Fine structure of the egg chorion in two anisopteran dragonflies from central India (Libellulidae)

2009·1.6 MB·English

by  Andrew, R J

Odonatologica38(4):359-363 December I,2009
SHORTCOMMUNICATIONS
Finestructure of theegg chorion intwo
anisopteran dragonfliesfromcentral India
(Libellulidae)
R.J.Andrew
Post-GraduateDepartmentof Zoology,HislopCollege,Nagpur440001,India
[email protected]
ReceivedJune13,2009 / RevisedandAcceptedJuly18,2009
The fine structure ofthe egg chorion in Brachydiplax sobrina(Ramb.) and Or-
thetrum s. sabina (Dru.),is described usingthe scanningelectron microscope. The
unwettedeggsofB.sobrinaarebluish-green andspindle-shapedwhile thoseofO.s.
sabina areoval andlightbrown incolour. Theeggchorion isdistinctlydivided into
anouterexochorion andaninnertoughendochorion. Theexochorionexpandsinto
athick,sticky,jelly-likestructure inwaterduringoviposition,whereastheendocho-
rion remains unchanged.Theendochorion isthinand smoothinO.s.sabina,but in
B. sobrina the undersurface ofthe endochorion ispittedand rough.Theapicalmi-
cropylarapparatusiscomposedofaspermstoragechamber(atrium)and amedian
projectingstalk,which possessesapairofsub-terminal orifices. The atriumin B.so-
brina isdomeshapedwith atinystalk whereasin O. s.sabina themicropylarappa-
ratus istriangularwith a longerstalk and apairofalmostapicallyplacedorifices.
Significantvariations occurinthe shapeand sizeofthemicropylarapparatus.The
functional interrelationshipofthemicro morphologicalmodifications in thechori-
onicstructuresisdiscussed.
INTRODUCTION
Scanning electron microscopic studies ofthe insectegg chorion haveyielded
importantinformationofgreattaxonomicandfunctionalsignificance (HINTON,
1981; MARGARITIS, 1985).InOdonata, however,such studiesare confinedto
only afewspecies(MILLER, 1987; IVEY,et. al.,1988;BECNEL&DUNKLE,
1990;TRUEMAN, 1990; 1991;SAHLEN,1995; MAY,1995), including fivefrom
India(ANDREW & TEMBHARE, 1992,95,96; ANDREW,2002;ANDREW
et. ah, 2006).
360 R.J.Andrew
Thepresentworkwas undertakentostudythefinestructureofpost-oviposition
changes in the eggchorionof the anisopteran, Brachydiplax sobrina (Rambur)
and Orthetrums. sabina(Dru.).
MATERIALANDMETHODS
Egg-layingfemaledragonflies werecollected duringthepost-monsoonperiod(July-Sept, 2002-
2005).The unwetted fertilizedeggs werecollected fromthe sub-genitalplatebyholdingthe wings
flatand mimickingthedippingaction orbypassinganeedle throughthethoraxwhichinitiated egg
shedding.Wet eggs wereobtained byplacingthe abdominal tipin watercontainers toinitiate egg
shedding(ANDREW&TEMBHARE, 1995).Someeggswerealsoobtainedbydissectingtheovi-
ductand vagina.Theexochorion ofafeweggs wasremoved usingfinetippedforcepsor 10%KOH
treatment(TRUEMAN, 1991).Theeggswereprocessed forSEMbythemethodsdescribedbyAN-
DREW&TEMBHARE(1992,1995).
RESULTS
BRACHYDIPLAXSOBRINA(Figs 1-4). - Theunwettedeggscollectedfrom
thesubgenital plate of thefemaleare small, bluish-green in colourand spindle
shaped with lineardimensionsof360±20 x230± 15pm. Inwater,theexocho-
rionswells intoaspongy,sticky, jelly-like structure andtheeggs nowmeasured
370±20 x270± 15pm.Theendochorionis tough and4.5pmthick. Its under-
surfaceispittedandrough. Themicropylarapparatusiswelldifferentiatedfrom
theexochorionby acircular depression(Figs 1,3)whichcorresponds tothepedi-
celoftheendochorion(Fig. 4).Themicropylarapparatusiscomposed ofa very
large dome-shaped spermstoragechamber(atrium)about55pmindiameterand
a micropylar stalk whichis flatandshort(10 pm), withapair of large (7pm in
diameter)orifices(Fig. 3). Theendochorioniccircular pedicel iswell developed.
Itisridged witha central humpandis locatedjust belowthemicropylar appara-
tus(Fig. 4).
ORTHETRUM SABINA SABINA (Figs 5-8). The unwetted fertilizedeggs
areoval, pale brownandlightly sticky withlineardimensionsof 370± 25 x260
± 15pm. Inwater,the exochorionswells intoa spongy,sticky structureand the
dimension ofthe eggchanged to 385 ± 20 x 310 ± 15 pm.The endochorionis
smooth andis5 ±2 pm thick(Figs 6, 7). Itisdevoidofarticulationsand bears
atinyapical pedicel. Themicropylarapparatusistriangular, nipple shaped, with
a diameterof38pm at thebase. Themicropylarstalk is 30pm long witha pair
of circularalmost apical orificeswhich are5.5pmin diameter(Figs 5, 8).
DISCUSSION
ThelibellulidsB.sobrinaand O. s.sabinaexhibit anexophytic modeofoviposi-
tion wheretheeggs duringoviposition extrudefromthegenital openingandac-
cumulateundertheninthabdominalsternum onthesub-genital plate andarethen
Eggchorion in Brachydiplaxsobrina and Orthetrumsabina 361
washed-offby flicking the abdomi-
naltipinthewater,asreported fora
