Conflicting Morphological and Reproductive Species Boundaries in the Coral Genus Platygyra

13 Pages·1997·5.4 MB·English

by  K Miller

Reference: Biol. Bull 192:98-1 10.(February. 1997)
Conflicting Morphological and Reproductive Species
Boundaries in the Coral Genus Platygym
KAREN MILLER1 * AND RUSSELL BABCOCK:t
1MarineBiologyDepl. James Cook UniversityofNorth Queensland, Townsville, QLD4811.
Australia:and2AustralianInstituteofMarineScience, PMB3, Townsville, QLD4810, Australia
Abstract. In mass-spawning corals, potential exists for junct distributions ofcertain morphospecies along lati-
gametes ofa number ofspecies to mix in the water col- tudinal and habitat boundaries, and the small levels of
umn. Theexistenceofmorphologically distinct sympat- variation in reproduction maybetwosuch mechanisms.
ric coral populations despite such an event implies the
presence ofisolating mechanisms to prevent hybridiza- Introduction
tionand maintain speciesboundaries. Over380fertiliza- Species boundaries in scleractinian corals are primar-
tion trials were conducted to determine the extent ofre- ily based on skeletal morphology (e.g.. Ellis and So-
productive isolation among the seven morphologically lander, 1786; Vaughan and Wells, 1943; Veron el a/..
defined species (morphospecies) of the scleractinian 1977; but see Lang, 1984). It is generally assumed that
coralgenusPlatygyra. on theGreat BarrierReef. Results morphological differences between coral species are
from these experiments demonstrate that fertilization highlycorrelated with reproductive incompatibility(i.e.,
between-morphospecies can occurat ratesequivalent to biologicalspeciesboundaries)(Willis, 1990)andthattra-
within-morphospecies fertilizations, indicating that no ditional theoriesofhierarchical evolution and speciation
gametic-level barriers to fertilization exist among these (e.g., Darwin, 1859)will apply tocorals.
morphological species. Observations ofspawning times Recognition that most ofthe corals on the Great Bar-
both in the field and in the laboratory have shown that rier Reef reproduce annually during a mass spawning
all seven morphospecies spawn on the same night and event (Harrison et a/., 1984; Willis et al., 1985; Babcock
thatthereisconsiderableoverlapinthehourofspawning el u/.. 1986) has prompted an examination ofassump-
among them. These data indicate that few, ifany, tem- tions that morphological species boundaries reflect bio-
poralbarrierstofertilizationexistamongmorphospecies logical species boundaries within the Scleractinia (Wal-
ofPlatygyra on the Great Barrier Reef. In addition, lar- lace and Willis, 1994). The persistence of morphologi-
vae resulting from between-morphospecies crosses are cally distinct species groups (e.g.. Veron et al., 1977)
capable ofsettlement and subsequent growth equivalent coupled with massspawningevents, wherebygametesof
to that of within-morphospecies larvae. Our results re- many species become mixed within the water column,
veal a discontinuity between reproductive and morpho- implies the presence ofsome mechanism to prevent hy-
logical species boundaries within the scleractinian genus bridization and hence maintain speciesboundaries(Wil-
Platygyra and challenge species definitions within the lis, 1990). The benefit of such mechanisms was sup-
Scleractinia. It is not yet clear what mechanisms might ported by results from experimental crosses in thegenus
maintain morphu!. /i^al boundaries in Platygyra in the Montipora. in which hybridization wasdocumented but
face ofthe clear pt raial for gamete mixing. The dis- considered to be maladaptive due to arrested develop-
ment in hybrid embryos (Hodgson. 1988). More re-
cently, however,fertilizationexperimentshaveindicated
sea*RrecPchr,eeisPveOentdBaIodSxdJrue1sl4sy-:90IN19a.9t6iK;oilnabanilrn1i1e,Wel,lNino'gvtWeoamnt.beerNreawn19dZ96eA.atlmaonsdp.hericRe- sthoamtesumcocrepshsofullogfiecratliliczoartailonspienciveistr(oWiisllipsosesliballe.. a1m9o92n)g,
tPresent address: University of Auckland. Leigh Marine Labora- although how experimental fertilization relates to field
tory.P.O. Box349,Warkworth. NewZealand. fertilization is unknown.
98
CONFLICTING SPECIES BOUNDARIES IN PLATYdYRI 99
The coral genus Platygyra includes morphological isolating mechanism during mass spawning is still spec-
species (morphospecies) from the Great Barrier Reef; ulative.
