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):
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.
Subscribe Today
