The Complex Inheritance of Disease Resistance in Mulberry Silkworms

Silkworm disease resistance is a crucial trait that determines the survival and productivity of silkworm rearing. This resistance, or the ability to fend off pathogens, is largely governed by the silkworm’s genetic makeup. The extent of resistance varies considerably between species and even within species, with some showing resistance to specific diseases and others demonstrating broader immunity. This article delves into the intricate inheritance patterns of resistance to various silkworm diseases.

1. Variability in Disease Resistance Across Silkworm Varieties

Studies have shown significant differences in disease resistance levels among silkworm varieties. Tests on multiple silkworm species revealed that resistance to nuclear polyhedrosis virus (NPV), cytoplasmic polyhedrosis virus (CPV), and densovirus (DNV) can differ dramatically, with differences of up to 875 times for NPV and 2000 times for CPV. However, there appears to be less variability in resistance to stroke disease and particulate disease. Interestingly, white-skinned varieties tend to exhibit stronger resistance to particulate disease. The table below summarises the resistance level across different diseases.

2. The Influence of Developmental Stage and Origin

The developmental stage of silkworms also plays a role in their disease resistance. Younger silkworms, known as “ant silkworms,” typically exhibit the lowest resistance, which gradually increases as they develop, reaching its peak in larger silkworms. Furthermore, the geographical origin of silkworm varieties impacts their resistance. For instance, studies on resistance to a specific viral softening disease (FV) showed that varieties from Japan, China, and Europe exhibited stronger resistance, followed by those from Central Europe, Europe, China, Japan and China, Japan and Europe, and finally, Japan.

3. Genetic Inheritance Patterns of Resistance to Different Diseases

The inheritance of disease resistance is complex and depends on the disease in question. Research indicates varying inheritance patterns:

3.1. FV Resistance

Resistance to FV is often a recessive trait. Studies suggest that a single gene pair might be the primary controller of this resistance, although multiple genes could be involved. Selecting for resistance through successive generations of silkworms has shown success, particularly in establishing homozygous recessive resistant lines.

3.2. Densovirus (DNV) Resistance

Similar to FV resistance, the resistance to DNV also appears to be recessively inherited and controlled by a single major gene.

3.3. Nuclear Polyhedrosis Virus (NPV) Resistance

The inheritance of resistance to NPV shows some disagreement between different studies. Some researchers suggest a significant paternal influence, controlled by a dominant gene and possibly modified by minor genes. Others have found a significant maternal effect, and a strong hybrid vigor for this resistance. It appears that results from different studies have shown different parent influences and inheritance patterns, which could be a result of the differences in the silkworm strains used in the studies.

3.4. Cytoplasmic Polyhedrosis Virus (CPV) Resistance

Research on CPV resistance reveals complex patterns. While one study indicated a simple dominant inheritance pattern controlled by a major gene, others suggested a partial maternal inheritance. Moreover, the disease resistance in susceptible varieties can be improved through several generations of selection, while it may not be possible to further improve the resistance of originally resistant varieties.

3.5. Fungal Disease Resistance (e.g. Beauveria bassiana)

The genetic control of resistance to fungal diseases, such as Beauveria bassiana, is complex, involving multiple major genes and minor genes. Two major genes, (mus) on chromosome 11 and (cal) on chromosome 7, are associated with susceptibility. These are recessive, with homozygous individuals showing high susceptibility. The heterozygous F1 generation, however, often exhibits a dominant effect, inhibiting disease onset.

4. Implications for Silkworm Breeding

Understanding the inheritance of disease resistance is essential for breeding programs aiming to create robust and productive silkworm strains. By carefully selecting and crossbreeding silkworms, breeders can develop varieties with improved resistance to specific diseases. The complexity of inheritance patterns highlights the need for continued research and sophisticated breeding strategies to enhance disease resistance in silkworms. This knowledge ultimately aids in ensuring a more stable and efficient silk production industry.