Platygyra daedalea, P. sinensis, P. pini. P. lanu-llina, P. Species-specific gamete-binding proteins have been
ryukyuensis. Platygyra'B', and Platygyra'H'(seeMiller, described in some marine invertebrates, including aba-
1994a, b). These morphological specieshavebeen recog- lone (Vacquier ct al.. 1990) and sea urchins (Palumbi,
nized by traditional taxonomy (e.g.. Chevalier, 1975; 1992; Metz and Palumbi, 1996). Similar proteins may
Veron ct ai. 1977; Veron, 1993) and numerical taxon- play an important role in the reproductive isolation of
omy(Miller, 1994a),althoughbothapproacheshaverec- many free-spawning invertebrates, including scleractin-
ognized a continuum ofseparate characters among the iancorals.
species. Based on theoccurrenceofmorphologicallydis- The high predictability ofspawning in Platygyra spe-
tinct species of Platygyra, it seems highly likely that cies on the Great Barrier Reef (Babcock et al., 1986)
mechanisms exist that promote reproductive incompat- makes it possible to explore reproductive relationships
ibility and subsequently maintain species differences aatndthpeotgeanmteiatleilseovlealt.inIgnmtehcishapanpiesrm,swbeetdwoeceunmtehnetserceosrualltss
within thegenus.
Contrary to expectations, genetic comparisons show from more than 380 trials that show potential for fertil-
no differentiation between the morphological species of ization between the morphological species ofPlatygyra.
gPtsstieplhemreeaninctclaimyeataogirtyrhviirpeatolayhsnyoo(,tlfMeaogivkPgeleelilnnnaecettraspyelalgxfanycsocrdhpelaealBc(nmoeiMwgoneiierzslniplgmeghe.raorwosyihuapnpinoespcdcc,rhcieBsueaesrlsn)ni.tzdnmtialTaeethh,yuegerneiarcnnhelefelilptygerithcebchtsgeesretd)en.imwcevoeetAerenlirn-t-c- aftfPmmraheoooreewrurapnplmnihlhdfooeooralnlspnoopfphyigreoedeicllti-cdiofezafegomyleisbgprscooesteaarnpirlnacecveldcassiptirebeisiescnoeopilnttreoaswhsotfe.eidoenufPaTgnclbsht-iapimlitmsaivyeotwgecryeynphvbroihaiaafnodr.nglrseiainptsFreceivrumecmsarsieetseeshuxfbsiegsresgruottcemgmwrsgoboeterwesseseisttn,ttewhhstetiehwehnratente-eo
rpehporleosgeinctalrebporuondduacrtiievseiinsotlhaitsiognenbuestwmeaeynsnpoetcineesc.eTsshaerirley- saegtitnlge,asnpdergmrodwi.luWtieona,lasondexpaarmeinntealdgtehneoetfyfpecetsonoffegratimleiztae-
lationshipbetween reproductiveand morphological spe-
tion success.
ciesboundarieswithin Platygyraand thepresenceorab- The discovery that successful fertilization can occur
senceofreproductive isolation between the morphologi- between morphological species of Platygyra calls into
cal speciestherefore needtobeestablished. question the previous assumption that morphological
A numberofmechanisms, includingpre-and post-zy- species boundaries equate with reproductive isolation
gotic barriers, could mitigate reproductive isolation be- and subsequently with evolutionary units. Our results
tween coral species. In Platygyra, there is some indica- promptareexamination ofspeciesdefinitionswithinthe
tion of temporal differences (in the order of hours) in Scleractinia and indicate that species groupings based
spawningtimesamongsomeofthemorphospecies(Bab- solelyon morphologyareclearly notalways"biological"
cock el al., 1986), although these observations involved species(Mayr, 1942). Ourdata, combined with otherre-
only a few colonies, and variability in spawning times cent findings in this field (Knowlton et al., 1992; Van
within morphospecies has not been documented. Coral Veghel and Bak, 1993; Van Veghel, 1994; Wallace and
gametes remain viable for at least 5 h in the water col- Willis, 1994), suggestthatspeciesdefinitionsintheScler-
umn (Oliver and Babcock, 1992), and it is not known actinia should perhaps be based on a range ofcriteria,
whethertheseshorttemporaldifferencesin spawningact ratherthan solely on reproductive or morphological evi-
asacomplete isolatingmechanism. dence. In addition, we suggest that in Platygyra, even
Sperm chemotaxis is used by a variety ofmarine in- with genetic exchange, discrete morphological groups
vertebrates to attract conspecific sperm (e.g.. Miller, will be maintained through acombination ofspatial and
1979, 1985), and may well be important in reducing po- temporalvariabilityinreproductiveprocessesandfitness
tentialforhybridization,forexample, inpelagiccommu- in a variety ofhabitats.