large numberof dragonflies (COR-
BET, 1999). In libellulidsthat ex-
hibitanexophytic modeofoviposi-
tion,theeggsaregenerallyoval/sub-
spherical to facilitatequick sinking
to avoid being consumedby fishes
andotheraquaticanimals.Further-
more, theseeggs can easily settlein
smallrecesses inthesubstrateofthe
water body (CORBET, 1962). The
exochorionof thelibellulidsB.sob-
rinaandO. s.sabinaexhibitstypical
libellulidfinemorphological chang-
es inwater by swelling intoasticky
spongyjelly-likemass(TRUEMAN,
1991; SAHLEN, 1995;ANDREW
&TEMBHARE, 1996;ANDREW,
2002). Theexpanded thick exocho-
rion protects the growing embryo
andfirmlyanchorstheegg.Itfurther Figs 1-4.SEM micrographsof Brachydiplaxsobrina
providesextendedareasfortinypar- (Rambur): (I) clusterof unwetted spindle-shaped
ticles to stick to it and formapro- eggs showingtheapical micropylarapparatus; —
tectivecamouflage. Theswelling in (2)fractured section oftheeggshowingthepitted
undersurface oftheendochorion; - (3)side view
B. sobrina is comparative less then ofthemicropylarapparatusshowingthemicropylar
that observed in O. s. sabina. This orifice onashort micropylarstalk; notethe dome-
is probably dueto differencesinthe shapedspermstoragechamber;- (4) eggwithout
waterbody usedforoviposition. O. exochorion showingtheridged,conspicuous,circular
pedicelwith aprominentcentralhump.
s.sabinaoviposits inshallowstillwa-
terorslow movingroadsidewater puddles whereasB.sobrinaoviposits inmuch
deeper water bodies(MITRA2006). SAHLEN(1995) observed thationsinthe
water and overwintering ofeggsalsoaffectthe degreeofexpansion oftheexo-
chorion.
Theendochorionofexophytic eggs isgenerally unsculptured as foundin O. s.
sabina, buttheundersurfaceinB. sobrinaispitted andrough. Asculptured out-
er surfaceofthe endochorionhas been reported inLadonadeplanta (IVEY et.
al., 1988)and Bradinopyga geminata (ANDREW & TEMBHARE, 1996). The
pedicel ofthe endochorionis well developed in both species studied but in B.
sobrinaitis projecting andheavily ridgedwithacentralhump, as foundin R. v.
variegata (ANDREW & TEMBHARE, 1994). Anendochorionicpedicle isob-
servedinmostofthelibellulideggsexaminedundertheSEM(IVEY et.al., 1988;
362 R.J.Andrew
TRUEMAN, 1991; ANDREW &
TEMBHARE, 1996) although
SAHLEN(1995) didnotobservethe
pedicel whilestudying libellulideggs
undertheSEM.
SEMstudiesofthemicropylarap-
paratushavedisclosedanevolution-
ary pattern in Anisoptera, since a
progressive reductioninthenumber
ofmicropylarorificesisnoticedfrom
14inPelaturidaeto2inLibellulidae.
This reductionrequires less sperm
material for fertilization and accel-
eratestherateofoviposition (IVEY,
et.al., 1988).Themicropylar appa-
ratus is also used as an important
taxonomictool in theclassification
ofcloselyrelatedspecies ofodonates
(MAY, 1995).
Inthe present study, the dragon-
fliesexhibita typical ‘libellulid’mi-
Figs 5-8. SEM micrographsof Orthetrum s.sabina
cropylar apparatus (TRUEMAN,
(Drury): (5)wetted eggshowingexpandedstickyex-
ochorion and anterior micropylarapparatus; — (6) 1991), but the large dome-shaped
unwetted eggwith theexochorion peeledoffshow- chamberandhumped pedicel of B.
ingthesmoothendochorion;- (7)fracturedsection sobrinaissimilarto thatofR. v. var-
showingthethin,toughendochorion;— (8)micro- iegata. The circular depression of
pylar apparatus with atriangularstalk and almost exochorion around the micropylar
apicalmicropylarorifice.
apparatusiswell developedinB. so-
hrinaand a similar depression is foundin theeggs of R . v. variegata and Tra-
mea virginia(ANDREW& TEMSHARE, 1994;ANDREW,2002). Theflatand
short micropylar stalk ofB. sohrinais also unusual sincemostofthe lihellulids
(excluding T. virginia)andO. s.sabinahavecylindrical stalks (BECNEL& DUN-
KLE, 1990;SAHLEN, 1995;MAY, 1995;ANDREW & TEMBHARE, 1996;
ANDREW, 2002).Themicropylar stalkismodifiedinaccordance withtheshape
andsize ofthefertilizationporeofthevagina forrapidandefficientfertilization,
as foundinthelibellulid. T. Virginia(ANDREW &TEMBHARE, 1994).
Inalmostallodonates, theeggcolourvariesfromyellowish browntodarkbrown
toblack. K. Inoue(pers. comm.) observedbluishgreeneggsof Macrodiplaxcora
at GuamIsland(USA) in thePacific Oceanand Hydrobasileus croceus atIshi-
gaki Islandin Ryukyu Archipelago, Okinawaprefecture, Japan. ThusB. sohrina
isthethirdodonatespecies intheworldtoexhibitbluishgreeneggsand thefirst
fromtheorientalregion.
EggchorioninBrachydiplaxsobrina and Orthetrumsabina 363
ACKNOWLEDGEMENT
Partial financial supportfromthe UniversityGrantsCommission,Pune,Indiaunder theMinor
Research Projectschemeisherebyacknowledged.
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