American Journal of Botany 88(11): 2113–2118. 2001.
POLLEN
LOAD SIZE, REPRODUCTIVE SUCCESS, AND
PROGENY KINSHIP OF NATURALLY POLLINATED
FLOWERS OF THE TROPICAL DRY FOREST TREE
PACHIRA QUINATA (BOMBACACEAE)1
MAURICIO QUESADA,2,4 ERIC J. FUCHS,3
AND JORGE
A. LOBO3
Estación de Biologı́a Chamela, Instituto de Biologı́a, Universidad Nacional Autónoma de Mexico, A. P. 21, San Patricio, Jalisco,
Mexico 48980; and
3
Escuela de Biologı́a, Universidad de Costa Rica, San José, Costa Rica
2
Several studies have demonstrated, using controlled pollinations, that the number and identity of pollen grains deposited onto a
flower’s stigma affect the reproductive success of plants. However, few studies have shown this relationship under conditions of natural
pollination. Using the tropical dry forest tree Pachira quinata, we evaluated the relationship between the number of microgametophytes
per pistil and the number of sires with respect to the production of fruits and seeds in a natural population of Pachira quinata. Our
study demonstrates that fruit and seed production are directly related to the number of microgametophytes per pistil in natural populations of P. quinata. Only 6% of the marked flowers developed into mature fruits and 10% of the marked flowers initiated fruits but
later aborted them. A mean of 23 pollen grains were required to produce a seed. Flowers with .400 pollen grains on the stigma
always developed into mature fruits, whereas flowers that received ,200 grains never matured fruits. Half of the pollen grains
transferred to a flower stigma germinated and developed pollen tubes to the base of the style. The number of pollen grains on a stigma
explained 34% of the variation in seed number per fruit, and the number of seeds produced per fruit is positively correlated with the
size of the seeds. The population of P. quinata studied is predominantly outcrossing, and seeds within fruits are sired by one or a few
donors. The total seed crop within trees was sired by three to five donors. Our study examined the implications of the variation in
size of microgametophyte loads per pistil with respect to the breeding system and the paternity of progeny under natural conditions.
The competitive ability of pollen and pollen tube attrition are important factors regulating fruit production in P. quinata.
Key words:
dry forest.
Bombacaceae; Pachira quinata; paternity; pollen competition; pollen–pistil interactions; self-incompatibility; tropical
Flowers pollinated under natural conditions differ in the
number and identity of pollen grains deposited onto their stigmas (Stephenson et al., 1995; Delph and Havens, 1998; Winsor, Peretz, and Stephenson, 2000). The size of the pollen load
deposited onto a stigma may affect the number and size of
seeds produced per fruit (Winsor, Davis, and Stephenson,
1987; Stephenson, Devlin, and Horton, 1988; Quesada, Winsor, and Stephenson, 1993), the probability of seed and fruit
abortion (Bawa and Webb, 1984; Stephenson and Winsor,
1986; Casper, 1988; Lee, 1988; Nakamura, 1988; Niesenbaum
and Casper, 1994; Niesenbaum, 1999), and the quality of the
progeny produced (Mulcahy and Mulcahy, 1975; McKenna
and Mulcahy, 1983; Ottaviano et al., 1988; Richardson and
Stephenson, 1992; Quesada, Winsor, and Stephenson, 1993,
1996a, b; Johannsson and Stephenson, 1997; Winsor, Peretz,
and Stephenson, 2000; but see Snow, 1990; Mitchell, 1997).
The genetic composition of the pollen load is also known
to affect the number and size of seeds produced per fruit. For
example, when mixtures of self and cross pollen are deposited
onto a stigma, the self pollen is less likely to achieve fertilManuscript received 25 July 2000; revision accepted 27 April 2001.
The authors thank Kathryn Stoner and Andrew Stephenson for valuable
comments on earlier versions of this manuscript, and also thank Kathryn Stoner, Florencia Quesada, Andrea Quesada, and Salomón Bermudez for assistance in the field. This research was partially funded by CONACYT research
grant (No. 31826N), PAPIIT, Universidad Nacional Autonoma de Mexico research grant (No. IN213999) awarded to Mauricio Quesada, Kathryn Stoner,
and Jorge Lobo, an IFS research grant (No. D/2617-2) awarded to Mauricio
Quesada, and Vicerrectoria de Investigacion, Universidad de Costa Rica research grant (No.111-99-319) awarded to Jorge Lobo.
4
Author for reprint requests (e-mail: [email protected]).