nities ofhydromedusae (Miller, 1979). Chemical sperm
attractants have also been implicated asa mechanism to Materialsand Methods
reduce hybridization in the coral genusMonripora (Coll
et al, 1994). However, even in Montipora, where sperm We conducted fertilization experiments and observed
attractants have actually been identified, chemotaxis is spawningsovera3'/2-yearstudyperiodduringsevencon-
not completely specific, and heterospecific attractants secutivespawningsatthreereefsinthecentralGreatBar-
maystill elicit adegree ofchemotactic behaviorbetween rier Reef: Magnetic Island 19 10' S, 14651'E (1990,
species (Coll et ai, 1994; Richard Miller, pers. comm.). 1991, 1992, 1993); Davies Reef 1850' S, 14739'E
Atthispoint,theroleofspermchemotaxisasaneffective (1990, 1991); and Orpheus Island 1835'S, 14629'E
100 K. MILLER AND R. BABCOCK
(1992). On the afternoon ofthe predicted mass spawn- witheven, symmetricalcleavagesandapparently normal
ing, gravid Platygyra colonieswj> osferred from the development (see Babcock and Heyward, 1986); or 'ir-
reefintoseparatecontainers; pi undernatural light regular' with uneven, asymmetrical cleavages and ab-
to simulate normal condiiio oloniesgenerally com- normal development.
;
menced spawning from k onwards. Once a colony Analysisoffertili:ation data
had spawned, gamete 's were collected from the Percent fertilization in crosses after 2-3 h and 6-8 h
surface ofthe water and sperm and eggs were separated
was based on the total number ofeggs censused in each
wbeyregesnttloeredagaittartoioonmatnedmpuesreatoufrea(1a0b5o-u/tum27sCie)vei.nSapceornm- vial. Percent fertilization after 6-8 h was calculated to
centrated form to reduce aging (see Chia and Bickell. detect any increase in fertilization over egg-sperm con-
tacttime. The percentageoffertilized eggsdevelopingto
1983; Oliver and Babcock, 1992) until required for fer-
blastulaebetween thetwocountsforeach vialwascalcu-
tilization. Eggs were taken through a series of 8 to 10
washes of'sperm-free water' (collected halfa kilometer lated as the number fertilized after 6-8 h (including
from the edge of the reef on the afternoon before the cleavages and embryos) expressed as a percentage ofthe
commencement ofspawning) to ensure the absence of number offertilized eggs after 2-3 h. Mean percent fer-
anytracesperm. Eggswerestored inglassjarswithgentle tilization ofthethree replicate vialsineachcross(colony
agiEtgagtsionanundtislpneeremdefdrfoomredxipffeerriemnetntcso.lonies were com- tphaiercionngt)rwoalsssuhsoedwefdorstpheeramnacloynsteas.miAnnayticornososfeesgfgosrdwuhriicnhg
bined experimentally in 20-ml glass scintillation vials to the washing process were not used in the analyses. Re-
sults from the seven spawnings at three reefs were com-
examine fertilization success between colonies ofdiffer-
bined at the level ofmorphological speciesforanalysis.
eonftalmlocroplhoonlioegsieuss.edMoirnphfeorltoigliizcaatilonspterciialesswiadesntbiafsiecadtioonn A two-way analysis of variance (ANOVA) based on
Miller(1994a). Concentrationsofthe stored sperm were the percent fertilization data after 2-3 h was carried out
to determine whether fertilization rates differed either
estimated by five replicate hemacytometer counts and,
between between-morphospecies and within-morpho-
just before fertilization, diluted to aconcentration ofap-
proximately 2.5 X 106 sperm/ml. This sperm concentra- species fertilizations or between the different morpho-
tion has been shown to give optimum rates for fertiliza- species (based on maternal colony type). The main
tion in trialswith Platygyra sinensisand othercoral spe- effects were fertilization type (2 treatments; within and
cies (Oliver and Babcock, 1992). About 100 eggs were between morphospecies) and egg species (7 treatments;
added to 20-ml aliquots ofdiluted sperm in a vial. Each Platygyra B, P. daedalea. Platygyra H, P. lamellina, P.
