Breeding method of fluoride-resistant and polyhedrosis-resistant bombyx mori strain

Abstract

The invention relates to a breeding method of a fluoride-resistant and polyhedrosis-resistant bombyx mori strain, and belongs to the technical field of bombyx mori genetic breeding. In order to solve the technical problems that an existing silkworm variety cannot resist fluoride poisoning and pyosis at the same time, and resistance is prone to loss, the method comprises the following steps: hybridizing a germplasm carrying a fluorine-resistant major gene and a germplasm carrying an antiviral dominant major gene to obtain an F1 generation, and backcrossing the F1 generation with a fluorine-resistant parent after fluorine-resistant screening to obtain a BC1 generation; and finally, fluorine-resistant and antiviral characters are synchronously screened and improved through selfing generations, and directional breeding is carried out by combining whole-course recorded development and cocoon quality characters. According to the method, a new silkworm strain with double resistance is successfully bred, and the method is mainly used for solving the problem of low survival rate of silkworm breeding in a fluorine pollution area and a virus high incidence area in industrial production.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of silkworm genetic breeding, and particularly relates to a breeding method of a silkworm strain resistant to fluoride and against blood type bacterial disease. BACKGROUND

[0002] In silkworm production, two major diseases, i.e., silkworm blood type bacterial disease and fluoride poisoning, have long been a threat. The silkworm blood type bacterial disease is caused by silkworm nuclear polyhedrosis virus (BmNPV), and after infection, the diseased silkworm usually develops disease in the next instar or pupal stage, with a very high mortality rate. The dead silkworms are prone to cross-infection in the population, leading to the death of the entire batch of silkworms and causing serious losses to production. Fluoride poisoning is caused by industrial pollution. After eating fluorine-containing mulberry leaves, the silkworms accumulate fluoride in their bodies, which leads to abnormal development, reduced silk production, and even no cocooning, etc. At present, there is no effective treatment for these two diseases, so breeding silkworm varieties with both resistances has become an urgent need in production.

[0003] Previous studies have shown that the silkworm fluoride resistance trait is controlled by a pair of major genes and several minor genes, showing partial dominant inheritance, and is closely linked to the 12th linkage group, with obvious additive effect and heterosis. The silkworm resistance to nuclear polyhedrosis virus is also controlled by a pair of dominant major genes and multiple minor genes, and shows the characteristics of partial paternal inheritance. Although cumulative selection by adding poison can improve the resistance of the variety, the existing varieties can only resist one of the diseases, and there is no practical variety with both fluoride resistance and virus resistance.

[0004] The main difficulty in breeding double-resistant varieties is that the two resistances are controlled by different genetic systems, fluoride resistance is partially dominant, while virus resistance is dominant inheritance, and they are located in different linkage groups. Traditional crossbreeding methods are difficult to stably aggregate two major genes in the same genetic background, and gene loss or resistance weakening may occur during the breeding process. In addition, the screening methods for the two resistances are different. Fluoride resistance requires phenotypic selection by continuously feeding fluorine-containing mulberry leaves, while virus resistance requires virus addition test. When these two selection pressures are applied at the same time, they may interfere with each other, increasing the difficulty of breeding.

[0005] In actual breeding process, there is also the problem of low screening efficiency. Due to the different genetic mechanisms of the two resistances, it is difficult to track the genetic dynamics of the two major genes during backcrossing and selfing, and effective genotyping and phenotypic screening methods need to be established. At the same time, in the high temperature and humidity environment, the growth status and resistance performance of silkworms will be affected, further increasing the complexity of breeding work. These factors have made it a technical problem to breed silkworm varieties with both fluoride resistance and virus resistance. SUMMARY

[0006] An object of the present application is to solve at least the above problems and provide at least the advantages to be described later.

