Structure and function of male genitalia in Libellula, Ladona and Plathemis (Anisoptera: Libellulidae)

15 Pages·2001·3.6 MB·English

by  Artiss, T

Odonatologica30(1): 13-27 March 1, 2001
Structure and function of male genitalia
in Libellula, Ladonaand Plathemis
(Anisoptera: Libellulidae)
T. Artiss*
Department ofBiology, Clark University, 950 Main Street,
Worcester, MA 01610-1477,United States
Received May 12, 2000 /Revised and AcceptedJuly28, 2000
Malegenitaliaof25 spp. are studied using scanningelectron microscopy, and
the structure of the fourth penile segment is described. Remarkable diversity
exists amongspp.,particularly in the size and shapeofthe lateral lobes, and the
morphology ofthe cornua.There are also differences in the number of cornua
among thetaxa surveyed. The surface ofthe lobes ofmany spp. is covered with
spines which anchor the penis during copulation, and may trap and remove
sperm when thepenis is collapsedand withdrawn following copulation.Spp. are
categorized according to the morphologyof the penis and inferred patterns of
sperm removal. Type 1 taxa possess relativelylarge,broad, flat lateral lobes, and
lack cornua, orpossess cornuathat are greatlyreduced in size. These spp. are
believed to displace sperm in thebursa copulatrix before depositing their own
sperm, therebygainingpositionalpriority duringoviposition.Type 2 spp.possess
elongated lateral lobes and/or cornua.These taxa are believed to engage in a
mixed strategy of sperm displacementand sperm removal. Optimization ofthese
characters on a phylogeny of the 3 genera indicates that the Type 1 sperm
displacementstrategy isancestral, and thattheType2 strategy was subsequently
derived within the majority oftheLibellulas.s. taxa.
INTRODUCTION
Among sexually reproducing organisms, there exists remarkablediversity of
intromittentstructures,evenamongclosely relatedtaxa (EBERHARD,1985).The
lock and key hypothesis, which holds that maleand femalegenitalia evolve to
ensure amechanicalfitreducing therisk ofhybridization, haslong beenusedto
*Current address and addtess for correspondence: Lakeside School, 14050 Is'Avenue NE,
Seattle,WA 98125-3099,United States; — e-mail: [email protected]
14 T. Artiss
explain genitalic diversity(e.g.PAULSON, 1974).However,EBERHARD (1985)
argues that there is littleevidence for lock and key reproductive isolation, and
insteadsuggests thatmalegenitalia are underintensesexual selectionby females.
The cryptic female choice hypothesis maintains that females may exercise
copulatory and post-copulatory mate choicedecisions based on themorphology
ofthe male genitalia. This hypothesis is intriguing, andthere have been a few
recent attempts to test its validity (ARNQVIST, 1997;TADLER, 1999).Another
alternativeis that genitalia may beevolving in response to spermcompetition to
manipulate orremove spermfrom previous copulations more effectively.
Odonategenitaliaare thoughttohaveevolvedrapidlyanddivergently inresponse
tointensespermcompetition. Thestructureandfunctionofodonategenitaliaand
intromittentorgans have been widely studied in males (PFAU, 1971;RESTIFO,
1972; MILLER, 1984, 1991, 1995;SIVA-JOTHY, 1984;WAAGE, 1979, 1984,
1986a)andfemales(MILLER,1984, 1988;SIVA-JOTHY, 1987).Themalepenis in
theLibellulidaeisafour-segmented structurewiththefourthdistalsegmentofthe
penis often bearing hooks, bristles, spines and inflatablestructures (MILLER,
1982).The femalegenital tractconsists ofalarge sac-like bursa copulatrix and
paired spermathecae (MILLER, 1982). The bursa is ventrally connected to the
vagina and oviducts by a valved opening. WAAGE (1979) first suggested that
malegenitaliahadtwo distinctfunctionsinodonates; spermtransfer,andremoval
ordisplacement of storedspermfromprevious copulations. Researchonanumber
ofodonate taxa hassubsequently confirmedthat male genitalia are theprincipal
organsof spermcompetition, andthatspermprecedence is greatestfor malesthat
are lastto mate with females priorto oviposition (FINCKE, 1984;McVEY &
SMITTLE, 1984;WOLFetal., 1989;M1CHIELS, 1992;HADRYSetal„ 1993).
