The Evolution of Distyly: Pollen Transfer in Artificial Flowers
Author(s): Judy L. Stone and James D. Thomson
Source: Evolution, Vol. 48, No. 5 (Oct., 1994), pp. 1595-1606
Published by: Society for the Study of Evolution
Stable URL: http://www.jstor.org/stable/2410250
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Evolution,48(5), 1994, pp. 1595-1606
THE EVOLUTION OF DISTYLY:
POLLEN TRANSFER IN ARTIFICIAL FLOWERS
JUDY L. STONE AND JAMES D. THOMSON
ofNew York,StonyBrook,New York11794
StateUniversity
ofEcologyand Evolution,
Department
fortheevolutionof
-A recentmodelbyLloydand Webbderivesconditions
necessary
Abstract.
We
ancestral
mutants.
morphsandinvading
basedon pollen-transfer
between
probabilities
distyly
ofthemodel.Our findings
to measuretheparameters
visiting
artificial
flowers
usedbumblebees
of a stigma-height
establishment
the firstevolutionary
stepproposedby them-odel,
supported
polymorphism
of an anther-height
establishment
Conditionsforthesubsequent
polymorphism.
stepare conforthefirst
pattemsalone.Becauseconditions
bypollen-transfer
werenotsatisfied
depression
sideredmoreonerous,however,
and becausethesecondstagedependson inbreeding
of the
theplausibility
we interpret
our resultsas supporting
as wellas pollen-transfer
patterns,
a
demonstrate
flowers
Lloyd-Webbmodel.Video imagesof bees visitingglass-sidedartificial
betweentheancestraland mutantmorph.In general,
fordisassortative
pollination
mechanism
betweenanthers
withtheheightdifference
correlated
werenegatively
probabilities
pollen-transfer
bee feeding
posturein sucha waythat
Stylelength
affects
ofthedonorandstigmaoftherecipient.
polymorphism.
couldfeasibly
disassortative
occurin theabsenceofan anther-height
pollination
pollination.
herkogamy,
heterostyly,
-Artificialflowers,
evolution,
Bombus,distyly,
Keywords.
ReceivedJune29, 1993. AcceptedJanuary
12, 1994.
Distylyis a balanced polymorphismin which
about halfoftheplantsin a populationhave long
stylesand shortstamens,whereasthe remainder
possess short styles and long stamens. It has
evolved independentlyat least 23 times (Ganders 1979; Lloyd and Webb 1992a). Since Darwin's early(1877) work on distyly,two quantitative models have been put forwardsuggesting
possiblestagesin itsevolution(Charlesworthand
Charlesworth1979; Lloyd and Webb 1992b).
Such models must explain two evolutionary
events: the developmentof the two morphological groups,and the dimorphicincompatibility
systemcharacteristicof distylousplants,wheremorphsare ferby onlycrossesbetweendifferent
tile.
The Charlesworths'model (1979) showed that
anychangein stigmaheightis unlikelyto become
established as a polymorphismin a self-compatiblespecieswithanthersand stigmaoriginally
borne at the same height.Any change that reduced pollen receipt would be lost, whereas a
change that increased receipt would sweep
throughthe population, preventingthe establishmentof a polymorphism.Accordingto this
view,thenecessaryfirststageis thedevelopment
which could
of dimorphic self-incompatibility,
become establishedifinbreedingdepressionand
selfingrates are high enough to favorthe developmentof a morphwithsterilepollen. A linked
mutationfora new stigmatypecompatiblewith
the new pollen type would complete the estabLatlishmentof dimorphicself-incompatibility.
er,themorphologicalgroupscould evolve, ifdistylyincreaseddisassortativematingbetweenthe
two matinggroups.
Most workershave acceptedthesequence outlined above, at least the propositionthat morphological groups evolved followingthe estabgroups.An important
lishmentofincompatibility
variation on the assumptionsof the model was
suggestedby Ganders (1979). He proposed that
the ancestral plants may have been herkogamous, thatis, the anthersand stigmawere spatially separated within flowers.He argues that
this morphologyis more oftenassociated with
outcrossingplantssuch as the putativeancestors
of distyloustaxa.
scenario, Lloyd
In a fundamentallydifferent
and Webb (1992b) proposed that distylycould
have evolved throughtwo morphologicalchanges
precedingthe establishmentof the incompatibilitygroups (reviewed by Barrett1990). As in
Ganders (1979), the ancestor is assumed to be
herkogamous.Specifically,the ancestor is postulatedto have "approach herkogamy,"in which
the stigma is borne at a higherlevel than the
anthers(Webb and Lloyd 1986). Then, a stigmaheight mutation produces a reverse-herkogamous morph(one withthe stigmalowerthanthe
anthers;fig.1), whichcan become establishedin
thepopulationbecause ofa frequency-dependent
1595
? 1994The SocietyfortheStudyofEvolution.All rights
reserved.
STONE AND THOMSON
1596
E1
1 111111 1, 111111 1 11111!
Approach
Reverse
Thrum
1. Longitudinalsectionsthroughartificialflowers,demonstrating
floralorganheights.Dimensions are
given in text.
FIG.
advantage in pollen transfer.Specific requirementsforthe establishmentof a polymorphism
depend on whetherseed set is pollen-limitedor
resource-limited.For the resource-limitedcase,
a polymorphism
could become establishedifeach
morphdonates pollen more successfullyto flowers of the opposite morph than to flowersof its
own morph.For thepollen-limitedcase, the sum
of pollen donation and receiptbetween flowers
of different
morphs must be greaterthan twice
the amount of intramorphtransfer.
In a second mutation,antherposition in the
reverse-herkogamousmorph is shiftedto the
heightof the highstigmas.The population now
consistsof threemorphs:approach,reverse,and
thrum. The thrum could replace the reverse
at crossmorpheitherbecause it is more efficient
pollinationwiththeapproach morph,or because
it has a lower selfingrate (if inbreedingdepression is greaterthan 1/2).Similarinequalitiesare
presumedto selectforthe loweringof approach
anthersto the level of the thrumstigmaand for
ancillaryfeaturesof distylysuch as morph-specificstigmaticsculpturing.Withthe morphological polymorphismestablished,incompatibility
groupsevolve in responseto selfingand inbreeding depression.