[0007] Another object of the present application is to provide a breeding method of a silkworm strain resistant to fluoride and blood type disease, which can combine traditional breeding methods of multiple generations of backcrossing, selfing and systematic breeding to stably integrate two independently inherited major genes controlling fluoride resistance and virus resistance into the same silkworm strain, so that the new silkworm strain obtained by breeding can exhibit significant resistance to fluoride and Bombyx mori nucleopolyhedrovirus (BmNPV) in a high-temperature and high-humidity environment.

[0008] To achieve these objects and other advantages of the present application, a breeding method of a silkworm strain resistant to fluoride and blood type disease is provided, which comprises the following steps: S1: crossing a germplasm RF carrying a major gene for fluoride resistance with a germplasm RN carrying a dominant major gene for resistance to Bombyx mori nucleopolyhedrovirus to obtain F1 generation; S2: screening for fluoride resistance from F1 generation, backcrossing the fluoride-resistant individuals with the germplasm RF to obtain BC1 generation; S3: randomly selecting 30 individuals from 2nd instar silkworms for BmNPV virus challenge test to detect the retention of the major gene for virus resistance, and continuously backcrossing BC1 generation with the germplasm RF for 2 times to obtain B1 generation; S4: selfing from B1 generation and adopting mixed area feeding and mixed area breeding, and simultaneously screening and improving the fluoride resistance and the resistance to Bombyx mori nucleopolyhedrovirus in the population of this generation; The method for screening fluoride resistance is to continuously feed the silkworms with fluoride-containing mulberry leaves from 2nd instar or 3rd instar feeding to the end of the instar period, and to screen individuals without fluoride poisoning symptoms; The method for testing and screening resistance to Bombyx mori nucleopolyhedrovirus is to quantitatively feed the silkworms with mulberry leaves impregnated with Bombyx mori nucleopolyhedrovirus at 4th instar feeding period, to replace the ordinary mulberry leaves after 24 hours, to count the mortality or to eliminate sick silkworms, and to select healthy individuals without infection; S5: recording the instar period, cluster duration, feeding performance and cocoon quality of silkworms throughout the breeding process, and eliminating bad eggs, weak silkworms, sick silkworms, bad pupae and bad moths, and selecting individuals with uniform development and excellent cocoon quality.

[0009] Preferably, the screening and improvement process of fluoride resistance and virus resistance is carried out in a high-temperature and high-humidity environment, the feeding environment temperature is 28-30℃, the environmental humidity is 80%-85%, full coverage breeding is adopted for 1-3 instars, semi-coverage breeding is adopted for 4th instar, ordinary breeding is adopted for 5th instar, and feeding is carried out 3 times a day.

[0010] Preferably, a sodium fluoride aqueous solution with a concentration of 200-400 ppm is prepared, mulberry leaves are soaked in the solution for 10-15 minutes, and then dried to obtain fluorine-containing mulberry leaves.

[0011] Preferably, the virus stock solution is diluted to a concentration of 1×10 6 ~5×10 6 / mL, mulberry leaves are soaked in the solution, and then dried to obtain mulberry leaves impregnated with Bombyx mori nucleopolyhedrovirus.

[0012] Preferably, when the mulberry leaves impregnated with Bombyx mori nucleopolyhedrovirus are fed, 3 pieces of impregnated mulberry leaves with a size of 4 cm×4 cm are fed to each 30 silkworms at a time within 24 hours.

[0013] Preferably, the germplasm RF is the parent of the variety Gui'can F95 carrying a fluoride-resistant major gene, and the germplasm RN is the parent of the variety Gui'can N2 carrying a dominant major gene resistant to Bombyx mori nucleopolyhedrovirus.

[0014] Preferably, in the process of screening against Bombyx mori nucleopolyhedrovirus, after each time of feeding the mulberry leaves impregnated with the virus for 24 hours and finishing feeding, fresh lime powder is immediately and evenly spread on the surface of the silkworm seat for disinfection, clean silkworm seat paper is replaced after disinfection, and normal feeding is started by replacing the ordinary mulberry leaves. The amount of lime powder spread is 10-15 grams per 0.1 square meter of silkworm seat area. The used silkworm seat paper needs to be removed together with the residues when the silkworm seat paper is replaced.