ccroonscsenwtarsatiroenplsicwaittehdineietxhpeerrtimweontoarltvhiraeles twiemrees.chSepcekremd ppfiuenraitt.heedPr.utsrwiyonn-kgwyitaihyeenAspieNsr,OceVPn.tAsfiwenraetsnislciiazsra)rt.iioeTndhdoeauttaantaoalfcytsoeirmsp6a-w8raesh.trheAe-
randomly and found to be within 10% ofestimated con-
development of cleaved eggs through to blastulae in
centrations.
within- and between-morphospeciescrosses. All ofthese
Experiments were designed in a matrix system in
analyses were done using both arcsine-transformed and
awcolhltiohcnohiueagslhl(cetogrlgiosanlAsiesbXewtsewpreeeercnmroBsssoaemndedreecmgigopsrrpoBhcaoxlllosygpwieicrtahmlaAls,lpoeettcchi.ee)rs, ronalAwydraetsau.ltsBfortAohNmmOaenVtahlAyosdessopfrroadwucdeadtasairmeilparresreenstueltdshearned.
one-way comparingfertilizationratesafter
have not yet been conducted due to the timeconstraints 2-3 h between morphospecies combinations was also
associated with coral spawning. All colonies were tested
fXorstpheeirrmabBi.liettyc.t)o.sRelefp-lfierctaitleizeco(netggrsolAs xwesrpeerprmeAp,areegdgsbBy cedrnoocnseessesti.onRdfeeeprttleiirlcimazitantiieoonnwwhbaeestthwteeoreontlhmoeowrretpowheoclrooemgpsiaicgranelifasilpcleamcntioerdsip-fphfaeoir-r-
placingeggsfromeachcolonyin 20 mlofsperm-freewa- species-pair crosses: consequently this analysis was car-
tertodetermine whetherthere had been any sperm con-
ried out only on crosses in which P. daedalea was the
tamination ofwashed eggs. Glass vials containing eggs maternal colony, and for the sperm ofP. daedalea, P.
gaenndtlsepaegritmatwieornetokeapt watfaermtbiilieznattiosneatotteamkpeerplaatcuer.e with lTahmeelilnicniad.enP.cepionfi.irrPe.gurlyairikaynudenrseigs.ulaP.resminbernysoissawnadscseolmfs-.
Fertilization inall icate vialswasrecorded after2- pared between crosses to determine whether irregular
3 hours and again after -J hours by censusing the first embryosweremorecommon in between-morphospecies
100 eggs sampled fron, one vial. Fertilization was
crosses.
scored as the initiation ofcleavage. 2-3 h after fertiliza-
tion(Babcockand Heyward. 1986). Afterabout 7 h most Effectojsperm concentration onfertilization rates
oftheembryoswereblastulae.Thecondition ofeggsand To determine whether fertilization success was an ar-
embryos in all crosses was recorded as either 'regular' tifact of using optimal sperm concentrations in expert-
CONFLICTING SPECIES BOUNDARIES IN PL.-lTYtiYRA 101
TableI
Meanfertilizationratesfr<nfenili:ationtrialsbetweenmurplwloxicalspecies<>/Platygyrabasedonallcrossestriedoverthe3'/r.mirstudyperiod
Sperm PD PH PL PP PR PS PB Sells
PD
102 K. MILLER AND R. BABCOCK
colonies were clonemates. Colonies were screened using TableII
allozyme electrophoresis at nine polymorphic loci Meanfertilizationrales( standarddeviation)betweenthreecolonies
(PGM*, MPI*. CK*. LP*, LT-1*, LT-2*. LG-1*. LG-2*. ntPlatygyradaedaleaandthreecoloniesofP.sinensisfrom
andGPI*; see Miller, 1994b; Millerand Benzie, in press, fertilizationtrialscarriedoutatMagneticIslandon 17October1992
forfurtherdetailsofelectrophoretictechniques). Thege-
netic difference between mated colonies, based on the Eggs
nine-loci genotype, wascalculated asa percentage based Sperm PDI PD2 PD3 PS1 PS2 PS3
on the numberofdifferent alleles(i.e.. alleles not shared
by the two colonies). The fertilization success between PDI 31.6 24.8 3.4 7.1 0.0
any two colonies and the genetic difference between PD2 9.8 (0.0) 6(71.41.07) 6(18..894) 4(03..182) (07..070)
them wasthen compared.
