Sexual dimorphism in the cocoon color of Bombyx mori (Bombycidae)

4 Pages·1996·1.7 MB·English

by  V Thiagarajan

350 Journal of the Lepidopterists' Society
mm
versebars outlinedinblackand are only4 inlength. Neitheragentis expectedto ef-
fect complete control ofM.faya because offayas exceptionallyinvasive traits such as its
ability to fix nitrogen, rapid growth rate, and high rate ofseed production (Vitousek &
Walker 1989).
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VITOUSEK, P. M. & L. R. WALKER. 1989. Biological invasion byMyricafaya in Hawaii:
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Rosemary Leen and George Markin, U. S. DepartmentofAgriculture, Forest Ser-
vice, Pacific SouthwestResearch Station, P. O. Box236, Volcano, Hawaii 96785, USA.
Receivedforpublication20March 1995; revisedandaccepted 7January 1996.
journaloftheLepidopterists'Society
50(4), 1996,350-353
SEXUAL DIMORPHISM IN THE COCOON COLOROF
BOMBYXMORI (BOMBYCIDAE)
Additional keywords: pupalcolor,voltinism.
The mulberry silkworm, Bomhyx mori L., is one ofthe most commercially exploited
lepidopteranspecies. Hybridsbetweencommercialsilkwormstrainsarerearedtoproduce
Volume 50, Number 4 351
|BfHlH«BB MLAILGEHTCYOECLOLOONWS FEDMAARLKEYCEOLCLOOOWNS, jIff^ilXVY^1.*\"HQ'fc!.
FlG. 1. Sexualdimorphism incocooncolorinBombyxmori.
cocoons for rawsilkproduction. In the production ofhybrids, sex separation is necessary
toprevent free matingbetween siblings. Inthe silkworm industryin India, sexseparation
is basedon the imaginal spot on the lowersegment ofpupae, whichprovides reasonably
reliable determinations but requires considerable labor (silkworm larvae have Herold's
imaginalbudin males and Ishiwadas glandin females on theventral side ofthe 8th and
9th abdominal segments; similarvisible external characters are known in Saturniidae; see
Milleret al. 1977, Miller& Machotka 1980). Pupal sexingalso requires cuttingopen co-
coons to confirm sex. In the preparation ofsilkworm hybrids, this method ofsex separa-
tionbecomes expensivewhen reciprocalcrosses are notdesired.
Severalstudieshavebeenconductedonsexdeterminationinparentalracesatdifferent
developmental stages in silkworms (Tazima 1978a, 1978b, Abadzhieva & Tanev 1987).
Sengupta (1968) reported sex-limitedcharacters oflarval markings in a Russian silkworm
race. Krishnaswami et al. (1981) observed sexual dimorphism in cocoon color in Hosa
Mysore, a multivoltine silkworm race. Nagaraj and Rao (1987) succeededin introducing
sex-limitedcocoon colorintobivoltine silkworm races CC1 and NB4D2. However, these
characterscouldnotbecommerciallyexploitedowingtothelowsurvivorshipandstability
intheseraces.
Inthepresent study,we attempted to fixabreedingline from abackcross of(Mysore
local x N4) x Mysore Localwith sexual dimorphism in cocoon colorto facilitate the pro-
cess ofhybrid silkworm seedproductionin commercial grainages with minimalwastage.
Segregation oflines was made after the F3 generation. In one ofthe sub-lines cocoon
colorwas found to exhibit sexual dimorphism. Studies ofthe sexual dimorphism began
with random cocoon samplingfrom fivebatches. Lightyellowanddarkyellowcoloredco-
coons were separatedvisuallyinto groups. These cocoonswere then cut open and sexed
onthebasis ofpupalmarkings. Thisattempttoclassifycocconswas repeatedfourtimes.
In an isolated sub-line from the threewaycross of(Mysore local x N4) x Mysore Lo-
