Abstract: The present study aims at shedding more light to larval duration and development of silkworm lines from Iranian silkworm gene bank and comparison of the results using statistical models for selection of the superior strains. Feeding and other conditions of larval rearing were conducted following the standard procedure and all germplasm strains were reared under standards protocols in all rearing steps. From obtained results, it is showed that the larval duration of the 101 (608.000 h), 5118x10133-3-3 (588.670 h), 307-300-2 (584.000 h), 105 (584.000 h) and 31 (584.000 h) strains remained significantly at upper level than other strains, respectively. The feeding larval duration in B2-09 (574.000 h), N19 (533.000 h), 1433-9 (525.000 h), BH-2 (517.330 h) and 1433-15 (511.330 h) strains increased significantly in comparison with other strains. Molting larval duration remained significantly at upper level in I 20 (197.670 h), 107-K (113.000 h), Black Larvae-White Cocoon (104.000 h), 101(104.000 h) and Shaki (103.000 h) increased significantly in comparison with other strains. From obtained results, it is showed the 1-3 instars larval duration of the Black-White (292.670 h), 101 (290.000 h), 1003-5 (288.670 h), 101xF6 (286.000 h) and 31 (286.000 h) strains remained significantly at upper level than other strains, respectively. Totally, 7409 (577.881), Black Larvae-White Cocoon (577.508), 236 (570.769), M-1-2(5) (568.583) and T5-M (566.602) showed higher evaluation index values. Also, 7409 (5.374), 236 (5.267), T5-M (5.183), 113-K (5.163) and White Larvae-Yellow Cocoon (5.027) showed higher sub-ordinate function values.
INTRODUCTION
The silkworm Bombyx mori is a domesticated insect reared by the farmers to produce silk. Sericulture is an agro-based cottage industry, which provides substantial income to the farmers and helps to produce high-quality raw silk (Velu et al., 2008).
Organization and maintenance of silkworm genetic resources as germplasm has become very important for meeting the desired objectives of the breeder for immediate or long-term utilization in silkworm breeding programmes. But, it is necessary to maintain them in their original form for their rational use in different breeding and other research purposes (Mukarjee et al., 1999; Basavaraja et al., 2003; Thangavelu et al., 2003; Yamaguchi, 2003; Rao et al., 2006).
Owing to the long history of sericulture practice and wide diversity of geographical conditions, there is a very rich resource of silkworm germplasm bank in Iran. At present there are 51 strains preserved in the Iran Silkworm Research Center (ISRC). As the progress in silkworm germplasm collection and investigation, many of the strains are used in silkworm breeding for commercial activity and played significant role in the advance of commercial silkworm varieties (Sohn, 2003).
As Zanatta et al. (2009) stated an extensive study is needed to improve existing lines for commercial purposes and to develop new strains through silkworm breeding programs aimed at improving silk productivity (Sen et al., 1999; Li et al., 2001). Several studies related to the use of productivity markers and morphological dissimilarity (Zanatta et al., 2009) as indicators of the best lines for breeding.
Iran Silkworm Research Center (ISRC), Rasht, Iran holds 51 silkworm varieties. These silkworm varieties show wide diversity in the phenotypic parameters. There is currently no immediately accessible data on peanut cocoon strains of Iranian silkworm germplasm. Therefore, the present study aims at shedding more light to larval duration and development of silkworm lines from Iranian silkworm gene bank and comparison of the results using statistical models for selection of the superior strains.
MATERIALS AND METHODS
This study was conducted in Islamic Azad University, Ghaemshahr Branch, Iran and Iran Silkworm Research Center from December 2008 till December 2009. Fifty one silkworm strains were used in the present study. These strains included 107-K, 119-K, 113-K, 105, 31, 51, 103, BH-2, B2-09, 1003-4, 1003-5, 1005, M2-6-22-2, M2-6-18(109), M-1-2(5), M2-6-22(107), M2-6-18.3, 307-300-2, 202A-204B, I 20, 101433-9-5, 101433-1-4, 101433-6-6, 1126 (111), 113 (2029), 151 (103xM-1-1), Xihang 2.3, Xihang 3.3, 153 (Xihang-1), 5118x10133-2-2, 5118x10133-3-3, Black-White, 101xF6, F6x101, Kinshu, M-1-1x31, 31xM-1-1, M-1-1x103, 103 Poly Marking, Shaki, 101, T1-J, T5-M, 236, 1524, 1433-15, 1433-9, 7409, N19, White Larvae-Yellow Cocoon and Black Larvae-White Cocoon.