The dragonfly family Libellulidaehas been the subject ofseveral studies of
male genitalia. KENNEDY (1922a, 1922b)surveyed the morphology of male
genitalia inthegenusLibellula, and inferredrelationships basedonsimilaritiesin
morphology. This genuswas examinedagain by RESTIFO(1972), who described
the genitalia in greater detail. MILLER (1991) examined the genitalia and
reproductive behaviorofseveral species of libelluliddragonflies,andcategorized
them according to copulatory activity, genital morphology and the suggested
mechanismsusedinspermcompetition. Inanotherstudy, MILLER(1995) found
thatalleleven species ofdragonflies studiedeitherremoved,displaced ordiluted
spermfromrivalmales,butthattheintromittentstructures usedtoaccomplish this
variedbetween species and genera.
The objectives ofthis study are (1) to examine and describe the functional
structures ofthe male genitalia in the generaLibellula, Ladonaand Plathemis
using scanning electron microscopy (SEM), (2) to categorize themajor types of
genitaliaandto discuss theirpotentialfunctionalsignificance, and(3) toexamine
patternsofsperm displacementinferredfromgenitalic morphologyinaphylogenetic
framework.Thisstudy is intendedtosupplement theprevious worksofKENNEDY
Malegenitaliain Libellulidae 15
Table I
Summaryofthe genitalstructures onthe fourth segment ofthe male penis in somelibellulid species
SSppeecciieess AAppiiccaall lloobbee LLaatteerraall1 lloobbeess MMeeddiiaall pprroocceessss
!M1 22 33 44 55 66 77 88 99 1100 1111
PP.. lloonnggiippeennnniiss xX xX xX 22
EE.. ssiimmpplliicciiccoolllliiss xX xX Xx 22
OO..ffeerrrruuggiinneeaa xX xX xX 22
LL.. aauurriippeennnniiss xX xX xX 11
LL.. aaxxiilleennaa xX xX xX 11
LL.. ccoommaanncchhee XX xX xX 11
LL.. ccoommppoossiittaa xX xX xX 11
LL.. ccrroocceeiippeennnniiss xX xX xX 22
LL.. ccyyaanneeaa XX xX xX 11
LLaaddoonnaa ddeeppllaannaattaa xX xX xX 00
LLaaddoonnaa ddeepprreessssaa xX xX xX 00
LLaaddoonnaa eexxuussttaa xX xX xX 00
LL.. ffllaavviiddaa xX xX xX 11
LL..ffoorreennssiiss xX xX xX 11
LLaaddoonnaa ffuullvvaa xX xX xX 00
LL.. hheerrccuulleeaa xX xX xX 22
LL.. iinncceessttaa xX xX xX 11
LL.. jjeesssseeaannaa xX xX xX 11
LLaaddoonnaa jJuuliliaa xX xX xX 00
LL.. lluuccttuuoossaa xX xX xX 11
PPllaatthheemmiiss llyyddiiaa xX xX xX 00
LL.. nneeeeddhhaammii xX xX xX 11
LL.. nnooddiissttiiccttaa xX xX xX 11
LL.. ppuullcchheellllaa xX xX xX 11
LL.. qquuaaddrriimmaaccuullaattaa xX xX xX 33
LL.. ssaattuurraattaa xX xX xX 33
LL.. sseemmiiffaasscciiaattaa xX xX xX 22
PPllaatthheemmiiss ssuubboomrnaaltaa xX xX xX 00
LL.. vviibbrraannss xX xX xX 11
5Descriptionsofthelobes: (1)large andspinose;- (2)small,reduced orabsent;- (3) large,roundand
flat;- (4) longand blade-like;- (5)long andspoon-like;- (6) reduced,rod-like; - (7)comuaabsent;
- (8) short and stout;- (9) longand straight;- (10)longand curved; - (11) number ofcomua.
(1922a, 1922b), RESTIFO(1972), MILLER(1991)andMAY(1992). Specifically,
this study provides acomprehensive survey ofthe morphology ofmalegenitalia
in three groups ofclosely related dragonflies. Scanning electron microscopy is
used to provide greater resolution and higher magnification images than most
previous studies. Finally, thisstudy is thefirst toexamine theevolutionofsperm
displacement strategies within a phylogenetic framework.
16 T. Artiss
METHODS AND MATERIAL
Penes weredissected from ethanolor dried preserved specimens ofmale libelluliddragonflies.
