Breeding is an age old practice. It started when man first started domestication of plant and animals and when particular animals and plants were selected for various reasons including productivity, quality of produce, easiness of domestication, healthiness etc.
Silkworm is one of the most genetically exploited animals. Silkworms were first domesticated during the ‘Han Dynasty’ in China about 5000 years ago. Since then the silk production capacity of the species has increased nearly ten folds. Silkworm is one of the few organisms were in the principles of genetics and breeding were applied to harvest maximum output. It is next only to maize in exploiting the principles of ‘heterosis’ and ‘cross breeding’
Silkworm breeding is aimed at the overall improvement of silkworm in a human-commercial point of view. The major objectives of silkworm breeding are: Improving fecundity, improving healthiness of larvae, improving quantity of cocoon and silk production, improving quality of cocoon and silk production, for specific purposes based on cocoon and silk production, for disease resistance etc. Let us discuss each of this in short.
Fecundity: it refers to the egg laying capacity of a breed. It is a very important factor since commercial sericulture is strongly dependent on silkworm egg.
Healthiness of larvae: healthy larvae lead to healthy cocoon crop. Healthiness is dependent on factors such as better pupation rate, less number of dead larvae in the mountage, shorter larval duration (shorter the larval duration lesser the chances of infection) and bluish tinged fifth instar larvae (it is observed that bluish colored fifth instar larvae are healthier than the reddish brown ones).
Quantity of cocoon and silk: Quantity of cocoon produced is directly related to the pupation rate and larval weight. Healthier the larva more will be the pupation rate and cocoon weight.
Quality of cocoon and silk: This depends upon a number of factors including genetic factors.
Specific purposes: Apart from commercial purpose advanced countries are giving attention to specific breed development for specific purposes like sericin production, sex limited breeds, thin/ thick filament production etc.
Disease resistance breeding: The major reason for crop losses is pathogen infection. Efforts are in vogue to evolve breeds which are tolerant or resistant to various pathogens.
‘Heterosis’ and its relevance in silkworm breeding
Heterosis is defined as the ‘extra vigour’ or superiority shown by the F1 progeny over either of its parents or the mid parental value. It is given by the formula
The phenomenon of heterosis was first observed and explained by Shull in 1909 in silkworm. The progenies obtained by crossing two inbred lines showed better performance and extra vigour.
The factors determining heterosis are: genetic relationship, Compatibility and Origin.
The theoretical explanation of the phenomenon of heterosis gives four theories as given below.
1) Genes control characters: Genes are the functional units of chromosomes. Certain characters are determined by single genes while others are determined by more than one gene. The output of single gene controlled characters is less. The resultant effect of multiple genes can be improved by their optimum combination.
2) Theory of dominance: This theory suggests that out of the various allele combinations the homozygous dominant is stronger. That is out of ‘AA’, ‘aA’ and ‘aa’ AA will have stronger effect.
3) Theory of over dominance: This theory says that the heterozygous dominant allele will have a stronger effect. That is to say the crossing of a homozygous recessive parent with a homozygous dominant parent out of the various combinations the heterozygous dominant combination will be stronger.
4) Epistasis: Epistasis is the interaction between genes. Epistasis takes place when the action of one gene is modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is said to be epistatic, while the phenotype altered or suppressed is said to be hypostatic.
Though the above theories are different they are not mutually exclusive.
The phenomenon of heterosis if fully exploited in silkworm and maize breeding. In the case of all other animal breeding, pure line selection method is made use of. Hence those breeding programs are deprived of the beneficial effects of heterosis. It was Toyama (1909) who discovered heterosis in silkworm. Till then pure line selection was the sole breeding method in silkworm. Since the finding of heterosis, Japanese scientists conducted trials by crossing pure lines in order to harvest the luxuriousness, robustness and superiority in production. During the late 20th century Japanese cocoon and silk production increased manifold, by the terminal cross breeding strategy.
Hybrid vigour and environment
Hybrid vigour refers to positive heterosis. It is the extra vigour or improvement in performance shown by the F1 progeny over either of its parents or mid parent value. Hybrid vigour is the manifestation of interaction between the genes or alleles of two purelines. However the phenotypic expression is always dependent on the interaction between genotype and environment. Even if the organism posses a very good genetic make up the phenotype is the result of its interaction with the environment. Thus, for optimum expression of the genotype congenial environment is necessary.
The phenotypic expression is also dependent upon the genetic plasticity or buffering capacity of the organism. The extra vigour shown by the out cross between two inbred lines of silkworm could be due to the extra buffering capacity of the resultant organism.