All silkworm germplasm rearing steps including egg, larvae, pupae and moth cycles were conducted at Iran Silkworm Research Center (ISRC) before this study as annual and routine germplasm conservation program. Their silkworm rearing technique included single batch rearing system. Feeding and other conditions of larval rearing were conducted following the standard procedure (ESCAP, 1993) and all germplasm strains were reared under standards protocols in all rearing steps. After hatching from the eggs, neonates were brushed and reared up separately on fresh leaves of mulberry (Morus alba). One-day-old 1st instar larvae from all strains were reared for experiment. Individual laying were prepared for each strain before rearing and each individual laying consisted of about 500 eggs taken from one disease free laying and decreased to 250 larvae at beginning 4th instar. The silkworm eggs had incubated in the controlled environment chamber. When there were 95% of eggs having little black dots on the surface of eggs, they were shaded with black gobo to prevent the light irradiation for about 48 h for making the larvae emerge form the eggs at one time. After most of them hatched, the silkworm larvae were fed on leaves of mulberry. Brushing was done carefully. The batches of 500 silkworm larvae were reared. The young larvae (1st-3rd instars) were reared at 27-28°C with 85-90% relative humidity and the late age larvae (4th and 5th instars) were maintained at 24-26°C with a relative humidity of 70-80%. The larvae were fed ad libitum mulberry leaves three times a day. Studied quantitative characteristics included larval duration (h), feeding larval duration (h), molting larval duration (h), 1-3 instars larval duration (h), 1-3 instars feeding larval duration (h), 1-3 instars molting larval duration (h), 4-5 instars larval duration (h), 4-5 instars feeding larval duration (h), 4-5 instars molting larval duration (h), 5 instar feeding larval duration (h) and cocoon spinning duration (h).
It was used for data analyzing from CRD model, GLM approach and SAS software (SAS, 1977). Under model was used for data analyzing for each strain: yij = μ + Gi + eij which yij was record or observation from trait, μ was trait average, Gi was group effect (strain) and eij was residual effects. Furthermore, it was used appropriate transformation like angle transformation for those data which did not followed by normal distribution. The DNMRT method was used for average compares (Duncan, 1951).
Also, evaluation index value and sub-ordinate function value were calculated for nutritional indices. Evaluation Index (EI) value for silkworm strains performance were calculated by using the following equation (Mano et al., 1993; Rao et al., 2006):
EI = [(A-B)/C]x10+50 |
where, A is mean of the particular trait in a strain; B is overall mean of particular trait in all strains; C is standard deviation of a trait in all strains; 50 is constant.
Sub-ordinate function is calculated by utilizing the following equation based on Gower (1971) and Rao et al. (2006):
Xu = (Xi-Xmin)/(Xmax-Xmin) |
where, Xu is sub-ordinate function; Xi is measurement of trait of tested strain; Xmin is minimum value of the trait among all the tested strains; Xmax is maximum value of the trait among all the tested strains.
The evaluation index (Table 2) and sub-ordinate function values (Table 3) for the all traits were calculated separately and average index value was obtained. Then studied silkworm strains are ranked based on average of evaluation index method and sub-ordinate function method (Table 4).
RESULTS AND DISCUSSION
From the obtained results, it was clear that different strains of silkworm Bombyx mori showed different performance based on larval development duration. The analysis of variance regarding to studied traits, showed that different strains have significant different for traits (p<0.01).
Based on Table 1 it is showed the larval duration of the 101 (608.000 h), 5118x10133-3-3 (588.670 h), 307-300-2 (584.000 h), 105 (584.000 h) and 31 (584.000 h) strains remained significantly at upper level than other strains respectively. The feeding larval duration in B2-09 (574.000 h), N19 (533.000 h), 1433-9 (525.000 h), BH-2 (517.330 h) and 1433-15 (511.330 h) strains increased significantly in comparison with other strains. Molting larval duration remained significantly at upper level in I 20 (197.670 h), 107-K (113.000 h), Black Larvae-White Cocoon (104.000 h), 101 (104.000 h) and Shaki (103.000 h) increased significantly in comparison with other strains. From obtained results, it is showed the 1-3 instars larval duration of the Black-White (292.670 h), 101 (290.000 h), 1003-5 (288.670 h), 101xF6 (286.000 h) and 31 (286.000 h) strains remained significantly at upper level than other strains, respectively (Table 1).
The 1-3 instars feeding larval duration in 105 (232.330 h), 101433-1-4 (232.000 h), 1003-4 (231.670 h), 236 (231.330 h) and 5118x10133-3-3 (230.330 h) strains increased significantly in comparison with other strains. The 1-3 instars molting larval duration remained significantly at upper level in the 107-K (81.000 h), 1126 [111] (72.000 h), 101xF6 (70.333 h), BH-2 (69.667 h) and 31 (69.000 h) increased significantly in comparison with other strains (Table 1).