In familiesin which distylyhas evolved, the
morphologyof nondistyloustaxa supports the
idea thatapproach herkogamyis a plausible ancestralcondition. In a surveyof 25 familiesin
whichheterostyly
has arisen,themajorityofspecies in all 25 have flowerswith stigmasand/or
anthersexsertedfromthe floraltube (Lloyd and
Webb 1992a). The majorityof species in 22 of
the 25 familieshave tubularcorollas (Lloyd and
Webb 1992a), a floralmorphologyknownto cooccurwithapproachherkogamy
(Webb and Lloyd
1986). Finally, in several cases cited by Lloyd
taxa have close
and Webb (1 992a), heterostylous
relativeswith approach herkogamy.
Determiningwhetherincompatibilitygroups
or morphologicaldistylyevolved firstis a more
proposition,althoughthe development
difficult
of reliable phylogeniesmay eventuallyhelp. At
present,we can at leastdeterminethemechanical
plausibilityof the Lloyd-Webbmodel. A critical
test of the model requires measuring pollentransferpatternsamong threefloraltypeswithin
one population: an approach-herkogamous(or
simply"approach") ancestor,theinitialreversethe
herkogamous("reverse")invader,and finally,
long-stamened"thrum" morph that supplants
the reversemorph.
Natural populations provide limited opportunityfortestingthe model because none contains all threemorphs.In a reviewof the LloydWebb model, Barrett(1990) suggestedthat"experimentalmanipulationof the floralmorphology of heterostylousand homostylousgroups"
would be necessaryto resolvethisproblem.Satthoughitwould be, thisapproachpresents
isfying
obvious technicalproblemsformostgroups:delicate floralparts cannot simplybe repositioned
at will. Partial emasculationsof flowersin a tristylous species could re-createthe three flower
types envisioned by the model; however, the
sourceof pollen frommidlevelantherswould be
impossible to distinguish.
The use of artificialflowersto examine pollenposed by
transfer
patternssurmountsdifficulties
attemptsto workwithnaturalpopulations.With
artificialflowers,thepositionofthe floralorgans
can be easily adjusted, and dyed pollen can be
used to differentiatedonor morphs. Having
bumblebeesvisittheflowersallows us to include
the most importantbiological determinantsof
pollen flow:the positionthe pollinatorstake relative to the reproductiveorgans,the specificity
of pollen placementon their
and reproducibility
bodies, and the redistributionof pollen by
grooming.Ifpositionsofanthersand stigmascan
be foundthatproducethe hypothesizedpatterns
followingpollinatorvisits,and ifthesepositions
are biologicallyreasonable,thenthe plausibility
of the mechanismis demonstrated.
Artificialflowerscan also be used to investigate the mechanism underlyingdisassortative
pollination.Lloyd and Webb (1992b) suggested
thatthe position of floralorgansaffectsthe free
pathtakenby theproboscisas it probes theflower. We recordedvideotapes of bees visitingartificialflowerswithglass sides to investigatethe
effectof floral organ placement on pollinator
feedingposition.
In summary,the goals of the study were to
bybees betweenartificial
measurepollentransfer
POLLEN TRANSFER IN ARTIFICIAL FLOWERS
flowersto test the plausibility of Lloyd and
Webb's (1992b) model. We compared pollen
transfer
firstbetweenapproach-herkogamous
and
reverse-herkogamous
flowers,and thenbetween
reverse-herkogamous
and thrumflowers.We took
video imagesofpollinatorsvisitingall threeflower typesto demonstratethe effectof floralmorphology on pollinator positioning within the
flowerand to investigatethe mechanismunderlyingany nonrandompatternsofpollen transfer.
a.
MATERIALS AND METHODS
Mechanics.-We kepta colonyofbumblebees
(Bombus impatiens)in a screenedflight
cage. The
bees wereallowed access to theoutdoorsthrough
a small openingin the cage. Flowers were constructedof 1.5 cm-square,1.4 cm-deepplexiglass
blockswith0.9-cmdiameterholesdrilledthrough
the center,mounted on polyethylenebases (fig.
1). Floral organs were formedby insertingsize
0 nylon-headedinsect pins throughthe central
well into the foam. Each flowercontained two
anthersand a style.We dipped antherpin heads
into small dishes of driedNarcissuspollen,dyed
with food coloring.The pollen adhered to the
nylonpin heads by electrostaticattraction.Stigmas weremade of 1.5 mm-squarepieces of double-sided stickytape, cut witha razor blade and
attachedto the centralinsectpin head. A series
ofjigs was used to set stigmaand antherheights.
We arrangedthe flowersin a series of small
arrayson a table in the flightcage (fig.2) and
pipetted50% honey solutionin the well of each
flower.The centralflowerin each arrayhad no
pollen so thatwe could measure outcrosspollen
flow to it. Each set of arrays remained in the
flightcage for1 h, resultingin a singletrial.Four
trialswereruneach day for7 d, withthepositions
of the arraysrotatedin each trial. One morph
bore red pollen and the otherbore greenpollen;
these colors were switched back and forthon
alternatedays. To estimatethe amount of pollen
presentedin thesetrials,we used an Elzone particlecounterto counttheamountofpollenplaced
on 15 each of red- and green-dyedanthers.We
measuredselfingratesby havingindividualbees
visit one flowerof each morph fora total of 30
trials.Red, green,and yellowpollen wererotated
across the threeflowertypes.
Bee feedingbehavior was recorded duringa
subset of trials.The time of landing,flowerposition, and time of departurefrom the flower
were recorded for each flowervisited fromthe
time focal bees left the hive to the time they
1597
A
A R A
A
A
A A A
A
R
R R R
R
R
R A R
R
H
H A H
H
R
R A R
R
R
R R R
R
H
H R H
H
b.
A= approachflower
R = reverseflower
l
I
H = thrum
FIG. 2. a. Array
arrangement
forstigma-height
polymorphism
Placement
experiment.
ofletters
represents
offlowers
placement
on table.The legendshowsrelativeorganheights
foreachmorph.Thecentralflower
in each subarray
containsno pollen,so thatoutcross
canbemeasured.
deposition
Flowers
ofdifferent
morphs
beardifferent
colorpollen.Subarrays
arerotated
through
thefourpositionsat eachtrial.b. Arrayarrangement
foranther-height
The top
polymorphism
experiment.
twosubarrays
allowcomparison
ofthrum
and reverse
donationtoapproachflowers,
whereasthebottomtwo
allow comparisonof thrumand reversedonationto
reverseflowers.
1598
STONE AND THOMSON
returned.For about half of these trials,50 Al of
nectarwas added to each flower.For the other
half,25 Al was used.
Afterthe arrayswere removed fromthe flight
cage, the pieces of tape wereremovedwithclean
forceps,mounted on slides, and covered with
cover slips. The numberof pollen grainsof each
color was counted witha dissectingmicroscope
at lOOx.