[0015] Preferably, in the step of disinfecting the surface of the silkworm seat by evenly spreading fresh lime powder, after the spreading is completed, the silkworm seat is left to stand for 10-15 minutes, then a soft hair brush is used to gently sweep along the longitudinal direction of the silkworm seat once, so that the lime powder fully penetrates into the gaps of the silkworm seat, after the sweeping is completed, the silkworm seat is left to stand for another 5-10 minutes, and then the clean silkworm seat paper is replaced; the length of the bristles of the soft hair brush is 2-3 centimeters, and the material is pig bristles or soft nylon; the present application at least includes the following beneficial effects: First, the silkworm strain selected by the present application can tolerate fluoride concentrations of 100 ppm or more and show high resistance to Bombyx mori nucleopolyhedrovirus (BmNPV) at the 2nd instar stage, and the median lethal concentration (LD 50 ) reaches the order of magnitude of 10 8 / mL, effectively solving the problem of simultaneously preventing and treating two diseases in production.

[0016] Second, the strain selected by the present application not only has the target disease resistance, but also significantly enhances the adaptability to adverse feeding environments, showing comprehensive stress resistance advantages.

[0017] Thirdly, the application adopts hybridization, backcrossing, selfing and systematic breeding and other traditional breeding methods, and does not need to rely on complex techniques such as genetic engineering, and the method is safe and reliable, the bred strain has good genetic stability, and is convenient for popularization and application in actual production.

[0018] Other advantages, objects and features of the present application will be apparent from the following description, and will be understood by those skilled in the art. DETAILED DESCRIPTION

[0019] The application will be further described in detail below, so that those skilled in the art can implement it according to the description.

[0020] It should be understood that the terms such as "have", "contain" and "include" used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0021] It should be noted that the experimental methods in the following embodiments are all conventional methods, and the reagents and materials can be obtained from commercial channels unless otherwise specified.

[0022] Example 1 Breeding of strain 96·NC99R The female strain 96 of the variety Gui'can F95 carrying the main gene of fluorine resistance was selected as the female parent, and the female strain NC99R of the variety Gui'can N2 carrying the dominant main gene of the Bombyx mori nucleopolyhedrosis virus (BmNPV) was selected as the male parent to carry out hybridization, and F1 generation was obtained. From F1 generation, fluorine resistance screening was started, and the fluorine-resistant individual was selected and backcrossed with the recurrent parent 96 to obtain BC1 generation. From BC1 generation, 30 individuals were randomly selected at the 2nd instar silkworm period to carry out the Bombyx mori nucleopolyhedrosis virus infection test to detect the retention of the main gene of the virus-resistant offspring. BC1 generation was backcrossed with the parent 96 for 2 times to obtain B1 generation. After B1 generation, selfing was used for propagation, and mixed area feeding and mixed area breeding were carried out. From B1 generation, the screening and improvement of fluorine resistance and virus resistance were carried out synchronously.

[0023] The silkworm feeding process adopts high temperature and high humidity breeding method, and the average temperature is controlled at 29℃, and the average humidity is 81%. The whole covering breeding is adopted at the 1st to 3rd instar, the semi-covering breeding is adopted at the 4th instar, and the ordinary breeding is adopted at the 5th instar, and the feeding is carried out 3 times a day.

[0024] The fluoride tolerance screening method is as follows: Accurately weigh 3 g of analytical grade sodium fluoride (NaF) reagent and add it to 10 kg of water to prepare a 300 ppm NaF solution (prepare immediately before use). Immerse fresh, ripe mulberry leaves in the solution for 10 minutes, then remove and air dry to obtain fluoride-containing mulberry leaves (actual fluoride content measured to be 237 ppm). From the second or third instar feeding stage, continuously feed the above-mentioned fluoride-containing mulberry leaves until the end of the molting period at that instar. During the screening period, cull individuals showing symptoms of fluoride poisoning and retain asymptomatic silkworms.