From obtained results, it is showed the 4-5 instars larval duration of the 101 (318.000 h), 1005 (312.000 h), N19 (312.000 h), Black Larvae-White Cocoon (310.000 h) and 307-300-2 (308.000 h) strains remained significantly at upper level than other strains, respectively. The 4-5 instars feeding larval duration in N19 (303.000 h), 1433-9 (297.000 h), M2-6-22-2 (288.000 h), 51 (288.000 h) and 307-300-2 (284.000 h) strains increased significantly in comparison with other strains. The 4-5 instars molting larval duration remained significantly at upper level in the 101 (41.000 h), 1005 (39.000 h), Black Larvae-White Cocoon (39.000 h), 5118x10133-2-2 (34.333 h) and White Larvae-Yellow Cocoon (33.667 h) increased significantly in comparison with other strains. From obtained results, it is showed the 5 instar feeding larval duration of the N19 (193.000 h), T1-J (187.000 h), M-1-2(5) (187.000 h), 1524 (187.000 h) and 1433-9 (187.000 h) strains remained significantly at upper level than other strains, respectively. The cocoon spinning duration in I 20 (21.000 h), N19 (19.333 h), 107-K (17.333 h), 51 (12.000 h) and M2-6-22-2 (11.000 h) strains increased significantly in comparison with other strains (Table 1).
Also, based on larval development of strains were assessed on different parameters including larval duration, feeding larval duration, molting larval duration, 1-3 instars larval duration, 1-3 instars feeding larval duration, 1-3 instars molting larval duration, 4-5 instars larval duration, 4-5 instars feeding larval duration, 4-5 instars molting larval duration, 5 instars feeding larval duration and cocoon spinning duration. Recorded characteristics of larval development using the evaluation index (Table 2, 4) and sub-ordinate function (Table 3, 4) methods and the details are as follows.
Based on Table 2 among germplasm strains, as per the evaluation index method, the all strains had equal score values for larval duration (40.230) and feeding larval duration (40.230) and 1-3 instars feeding larval duration (49.080), 4-5 instars larval duration (68.742), 4-5 instars feeding larval duration (51.999).
Meanwhile, as per the evaluation index method, the strains N19 (79.279), 1433-9 (75.646), M2-6-22-2 (61.113), 101 (61.113) and 1433-15 (61.113) showed higher evaluation index values for molting larval duration. Among germplasm strains, as per the evaluation index method, the strains 119-K (56.234), 51 (56.234), 103 (56.234), BH|-|2 (56.234) and 31 (56.234) showed higher evaluation index values for 1-3 instars larval duration. Meanwhile, as per the evaluation index method, the strains 7409 (65.115), 1433-15 (63.031), 1433-9 (63.031), 1524 (60.948) and 105 (56.781) showed higher evaluation index values for 1-3 instars molting larval duration (Table 2). Meanwhile, as per the evaluation index method, the strains 1433-9 (80.484), N19 (80.484), M2-6-22-2 (62.832), T5-M (54.889) and 236 (54.889) showed higher evaluation index values for 4-5 instars molting larval duration. Among germplasm strains, as per the evaluation index method, the strains 113-K (74.323), White Larvae-Yellow Cocoon (74.323), 107-K (52.936), 119-K (52.936) and 105 (52.936) showed higher evaluation index values for 5 instar feeding larval duration. Also, as per the evaluation index method, the strains M-1-2[5] (110.853), 31 (76.430), 101xF6 (76.430), 1005 (72.267) and M2-6-18.3 (70.346) showed higher evaluation index values for and cocoon spinning duration (Table 2).
| Table 1: | Mean±SD performance of larval traits in studied silkworm pure lines of gene bank |
| Means in each column followed by the same letters superscripted are not significantly different at α=0.01 | |
| Table 2: | Evaluation index values for larval traits in studied silkworm pure lines of gene bank |
| Table 3: | Sub-ordinate function values for larval traits in studied silkworm pure lines of gene bank |
| Table 4: | Ranking of studied silkworm germplasm based on average of evaluation index method and sub-ordinate function method for larval traits |
Totally, 7409 (577.881), Black Larvae-White Cocoon (577.508), 236 (570.769), M-1-2(5) (568.583) and T5-M (566.602) showed higher evaluation index values (Table 4).
Based on Table 3 among germplasm strains, as per the sub-ordinate function method, the all strains had equal score values for larval duration (0.000), feeding larval duration (0.000), 1-3 instars larval duration (1.000), 1-3 instars feeding larval duration (0.000), 4-5 instars larval duration (1.000), 4-5 instars feeding larval duration (1.000) (Table 3).