The firstpartoftheexperimentcomparedpollen transferbetween approach-herkogamous
flowersand reverse-herkogamous
flowers(fig.2a).
Both typesof flowershad stamens 9 mm high,
whereashighstigmaswere 13 mm and low stigmas were 5 mm. The second part compared reverse-herkogamous
flowersand a "thrum"flower withstigmasat 5 mm and stamensat 13 mm
(fig.2b). The top two arraysin figure2b. allow
comparison of thrumand reverse donation to
approach flowers,while the bottom two allow
comparison of thrumand reverse donation to
reverseflowers.
For the video work,we constructedartificial
flowerswithone glass side, and a 25 x 25 x 25cm box witha clearplexiglasswall forconducting
thetrials.A table-mountedvideo camera at high
magnification(1Ox) was focused on the flower,
which filledthe entirescreen.We placed a previouslytrained,chilledbee on the flower,which
contained20 ,l of 50% honeysolution.We took
footageof both small and largeworkersfeeding
on all threeflowertypes,with floralorgans arrangedboth perpendicularto, and in line with,
the line of sightof the camera. The video image
contained a timer including frame number (1
frame/2s). We viewedtheimagesframebyframe
to record typeof floralorgan contacted(anther
or stigma),duration of contact,and portion of
bee's body making contact (distal, ventral,or
lateral;proboscis,head, thorax,or abdomen).
Small bees contactedthe flowersin a greater
varietyof ways; thus,only data fromsmall bees
are discussed here. We tested forlack of independence of floralmorph,floralorgan,and bee
body partusinga G test(Sokal and Rohlf 1981).
TestingConditionsof the Lloyd-Webb Model. -In each feedingbout,pollen fromthe donor
morphs can be counted on the stigmasof each
of the recipientmorphs. These transferprobabilities are critical parameters in Lloyd and
Webb's (1992a) model. Lloyd and Webb specifiedconditionsforthe invasion of new morphs
in termsof a matrixof qj, definedas thenumber
(quantity)of grainsfrommorphi thatare trans-
ferredon the stigma of morphj in successive
visitsto i,j flowers.The firststage of the model
presentsnecessaryconditionsfora stigma-height
polymorphism to become established, given
presenceor absence of pollen limitationof seed
set. If seed set is solely resource-limited,these
conditions must be met: qra > qaa and q, > q1,
where a and r representapproach and reverse
morphs,respectively.That is, donation by each
morph to the opposite morph must be greater
thandonationto otherplantsofthesame morph.
Because pollen receiptis always sufficient
forfemale function,
onlydifferential
male function(i.e.,
donation success) is necessary for a polymorphism to establish.If seed set is pollen limited,
qar
+
qra
>
2q. and
qar
+
qra
>
2q,r must both
be fulfilledforthe polymorphismto become established;i.e., morphsmust on average transfer
to and fromflowersofthe
pollen moreefficiently
othermorphthanwithflowersoftheirown type.
Inspection of the data (table 2) indicated that
therewas substantialvariation among different
trialsin theamountsofpollendeposited.To avoid
pooling heterogenousdata sets, we examined
theseinequalitieswithineach trialand used sign
teststo determinesignificant
ofthe
directionality
inequalitiesacross the entireset of trials(Sokal
and Rohlf 1981).
Conditionsfortheestablishmentoftheanther
heightpolymorphismare somewhatmore complicated.Pollen transferis onlypartof the story,
as morph-specific
selfingratesand inbreedingdepressionare also important.Regardingthe pollen-transfer
components,it is assumed that the
increase in anther heightwill cause the thrum
morph to be less successful than the reverse
morphat donatingpollen to reverseflowers(qrr
> qhr, where h representsthe thrummorph). If
this reduced success is more than offsetby an
increase in donation to the approach-herkogamous morph
(qha
> qra), then the anther-height
polymorphismwould be favoredpurelyby polbetween
len-transfer
asymmetry.The difference
the two inequalitiesmustbe positive,indicating
an increasein other-morph
pollinationthatmore
than compensates for the reductionof withinmorphpollination.Again, the experimentaldesign allowed the comparisons of interestto be
made foreach trial.Sign testswereused to compare the numberof trialsin whichthe difference
betweentheinequalitieswas positiveto thenumber wherethe difference
was negative,as well as
to ascertainwhetheror nottheinequalitiesthemselves were validated.
1599
POLLEN TRANSFER IN ARTIFICIAL FLOWERS
IA. Mean (sample size, standarderror)numberof seconds spent foragingper flowerby bumble bees.
flower;H, thrum-type
flower.Flowerscontained25
A, approach-herkogamousflower;R, reverse-herkogamous
or 50 ,dlnectar. Video flowerscontained variable amounts of nectar,since re-fillingbetween visits was not
possible duringfilming.
TABLE
25 ,dl
50 ,dl
Variable
A
H
R
31.8 (6, 16.32)
150 (3, 66.90)
20.0 (27, 4.71)
21.6 (22, 6.06)
42.6 (15, 12.06)
12.4 (19, 2.36)
21.6 (22, 3.84)
43.2 (28, 9.90)
11.9 (22, 2.80)
l B. Two-wayANOVA formean foraging
time/ of the central,pollen-freeflowers(n = 216, SD
flowerforflowersin arrays.Volume is amountofnectar = 45.67). Because theratio ofpollen-bearinganin flower.Morph is approach, reverse,or thrum.
thersto stigmaswas 4:1, an average of 0.286%
TABLE
Source
df
SS
1 11.02
Volume
2
7.41
Morph
2
4.79
Vol x morph
90 45.30
Error
MS
F
pr > F
11.02
3.70
2.39
0.50
21.88
7.35
4.75
0.0001
0.001
0.01
In thedevelopmentof theirmodel, Lloyd and
Webb (1 992b) assumed thatself-depositionrates
are equal among the approach-and reverse-herkogamous morphs (Ua = ur). We examined this
selfingrates
assumptionbycomparingsingle-visit
to all threemorphsby individualbees. Wilcoxon
signed-ranktests were used to compare selfing
rates.
of thepollen originallyon the antherswas delivered to stigmas.This fractionof productionto
deposition is withinthe range of estimates for
natural systems(e.g., Ganders 1976; Thomson
and Thomson 1989).
