The Genetic Inheritance of Dormancy in Silkworms

Silkworm dormancy, a period of inactivity before molting, is a crucial trait impacting their development. This dormancy, also known as ‘sleep,’ is genetically determined, with varying numbers of sleep cycles (3, 4, or 5) inherited across generations. This article explores the inheritance patterns of these dormancy traits, highlighting the complexities and genetic mechanisms involved.

1. Basic Inheritance Patterns of Dormancy

The number of dormancy periods (sleeps) in silkworms is a heritable trait. Research by Saburo Ogura and Tetsuo Nakamura revealed distinct inheritance patterns:

These results suggest a dominance hierarchy where 3 sleeps are dominant over 4 sleeps, which are in turn dominant over 5 sleeps. The F2 generation’s separation ratios provide evidence of Mendelian inheritance patterns.

2. Variations in 3-Sleep Inheritance

Further studies by Moroshishi Shiujiro revealed that 3-sleep inheritance isn’t monolithic; it can be categorized as either strong or weak:

Strong 3-Sleep System:

In the strong system, the results for 3 sleeps x 4 sleeps and 4 sleeps x 5 sleeps align with the initial studies by Ogura. However, 3 sleeps x 5 sleeps exhibits a unique 11:3:2 ratio in the F2 generation.

Weak 3-Sleep System:

The weak 3-sleep system displays more complex inheritance patterns:

• 3 sleeps x 4 sleeps: F1 ratios vary significantly (from 1:0 to 1:0.1). F2 offspring from 3-sleepers show separation ratios like 1:0.8, 1:5, and 3:1, while F2 from 4-sleepers separates into a 1:11 ratio.
• 3 sleeps x 5 sleeps: F1 shows diverse ratios of 3 sleeps to 4 sleeps (1:1.1 to 1:1.7). F2 generations from 3, 4, and 5 sleepers only separate in a 1:11:4 ratio.

These variations indicate the presence of additional factors influencing dormancy, possibly involving modifiers or multiple genes.

3. Sex-Linked Inheritance of Dormancy

Dormancy inheritance also has sex-associated phenomena, as observed by Nagatomo and Shijiro. It was found that when a four-dormant female silkworm was crossed with a male silkworm, including one or multiple Cambodia species, three-dormant female silkworms appeared in the offspring. However, this three-dormant phenotype was not observed in the reciprocal crosses, indicating a sex-linked pattern of inheritance.

4. Genetic and Hormonal Regulation of Dormancy

The dominance relationship of dormancy genes is generally 3 sleeps > 4 sleeps > 5 sleeps. These dormancy genes are believed to influence juvenile hormone secretion in the pharynx. The 3-sleep genes enhance hormone secretion, which impacts the duration of dormancy. In addition to direct genetic control, factors such as sexual maturation genes (controlling brain function), temperature, and light also contribute to dormancy and adaptation.

5. Communication Pathways in Dormancy Regulation

Shishijiro’s research identified communication pathways crucial for dormancy regulation:

• Brain-pharyngeal body-prothoracic system
• Brain-hypopharyngeal ganglion system
• Brain-cardiac system

These systems, along with environmental factors like temperature and light, collectively regulate hormone secretion from endocrine organs through the brain’s neurosecretory cells. This comprehensive regulation impacts the expression of quantitative traits, including dormancy, degeneration, cocoon weight, and oviposition, highlighting the intricate relationship between dormancy, genetics, and other quantitative traits.

Conclusion

The inheritance of dormancy in silkworms is a complex genetic trait influenced by multiple genes, including major genes and sex-linked genes, environmental factors, and hormonal pathways. The classical Mendelian ratios observed in some crosses are modified by other genetic components and environmental interactions, demonstrating the multifaceted nature of this essential silkworm characteristic. Understanding the inheritance patterns and regulatory mechanisms of dormancy is critical for developing improved silkworm strains with desirable traits.