybrids. Fish culture is primarily concerned with
hybrids which show in the most conspicuous way the merits of the hybrid
organism; i.e. higher viability, accelerated rate of growth, and greater adaptability.
It has to be emphasized, nevertheless, that hybridization does not always lead
to the improvement of qualities in offspring. The results of crossings can be
quite variable and are conditioned by numerous factors. The results depend on
the biology of parental lines (including their morphological and karyological
characteristics), magnitudes and genetic structure of populations of initial
groups, degree of remoteness, and the level of heterozygosity of the crossing
forms. The success of crossings will be determined to a great extent by the
knowledge of these particulars and a skillful selection of groups for obtaining
a maximum possible effect of heterosis from crossing.
Opportunities of
hybridization:
Commercial
hybridization implies crosses of species, subspecies, breeds and strains for
the purpose of obtaining marketable hybrids of the first generation. These
methods are now used with advantage in fish culture. In most cases, such
crossings have yielded valuable hybrid forms, some of which are rather
promising from the view point of economy for commercial rearing. They comprise
both heterosis hybrids and hybrid forms with a favorable combination of
parental features.
v Higher
viability.
v Accelerated
rate of growth.
v Greater
adaptability.
v Improvement
of qualities in offspring.
v Fishes
manifest high fecundity.
v Considerable
magnitude of population.
v High
level of natural heterozygosis.
v Many
hybrids are more resistant to parasitic diseases. Hybrids of salmon and
sea-trout, for example, are less susceptible to ichthyophthiriasis and bear it
in a less severe form. Hybrid carp breed at the rearing farm are more resistant
to infectious dropsy.
v Acceleration
of growth and higher viability, which are characteristic of hybrids, are most
distinct in the first months of rearing. The ability of hybrids to grow rapidly
is especially conspicuous when conditions of nursing become unfavourable
(abrupt variations in water temperature, over stocking, or malnutrition). In
such circumstances the rate of growth (relative weight increment) of hybrid
specimens becomes higher than of the fastest growing parents, even in cases where
the young hybrids initially do not exceed the parental forms. Such a general,
non-specific nature of heterosis is mainly characteristic of hybrids produced
by moderately distant hybridization and mostly in the case of intraspecific
crossings (subspecific, interpopulation, race, interstrain and interbreed). In
interspecific hybrids, advantages are chiefly expressed in accelerated growth
rather than in higher viability, although the combination of these features in
hybrids is observed in some cases.
v Commercial
crossings can be successful only in the case of well-controlled groups of
parental stocks; such control can easily be exercised if fish farms receive
ready spawners instead of young replacement stock. Fish farms should also be
supplied with hybrid larvae reared at specialized selection farms.
v The
establishment of a permanent regime of hatching (water temperature, content of
oxygen and metabolic products in water), a precise count of the initial number
of eggs, and measurements of levels of waste in the process of embryonic and
post-hatching development are of great importance. The stocking for rearing
requires equalization of initial weight of groups to be compared, otherwise the
results of rearing would not be representative. It is desirable to apply both
separate and joint schemes of rearing of all comparable combinations. Separate
rearing should be effected in serial ponds with no less than three
replications.
Problems of
hybridization:
v Explicit
inbreeding depression in close breeding.
v In
all probability, heterosis of hybrids of freshwater fishes is due to
overdominance. As experimental evidence of this, one can apparently consider
demonstrated cases of heterosis on the molecular level in salmon hybrids, in
whitefish hybrids and in sunfishes. All authors revealed changes in the
structure and properties of protein molecules of hybrid specimens. In salmon
and whitefish the change was expressed in a non-specific rise of resistance of
some hybrid proteins to the lethal effect of heat and alcohol, whereas in
sunfishes it resulted in increased oxygen-combining properties of haemoglobin
in the hybrids.
v As
indirect evidence of the significant role of the overdominance mechanism, one
can compare the cases of obtaining a considerable heterosis effect in interstrain
crosses with a low degree of inbreeding of initial lines, and the crossing of
representatives of different species, subspecies and natural populations.
v In
case of remote hybridization, uncontrolled crossings of highly specialized
species and the introduction into the parental stocks of hybrid fish can lead
to loss of valuable properties of parents. It is especially dangerous when
fertile hybrids have the opportunity (and even allowed) to go into natural
water bodies.
Conclusion
Industrial hybridization
is and evidently will be of great importance in fish culture. It must be noted,
however, that the experience accumulated in fish hybridization makes it
necessary to be careful in recommending a method of commercial crossing. The
main reason is the possible introduction of spawners of hybrid origin into the
parental spawning stocks. In intraspecific crossings, mixing can easily occur
due to a lack of phenotypic differences between initial groups. Thus, in many
fish farms, some years after the beginning of work on hybrids of eastern carp,
it was not possible to distinguish eastern carp from hybrids, and industrial
hybridization lost its meaning.
References
üCommercial
hybridization and heterosis in fish culture
http://www.fao.org/docrep/005/b3310e/b3310e18.htm
ü Dna
hybridization - glossary entry - genetics home reference
http://ghr.nlm.nih.gov/glossary=dnahybridization
ü Andriasheva,
m.a., 1966 heterosis at inter-specific crossings of carp.
izv.gosud.nauch.-issled.inst.ozer.rech.ryb.khoz., 61:62–79
ü Armbruster,
d., 1966 hybridization of the chain pickerel and nothern pike. progve fish
cult., 28 (2):76–8
ü Black,
j.d. and l.o. williamson, 1947 artificial hybrids between muskellunge and
northern pike. trans.wis.acad.sci.arts lett., 38:299–314
Chaudhuri, h., 1959
experiments on hybridization of indian carps. proc.indian sci.congr., 46(4):20
