Diapause of Mulberry Silkworm

A physiological phenomenon in which silkworms temporarily arrest cell division, differentiation and growth in the early stage of embryonic development. On the one hand, it is to make physiological preparations for the next step of development, so that its life cycle is consistent with the seasonal changes in obtaining food, and on the other hand, it is also a means to survive the severe winter and low temperature. Diapause is different from dormancy. Dormancy is a phenomenon in which insects temporarily stop their development in a bad environment. When the environmental conditions that inhibit the normal progress of life activities are removed, they will soon wake up; while diapause occurs periodically and is deeper than dormancy. The physiological state of inhibited metabolism is a historical response to the rhythmic and repeated arrival of adverse environmental conditions, and is the result of long-term adaptation of silkworms to environmental conditions. Under natural circumstances, the release of diapause requires a certain time and certain conditions, and is controlled by hormones.

Before diapause occurs in silkworms, there are often one or several generations that are not diapause. In the natural state, the number of generations that occur within one year is symmetrical. There are varieties that only occur once in a year for 1 chromatism, 2 generations for 2 generations, and more than 3 generations for diversification. There are even varieties with or without diapause. There are differences in the development process, physique, and quality of cocoon silk of different chemical sex silkworms. 1. The chemical silkworm has a large body, eats more mulberries, has a long elapsed time, is weaker, and has poor resistance to adversity; 2. Compared with a chemical silkworm, the chemical silkworm has a shorter period of development, has fewer mulberries, has strong resistance to stress, and has a heavier cocoon. The weight of the cocoon layer is slightly lower; the diversified silkworm has a small body, less eating mulberry, rapid growth, and higher temperature resistance, but the cocoon is small, the silk is less, and the sericin is more.

The mechanism of diapause Whether the eggs laid by the silkworm is diapause depends on the amount of diapause hormone secreted by the neurosecretory cells of the hypopharyngeal ganglia, and the secretion of the lateral body of the heart plays an auxiliary role. And this secretion is partly determined by genetic differences, and partly by environmental conditions, especially temperature and light. Environmental stimuli are stored in the brain as a kind of information, instructing the endocrine organs to release relevant hormones at a certain time. Gene as a kind of genetic information, its performance depends on the synthesis of a series of enzymes and hormones and other active substances, thereby affecting the growth and development of silkworms. Enzymes and hormones can conversely restrict gene expression through a series of physiological and biochemical processes, and they are interrelated and mutually restricted. Japanese scholar Zhuxing Shijiro et al. (1956) confirmed that the main diapause gene on the 6th chromosome ± 21.5 locus directly controls the function of the hypopharyngeal ganglion to secrete diapause hormones, and its role is V ¹ (dominant 1 sex gene )>V ² (dominant gene)>V ₃ (dominant gene). The secretory function of the swallowing ganglion is largely affected by the sex-linked maturity gene on the Z chromosome 2 locus that controls brain function. The mature genes include the early maturity gene (Lme), the middle maturity gene (+Lm) and the late maturity gene. (Lm). The brain with the late maturity gene (Lm) has the strongest effect on the hypopharyngeal ganglion stimulation and promotes the secretion of diapause hormone, followed by the middle maturity gene (+Lm), and the precocious gene is the weakest. Generally, the Lm gene is mainly possessed by the homologous system. If the diversification (V ³ ) has this gene, it is easy to change towards the homologous (V ₂ ) direction, and if it has this gene, the gene is easy to homologous (V ₁ ). Change of direction. The Lme gene is mainly possessed by the polymorphic system. Because it stimulates the secretion of hypopharyngeal ganglion hormones the weakest, it does not have that degree of influence on the change of chemical properties. The hypopharyngeal ganglion innervated by the V ₃ gene secretes diapause. Hormone, but due to the small amount, non-diapause eggs are produced; V ₁The gene-innervated hypopharyngeal ganglia lay diapause eggs due to the large amount of hormones they secrete. The function of the hypopharyngeal ganglion under the control of the +V gene to secrete diapause hormone varies depending on the environment. During low temperature and dark stimulation, the brain’s function changes from +Lm to Lm ^e , and non-diapause eggs are produced; when high temperature is bright , the function of the brain changes from +Lm to Lm ^e . +Lm changes in the direction of Lm, and diapause eggs are laid.