[0025] The antiviral test method is as follows: Dilute the purified silkworm nucleopolyhedrovirus stock solution to 3×10⁻⁶. 6 / mL (prepare fresh for immediate use). Soak fresh, ripe mulberry leaves in the virus solution until fully and evenly absorbed. Remove the leaves and allow the surface moisture to evaporate naturally. Cut the leaves into 4 cm × 4 cm cubes. Add the virus quantitatively during the first feeding of leaves at the 2nd instar. Feed 3 virus-soaked mulberry leaves to every 30 silkworms within 24 hours. After 24 hours, disinfect the silkworm rearing table with fresh lime powder, replace the table paper, and switch to normal feeding with regular mulberry leaves. At the start of the 3rd instar, count the number of healthy silkworms and the number of silkworms that died from virus infection to calculate the morbidity rate.

[0026] The following methods can be used to improve antiviral screening: dilute the original virus solution to 10... 6 Prepare virus-infused mulberry leaves at a concentration of [number] cells / mL (prepare fresh for immediate use) according to the above method. Add an appropriate amount of virus-infused leaves to the feed of 4th instar silkworms. Immediately after 24 hours, disinfect the surface of the silkworm rearing bed by evenly sprinkling fresh lime powder. After disinfection, replace the bed paper with clean paper, and then resume normal feeding with regular mulberry leaves. The amount of lime powder used is 12 grams per 0.1 square meter of the rearing bed area. When replacing the bed paper, remove the used paper along with any residue. Throughout the developmental process, continuously cull silkworms, pupae, and moths exhibiting symptoms of septicemia.

[0027] Throughout the breeding process, the instar, clustering, feeding performance and cocoon quality of silkworms are systematically recorded. Poor eggs, weak silkworms, diseased or dead silkworms, poor pupae and poor moths are eliminated generation by generation, and individuals with uniform development, robust physique and excellent cocoon quality are selected.

[0028] Example 2 In this embodiment, the breeding of strain 96·NC99R, the silkworm rearing process, and the fluoride tolerance screening method are all the same as in Example 1. The difference is that the following disinfection management plan was adopted in the antiviral screening process: Each time the silkworms are fed mulberry leaves soaked with the virus, disinfection should be carried out immediately after the leaves have been consumed within 24 hours. First, fresh lime powder is evenly sprinkled on the surface of the silkworm rearing tray at a rate of 12 grams per 0.1 square meters. After sprinkling, let it stand for 12 minutes to allow the lime powder to fully contact the pathogens. Then, using a soft-bristled pig bristle brush with bristles 2.5 cm long, gently sweep along the longitudinal direction of the tray once to allow the lime powder to penetrate into the crevices. After sweeping, let it stand for another 8 minutes.

[0029] Subsequent antiviral testing, screening enhancement methods, and data recording and individual selection criteria throughout the breeding process were all the same as in Example 1. The implementation of the above disinfection management plan effectively controlled cross-infection of the virus during the screening process, improving the accuracy and reliability of antiviral screening.

[0030] Example 3: Breeding of strain 97·NC9R A female parent, 97(A), of the Guichan F95 variety carrying the major gene for fluoride tolerance, was selected as the maternal parent and crossed with NC99R, the female parent of the Guichan N2 variety carrying the dominant gene for resistance to silkworm nucleopolyhedrovirus (BmNPV), to obtain the F1 generation. Fluoride tolerance screening began from the F1 generation, and fluoride-tolerant individuals were backcrossed with the paternal parent 97(A) to obtain the BC1 generation. From the BC1 generation onwards, 30 individuals were randomly selected during the second instar silkworm rearing period for BmNPV infection testing to detect the retention of the major antiviral gene in the offspring. The BC1 generation was backcrossed twice with the parent 97(A) to obtain the B1 generation. From the B1 generation onwards, self-pollination was used for breeding, and mixed-area rearing and seed production were conducted. From the B1 generation onwards, screening and improvement of fluoride tolerance and antiviral resistance were carried out simultaneously.