(0.21) (3.96) (0.0) (3.18) (1.98)
PD3 27.5 47.5 70.6 43.1 16.7
Larvalrearing (6.51) (15.63) (1.13) (15.91) (23.55)
PS1 72.0 67.7 91.6 45.1 7.15
In addition to the fertilization experiments described (5.66) (1.34) (4.74) (3.04) (10.11)
above, large numbers (>1000) of larvae from both PS2 73.2 56.1 95.4 73.3 0.0
within- and between-morphospeciescrosses were reared (3.25) (6.79) (0.0) (2.69) (0.00)
PS3 23.1 18.2 74.9 81.2 10.7
and allowed to settle so that their subsequent develop-
(11.24) (4.31) (0.71) (2.05) (7.00)
ment could be monitored through planulae to polyp.
Larvae were raised usingthe methodsdescribed by Bab- Self-fertilization rates (eggs and sperm from the same colony, de-
ctroacnksfaenrrdedHefyrowmarpdlas(t1i9c86j)a.rsA(f2t-e4r 14)-t5odcalyoss,edlaarqvuaaeriwaerien Anloltecdonbtyro"l-s-("e)ggwseirnesp<e1r%m-firneaellwactoelro)niheasdex0c%epfterPtiDliIzawtihoinc.h was <3%.
which settlement substrata (either coral rubble or terra-
cottapavingtiles)wereprovided. Onceall larvaehadset- II). No colony had consistently low fertilization with all
tled, settlement plateswere transferred to a flow-through othercolonies (e.g.. Table II), so variability was not due
aquarium system where they were maintained to obtain simply to low overall viability ofthe gametes in any one
specimensforexamination ofcolony morphology.
colony.
In crosses forwhich P. daedaleawasthe maternalcol-
Results ony, ANOVA indicated significant variability in fertil-
ization rates with different sperm types (df = 5, MS =
Fertilisation trials 19043.31, F= 25.65, P<0.001). However, thesourceof
The fertilization trials among colonies of Platygyra mostofthisvariability lay indifferencesbetween ratesof
showed that fertilization took placebetween all the mor- self fertilization rather than in crosses within and be-
phological species tested (Table I). The gametes of the tween morphological species. Tukey's HSD test showed
seven morphological species crossed were highly com- nodifferencesbetween fertilization ratesforwithin-mor-
patible, and mean fertilization rates among different phospecies crosses [PD X PD (x = 45.2, n = 49)] and
morphospecieswere similar(between 50% and 70%, Ta- between-morphospeciescrosses [PD X PS(.Y = 71.7, n =
ble 1). In total, 389 crosses were conducted including 50), PD X PL(x_= 68.2, n = 13), PD X PP(x = 66.6, n =
1 140separatevials. Although variabilitywithin replicate 10), PD X PR(.Y = 66.1, n = 8)]. Thesefertilization rates
vials was sometimes high (Table II), the controls indi- were, however, significantly different from rates ofself
cated no contamination oftrials by foreign sperm. The fertilization in P. daedalea(x = 3.1, /; = 28).
high levels of fertilization (>50%) measured in crosses Fertilization success between some morphological
demonstratesthat colonieswere reproductively compat- species was low. Crosses between Platygyra H eggs and
ible. Interestingly, self-fertilization rates were low in all P. daedaleasperm (n = 5 pairs), Platygyra H eggsandP.
morphospecies(Table I). pini sperm (/? = 3 pairs), P. pini eggs and P. sinensis
Fertilization success was highly variable both among sperm (n = 2pairs)and Platygyra BeggsandP. daedalea
and within morphological species, with most differences sperm (/; = 1 pair) had fertilization ratesbelow 20% (Ta-
occurringatthe level ofindividual crosses. Forexample, ble I). However, the low fertilization ratesbetween these
fertilization success between any two P. daedalea colo- morphospecies are within the range observed between
niesranged from 9.8% to67.1% in crossescarriedouton colonies ofthe same morphospecies (Table I) and may
17 October 1992 (Table II) and between 0% and 81% not reflect species incompatibility. Larvae produced by
betweenP. sinensiscoloniesonthesamenight(Table II). crosseswithlowfertilizationrateswereregularinappear-
Similarly, the successofbetween-morphospeciescrosses anceand seemed todevelop normally.