The neurosecretory cells of the hypopharyngeal ganglion are distributed on both the dorsal and ventral sides of the ganglion and the part connecting the nerve cord. The number and size vary with silkworm species. Diapause hormones start to be produced at the 5th age, and the amount of production increases significantly after pupa molting. After the 3rd day of pupation, secretion activities prevail, and the secretion method is directly released into the blood. The target organ of diapause hormone is the ovary. The effect of diapause hormone on egg cells is limited to a certain period. Only when the egg cell develops to about 500 micrograms, can it accept the effect of diapause hormone, but each egg cell in the ovary develops successively, so all eggs must be diapause Eggs must have diapause hormone secretion throughout the pupal stage of ovarian development.

Physiological and biochemical silkworms in diapause enter diapause at the early stage of embryonic development, and the eggs develop rapidly in the pupal stage, and diapause hormones are also prevalently secreted at this time, so the physiological and biochemical characteristics of the eggs should be basically determined in the development of the ovary. Diapause hormones are closely related to the material metabolism of the ovaries and eggs. The newly laid eggs contain a lot of glycogen. The diapause eggs are about 1.7 times that of the non-diapause eggs, and the blood sugar level changes in the opposite way. Diapause hormone acts on the ovaries, and has the effect of promoting the use of blood trehalose to synthesize ovarian glycogen. Among the enzymes that synthesize glycogen from trehalose, the activity of trehalase is most affected by the diapause hormone. Other enzymes The activity is almost unaffected. On the second day after the diapause eggs were laid, the glycogen content in the eggs was significantly reduced. After two weeks, it was reduced to less than 1/10 of the initial content. Since then, it has remained at such a low level. Its destination is to convert into sorbitol and Glycerin, this mixture of sorbitol and glycerin has an antifreeze effect, so the reduction and transformation of glycogen content in diapause eggs is an adaptation phenomenon for silkworms to gain resistance to the severe cold in winter.

In addition to affecting the carbohydrate metabolism of eggs, diapause hormones have a certain relationship with pigment metabolism, lipid metabolism and protein metabolism. The coloration of diapause eggs is due to the accumulation of a large amount of 3-hydroxycanine urea in the diapause ovaries. It is caused by the synthesis of ocular pigments such as Omin and Omartin in the laid eggs, and the diapause hormone can promote the penetration of 3-hydroxycanine urea into the ovaries. Diapause eggs have more lipid content than non-diapause eggs. Diapause hormones can promote the accumulation of lipids in the ovaries in the middle of the pupal stage, especially to increase the content of triglycerides. Although the protein content in the ovary and eggs has not been found to change significantly due to diapause, a certain protein that is insoluble in trichloroacetic acid but soluble in acid ethanol has a significant increase in the content of non-diapause eggs (for diapause eggs) 1.5 to 4 times of the diapause hormone) has the effect of reducing this special protein in the ovary and egg. It is known that this protein is a kind of albumin with a small molecular weight, mainly esterase A. The diapause hormone not only inhibits the esterase The synthesis of A also inhibits the activity of esterase A.

Diapause hormones also affect a series of physiological and biochemical characteristics in mature maternal eggs: oxygen consumption is reduced by 50.18%, and the DNA synthesis process of diapause eggs is very weak 48 hours after fertilization (24～25℃). Before the embryo enters the diapause state, The synthesis of silkworm egg DNA is actually difficult to detect. The content of DNA and RNA are reduced by about 25%, but the ratio of DNA/RNA is the same as that of non-diapause eggs, which is maintained at a low level (0.046). The reduction of nucleic acid metabolism in silkworm eggs directly affects the progress of cell division and becomes the cause of embryonic diapause. Unlike the diapause eggs, the non-diapause eggs produced by the female moths whose hypopharyngeal ganglia have been removed have almost a linear increase in DNA growth after fertilization, and there is no tendency to stop or decrease. The intense DNA synthesis process is a prerequisite for cell division and the continuous development of silkworm egg embryos. The diapause hormone secreted by the hypopharyngeal ganglion does not directly inhibit embryonic development, but mainly by changing the metabolic system of the egg, especially through the nucleic acid metabolic system, controlling the process of cell division and embryonic morphology, thereby determining the egg state of the silkworm. The occurrence of diapause.