[0031] The high-temperature and high-humidity rearing method, as well as the methods for poisoning tests and selection and improvement of fluoride resistance and NPV resistance, are the same as in Example 1.

[0032] Comparative Example 1 In this comparative example, the breeding of strain 96·NC99R, the silkworm rearing process, and the fluoride tolerance screening method were all the same as in Example 1. The difference was that the following disinfection management plan was adopted during the antiviral screening process: During the screening process against silkworm nucleopolyhedrovirus, after each feeding of mulberry leaves soaked in the virus for 24 hours and after the leaves were consumed, fresh lime powder was not evenly sprinkled on the surface of the silkworm bed for disinfection. Instead, the clean silkworm bed paper was directly replaced, and then the silkworms were fed with ordinary mulberry leaves for normal feeding.

[0033] Test results I. Summary of Feeding Results in the Screening Processes of Examples 1 and 3 Table 1. Feeding performance of 96·NC9R F1 is the F1 generation obtained by crossing the parent 96 with the parent NC99R. BC1 is the BC1 generation obtained by backcrossing the F1 generation with the parent 96. BC2 is the BC1 generation continuously backcrossed with the parent 96 once. BC3 is the BC1 generation continuously backcrossed with the parent 96 twice. B1-7 is each generation after the BC1 generation propagated by self-crossing.

[0034] From the analysis of the feeding results of each generation of the 96·NC9R strain in Table 1, it can be seen that the main economic traits of the strain show a positive optimization trend during the breeding process. The full-age time gradually shortens from 21 days and 6 hours of the F1 generation to 19 days and 9 hours of the B7 generation, indicating that the development speed is improved. From the B1 generation to the B6 generation, the full-age disease silkworm rate is stable at about 1%, and the pupa unified survival rate is 80%-90%, showing that the strain has obtained stable and enhanced disease resistance and survival ability through breeding. In terms of cocoon quality traits, the full cocoon weight remains at a high level of 1.2-1.585 g, and more importantly, the cocoon layer rate significantly increases from 21.48% of the F1 generation to 24.94% and 25.91% of the B4 and B6 generations, indicating that while achieving the goal of disease resistance breeding, the silk production capacity of the strain has also been improved synchronously. The B7 generation is special, the whole silkworm room is contaminated by NPV, and the non-resistant NPV variety 932 of the same period is close to extinction, but the 96·NC9R strain can also survive, and the unified survival rate of the pupa reaches 43.9%. Although the full cocoon weight and cocoon layer weight have decreased, the cocoon layer rate still maintains above 22.80%, indicating that the new strain has obtained the ability to resist NPV. In summary, these data show that through the breeding method described, a new silkworm strain with uniform development, strong resistance and excellent cocoon quality has been successfully obtained.

[0035] Table 2 Feeding results of 97·NC9R Similar to Example 1, F1 is the F1 generation obtained by crossing the parent 97 (A) with the parent NC99R. BC1 is the BC1 generation obtained by backcrossing the F1 generation with the parent 97 (A). BC2 is the BC1 generation continuously backcrossed with the parent 97 (A) once. BC3 is the BC1 generation continuously backcrossed with the parent 97 (A) twice. B1-7 is each generation after the BC1 generation propagated by self-crossing.

[0036] The analysis of the feeding performance of each generation of 97·NC9R strain in Table 2 shows that the main economic traits of the strain during the selection process showed a positive optimization trend. The total age time gradually shortened from nearly 22 days in F1 generation to 19 days and 4 hours in B7 generation, indicating that the development speed was improved. After B1 generation, the disease rate was stable within 3%, and the pupa uniformity rate was not very stable, with the lowest being 76% and the highest being more than 95%. In terms of cocoon quality traits, after 7 generations of selection, the cocoon layer rate could be stabilized at about 23%, which belonged to a high cocoon layer rate level. B7 generation was a special generation, the whole silkworm room was contaminated by NPV during that silkworm period, and the 932 strain, which was not resistant to NPV, was close to extinction in the same silkworm room at the same time, but 97·NC9R could also survive, and the pupa uniformity rate reached 56.87%. Although the total cocoon weight and cocoon layer weight decreased, the cocoon layer rate still maintained more than 23%, indicating that the new strain had the ability to resist NPV. In summary, these data show that through the selection method described, a new silkworm strain with uniform development, strong resistance and excellent cocoon quality has been successfully obtained.