Meanwhile, as per the sub-ordinate function method, the strains N19 (1.000), 1433-9 (0.933), M2-6-22-2 (0.667), 1433-15 (0.667) and 7409 (0.667) showed higher sub-ordinate function values for molting larval duration (Table 3). Meanwhile, as per the sub-ordinate function method, the strains 7409 (1.000), 1433-15 (0.958), 1433-9 (0.958), 1524 (0.917) and 105 (0.833) showed higher sub-ordinate function values for 1-3 instars molting larval duration (Table 3). Meanwhile, as per the sub-ordinate function method, the strains N19 (1.000), M2-6-22-2 (0.540), T5-M (0.333), 236 (0.333) and White Larvae-Yellow Cocoon (0.069) showed higher sub-ordinate function values for 4-5 instars molting larval duration (Table 3). Among germplasm strains, as per the sub-ordinate function method, the strains 113-K (1.000), White Larvae-Yellow Cocoon (1.000), 107-K (0.500), 119-K (0.500) and 105 (0.500) showed higher sub-ordinate function values for 5 instar feeding larval duration (Table 3). Also, as per the sub-ordinate function method, the strains M-1-2[5] (1.000), 31 (0.499), 101xF6 (0.499), 1005 (0.438) and M2-6-18[109] (0.410) showed higher sub-ordinate function values for and cocoon spinning duration (Table 3).
Based on Table 4 totally, 7409 (5.374), 236 (5.267), T5-M (5.183), 113-K (5.163) and White Larvae-Yellow Cocoon (5.027) showed higher sub-ordinate function values (Table 4).
The results on germplasm evaluation of the different silkworm strains tested in the present study indicate genotype significant effects on performance evaluation. To date there is not report regarding investigation and assessment of larval development duration of Iranian peanut germplasm silkworm strains using evaluation index method and sub-ordinate function method. Hence, it can claim this report is the first report regarding application of these methods for comparison of larval development traits in Iran silkworm germplasm.
The obtained results relate to earlier findings and supported many previous reports regarding performance differences of various silkworm strains. For example, Ramesha et al. (2009) evaluated various silkworm strains and stated selection of suitable parents and information on nature and magnitude of gene action of traits of economic importance determine the success of any crop. They believed critical assessment of variability present in the breeding materials is one of the pre-requisites for paving the way of combining most of the desirable traits present in different genotypes into a single hybrid combination. However, the per se performance of parental breeds is not always be the good reflection of the combining ability and its analysis therefore helps the breeders to understand the nature of gene action to identify prospective parents/hybrids (Ramesha et al., 2009).
Enguku et al. (2007) also compared performance of various silkworm strains in germplasm. Meanwhile Malik et al. (2005) evaluated some silkworm strains and stated there are different performance among various strains.
Of course, our findings added to data also, since, there is not any report regarding Iranian peanut silkworm germplasm to date.
Most of the quantitative traits of commercial importance in silkworm are under complicated polygenic control under the influence of the environment (Rao et al., 2006). For synthesizing the potential polyvoltine cross breeds, usually, the high yielding traits of bivoltine varieties and fitness traits of strains are hybridized as proper selection of potential and homozygous parents is very important (Rao et al., 2006).
As Kumaresan et al. (2007) presented there is an optimum level of genetic divergence between parents to obtain heterosis in F1 generation and it may not be logical to advocate the use of extreme diverge parents to obtain heterotic combination (Arunachalam et al., 1984; Kumaresan et al., 2007).
As Mirhoseini et al. (2004) stated the cocoon characteristics are important economical characteristics of silkworm and due to their high heredity, the efficiency of direct selection of them is very high. Efficiency of heterosis in the improvement of the mean of cocoon characteristics in the hybrids will be manifold than the inter-strain selections.
Other reports clarified the undeniable role of heterosis in the technology of silkworm egg production. As a result, the better hybrid must be determined from adding the amounts of the heterosis of the characteristics related to cocoon and resistance and with using other information like GCA and SCA, evolvement of appropriate maternal bases to produce commercial silkworm eggs could be conducted (Mirhoseini et al., 2004).
ACKNOWLEDGMENTS
This manuscript is obtained from MSc. Thesis of Morteza Salehi Nezhad at Islamic Azad University, Ghaemshahr Branch, Ghaemshahr, Iran. We are grateful to the Iran Silkworm Research Centre for providing silkworm data and Islamic Azad University, Ghaemshahr Branch, Iran for support. This work was supported and financed by the Iran Silkworm Research Center (ISRC) mainly. We thank Mrs. K. Taieb Naeemi, Mr. M. Naserani and Mr. Y. Kheirkhah for technical assistances.