seems largely dependent on colony compatibility (Table Reciprocal crosses between different morphological
CONFLICTING SPECIES BOUNDARIES IN PLATYdYRA 103
TableIII
ResultsIrommultipletwo-\vayanalysesojvariancetocompare
fertilizationsuccess(inbothwithin-andbetween-morphospecies
crosses)forthesevendifferentmorphologicalspeciesofPlatygyra
Experimental
dataanalyzed
104 K. MILLER AND R. BABCOCK.
parent coloniesbut may be aconsequenceoftheexperi- Temporalseparation in.spawningtimes
mental method(seeOliverand Babcock, 1992). There was some evidence of temporal separation of
spawning times between some morphological species
Effectsofsperm concentration onfertilisation rates of Platygyra (Fig. 6). However, this separation was
Therewerenoeffectsofspermconcentrationonfertil- insottedclebaertlwyedeenfintehde adnifdfearnenotvemrolrapphionspsepcaiwensi.ngAtlitmheouegxh-
ization rates in either P. daedalea or P. ryukyuensis Platvgyra daedalea colonies have been observed spawn-
crosses. Fertilization rates forP. daedaleacoloniesfertil- ingthroughoutthe night, P. lamel/inaandP.pinitendto
izedwith conspecincsperm did notdiffersignificantly at spawn earlier in the evening (spawning 50 to 200 min-
sperm concentrations ranging from 101 to 105 per ml utes after sunset), while P. sinensis. P. ryukyuensis. and
(Fig. 3). Similarly, sperm concentration did not affect Platygyra H generally spawn later(more than 200 min-
fertilization rates in crosses ofP. daedalea eggs with P. utesafter sunset) (Fig. 3). The ranges ofspawning times
rvtikvuensissperm (Fig. 3)orincrossesofP. ryukyuensis observed in the field and laboratory show that therewas
eggswith conspecific sperm and with P. daedalea sperm considerable overlap in spawning, and that the gametes
(Fig. 3). ofdifferent morphological specieswereoften released si-
multaneously.
Effectsofeggageonfertilization rates
The ability ofan egg to be fertilized by sperm from a Larvalrearing
different morphological species does not appear to be a The embryos ofwithin- and between-morphospecies
function ofegg age. There was no significant change in crosses developed into ciliated planulae larvae within 2
within-morphospecies or between-morphospecies fertil- days of fertilization and were competent to settle 4-
ization rates with egg age in either P. daedalea or P. si- 5 daysafterspawning. Metamorphosedlarvaefromboth
nensis. Regression analysis ofpercent fertilization with fertilization typesgained zooxanthellaewithin 2 daysaf-
egg age was not significant for either P. daedalea or P. tersettlementand subsequently began skeletal construc-
sinensis for any fertilization type (Fig. 4): within-mor- tion. After 5 months, recruits ranged in diameter from 2
phospecies(r = 0.005,0.05 respectively), between-mor- to 10 mm. In March 1993 the oldest colony from a be-
phospecies (r = 0.04, 0.1 respectively), self(r = 0.07. tween-morphospeciescrosshad reached 3'/2 yearsofage,
0.1 respectively), or controls (r = 0.0004. 0.0 respec- and colony diameters ranged between 2 and 5 cm. Un-
tively). Although r was not significant (possibly due to fortunately, all colonies died in 1993 because ofa prob-
high variation in fertilization rates; see Fig. 4), fertiliza-
tion (both within- and between-morphospecies) showed
an overall decrease with egg age in P. daedalea and a
60
general increase with egg age in P. sinensis (Fig. 4). Be-
tween-morphospecies fertilization rates were generally
higher than within-morphospecies fertilization rates in T I
P. daedalea, whereas between-morphospecies fertiliza-
tion rateswerelowerthan within-speciesfertilizationsin in 40 -
o>
P. sinensis (Fig. 4); however, neither difference was sig- O)
nificant.
5
Fertilizationsuccessasajunction ofgenotype 20
Fertilization success in colonies ofPlatygyra was not
relatedtothegeneticdifferencebetween parentcolonies.
No two colonies used in fertilization trials had identical
nine-locusgenotypes(i.e.. nonewereclonemates). Colo-
niesthatwerebetween40%and60%differentgenetically Between Within Between Within
had fertilization rates varying between 0% and 100%' 2-3hrs 6-8hre
(rFeicgo.gn5)i.tiNoon aelvliedleen(cGeroosfbwerhga.t1m9i8g8h)twbaescsoenesnidoenretdheabsaeslifs- emFbirgyuorsei2n.botEhmbwirtyhoin-coanndditbieotn.weOecnc-umrorrepnhcoespoefcireegsuflearrtialnizdatiirornetgruilaalrs
of the nine-locus genotypes of incompatible colonies. expressedasapercentageofthefirst 100eggscountedpervial(allmor-
Colony pairsexhibitinglowlevelsoffertilization did not phospeciespooled). Barsrepresent 95% confidence limits. regular
possessconsistentallelic orgenotypiccharacteristics. embryos; irregularembryos.