[0037] II. Resistance test Fluorine tolerance: 237 ppm fluorine-containing mulberry leaves were fed to 2nd instar larvae.

[0038] BmNPV resistance: 3x10 6 PIBs / mL were fed to 2nd instar larvae. High temperature and humidity adaptability: 29±1℃, RH 82±3%.

[0039] The resistance test results of B7 generation 96·NC9R strain are shown in Table 3.

[0040] Table 3 Resistance test results As shown in Table 3, the B7 generation 96·NC9R strain showed excellent and stable performance in three key resistance tests. In the fluorine tolerance test, the survival rate of the strain was stable at more than 85% after feeding 237 ppm high-concentration fluorine-containing mulberry leaves to 2nd instar larvae, showing strong tolerance to fluorine stress. In the BmNPV resistance test, the half lethal concentration (LD50) was greater than 10 8 PIBs / mL, reaching a very high order of magnitude, which indicates that the strain has very strong resistance to silkworm nuclear polyhedrosis virus. In addition, in a high temperature and humidity environment of 29±1℃ and relative humidity of 82±3%, the strain still maintains uniform development and stable survival rate, fully demonstrating its good environmental adaptability and comprehensive resistance advantage. These results together show that the 96·NC9R strain successfully cultivated by the selection method effectively aggregates the excellent traits of fluorine tolerance, virus resistance and adaptation to high temperature and humidity environment.

[0041] III. Cross-infection incidence After disinfection, the surface of the silkworm seat and the silkworm board was smeared with a cotton swab, respectively, and the viral nucleic acid copy number was detected by real-time fluorescent quantitative PCR (qPCR). The results are shown in Table 4.

[0042] Table 4 Viral nucleic acid copy number (log10 copies / cm²) Example 1 used lime powder disinfection but did not perform the brushing penetration step, and the viral residual amount was low but still detectable. Example 2 used the "lime powder spreading + standing + brushing + re-standing" intensified disinfection process, and the viral residual amount was significantly reduced, indicating that the disinfection method effectively controlled the spread of the virus. Comparative Example 1 did not use lime powder disinfection, and the viral residual amount was extremely high, which easily led to cross infection.

[0043] Although the embodiments of the present application have been disclosed as above, it is not limited only to the application listed in the specification and the embodiments, and it can be fully applied to various fields suitable for the present application, and additional modifications can be easily made by those skilled in the art, and thus the present application is not limited to specific details, without departing from the general concept defined by the claims and the equivalent scope.

Claims

1. A method for breeding a silkworm strain resistant to fluoride and anti-hemolytic type of pyemic disease, characterized in that, The method comprises the following steps: S1: crossing a germplasm RF carrying a dominant fluoride-resistant gene with a germplasm RN carrying a dominant gene against Bombyx mori nucleopolyhedrovirus to obtain F1 generation; S2: selecting and keeping fluoride-resistant individuals from F1 generation and backcrossing them with the germplasm RF to obtain BC1 generation; S3: selecting 30 individuals at random from BC1 generation at the 2nd instar to perform a Bombyx mori nucleopolyhedrovirus infection test to detect the retention of the dominant virus-resistant gene, and continuously backcrossing BC1 generation with the germplasm RF for 2 times to obtain B1 generation; S4: self-crossing B1 generation and adopting mixed area feeding and mixed area breeding, and simultaneously performing selection and improvement of fluoride resistance and resistance to Bombyx mori nucleopolyhedrovirus in the generation population; The fluoride resistance screening method is to continuously feed the fluoride-containing mulberry leaves from the 2nd instar or the 3rd instar to the end of the instar, and to select individuals without fluoride poisoning symptoms; The Bombyx mori nucleopolyhedrovirus resistance screening method is to quantitatively feed mulberry leaves impregnated with Bombyx mori nucleopolyhedrovirus at the 4th instar feeding period, to replace the ordinary mulberry leaves after 24 hours, to count the mortality or to eliminate sick silkworm individuals, and to select healthy individuals without infection; S5: recording the instar progress, cluster progress, feeding performance and cocoon quality of silkworms during the whole breeding process, and eliminating bad eggs, weak silkworms, sick silkworms, bad pupae and bad moths, and selecting individuals with uniform development and excellent cocoon quality.