Nineteen species from the genus Libellula s.s. were examined together with members of the
allied generaLadona (inchfulva and depressa;ARTISS et al., 2000 ) and Plathemis.Although
thetaxonomic status ofthese genera is the subjectofsome controversy, they shall bereferred to
by separate genericrank in this study. Three other libellulids were also examined;Pachydiplax
longipennis,Orthemisferruginea,andErythemissimplicicollis. Datafrom these specimens were
used to describe components of the fourth segment(Tab. I), However, in somecasesdata were
Fig. 1. Scanning electron micrographs ofpartially inflated penes of libellulid dragonflies:(A)
Libellula croceipennis;-(B) L. herculea;;- (C) L. saturata, - (D)L. forensis; -(E) L.pulchella;
- (F)L. composita. —[Abbreviations:A, apical lobe;- C, cornua;- H,hood;- LL, lateral lobe;
- M,medial lobe;- MP, medial process; - P, posterior lobe]
Male genitaliain Libellulidae 17
obtained from previous studies (KENNEDY, 1922a, 1922b; RESTIFO, 1972; WAAGE, 1984;
MAY, 1992;MILLER, 1991).
Duringcopulation,the fourth distal segment ofthe penisis swung outca 180° ventrally,and
genitaliclobes on this segment are exposed and inflated. To simulate genitalicinflation,penes
weremanipulatedwith dissectingpins to expose the ventralsurface ofthe fourth distal segment.
In somecases, penes were soakedin double distilledHG and glyceroltofacilitate manipulations.
2
Specimens were air dried,or dried in hexamethyldisilizane(HMDS), mounted on a stub with
carbon adhesive, sputter coated with goldpalladium in a Polaron E5100 sputter coaler for 1
minute, and examined in a Zeiss DSM982 field emission scanning electron microscope. All
images were savedto disk as TIFF images,and printedonan Epson inkjetprinter (600dpi).
To examine the ofevolution ofpatterns ofsperm displacement,aphylogenyofthe groups of
interest is needed, A phylogenyof the odonate genera Libellula, Ladona, and Plathemis based
onDNA sequence data fromthe mitochondrial genes, cytochromeoxidase Iand 16SrRNA was
generated (ARTISS, 1999;ARTISS et al., 2000). This phylogeny is largely consistent with the
phylogeny of these groups produced by KAMBHAMPATI & CHARLTON (1999), but the
formerphylogeny includes moretaxa. A maximum likelihood analysis of the combineddata set
using the general time reversible substitution model with rate heterogeneity accommodated
using invariant sites plus gamma-distributedrates (GTR +I + G) produced a single tree (log
likelihood of-7198.05338).This tree was used in subsequentanalyses. For specific details on
analyses used in phylogeneticreconstruction, seeARTISS etal. (2000).
To examine patterns ofevolution in sperm manipulationstrategies, species were categorized
according to their morphology, and inferred patterns ofsperm displacement (see SIVA-JOTHY,
1984). Inferred patterns wereoptimizedonthe phylogenetictree using MacClade (MADDISON &
MADDISON, 1992). Changes in patterns wereunordered (changes from onestateto another are
equally probable, andequally weighted;viz. Fitch parsimony, FITCH, 1971;HARTIGAN, 1973),
RESULTS
General descriptions ofthegenitalia ofLibellulacan be found in KENNEDY
(1922a, 1922b) andRESTIFO (1972). Only themain featuresand structures not
obviousfromprevious studieswillbepresented here.Descriptions willbelimited
to lobes on the fourth segmentas they are interspecifically variable, and have
formed the basis of previous taxonomic studies (KENNEDY, 1922a, 1922b;
RESTIFO, 1972).Generalfeaturesofthemalegenitalia areseeninFigures 1-5.
Thefourthsegmentis comprised ofasclerotizedhoodanda distalcomponent
consisting ofseveralinflatablelobes.In somespecies, thehoodis comparatively
small (L. semifasciata, saturata), whilein others it is prominent, andpossesses
numerousshort spines (L. luctuosa, incesta). The distal componentofthe penis
comprises an apical lobe found on the ventral surface, a medialprocess lying
dorsalto theapical lobe, andpaired laterallobes.Each componentofthefourth
segmentis describedingreaterdetailbelow(seeTab.I). Descriptions ofpresumed
functionsoflobes ofinflatedpenesfollowMILLER(1991).