Acrossall trials,stigmaticpollen loads differed
dependingon the combinationof donor and recipient(fig.4). High stigmasreceived more pollen than low stigmas. Considering only inter-
a 50 tl|
80
nectar
E
60 -
DONORTYPE
4
*
RESULTS
The majorityofbee movementsbetweenflowers were betweenflowerson the same subarray
(83.6%, n = 72, P < 0.01, G test).
bees in arraysspent about twice
Surprisingly,
as long foragingon flowerswith high stigmas
than on eitherof the morphs with low stigmas
(table 1). Bees feedingon video flowers,where
they were only presentedwith a single flower,
also spent more time feedingon approach than
on reverseor thrumflowers(table 1). Bee foragingbehaviorand theamountofpollen transfer
also changeddependingon theamount ofnectar
present.Largernectarloads caused longervisits
and less pollen depositionbut not did not change
thepatternofpollen transfer
(table 1, fig.3). Bees
visited an average of 5.0 flowerson single foragingbouts if flowerscontained 50 Al (n = 9, SE
= 0.83) and 7.0 flowersif theycontained 25 Al
(n = 7, SE = 0.76; P > 0.05).
Pollen-bearinginsect pins carried a mean of
4474.5 grains(n = 22, SD = 1699.9), whichwas
independentof dye color. An average of 51.13
pollen grainsper bout was foundon the stigmas
a,c 20 -
Reverses
|VIThrum
l
14
13
i4
14
Reverse
Approach
TYPE
RECIPIENT
b. 25 tl
nectar
80 -
E0 -60
DONORTYPE
40
U Reverse l i
w 20
_ 20 -
V
1
14
14
14
Thrum
14
Reverse
Approach
TYPE
RECIPIENT
from
bybumblebees
3. Pollengrainstransferred
and thrumto approach-and rereverse-herkogamous
Verticallinesinflowers.
artificial
verse-herkogamous
Numbersbelowbarsrepresent
errors.
dicatestandard
numberoftrials.
FIG.
STONE AND THOMSON
1600
2. Pollen counts on stigmas. qaa indicates
numberofgrainsfromapproach-herkogamousflowers
that were counted on an approach-herkogamousstigma withina single trial,qra is the numberof reverse
grainscountedon an approachstigma,and so on. Trials
consisted of a set of fourarraysvisited by bees for 1
h (See fig.2).
TABLE
60 -
E
U
e40
DONORTYPE
Reverse
Iil
20
Trial
1
2
25
26
27
27
3
Reverse
Approach
4
RECIPIENT
TYPE
5
6
7
8
9
80
10
11
60
12
13
1Y'PE
l 111
~~~~~~~~DONOR
uc
14
Reverse IM
*~~~~~~
40- 1 |
@
15
16
Thrum jjl
r20 _El
L
17
18
19
25
27
27
28
20
Reverse
Approach
21
TYPE
RECIPIENT
22
by bumblebees 23
FIG. 4. a. Pollengrainstransferred
and reverse-herko-24
betweenapproach-herkogamous
Verticallinesindicatestan- 25
flowers.
gamousartificial
26
numberof 27
darderrors.
Numbersbelowbarsrepresent
are meansand obscure
trials.The valuesrepresented
acrosstrials.b. PollengrainstransferredMean
heterogeneity
flow- SD
andthrum
toapproachandreverse
fromreverse
ers.
highstigmasreceivedan average
morphtransfer,
of 60.6 to 88.2 grains,dependingon the donor
type(fig.4, tables 2, 3). In contrast,intermorph
donation to low stigmasaveraged only 25.1 to
36.1 grains(fig.4, tables 2, 3).
The videos revealed that bees' bodies contactedfloralorgansin a structuredway (table 4).
Tall organs(stigmaon approach flowers,anthers
on thrums)tendedto contactthe thoraxand abdomen, whereas shortstigmasand antherscontactedprimarilythehead and thorax(fig.5). Style
position affecteddorso-lateralpositioningof the
bee such thatsome floralorgansforsome morphs
tended to be contactedlaterally,whereas others
tended to be contacteddorso-ventrally(fig.6).
The plausibilityof the evolution of a stigmaheightpolymorphismwas upheld by the pollentransferlevels thatwe found.Of the inequalities
requiredforthe resource-limitedcase, qra > qaa
qaa
11
55
16
30
22
50
37
15
11
18
46
74
33
21
35
10
41
27
15
43
37
21
53
30
14
74
23
31.9
17.58
qra
17
42
52
79
22
28
38
16
35
27
63
27
65
24
60
51
104
81
135
91
43
46
100
65
72
165
89
60.6
35.55
qar
24
15
*
6
28
17
36
8
24
28
70
16
25
16
86
51
56
48
26
54
3
17
47
53
82
93
45
36.1
24.92
qff
14
9
19
51
17
0
11
2
119
55
59
12
23
11
13
*
1
57
16
190
*
21
0
32
69
60
21
32.7
40.71
* Missing values.
was satisfiedin 23/27 trials(P < 0.01, signtest),
and qar> q1,was satisfiedin 18/24 trials (p <
0.05; sign test). In other words, both morphs
were more successfulat donatingpollen to flowers oftheoppositemorphthanto flowersoftheir
own morph.Consideringthepollen-limitedcase,
qar + qra > 2q. was satisfied in 20 of 27 trials
(P < 0.05; sign test), and qar + qra > 2q,r was
satisfiedin 20 of 24 trials(P < 0.01, sign test).
Thus, the sum total of intermorphtransferwas
greaterthan twice the intramorphtransferfor
eithermorph,establishingconditionsfora protected polymorphismif both female and male
reproductivesuccess are limitedby pollen transfer.
In contrast,the establishmentof an antherheightpolymorphism,based on pollen-transfer
patternsalone, was only partiallysupportedby
the data. One of the necessaryinequalities was
1601
POLLEN TRANSFER IN ARTIFICIAL FLOWERS
3. Pollen counts on stigmas. qra indicates
number of grains fromreverse-herkogamousflowers
that were counted on an approach-herkogamousstigma withina single trial,qha is the number of thrum
grainscountedon an approach stigma,and so on. Trials
consisted of a set of fourarraysvisited by bees for 1
h (fig. 2). Flowers in trials 1-16 contained 50 ,l of
nectar.Flowers in trials 17-28 contained25 Al of nectar.