2. The method for breeding the silkworm strain resistant to fluoride and anti-hemolytic type of pyemic disease according to claim 1, wherein The selection and improvement of fluoride resistance and virus resistance are performed in a high-temperature and high-humidity environment, the feeding environment temperature is 28-30℃, the environment humidity is 80%-85%, full coverage rearing is adopted at the 1st-3rd instars, semi-coverage rearing is adopted at the 4th instar, ordinary rearing is adopted at the 5th instar, and feeding is performed 3 times a day.

3. The method for breeding the silkworm strain resistant to fluoride and anti-hemolytic type of pyemic disease according to claim 1, wherein, A sodium fluoride aqueous solution with a concentration of 200-400 ppm is prepared, mulberry leaves are impregnated in the solution for 10-15 minutes, and the fluoride-containing mulberry leaves are obtained after air drying.

4. The method for breeding the silkworm strain of claim 1, wherein the silkworm strain is resistant to fluoride and anti-hemolytic serotype disease. The virus stock solution was diluted to a concentration of 1 x 10 6 5 x 10 6 pieces / mL, and the mulberry leaves were dipped in the solution and dried to obtain mulberry leaves impregnated with the Bombyx mori nuclear polyhedrosis virus.

5. The method for breeding the silkworm strain of claim 1, wherein the silkworm strain is resistant to fluoride and anti-hemolytic serotype disease. When the mulberry leaves impregnated with Bombyx mori nucleopolyhedrovirus are quantitatively fed, 3 pieces of impregnated mulberry leaves with a size of 4 cm×4 cm are fed to every 30 silkworms within 24 hours.

6. The method for breeding the silkworm strain of claim 1, wherein the silkworm strain is resistant to fluoride and anti-hemolytic serotype disease. The germplasm RF is the parent of the variety Gui'can F95 carrying a dominant fluoride-resistant gene, and the germplasm RN is the parent of the variety Gui'can N2 carrying a dominant gene against Bombyx mori nucleopolyhedrovirus.

7. The method for breeding the silkworm strain of claim 1, wherein the silkworm strain is resistant to fluoride and anti-hemolytic serotype disease. During the Bombyx mori nucleopolyhedrovirus resistance screening process, fresh lime powder is uniformly spread on the surface of the silkworm seat immediately after the mulberry leaves impregnated with the virus are fed for 24 hours and eaten up, and the silkworm seat paper is replaced after disinfection, and then normal feeding is started by replacing the ordinary mulberry leaves; The spreading amount of the lime powder is 10-15 grams per 0.1 square meter of silkworm seat area; The used silkworm seat paper needs to be removed together with the residues when the silkworm seat paper is replaced.

8. The method for breeding the silkworm strain of claim 7, wherein the silkworm strain is resistant to fluoride and anti-hemolytic serotype disease. In the step of uniformly spreading fresh lime powder on the surface of the silkworm seat for disinfection, after the spreading is completed, 10-15 minutes of standing is performed, then a soft hairbrush is used to gently sweep along the longitudinal direction of the silkworm seat once, so that the lime powder fully penetrates into the gaps of the silkworm seat, after the sweeping is completed, 5-10 minutes of standing is performed, and then the operation of replacing the clean silkworm seat paper is performed; the length of the bristles of the soft hairbrush is 2-3 cm, and the material is pig bristles or soft nylon.

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