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The early embryonic mitosis in normal and cooled eggs of the silkworm, Bombyx mori
1Department of Sericulture, Faculty of Agriculture, Hokkaido University, Sapporo, Hokkaido 060, Japan.
Abstract:
Early embryonic mitosis of the silkworm, Bombyx mori, was morphologically studied in the normal eggs and in the eggs treated by low temperature (-10°C). The first embryonic mitosis is observed in the eggs at 120 to 150 minutes after deposition at 26°C. After egg and sperm pronuclei unite, a spindle is formed in each of the pronuclei independently. At metaphase and anaphase paternal and maternal chromosomes are in separate groups on a spindle (gonomeric) and karyogamy takes place at telophase when they reach the poles. The second embryonic mitosis is shown in the eggs at 180 to 210 minutes after deposition. The division of two nuclei is not synchronous in the silkworm, and the mitosis is not gonomeric. In the eggs treated by low temperature, spindle fibers are not observed at all at -10°C, and chromosomes, which form two deeply stained masses of irregular shape, are seen in the less stained area of spindle shape. When the eggs are returned to 26°C, some eggs go into normal gonomeric division, while some form two small and compact spindles, which seem to be derived from each of the pronuclei. It was observed that these compact spindles are able to continue mitosis.
Insights
Early silkworm embryonic mitosis shows distinct gonomeric division in normal eggs. Low temperatures disrupt spindle formation, but some embryos recover normal or alternative mitotic patterns upon rewarming.
Area of Science:
- Developmental Biology
- Cell Biology
- Entomology
Background:
- Early embryonic development in insects, particularly silkworms (Bombyx mori), involves precise cell division processes.
- Understanding the mechanics of early mitosis is crucial for comprehending developmental pathways and potential disruptions.
Purpose of the Study:
- To morphologically investigate early embryonic mitosis in Bombyx mori under normal conditions.
- To examine the effects of low-temperature stress (-10°C) on early embryonic mitosis in Bombyx mori.
- To observe the recovery of mitotic processes after low-temperature exposure.
Main Methods:
- Morphological observation of early embryonic cell division in silkworm eggs.
- Application of low-temperature treatment (-10°C) to stressed eggs.
- Microscopic analysis of spindle formation, chromosome behavior, and nuclear fusion.
Main Results:
- The first embryonic mitosis in Bombyx mori occurs 120-150 minutes post-deposition, characterized by independent pronuclear spindle formation and gonomeric chromosome arrangement.
- The second mitosis (180-210 minutes) is asynchronous and non-gonomeric.
- Low-temperature treatment (-10°C) completely inhibited spindle fiber formation, resulting in irregularly shaped chromosome masses. Upon rewarming, some embryos resumed normal gonomeric division, while others formed compact spindles capable of further mitosis.
Conclusions:
- Early embryonic mitosis in Bombyx mori exhibits distinct gonomeric and non-gonomeric phases.
- Low temperatures severely disrupt mitotic machinery, specifically spindle formation.
- Silkworm embryos demonstrate a capacity for mitotic recovery and progression even after significant cold shock, forming alternative spindle structures.
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