TABLE
Trial
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
Mean
SD
qra
149
134
97
137
34
2
25
0
25
0
104
57
7
71
61
63
33
101
135
123
30
37
69
71
142
35
206
219
77.4
60.36
qha
41
59
144
*
105
73
27
11
99
0
104
36
44
232
122
108
114
150
110
138
43
93
96
61
137
138
103
82
88.2
52.58
qrr
78
26
68
27
3
19
6
16
87
1
13
7
53
100
30
68
106
43
84
201
7
0
9
54
47
37
94
65
48.2
44.78
qhr
31
112
67
55
12
29
3
10
13
6
0
0
10
22
10
61
18
22
2
80
26
32
29
2
22
20
0
10
25.1
27.06
* Missing values.
upheld: qr > qhr for20/28 trials(P < 0.05, sign
test). The otherwas not: qha > qraforonly 17/
27 trials(P > 0.05, signtest).Althoughthe median value forqha was 99 and the median forqra
was 61, the distributionsof the values were not
significantlydifferent(Mann-Whitney U test).
Therefore,the increase in donation to the approach morph exceeded the loss in donation to
These
the reverse morph but not significantly.
resultsdo not refutethe possibilityof spread of
an anther-height
mutation,but neitherdo they
provide strongsupport.
probabilitiesare not
However, pollen-transfer
the only parametersneeded to predictthe likevariant.
lihood ofestablishmentof a thrum-type
Morph-specificselfingrates and inbreedingdepression are also important(Lloyd and Webb
by
modelfornumberofcontacts
TABLE 4. Log-linear
bee bodypartsto floralorgansforthreetypesofartificialflower.Morphis approach,reverse,or thrum.
Beebodypartis head,
orstigma.
Floralorganis anthers
weresummed
orabdomen.Numberofcontacts
thorax,
22 visitsto reverse
over27 visitsto approachflowers,
flowers,
and 19 visitsto thrumflowers.
Effect
G
df
P
Morphx floralorganx
bodypart
Morphx floralorgan
Morphx bodypart
Floralorganx bodypart
181.24
66.74
37.18
17.66
12
3
4
3
.001
.001
.001
.001
APPROACHFLOWERS
>
l
(n=27)
E
1
0
head
0M thorax
1|81 abdomen
co
E
_RE
E
stigma
anthers
REVERSEFLOWERS
3-
(n=22)
El head
Eb
El0
~ ~~~~~~~~~~Eabdomen
c
E
:_I
0
4
E
o
stigma
3~~~~~~~
0
C
thorax
anthers
THRUMFLOWERS
9)
g
~~~(n=1
2head
2i
1-
A
1
g
g2El
co
thorax
abdomen
E
stigma
anthers
FIG. 5. Mean numberof contactswithanthersand
stigmasby variousbodypartsof bees visitingthree
havehigh
flowers.
Approachflowers
typesofartificial
have
reverseflowers
stigmasand midheight
anthers,
flowers
andthrum
lowstigmas
andmidheight
anthers,
have low stigmasand highanthers.Verticallinesinto number
dicatestandarderrors.Samplesize refers
ofvisits.
STONE AND THOMSON
1602
STIGMA
ANTHERS
12-
12-
0
o0
4-'84-
~~~~~~~~0
0
4-
4
-
8/
0
8
-10
Approach
Reverse
|
laea
llE
Approach
Thrum
Reverse
otc
Thrum
30
dorsal contact
30 artificial flowers. Approach flowers have high stigmas and midheight anthers, reverse flowers hlateral contact
E ventralcontact
f
20
207
0c
o
0
o
-
4-,
0
4-'
10
4-
Approach
Reverse
Thrum
10-
Approach
Reverse
Thrum
of bees' heads and abdomensas theycontactedanthersand stigmasof threetypesof
FIG. 6. Orientation
havelow stigmas
reverseflowers
havehighstigmasand midheight
flowers.
artificial
anthers,
Approachflowers
ofcontacts
Valuesaretotalnumber
havelowstigmas
andhighanthers.
andthrum
flowers
andmidheight
anthers,
22 visitsto reverseflowers,
and 19 visitsto thrumflowers.
from27 visitsto approachflowers,
envisioned by the Charlesworths,with a nonherkogamousancestor a style-lengthpolymorphism is unlikelyto be favoredby selectionfor
pollen receipt.Lloyd and Webb came to the oppositeconclusionby consideringpollendonation
(male function)and invokinga herkogamousancestor. Of this crucial step in the Lloyd-Webb
scenario,Barrett(1990) commentedthat"While
it is well known that even low levels of disassortativematingresultin polymorphicequilibria
in self-compatiblepopulations, it is less clear
whethera simplechangein stigmaheightis likely
to give riseto thispatternofmating."Our results
certainlydemonstratethat such disassortative
DISCUSSION
matingis possible. That we obtained those reEvolutionary Interpretation.-Bumble bees sults with the firstflowerdesign chosen might
transferredpollen among these flowersin pat- even indicate that it is likely,although further
ternsthatreinforcethe plausibilityof a reverse- exploration of various artificialmorphologies
herkogamousvariant being able to invade an would be necessaryfor a formalassessment of
ancestralpopulation of approach-herkogamous likelihood.Our resultsalso suggestthatselection
flowers.This is perhaps the most importantas- throughmale functionis sufficient,
whetheror
pect of the Lloyd-Webb model, and the aspect not selection throughfemale functionis operthatdiffersmost fromthe viewpointof Charles- ating,thatis, whetherseed productionis pollenworthand Charlesworth(1979). In the scenario limited or resource-limited.
1992b). For example,ifinbreedingdepressionis
weak, a reductionin self-pollinationby the new
morphwould decreaseitslikelihoodofspreading
in the population. If inbreedingdepression is
strong,the thrummorph could replace the reverse morphifit had a lower selfingrate. Selfing
rates did not differforthe threemorphs. Mean
per-visitselfingrateswere 28.1 forthe approach
morph(n = 31, SE = 4.72), 31.0 forthe reverse
morph (n = 31; SE = 3.87), and 32.6 for the
thrummorph (n = 26; SD = 6.39). We did not
finddifferential
selfingratesthat mightincrease
supportforthe second stage of the model.
1603
POLLEN TRANSFER IN ARTIFICIAL FLOWERS
The second stage of the Lloyd-Webb model,
dimorphism,
the evolution of an anther-height
was seenbyLloyd and Webb as "easy to achieve"
dimorphismhas evolved.
afterthestigma-height
Barrett(1990) also raised no objection to this
stage of the model, in contrastto his agnostic
position (above) regardingthe establishmentof
thestylardimorphism.For our designofflowers,
however, support for this phase is weak. Alpatternswent in the
thoughthe pollen-transfer
rightdirectionmoreoftenthannot,thistendency
fellshortof significance.