The apical lobe is located onthe ventral surfaceofthefourthsegment.It often
possessesnumerousspines(Fig.5A).Wheninflated,thespines areerectedatright
angles or proximally. Inmost species, this lobe is small (L. pulchella, P. lydia);
howeverinothersitis large,as inL. quadrimaculata, whereitisthelargest ofthe
18 T. Artiss
lobes(Fig. 4F).
The laterallobesare wellsclerotized, andduringinflationthey generally swing
outwards and rotate ventrally. The lateral lobes are highly variable among the
three genera. In some species they are greatly reduced (L. quadrimaculata,
croceipennis).Inothers, thelaterallobesare large,oval, concavedisks (Plathemis
spp.; Figs 4A,B). In contrast, thelaterallobes ofsomespecies arequitelong, and
thetipsare stronglycurvedresembling cornua(L.composita, cyanea) orflaredand
curvedresembling scoops (L.luctuosa).Thelaterallobesofseveralspeciespossess
anarrow grooverunning mediallyalong thelength oftheinterioredge (L.forensis;
Fig. 5B).
Fig. 2. Scanning electron micrographs ofpartially inflated penes of libelluliddragonflies: (A)
Libellula axilena;: - (B) L.auripennis; - (C)L. needhami; —(D) L. luctuosa; - (E) L. incesta;
(F) L. vibrans. - [Abbreviations asin Fig. 1]
Male genitaliain Libellulidae 19
The medial process bears several structures thatpivot dorsally with inflation.
Mostconspicuously, themedialprocess bears the inner(or posterior) lobes,and
oneor more cornua. In some libellulidgenera, the medialprocess may possess
flagella(MILLER, 1991),butflagella wereabsentinall species inthisstudy. The
surfaceofthemedialprocessmaybe coveredwithnumerousshortspines(L.cyanea;
Fig. 5D).
Fig. 3. Scanning electron micrographs ofpartially inflated penes of libellulid dragonflies: (A)
Libellula jesseana; - (B) L. comanche; - (C) L. cyanea; - (D) L. flavida; - (E) Orthemis
ferruginea;-(F) Erythemissimplicicollis. - [AbbreviationsasinFig. 1]
20 T.Artiss
The cornua are typically long, curved, rod-like structures, although in some
species they are greatly reduced (L. quadrimaculata). The cornua are generally
smooth, slenderextensionsofthedistalportion ofthemedialprocess, although in
some cases, the comua may possess short, proximally orientedor interlocking
spines(L.herculea,O.ferruginea;Fig. 5E). Thenumberofcomuavariesbetween
species. Insomespecies, thecomuaareabsent(Plathemisspp.,Ladonaspp.;Figs.
4A-E),whileotherspossess uptothreecomua(L.saturata). Themajorityofspecies
possess asingle,relativelythickcornuwhichmaybeslightlyforked(L. auripennis)
or cupped (L. vibrans; Fig. 5F) atthetip. Libellulacroceipennis andL. herculea
possess shortspines atthebase ofthecomua(Figs 1A,B).
Inthemajorityofspecies examined,theposterior orinnerlobes were notvisible
as they wereobscuredby thelaterallobes or medialprocess. Theposterior lobes
aretypically highly inflatable,andare visibleoncethepenisis inflated.Inthefew
cases wherethe lobes were easily visible, they oftenbristledwitha numberstout
spines ontheirventralsurface(L. auripennis.Fig. 5D).
DISCUSSION
FUNCTIONAL SIGNIFICANCE
Thesize, shape andfeaturesofthelobeson thefourth segmentofthe penis are
highly variableamong thespecies studied. Therapid and divergent evolutionof
genitalia in other odonate groups has been attributed to sperm competition
(WAAGE, 1984;MILLER, 1991, 1995)and I am assuming this to explain the
diversificationof genitalic forms amongspecies in the current study. However,
EBERHARD’s (1985)femalechoicehypothesis may alsoexplain the diversityof
genitalia amongodonates(seeSIVA-JOTHY& HADRYS, 1998).Hemaintainsthat
male intromittentorgans are thetargetoffemalecopulatory andpost-copulatory
mate choice,andthatfemalesexualselectionhasproduced thevarietyofgenitalia
seen inmany organisms. This hypothesis is unlikely toexplain genitalic variation
amongtaxa inthisstudyas thestructuresconsideredhereinarebelievedtofunction
as spermremoving mechanisms(reviewedinCORBET, 1999).Moreover,thereare
severalfeaturesthatfemalesmayuseasthebasis ofmatechoicethatwouldpreempt
post-copulatory mate choice such as territories and differencesamong males in
body sizeandcolor(WOLF&WALTZ, 1988;MOORE, 1990;reviewedinCORBET,
1999).