Removal of Diapause Once the silkworm embryo enters diapause, it will not be released immediately even if given suitable living conditions. Before terminating diapause, a series of physiological preparations are required to restore embryo morphogenesis, called diapause development. Low temperature contact for a certain period of time is a necessary condition for diapause development. The effective temperature to relieve diapause is below 15 ℃, and the best effect is 5 to 7.5 ℃. In addition, the diapause eggs can also be relieved of diapause by giving artificial physical or chemical stimulation at an appropriate period after they are laid. The production is called artificial incubation.

When the silkworm eggs are in diapause, glycogen in the eggs suddenly disappears and sorbitol and glycerin appear. When the diapause is relieved, glycogen is synthesized from these sugar alcohols. The Japanese scholar Kaiying (1978) investigated the activity of esterase A in the egg to solubilize yolk cells and believed that the increase of esterase A activity is closely related to embryonic development. If diapause eggs with extremely low esterase A activity 20 hours after spawning are treated with hydrochloric acid, the activity of esterase A will appear after half an hour, and the activity will rise to the same level as non-diapausing eggs after 2 hours. On the other hand, low temperature can also increase the activity of esterase A. Soon after the diapause eggs were refrigerated, the activity of esterase A began to rise and reached the peak activity after 50 days. This is consistent with the gradual activation of diapause eggs when refrigerated at 5°C and full activation after about two months.

In addition, Kurata Keier et al. (1977) found that the size and number of nucleoli of silkworm eggs before and after diapause are significantly different, and rRNA is no longer increased in the diapause eggs, while the rRNA synthesis activity of non-diapause eggs increases sharply. The action of nucleolus is compatible with the synthesis activity of rRNA, that is, the nucleolus of diapause embryos is smaller. Diapause rRNA genes are inhibited in both nucleolus formation and rRNA synthesis. The inhibition of the expression of this gene is believed to be the cause of diapause. After being treated with low temperature or other conditions, its rRNA synthesis activity will be restored and reach the same level as that of non-diapause eggs.

The variability of chemistry is not fixed, and the decisive reasons for chemistry are not only internal factors, that is, heredity, but also external factors such as temperature and light can also cause chemistry changes. The effect is particularly significant in embryonic development. 1. Regardless of the temperature during the incubation period, the generalization does not change. In the late stage of incubation, when protected at a high temperature above 25°C, the bisexual species will develop into a metamorphic silkworm and lay eggs; if protected at a low temperature of about 15°C, it will develop into a metamorphic silkworm and will not produce more. Annual eggs; if protected at an intermediate temperature of 20 ℃, the fertile eggs and non-yield eggs will be mixed. The influence of light and humidity is less important than temperature, that is, under the condition of cyanosis at intermediate temperature, 17 to 18 hours of ordinary light per day can promote the laying of eggs. There is also a tendency to lay eggs when the humidity is high. Therefore, using this characteristic, the silkworm eggs of the binary species can be artificially produced to produce a single or a binary silkworm.

In addition to the effects of environmental conditions such as temperature during embryonic development on chemical changes, the temperature and light in the larval and pupal stages also have a certain effect on chemical changes under the condition that the egg stage has not yet determined its chemical characteristics, that is, the high temperature in the small silkworm stage. , Bright, low temperature (below 25℃) and darkness in the large silkworm stage, more fertile eggs will be produced, on the contrary, it is easy to lay non-aging eggs. The quality of mulberry leaves during feeding also affects the chemistry of laying eggs. In general, when the nutritional conditions are poor, there is a tendency to produce more eggs without aging. The effect of nutritional conditions on the chemical properties not only affects the chemical properties of the next generation of eggs, but also extends to the chemical properties of the eggs laid by the next generation of silkworms.