In contrastto the firststage, in which interferencebetweenstyleand stamensdictatesthat
only a mutation of large effectwould make a
difference,
the second stage in the evolution of
fulldistylymightbe morelikelyto proceed from
a seriesof small changesin antherheightoccurringbecause ofperformanceadvantagesforflowers with anthersmore similar in heightto opposite-morphstigmas.If this is the case, it is to
be expected that selectionpressureson the second stage are less strongand would not be revealed withoutlargersample sizes. The greater
separation between stigma heightsthan anther
heightsin many species supportsthispossibility
Phillipand Schou 1981;
(e.g.,Anchusaofficinalis,
Psychotriaspp., Hamilton 1990).
Robustnessof Pollen-TransferInequalities.We chose the single feedingtrial as our unit of
comparison,because therewas considerableheterogeneityof pollen deposition across trials. If
pollen-transferprobabilities are pooled across
trials,some ofthemare morerobustthanothers.
In general,all of the patternshold, withthe exception of inequalitiesinvolvingpollen transfer
betweenreverseflowers(q4). Figure4 showsthat
the mean forqar = qr, in contrastto the sign-test
results,which show qar > q1.. Examination of
table 2 shows thatseveralextremelyhighvalues
between
forq,. are responsibleforthe difference
the two types of comparisons. Using the Wilcoxon signed-rankstest,qar > qrrat P < 0.06,
veryclose to the conventionalcriterionforstatisticalsignificance.
Which statisticalapproach is most appropriate? Stigmaticpollenloads in ourdesignare analogous to lifetimefloralstigmaticpollen loads in
nature.Means are not appropriate,because they
imply that excess pollen donated to one flower
to another. If seed set is recan be transferred
source-limited(as in thiscomparison),thenumber of flowersforwhich a morph wins the majorityof ovules should be more importantthan
5. Mean and mediannumberofpollengrains
number
twomorphs.qraindicates
between
transferred
counted
flowers
of grainsfromreverse-herkogamous
stigma,qha is thenumon an approach-herkogamous
berof thrumgrainscountedon an approachstigma,
and so on. Separationis thedistance(mm)between
theantherheightof thedonorand thestigmaheight
oftherecipient.
TABLE
qha
qra
qrr
qar
qaa
qhr
88.2 69.2 39.6 36.1 31.9 25.1
Mean
19
30
28
26
61
99
Median
8
4
4
4
4
Separation 0
betweenflowersin
the magnitudeof differences
whichit was in the majorityversusthe minority.
The signtestis moreappropriatethanthesignedranks test under these conditions. The results
hold,althoughit is worthnotingthatinequalities
involvingapproachflowersas recipientsare more
robustthanthoseinvolvingthemas donors.This
patternis consistentwiththe video footageand
withobservationsfromnaturalflowersdiscussed
below.
Mechanism.-In general,the resultsconfirm
Darwin's (1877) predictionthat pollen transfer
betweenanthersand stigwould be mostefficient
ma presentedat the same height.The median
transfervalues were lowest between flowersin
whichanthersand stigmawere8 mm apart,highest between flowerswith anthersand stigmaat
the same height,and intermediatebetweenflowersin whichantherand stigmaswere4 mm apart
(table 5). Video images of bees contactingfloral
partsdemonstratethatcontactsbetweenlongand
shortfloralorgansare segregatedon bees' bodies,
in accordance with studies of wild-caughtpollinators of heterostylousplants (Darwin 1877;
Olesen 1979; Lewis 1982).
For the stylarpolymorphismto become established,however,the positionof the stigmaof
the invading morph must create asymmetrical
pollen flow in the absence of an anther-height
height
thatis,whenanther-stigma
polymorphism,
betweenintra-and inseparationdoes not differ
termorphpollinations.What is the mechanism
forthisasymmetrythatwe observed?Lloyd and
Webb (1992b) postulated that the "free path"
taken by the proboscis of the pollinatordiffers
between the two morphs. They suggestedthat
the positions of the floralorgans constrainthe
path available forprobingby the pollinator,affectingwhichportionsoftheprobosciscome into
contact with floralorgans. The presence or ab-
1604
STONE AND THOMSON
TABLE6. Probabilityof individual pollen grains being transferred
between and withinmorphs in distylous
species. Probabilitycalculated from Tij = rijfj/pifi,
where T is the probabilityof a pollen grain produced by
morphi landingon a stigmaof morphj, r is the mean stigmaticpollen load forthistransfertype,pi is the pollen
production of the donating morph, and Ji,fj are the population frequenciesof the donor and the recipient
(equation fromLloyd and Webb 1992b). All values are multipliedby 10-3. The superscriptE indicates that
recipientswere emasculated; intramorphvalues withoutthis superscriptare included forcomparativepurposes
but are not actual transitionprobabilities,since theyinclude self pollen. Sample sizes followingthe reference
indicate thatthe values representmeans froma numberof populations or years.
Species
Tpp
Amsinckia
douglasiana
A. grandiflora
A. vernicosa
var. furcata
Cratoxylumformosum
Hedyotiscaerulea
Hypericum
aegypticum
Jepsoniaheterandra
J.parryi
Lithospermum
caroliniense
Lythrum
californicum
L. lineare
Primulaelatior
P. veris
P. vulgaris
P. vulgaris
Pulmonariaobscura
Turnerasubulata
3.0E
13.4
I.OE
13.0
Ttp
Tpt
Ttt
3.9
3.7
4.0
6.1
36.1
8.2
4.5E
7.7
0.7
1.4
12.8
58.1
10.3
2.4
2.4E
3.1
10.8
0.4
31.0
4.0
2.7E
2.7
0.7
0.5
4.2
1.6
5.0
1.3
0.7E
2.1
1.6
3.1E
3.0
1.7
6.2
1.7
0.7
0.8E
0.7
1.9
2.1
5.5E
1.3E
2.1
4.0
4.0E
1.7E
6.8
Reference
Ganders 1976
Omduff1976 (n = 3)
Ganders 1976
Lewis 1982*
Ornduff1980a (n = 12)t
Ornduff1975
Ganders 1974
Omduff1970t
Weller 1980, 1985 (n = 9)?
Ornduff1978 (n = 6)
Ornduff1978j
Schou 1983#
Ornduff1980b (n = 3)
Ornduff1979 (n = 4)
Piper and Charlesworth
1986**
Olesen 1979tt
Rama Swamy and
Bahadur 1984 (n = 5)tt
Assumedpin: thrum
ratio50:50.Stigmatic
pollenload valuestakenfromabstract.
forwhichstigmatic
datataken
t Morphratiodatatakenfrom12 populations
pollenloadswereavailable.Pollenproduction
from4 populations
reported.
t Morphratiosaremeansfrom8 populations.
werefirst
? Mean stigmatic
loads/population
calculated
datafromold versusyoungflowers
(combining
and/orsmallversus
were
largeplants).Thesewerepooled,excluding
a MillerDunes 1978 report(Weller1980)sincedata fromthispopulation
includedin bothreferences.
