Exploring Analogous Genes in the Mulberry Silkworm

The mulberry silkworm exhibits fascinating genetic diversity, with multiple genes influencing similar traits. A prime example of this is the “oily silkworm” phenotype, characterized by transparent skin due to reduced uric acid content in dermal cells. This article delves into the complexity of these analogous genes, exploring their inheritance, effects, and mechanisms.

1. Multiple Genes, Similar Traits: The Oily Silkworm Phenomenon

The oily silkworm trait is not controlled by a single gene with multiple alleles, but rather by over 20 distinct genes, each with multiple alleles, spread across various loci on the silkworm chromosomes. These genes, except for one dominant gene, Obs, that causes short sectioned oil, are predominantly recessive. When two different recessive oily silkworm strains are crossed, the resulting first generation offspring will show a normal, opaque skin. However, in the second generation, oily silkworms will reappear, suggesting that these are indeed controlled by multiple loci.

2. Varying Degrees of Transparency and Interactions

The degree of skin transparency in oily silkworms varies considerably, ranging from subtle differences barely distinguishable from normal silkworms to highly transparent skin where internal organs are easily visible. This variation results from different genes having differing effects on uric acid production, and from potential additive or synergistic effects.

When two oily silkworm strains are crossed, the double homozygotes may express a level of transparency that is dominated by the gene with a higher effect. In some cases, the transparency may be intermediate, or in the case of combined low and high degree transparent strain, the effect may even be synergistic.

3. Uric Acid and the Mechanism of Transparency

The fundamental difference between normal and oily silkworms is the reduced amount of uric acid present in the dermal cells of the latter. Uric acid acts as a pigment in normal silkworms, contributing to their opaque skin. The degree of transparency is inversely proportional to the amount of uric acid in the dermal cells. The oily silkworm genes prevent uric acid from being properly absorbed in these cells, leading it to be excreted into the blood stream and out of the body. This is due to an abnormality in certain proteins, especially one involved with the pigment production.

4. Pleiotropic Effects of Oily Silkworm Genes

The oily silkworm genes have a wide range of pleiotropic effects, extending beyond skin transparency. These effects can include:

• Weakness and Prolonged Development: Oily silkworms are often weaker than normal silkworms and may experience an extended developmental period.
• Reproductive Issues: Some strains of oily silkworms exhibit reduced fertility or produce white eggs.
• Other Pigmentation Changes Oily silkworm strains may also exhibit lighter serous membrane pigmentation, and changes in the pigments of dermal cells, also affecting the levels of vitamin B2 in the martensian tube cells.
• Cellular Changes: Oily silkworm genes can cause cellular changes, such as the reduction in size of the silk glands and other cells.

5. Inducing Mimic Oily Silkworms

The relationship between uric acid and the oily silkworm phenotype is further highlighted by experiments where melamine was fed to normal silkworms. Adding melamine to the mulberry leaves of normal silkworms can induce the oily silkworm phenotype. This effect is observed not only in young silkworms but also in the fifth-instar larvae, which usually have a high concentration of uric acid in their skin. Within a few days of melamine feeding, these silkworms exhibit the typical transparency of oily silkworms, due to the absence of uric acid in their skin, which is now detected in the feces instead. These results strongly support the idea that the oily silkworm phenotype is a result of the abnormality in uric acid accumulation. Additionally, the appearance of the mimic oily silkworm phenotype is also associated with the reduction of major proteins in the dermal cells. The pigment particles also collapse and decrease over time. The mimic oil silkworm experiment further confirms the relationship between the reduction of uric acid in dermal cells and the transparency of the skin.

Conclusion

The oily silkworm presents a complex genetic scenario where multiple genes, not alleles of a single gene, independently contribute to the same phenotype. This makes the oily silkworm phenotype an excellent example of analogous genes and their interactions. The study of the mechanisms behind these genes, particularly how they impact uric acid accumulation, offers valuable insights into the developmental and physiological processes of the silkworm.