Functionalexplanations ofparts ofthe fourth segment ofthe male penis are
offeredbelow.
APICALLOBE. - The apical lobeis largeand blunt in the majority of species
studied. Uponinflation, the apical lobeswings ventrally, helping to position the
penis dorsallytowards thebursa communis(MILLER, 1991).Thesurface ofthe
apical lobe is generally covered with spines which presumably helpanchor the
Male genitaliainLibellulidae 21
penis during copulation (Fig.5A). MILLER(1991) suggeststhatthesespines may
alsoremove rivalspermdisplaced intothevagina.
LATERAL LOBE. - Atrest, the laterallobes are positioned on either sideofthe
medialprocess (MILLER, 1991). Upon inflation, these lobes enter the genital
canal forcing theventral valveon thebursa communisopen. When thepenis is
Fig. 4. Scanning electron micrographs ofpartially inflated penes oflibellulid dragonflies: (A)
Plathemis lydia;-(B) P. subornata;— (C) Ladonajulia;- (D) L. depressa;- (E) L.fulva; -(F)
Libellulaquadrimaculata.—[Abbreviations as in Fig. I]
22 T.Artiss
deflatedandremoved, theinner surfacesofthelaterallobes may remove sperm
(WAAGE, 1986b;MILLER, 1991).In several ofthe species studied, the lateral
lobeswere reduced,andtheir functioninprotecting thepenis or inassisting with
spermremovalmay belimited(e.g.L. quadrimaculata). However,themajorityof
thespecies studiedhadelongated laterallobes.Giventheabsenceorreductionin
sizeofthe cornua in manytaxa, it is likely thatthe elongated laterallobes have
evolvedto facilitatespermremoval. Moreover, the laterallobesof some species
areenlargedattheproximal end(e.g. L.luctuosa, Fig.2D),possessamedialgroove
thatmay serveasacanalalongwhichpre-existing spermmaytravel(e.g.L. forensis;
Fig. 5B), and/orhave pitted surfaces (e.g. L. composita Fig. 5C). All ofthese
,
features would facilitatesperm removal ifthey accessed thebursacopulatrix or
spermatheca.
MEDIALPROCESS. - The medialprocess is theportion ofthe penis thathas the
most direct access tothe bursa copulatrix and spermatheca making it thelikely
agentofspermremoval.Unfortunately, theventralsurfaceofthemedialprocess is
not easily visibleunlessinflationisachievedratherthan simulated,so descriptions
ofthisstructure aregenerally lacking inthisstudy. However, insomecases where
thesurfacewas visiblepenesinwhichinflationwas simulated, theventralsurface
containednumerousshortspines (e.g.L. auripennis. Figs 2B, 5D). Inadditionto
anchoring thepenis duringcopulation, these spines may displace spermfrom the
bursa intothe spermatheca, and catchand removerival spermwhen thepenis is
withdrawn(WAAGE, 1986b;MILLER,1991;CORBET,1999).
POSTERIOROR INNER LOBES. - The structure ofthe innerlobes is similarly
difficultto observe without genitalic inflation. However, in species where they
were visible, they often appeared to be long, robust structures, bearing coarse
spines on their ventral surface. Theselobes may reposition spermby moving it
distallyaway fromtheregion offertilization, andthespines may trapandremove
spermuponwithdrawal(SIVA-JOTHY, 1984;MILLER, 1991).
CORNUA. - Cornua are generally long, smooth, slender structures. In many
odonates,flagella areusedtoremove spermfromthespermatheca (MILLER, 1982,
1990, 1991, 1995;SIVA-JOTHY, 1984).However,noneofthespecies inthisstudy
possessed flagella,anditis likely thatthecornuaperform thisfunction.Thegross
morphology ofcornuawassimilarinmanyspecies. Howeversomespecies possessed
numerousspines (Fig. 5E), or hadatip thatformedafork ora cup(Fig. 5F).Allof
these structures could facilitatesperm removal. MILLER(1991) suggested that
thestiff, curved cornua mayalsoanchorthepenis withinthefemalegenital tract
during copulation.
PATTERNS OFSPERM REMOVAL
Anisopteran penes have been categorized according to their morphology, and
inferredpatterns ofsperm displacement (SIVA-JOTHY, 1984).Two basic sperm