Pollenproduction
and morphratiosaremeansfromthreepopulations
(Weller1980).
valuesstrongly
influenced
1Pollentransfer
bya pin: thrum
morphratioof0.85:0.15 basedon a samplesizeof20 plants.
#At 12 d afteranthesis,
whenstigmatic
pollenloadswereat a maximum.
**The ratioofplantsin thepopulation
was 50:25:25pin: thrum:long-homostyle.
Long-homostyles
possessedthrum-type
I haveassumedthatthep:tratioforthrums
as donorsis 50:50, buttheratioforpinsas donorsis 50:25.
pollen.Therefore,
surface
was counted.
ttA sampleofthestigmatic
ttAssumedpin: thrumratio50:50.
*
sence of the long styleleads to different
patterns
of pollen pickup,even thoughantherheightsdo
not differbetweenthe two morphs.
Videotapesprovideevidencethatthelongstyle
does dictatebee posturein sucha way thatpollen
fromlong-styledmorphs is picked up on body
regionsdistinctfromtheareas wherepollen from
short-styledmorphs is picked up. Bees feeding
on approach flowerstend to enterthe flowerso
that the long stylelies along the midline of the
body,withantherscontactingthethoraxlaterally
(fig.6). Bees feedingon reverseor thrumflowers,
in contrast,oftenenterperpendicularly,so that
the antherscontactprimarilythe ventralor dorsal thoracicregions(fig.6). Stigma contactsare
also affectedby bees' posture.Bees contact approach stigmaspredominantlywith the ventral
surfaceof the thorax (fig.6), providingan excellent mechanism for pollen transferfromreverseand thrumto approach flowers.The mechanism for transferfrom approach to reverse
flowersis notso apparent.Bees contactedreverse
stigmas relativelyinfrequently,and predominantlywith the dorsal side of the head, an area
which contacted anthers of all flowertypes in
similarproportions(fig.6).
withNaturalFlowers.
-PollenComparisons
transfer
patternsfortheartificialflowersare consistentwith those reportedfromnatural populations.Studies of open-pollinatedstigmasallow
1605
POLLEN TRANSFER IN ARTIFICIAL FLOWERS
a comparisonof pollen flowbetweenmorphsbe- the particularfloraldesign we chose? The size
cause of the pollen-size polymorphismasso- and anther-stigmaseparation in our flowersis
ciated with distylyin many species (Ganders withintherangefoundin nature(Ganders 1979);
1979; Dulberger 1992). In 13 of 17 cases sur- however,several otherfeaturescould be modiveyed,pollen flowwas greaterfromthrumto pin fiedto morecloselyapproximatenaturalflowers.
flowersthanfrompin to thrumflowers(table 6). First,the sides of the corolla were parallel, inThis resultreinforcesa generalimpressionthat stead of taperingtowardsthebase. It is probable
pollen transferis higherbetween high anthers that a taperingcorolla would increase contact
and longstigmasthanbetweenorgansplaced deep withthe low stigmaon reverseand thrumflowwithinthe corolla tube (Levin and Berube 1972; ers. Second, theinsectpins used forfloralorgans
are muchstiffer
thannaturalstylesand filaments,
Beach and Bawa 1980).
Tristylousspecies have similarpollen-transfer perhaps dictatingpollinator posture more expatterns.In Pontederiacordata,midlevelanthers tremely.Third, the unrealisticallyhigh nectar
to mid stigmashave thehighesttransitionprob- volumes used to induce bees to feed resultedin
abilityof any of the nine possible combinations unnaturallylong visits. Prototypeflowersthat
in fiveof nine experiments,and long anthersto avoid thesedrawbacksare in theworks.It would
longstigmashave thehighestprobabilityin three be interesting
to constructa seriesofsuch flowers
ofnine experiments(Harder and Barrettunpubl. to see how well asymmetricalpollen flowpersists
calculations; data fromPrice and Barrett1982, over a rangeof morphologies.
1984; Barrettand Glover 1985; Glover and Barrett1986). The midheightorgans in Pontederia
ACKNOWLEDGMENTS
are placed near the mouthof the corolla, implyWe thank D. G. Lloyd, C. J. Webb, L. D.
ingthatthismaybe theoptimalpositionforboth
pollen donationand receipt.However,as Harder Harder, and S. C. H. Barrettforprovidingunand Barrettpointedout,thetransfer
probabilities published manuscripts,and D. G. Lloyd, S. C.
are strongly
influencedbytheratioofthemorphs H. Barrett,and two anonymous reviewersfor
in the population. Measurementof pollen stig- commentson an earlierversion of the paper. J.
maticloads followingknownsequences ofvisits, Yen provided video expertise and equipment
or constructionofpopulationswithequal morph (supported by Office of Naval Research
ratioswould be the best way to measuretransfer NOOO1492J1690), and J.Levintonloaned us video playback equipment. Financial supportwas
probabilities.
With experimentalpopulations of the self- provided by National Science Foundation grant
BSR 90-06380 to J.T. and BSR 91-22752 to J.T.
compatible tristylousEichhornia paniculata,
Kohn and Barrett(1992a) foundthatthe short- and J.S. This is contribution897 in Ecologyand
styledmorph had the highestmale fitnessand EvolutionfromtheStateUniversityofNew York
the long-styledmorph was relatively female. at StonyBrook.
Subsequent experiments (Kohn and Barrett
1992b) revealed thathigh-levelanthersdonated
pollento compatiblestigmasmoreefficiently
than
low-levelanthers.In Lythrumsalicaria, O'Neill
(1992) found that dye transferamong morphs
was asymmetric,withmore thanrandomexpectation of transferfrom short-styledmorphs to
mid-and long-styledmorphs.Once again,pollen
transferbetween higher-levelorgans predominates. These experimentssupportthe argument
for the introductionof a reverse-herkogamous
mutantin orderto increase male fitness.
Toward Increasing Realism. -Our artificial
flowersdemonstratedthat the presence or absence of a long stylecan affectthe pollinator's
stance in a stereotypicalway, resultingin disassortativepollinationbetweenmorphs. How robust is this result,and how dependent is it on
LITERATURE
CITED
Barrett,S. C. H. 1990. The evolution and adaptive
Trends in Ecology and
significanceof heterostyly.
Evolution 5:144-148.
