Silkworm Biology

Stages of life cycle

The domestic silkmoth, Bombyx mori, is a member of the family Bombycidae of about 300 moth species under the order Lepidoptera. The life cycle of silkworm represents the most advanced form of metamorphosis. Termed holometabolous, the silkworm completes life cycle through serial progression of four distinct stages of development; egg, larva, pupa and adult. The number of life cycles (generations, which is termed as voltinism) per year depends on the silkworm strain and it varies with the environmental conditions particularly temperature. Silkworm strains which go through multiple generations (5-6) in a year are polyvoltines or multivoltines. These strains do not undergo egg diapause, which is an adaptation to tropical condition in which there is no severe winter. Under natural conditions, silkworm strains which undergo only one generation in a year are univoltine strains. This is an adaptation to overcome harsh winters in temperate countries. Artificially, these eggs which hibernate during winter are stored at 4^oC. After removal from cold storage to room temperature (25^oC), about two weeks later ova in diapause eggs begin final development until hatching. The egg is about the size of a pinhead and resembles a poppy seed. The eggshell provides a protective covering for embryonic development. When first laid, an egg is light yellow. Fertile ova darken to a blue-grey within a few days. The larva is an elongated caterpillar ,the only feeding stage in the life cycle. The larva is monophagous, feeds only on mulberry (Morus alba). During larval life, the larva sheds its skin (molt) 4 times to accommodate growth. The period between successive molts is called an instar. At the end of the 5^th instar, the larva spins a silk cocoon of one continuous fibre within which it undergoes pupation. Silk cocoons are the commercial source of silk. From the time larva hatches out from the egg up to the time of spinning silk thread at the end of larval life, grows about 10,000 times. Bred in captivity for thousands of years on trays of mulberry leaves, B. mori is fully domesticated and cannot survive without the assistance of man. The silk cocoon serves as protection for the pupa. Cocoons are shades of white, cream and yellow depending on silkworm variety. After a final molt inside the cocoon, the larva develops into a brown, chitin covered structure called the pupa. Metamorphosis of the pupa result in an emerging moth or adult. The moth is covered with heavy, round, furry scales and lacks functional mouthparts, so are unable to consume food. The forewing has a hooked tip, which is a characteristic feature of this family; however it is flightless. Wings and body are usually white, but may vary in shades of light brown. Wingspan is 1.5 to 2.5 inches. (4-6 cm). It is the reproductive stage where adults mate and females lay eggs. Adult is the final stage in the life cycle of B. mori.

Chromosomes and their behaviour

As in most Lepidoptera, chromosomes of the silkworm are holocentric, i.e. they possess centromeres throughout the chromosome body. This was shown by (i) persistence of chromosome fragments after irradiation during cell division, (ii) the dispersion of microtubule attachments, (iii) the absence of recombination nodules in females, (iv) the presence of supernumerary chromosomes in some close relatives of the silkworm, and (v) the chromosome pairing behaviour and fertility of interspecific hybrids of two Saturniid species,Antheraea pernyi (Chinese oak tasar silkmoth) and Antheraea roylei (Indian oak silkmoth) with diverse chromosome number. B. mori chromosomes are highly condensed and appear dot-shaped at most meiotic and mitotic metaphase stages. Diffused centromeres and lack of special features make them difficult to identify individually. This has resulted in limited application of modern cytogenetic tools for genetic and molecular studies.

Genetics

The silk moth, B. mori has been used as a model system for formal genetic studies since the discovery of Mendelian inheritance at the turn of the century because of its large size, ease of rearing in the laboratory and economic importance. The well-developed genetics of this species includes more than 400 mutations, which have been mapped to 28 linkage groups or chromosomes. In addition, the existence of hundreds of geographic races and genetically improved strains used for silk production which differ not only in Mendelian traits but also in quantitative traits such as body size, feeding duration, thermal tolerance and disease resistance. These traits remain to be subjected to systematic analysis using modern genetic tools.

Sex determination

In silkworm, females are heterogametic (ZW) and males are homogametic (ZZ) in sex chromosome constitution. The change in ratio of sex chromosomes to autosomes has no effect on sex determination and gynandromorphs, that is, sexual mosaics, rather than morphologically blended intersexes were produced in such studies, suggesting that sex determinants are active on both Z and W-chromosomes. The triploid and tetraploid silkworms, which carry ZZW and ZZZW chromosome constitution invariably, develop into females suggesting that female determinants are possibly localized on W-chromosome. In the light of detailed investigations on sex-determination mechanism in Drosophila, it is worth investigating primary signals in Bombyx sex determination and degree to which underlying genetic and molecular mechanisms are conserved.

In silkworm, there appears to be no dosage compensation. This has been demonstrated using an anonymous Z-specific transcript and Bmkettin, homologous of the X-linked Drosophila kettin gene. In both cases, the level of transcripts in males was two times greater than that in females unlike dosage compensated species such as mammals, in which one copy of the homogametic chromosome pair (XX) is inactivated in somatic tissues by heterochromatinisation. In silkworm and other lepidopteran insects, heterochromatic body is shown to be present only in females in somatic tissues such as silk glands, malphegian tissues, epidermal cells, etc. The sex chromatin body has been correlated with the W-chromosome in endomitotic tissue such as silk gland, artificially induced polyploids and in mutants carrying translocations between the W and various autosomes of silkworm.

In the light of the findings that W-chromosome is strongly female determining and there is no dosage compensation in Bombyx, it would be of interest to investigate whether heterochromatin of W-chromosome specific sequences shows any features with well-studied phenomenon of X-chromosome inactivation in mammals. The identification of W-chromosome specific sequences would facilitate further investigation of this process at the molecular level.

Repetitive DNA

B. mori was the first insect shown to have an interspersed pattern of repetitive and non-repetitive sequences typical of mammalian genomes. Molecular characterization of the repetitive elements from B. mori has revealed the presence of transposable elements typical of the Drosophila genome as well as retroposons typical of the mammalian genomes. The B. mori genome harbours abundant retroposons (SINEs, for short interspersed nucleotide elements repeats) like Bm₁ and Bm₂ elements which represent 5 to 10% of the total genomic mass, similar to the Alu and Alu-like elements found in the mammalian genomes. Interestingly, the fruitfly genome is devoid of such SINE elements.