CONFLICTING SPECIES BOUNDARIES IN PLATYdVRA 105
P. daedalea eggs
100 -i
106 K. MILLER AND R. BABCOCK.
100 P daedaleaeggs opment ofPlatygyra larvae orthe competence oflarvae
to settle, metamorphose, and grow. Sterility ofthese off-
spring may be a post-zygotic isolating mechanism be-
60 tween morphologicalspeciesofPlatygyra, althoughcosts
40 associatedwithsterileprogeny(e.g., gametewastage, loss
ofresources) maybehigh. However, in histological stud-
20
ies all mature-sized colonies of P. sinensis had gonads
(Babcock, 1986)and inthe 389 fertilizationtrialscarried
0-22-44-66-8 out, no colony wasever found that did not fertilize with
atleastoneothercolony,suggestingthatsterilePlatygyra
areeitherrare or nonexistent.
Resultsfrom thisstudy,combinedwithgeneticstudies
100
P smensiseggs (Miller and Benzie, in press), lead us to believe that fer-
80 tilization and gene flow will occurbetween morphologi-
cal "species" ofPlatygyra. This raises a number ofper-
is eo plexing questions. First, should these morphological
40 groups within the genus Platygyra (Miller, 1994a) be
considered true species, or is there simply one highly
20
polymorphic species (e.g., Skulason and Smith, 1995)
o within the genus? And second, ifgene exchange occurs
0-2 24 4-6 6-8 freelybetweenall morphologicaltypes,howcanthemor-
Eggageatfertilization (hours) phologicallydistinctgroupsbe maintained in thisgenus?
Figure4. Effectsofeggaging. Meanfertilization rateswithincreas- Morphological species boundaries within the genus
ingeggageincrossesbetweenthreecolonieseachofPlotygyradaedalna Platygyra are clear (Miller 1994a), and morphological
andf .v/Hivj.y/.y. Barsrepresent95"i confidencelimits. within-mor- differences appear to be far greater between Platygyra
phospeciescrosses. between-morphospeciescrosses. spp. than between othercontroversial speciesgroups: for
example, the three morphological variants of Montas-
traea annularis that are now considered sibling species
crosses apparently did not have reduced survivorship (Knowlton et ai. 1992; Weil and Knowlton, 1994); the
(Fig. Ic). two morphsofAfontiporadigitatathat havebeen identi-
Percent fertilization was high after 2 h (the minimum fied as reproductively isolated species (Stobart and Ben-
time after fertilization in which cleavage becomes evi- zie, 1994); and Acropora cuneata and A. palifera, which
dent), and fertilization rates did not increase with egg- aredifficult todistinguish by morphology butaregeneti-
sperm contact time. This suggests that fertilization be- cally distinct (Ayre el a/., 1991). Hence it is not surpris-
tween gametesoccurred promptly aftertheeggswere in- ing that the seven morphological variants ofPlatygyra
troduced intothevials, irrespectiveofthe morphotypeof have been considered separate species in the past.
the parent colonies. Variations in egg age, sperm dilu- However, genetic and reproductive data indicate that
tion, and parental genotype failed to affect patterns of Platygyra morphospecies are much more closely related
fertilization in crosseswithin oramong morphospecies. than theMontaslreaspp., theMontiporaspp., orthetwo
Noevidenceofpre-zygotic barriersto fertilization was Acropora species. Furthermore, many groups of corals
detected among the morphological species ofPlatygyra. showdiscrepanciesbetween speciescharactersincluding
The seven morphological species have overlapping morphology, genetics, and reproduction (Potts and
spawningtimes(Fig. 6), and high levelsoffertilization in Garthwaite, 1986; McMillan et at., 1991; Willis el ai.