Barrett,S. C. H., and D. H. Glover. 1985. On the
Darwinian hypothesisof the adaptive significance
of tristyly.
Evolution 39:766-774.
Beach, J. H., and K. S. Bawa. 1980. Role of pollinatorsin theevolutionofdioecy fromdistyly.Evolution 34:1138-1142.
Charlesworth,
D., and B. Charlesworth.1979. A model
for the evolution of distyly.American Naturalist
114:467-498.
formsof flowers
Darwin, C. 1877. On the different
on plants of the same species. J. Murray,London.
Dulberger,R. 1992. Floral polymorphismsand their
functionalsignificancein the heterostyloussyndrome. Pp. 41-84 in S. C. H. Barrett,ed. The evoSpringer,Berlin.
lutionand functionofheterostyly.
Ganders, F. R. 1974. Disassortative pollination in
1606
STONE AND THOMSON
thedistylous
plantJepsoniaheterandra.
Canadian
speciesofLythrum
sectionEuhyssopifolia.
AmerJournal
ofBotany52:2401-2406.
icanJournal
ofBotany65:1077-1083.
* 1976. Pollenflowindistylous
* 1979. Pollenflowina population
populations
of
ofPrimula
Amsinckia(Boraginaceae).Canadian Journalof
vulgarisHuds. BotanicalJournalof the Linnean
Botany54:2530-2535.
Society78:1-10.
* 1979. The biologyofheterostyly.
* 1980a. Heterostyly,
New Zeapopulation
composition,
landJournal
ofBotany17:607-635.
and pollenflowin Hedyotiscaerulea.American
Glover,D. E., and S. C. H. Barrett.1986. Stigmatic
ofBotany67:95-103.
Journal
pollenloads in populations
* 1980b. Pollenflowin Primulaveris(PrimofPontederia
cordata
from
thesouthern
U.S. American
Journal
ofBotany
ulaceae).PlantSystematics
andEvolution135:8973:1607-1612.
93.
Hamilton,
C.W. 1990. Variations
ona distylous
theme Phillip,M., and 0. Schou. 1981. An unusualheterin Mesoamerican
Psychotria
subgenusPsychotria
omorphic
self-inincompatibility
system.
Distyly,
(Rubiaceae).MemoirsoftheNew YorkBotanical
compatibility,
pollenloadandfecundity
inAnchusa
Garden55:62-75.
officinalis
(Boraginaceae) New Phytologist89:693Kohn,J. R., and S. C. H. Barrett.1992a. Experi703.
mentalstudiesonthefunctional
significance
ofhet- Piper,J.,and B. Charlesworth.
1986. The evolution
erostyly.
Evolution46:43-55.
ofdistyly
inPrimulavulgaris.
BiologicalJournal
of
1992b. Floralmanipulations
revealthecause
theLinneanSociety29:123-137.
of male fitness
variationin experimental
popula- Price,S. D., and S. C. H. Barrett.1982. Tristyly
in
tionsof Eichhorniapaniculata(Pontederiaceae).
Pontederia
cordataL. (Pontederiaceae).
Canadian
Functional
Ecology6:590-595.
ofBotany60:897-905.
Journal
Levin,D. A., and D. W. Berube. 1972. Phloxand
1984. Thefunction
andadaptivesignificance
ofa pollination
Colias:theefficiency
Evosystem.
of tristyly
in PontederiacordataL. (Pontederilution26:242-250.
aceae). BiologicalJournalof theLinneanSociety
Lewis,D. 1982. Incompatibility,
stamenmovement
21:315-329.
ina heterostyled
andpolleneconomy
forest RamaSwamy,N.,andB. Bahadur. 1984. Pollenflow
tropical
tree,Cratoxylum
Proceedindimorphic
formosum
(Guttiferae).
Turnera
subulata(Turneraceae).
New
ingsoftheRoyalSocietyofLondonSeriesB 214:
Phytologist
98:205-209.
273-283.
in Primulaelatior(L.)
Schou,0. 1983. The distyly
Lloyd,D. G., andC. J.Webb. 1992a. The evolution
Hill (Primulaceae),
witha studyofflowering
pheofheterostyly.
Pp. 152-178in S. C. H. Barrett,
ed.
nologyand pollenflow.BotanicalJournalof the
Theevolution
andfunction
ofheterostyly.
Springer,
LinneanSociety86:261-274.
Berlin.
Sokal,R. R., andF. J.Rohlf. 1981. Biometry,
2d ed.
. 1992b. Theselection
ofheterostyly.
Pp 179W. H. Freeman,San Francisco.
207 inS. C. H. Barrett,
ed.Theevolution
andfunc- Thomson,
J.D., andB. A. Thomson.1989. Dispersal
tionofheterostyly.
Berlin.
Springer,
of Erythronium
grandiflorum
pollenby bumbleand pollen
Olesen,J. M. 1979. Floralmorphology
bees:implications
forgeneflowand reproductive
flowin theheterostylous
speciesPulmonariaobsuccess.Evolution43:657-661.
scuraDumort(Boraginaceae).
NewPhytologist
82: Webb,C. J.,andD. G. Lloyd. 1986. The avoidance
757-767.
ofinterference
betweenthepresentation
ofpollen
O'Neil, P. 1992. Variationin male and femalereand stigmain angiosperms.
II. Herkogamy.
New
productive
successamongfloralmorphsinthetrisZealandJournal
ofBotany24:163-178.
salicaria(Lythraceae).
Amer- Weller,S. G. 1980. Pollenflowandfecundity
tylousplantLythrum
inpopicanJournal
ofBotany79:1024-1030.
ulationsofLithospermum
American
caroliniense.
R. 1970. Incompatibility
and the pollen
Ornduff,
ofBotany67:1334-1341.
Journal
economyofJepsoniaparryi.AmericanJournalof
1985. The lifehistory
ofLithospermum
carBotany57:1036-1041.
oliniense:a long-lived
herbaceoussand dunespeand pollenflowin Hyper1975. Heterostyly
cies.EcologicalMonographs
55:49-67.
icumaegypticum
BotanicalJournal
of Wolfe,L. M., and S .C. H. Barrett.1989. Patterns
(Guttiferae).
of
theLinneanSociety71:51-57.
pollenremovaland depositionin tristylous
Pon1976. Thereproductive
ofAmsinckia
tederia
system
cordata(Pontederiaceae).
BiologicalJournal
a distylous
grandiflora,
species.Systematic
oftheLinneanSociety36:317-329.
Botany
1:57-66.
1978. Featuresofpollenflowin dimorphic
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