Sericulture is an important rural agro-industrial practice in India and all the major commercial silks are produced in the country [1]. Sericulture, the practice of breeding silkworms for the production of raw silk, has been underway for at least 5000 years in China, from where it was spread to Korea and Japan and later to India and the west. The prestigious silk fiber is produced by the monophagous insect, the silkworm Bombyx mori, a Lepidopteron; whose sole food is mulberry. Silkworm is a holometabolous insect, which completes its life cycles of four different metamorphosing phases i.e., egg, larva, pupa and adult 50–55 days.
More than 4000 Bombyx mori strains are currently available throughout the world and these strains are maintained by continuous sibling mating. At Central Sericultural Germplasm Resources Centre (CSGRC), Hosur, Tamil Nadu, where it was undertaken around 450 strains of silkworm are maintained and conserved.
The gene pool available in the country can be broadly divided in to two groups, the low yielding stocks characterized by high adaptability to tropical conditions and the yielding stocks exhibiting regular diapause, but suffer from the low adaptability to the highly variable tropical agro-climatic conditions [2, 3] and are mainly developed from Japanese hybrids[4]. The proposed ‘gene tagged breeding’ to combine the best qualities of the temperate high yielding stocks with tropical hardy ones [5, 6].
The analyzed many cDNA libraries prepared from various tissues[7] and at different developmental stages that cover almost entire set of Bombyx genes, comprising 35,000 expressed sequence tags (EST) from 36 cDNA libraries. Expressed sequence Tags or ESTS are small pieces of DNA sequences that are generated by sequencing either one or both ends of an expressed gene. Once generated, they are useful in cloning specific genes of interest and mapping of functional genes in various related organisms. Such markers are obtained by partial sequencing of random clones. ESTs are popularly used in full genome sequencing and mapping program underway for a number of organisms and for identifying active genes thus helping in identification of diagnostic markers. EST is very simple tool for genome analysis. ESTs provide researchers with a quick and inexpensive route for discovering new genes, for obtaining data on gene expression and regulation, and for constructing the genome maps. EST database was also used to develop simple sequence repeat (SSR) markers for various species.
To build a foundation for the complete genome analysis of the silkworm, Bombyx mori, EST database has been constructed [8] covering about 55% of all genes of silkworm. EST’s (Expressed Sequence Tags) are small pieces of DNA sequence, usually 200–500 nucleotides long, that are generated by either one or both ends of a cDNA. ESTs are presently being used as markers for genome mapping and identification of expressed genes. ESTs libraries and databases have proven to be powerful tools for gene discovery, gene mapping, and for the analysis of quantitative traits. ESTs are generated by large-scale sequencing of randomly picked clones from cDNA libraries constructed from mRNA isolated at a particular development stage and/or tissue. Japanese scientists are actively involved in the genome analysis in the mulberry silkworm. It has a large collection of cDNA libraries prepared from various and different developmental stages to cover the entire set of silkworm’s genes. It is also engaged in construction of BAC (Bacterial Artificial Chromosome) library and making BAC contigs based on DNA fingerprinting and EST markers anchored to linkage maps.
Vitellogenins, the precursors of major yolk proteins of oviparous animals, Vitellogenin of the silkworm, Bombyx mori ,is a tetramer with molecular weight of 440K,composed of each two molecules of non-identical subunits termed heavy chain and light chain [9].
The prothoracic gene of Bombyx mori is a polymorphic one. The gene for the prothoracicotropic hormone (Ptth) in Bombyx mori has been already cloned and characterized. Three alleles a,b,c were identified in the Ptth locus which encodes PTTH in the silkworm ,Bombyx mori. These three alleles can be easily diagnosed by using the PCR technique because their length of the 3rd and 4th introns varies depending on the genotype [10].
Silk fibroin is secreted into the lumen of the posterior silk gland. Silk fibroin is composed of one heavy chain (≈ 350 KDa) and one light chain (25 KDa) [11, 12]. The identified a new mRNA species in the posterior silk gland encoding a silk protein of 25.197 KDa which was termed as P25. P25 protein is produced in equimolar concentrations to major Fibroin [13, 14].
Therefore, in the present study, organization of yolk protein gene and PTTH gene genetic diversity were analyzed and their variations associated with productive traits in silkworm germplasm