trialsindicatethatnospecies-specificgameterecognition 1992; Wallace and Willis, 1994; Romano and Palumbi,
systems (e.g., Uehara el a/., 1990; Vacquier el ai. 1990; 1996). Defining species boundaries in corals is clearly a
Palumbi, 1992) are operating in Platygyra. Preliminary controversial issue. Nonetheless, ourresultsshowthat in
investigationsofsperm chemotaxis in Platygyra showed the case ofPlatygyra, it is inappropriate to consider the
limited evidence ofsperm attraction, but noevidence of taxonomic unit as the minimal evolutionary unit and
species-specificity of sperm attraction in Platygyra (K. that evolutionary processes in different morphological
Millerand R. Miller, unpubl. data). Even ifpresent, spe- speciesofPlatygyrawill be linked.
cies-specific sperm chemotaxis could not be completely The maintenance of seven morphologically distinct
effective as a barrier to fertilization without membrane- entitiesin Platygyra in spiteofgeneticinterchange poses
level recognition orselectivity. a challenging question. Progeny from between-morpho-
Mixed parentage does not appear to affect the devel- species crosses may be expected to have morphological
CONFLICTING SPECIES BOUNDARIES IN I'l.ATYdYRA 107
100 100 B
80 80
60 60
40 40
20 20
'
.
i : i . .
20 40 60 20 40 60 80
100 100 D
: i
80 i - 30
60 60
40 40
20 20
20 40 60 80 20 40 60 80
%difference(genetic) %difference(genetic)
Figure5. Fertilizationsuccessasafunctionofgenotype.Comparisonofpercentfertilization(basedon
themean ofthree replicates)and allelicdifferencesbetween coloniesused in fertilization trials.(A) Mag-
neticIsland, 1991:(B)MagneticIsland, 1992;(C)DaviesReef, 1991;(D)OrpheusIsland, 1992.
characters intermediate between those ofthe parent col- unlike Goniastreafavulus, which commonly self-fertil-
oniesand, withcontinued interbreeding,distinctionsbe- izes(Heywardand Babcock, 1986;Stoddarte/tf/., 1988).
tweenthe morphologicalspecieswouldbe likelytobreak In addition, thehigh variability in fertilization ratesboth
down. However, nothing is known ofthe mechanisms withinand between morphological speciesalsoindicates
that produceskeletal differencesincorals; ifmorphology some level ofincompatibility or individual recognition
is linked to dominance alleles, then between-morphos- between certain pairs of individuals. This degree ofre-
peciesprogeny maydisplaythe skeletal characteristicsof productive separation in combination with small differ-
only one parent (e.g., Strathmann, 1981; Byrne and An- ences in the timing ofspawning is probably insufficient
derson, 1994). Alternatively, processessuch asintrogres- to produce fixed allelic differences in the populations,
sion may play a role in the maintenance ofmorphologi- but it may produce the indications ofnonrandom mat-
caldifferences in Platygyra(Millerand Benzie, in press). ingpresent in thegenotypic structureofPlatygyrapopu-
Reproductive and distributional variation may also lations(Millerand Benzie, in press).
provide some level ofsegregation between the morpho- No clear-cut habitat separation is apparent in
species, thus helping to maintain morphological differ- Platygyra, whereall morphospeciescan be found in var-
ences. There is some separation among spawning times ious habitats on a single reef (Miller, 1994a). At large
ofPlatygyra morphospecies on the Great Barrier Reef, scales, however, trends in the distribution of morpho-
but these do not constitute complete temporal barriers speciesareapparent. Forexample, P. lamellinaisrareon
to fertilization (Fig. 6). Nevertheless, mostegg-sperm in- mid-shelfandshelf-edgereefsinthecentralGreatBarrier
teractionstake place in thefirst fewsecondsaftermature Reef; P. pint is rare on coastal reefs; P. ryukyuensis is
eggs are in contact with a sperm suspension (Fig. la; most common on coastal fringing reef flats; and P. la-
Denny and Shibata. 1989; Mundy et a/., 1994), and col- mellina, P. ryukyuensis. and Platygyra H are rare or ab-
onies are more likely to be fertilized by close neighbors sent on the reefs from the southern Great Barrier Reef
(Oliver and Babcock, 1992). These factors in combina- (Miller, 1994b). Acrossevenlargerscales, furtherdiscon-
tion could result in a degree of temporal reproductive tinuities in species distributions are evident (Veron,
segregation, ifnottotal isolation. 1993), and observations ofspawning times ofPlatygyra
Sophisticated egg-sperm recognition systems are in other regions (e.g., Okinawa: Heyward et ai, 1987;
clearly operational at several levels in Platygyra. Colo- Hayashibara et ai. 1993) indicate that on geographical
niesofPlatygyradonotfrequentlyself-fertilize(Table I), scales segregation of spawning times among Platygyra