cal, sexual dimorphism in cocoon colorwas observed in the F12 generation (Fig. 1; male
cocoons are lightyellowin color, whereas female cocoons are darkyellow). Invisuallyse-
lectedlightyellowcocoons 96.5% were male and in darkyellowcocoons 92.5% were fe-
352 Journal of the Lepidopterists' Society
TABLE 1. Results ofattempts to segregate Bombyx mori cocoons bysexusingcocoon
color.
CocoonColor SampleSize Replications PercentMale PercentFemale
lightyellow 400 4 96.5 ± 4.7 3.5 ± 4.7
darkyellow 400 4 7.5 ± 9.6 92.5 ± 9.6
male(Table 1).Thefemalecocoonsweremoreelongate,lessflossyandslightlylargerthan
the malecocoons. Slightlyoval, spindle-shapedcocoonswerecommoninbothsexes.
Tazima (1978a) has explained the phenomenon ofdifference in cocoon colorbetween
sexes and reported that cocoon colorin most ofthe colored cocoon producing silkworm
racesisdirectlyrelatedtohemolymphcolor,exceptinsome Europeanraceswheretheex-
pression ofcocoon coloris influenced by an inhibitor gene. The presence ofthe yellow
blood gene (Y) in silkworms is responsible foryellowcolorin cocoons. The allele +C in-
hibits transmissionofpigmentsinto anypartofthe middle silkgland, resultinginproduc-
tion ofwhite cocoons in some silkworm genotypes. Presence ofY and +C alleles in tan-
dem is stated to be responsible forintermediate color in cocoons and polymorphism in
hemolymph color (Tazima 1978a; Ford 1975 reported that hemolymph color is always
darkerin females thanin males ofChoristoneurapinus (Tortricidae)).
Itisinterestingtonotethatthe coloredcocoonproducingraces ofBombyxmori in In-
dia, such as Nistari (golden yellow) and Mysore Local (greenish yellow), do not exhibit
clear sexual dimorphism in cocoon color. However, use ofthese in the formation ofnew
racesthroughhybridizationresultedinsexualdimorphismintheisolatedsub-lines (Krish-
naswamy et al. 1981, Nagaraj & Rao 1987, Singh et al. 1992). Similarly, in the present
study, two parental races (Mysore Local multivoltine race with greenish yellowcocoon,
and N4bivoltine racewithwhite cocoon)were combinedin athreewaycross toobtain a
breedinglinewith sexual dimorphism incocooncolor. The relativeimportance ofgenetic
and environmental factors in the expression ofsexual dimorphism in cocoon colorcould
notbe determinedin the present study. Furtherworkis ongoingto obtain detailedinfor-
mationon sexualdimorphism andits relationtoclimaticandotherecologicalconditions.
We thank S. Sekar, stenographer of the Regional Sericultural Research Station,
Coonoor fortypingthis manuscript andto the technical staffwho assistedinthe comple-
tionofthis study.
Literature Cited
Abadzhieva, R. & G. Tanev. 1987. Agenetic markerforsexinthe silkworm. Genet. Sel.
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SENGUPTA, K. 1968. Sexlinked (sexlimited) characters intwo races ofsilkworm Bombyx
mori L. maintainedbythe Central Silkworm Seed Station, Srinagarand the method
Singfho,riKts.,trTa.nsPf.erS.toCohtaheurhraances&foMreceomrmaerVceiramlae.xpl1o9i9t2a.tioSne.xuIanldidanimJ.orSperhiicsulm.i7n:7c9o-c85o.on
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Volume 50, Number 4 353
Tazima, Y. 1978a. The silkworm - an important laboratory tool. Kodamsha Ltd., Japan.
307pp.
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V. Thiagarajan, T P. S. Chauhan and E. Rajalakshmi, Regional Sericultural Re-
search Station, Central Silk Board, Coonoor-643 101, TamilNadu, India.
Receivedforpublication 30June 1994; revisedandaccepted 7January 1996.