1
ization in species with cryptic, partial, or complete self-incompatibility systems (Bertin and Sullivan, 1988; Lloyd, 1992;
Plitmann, 1993; Cruzan and Barret, 1996; Levin, 1996; Stephenson, Good, and Vogler, 2000). This may lead to reduced
seed production if there is insufficient cross pollen to fertilize
all of the ovules. In self-compatible species, self-fertilization
and other forms of inbreeding are known to result in higher
levels of seed abortion and/or smaller seeds, due to homozygosity of rare lethal and defective alleles (e.g., Darwin, 1876;
Charlesworth and Charlesworth, 1987; Husband and Schemske, 1995; Bosch and Waser, 1999). Late-acting self-incompatibility (Seavey and Bawa, 1986) and other forms of selfincompatibility (Bertin and Sullivan, 1988; Becerra and Lloyd,
1992) can also lead to reduced seed production following selffertilization. Reduced seed production per fruit, in turn, adversely affects the probability of fruit maturation by reducing
the sink strength of the developing fruit relative to developing
fruits with a full complement of seeds (see Stephenson et al.,
1995).
Only a few studies have evaluated the implications of natural variation of pollen load size on the reproduction of plants
and these have been mostly limited to herbaceous species of
temperate regions (Cruzan, Neal, and Wilson, 1988; Stephenson et al., 1995; Delph and Havens, 1998; Winsor, Peretz, and
Stephenson, 2000). In this study we evaluate the relationship
between the number of microgametophytes per pistil, the incompatibility system, and the number of sires with respect to
the production of fruits and seeds in a natural population of
the tropical tree Pachira quinata. We specifically address the
following questions: (1) What is the relationship between the
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AMERICAN JOURNAL
amount of microgametophytes per pistil and the production of
fruits and seeds under natural conditions? (2) What is the
breeding system of Pachira quinata, based on experimental
crosses and genetic analysis? (3) Is fruit set affected by the
breeding system? (4) What is the relationship between the
breeding system and the genetic relatedness of the progeny
from the flowers that set fruit under natural conditions?
MATERIALS AND METHODS
Study site—This study was conducted at Palo Verde National Park within
the lower Tempisque depression in northwest Costa Rica (geographical coordinates are 108199 to 108249 N and 858189 to 858259 W). Palo Verde is a
dry tropical forest with a mean annual rainfall of 1500 mm and a mean annual
temperature of 308C. This area is characterized by an extended dry season
from December to April and a rainy season from May to November.
Study species—Pachira quinata (Bombacaceae) (Jacq.) Alverson [5 Bombacopsis quinatum (Jacq. Dugand)] (Alverson, 1994) is a Neotropical species
that is distributed in the dry forest from Honduras to northern South America.
It is an important timber species that has been heavily exploited because of
its durable and resistant wood. Pachira quinata trees may grow up to 35 m
in height and 2.5 m in diameter under natural conditions. Trees normally have
big buttresses at the base, a gray, thorny bark, and a deep central root system.
Leaves are digitate compound, with glabrous, oblong leaflets. Pachira quinata
is deciduous: leaf drop occurs in late November and new leaves are produced
in early May. The flowering period extends from January to March, immediately followed by the fruiting period, from April through May. Flowers are
hermaphroditic, and protandrous, with multistaminate white filaments and a
single central style. Anthesis takes place at 1900 and flowers last for one
night. Flowers are pollinated by bats (Glossophaga soricina) and sphingid
moths. Fruits are dehiscent, dry, and woody. Each ovary contains an average
of 160 ovules and fruits mature an average of 20 seeds, which are small and
wind dispersed. Pachira quinata has been classified as a self-incompatible
species based on hand pollination experiments (Kane et al., 1993).
Natural variation of pollen load size and fruit and seed production—Nine
adult P. quinata trees were selected based on accessibility to flowers. To
estimate the relationship between pollen load size on fruit and seed production, a sample of flowers from each tree was marked and followed until fruit
maturation for the entire reproductive period of 1998. A total of 630 flowers,
equally distributed on the nine trees sampled, were marked and followed to
fruit maturation. Approximately 14 flowers were marked daily during 45 consecutive days from February to mid-March. Flowers were marked during the
flowering peak of trees in the population to insure that the flowers most likely
to be pollinated were sampled. To determine the flowering peak of the population, we followed the flowering phenology of 40 trees by counting the
number of flowers produced by each individual daily during the entire reproductive season. Flower styles were excised 48 h after anthesis. Preliminary
results showed that pollen tubes grow from the stigma to the ovary in ,24
h. Therefore, even if flowers were pollinated at sunrise, the pollen tubes had
sufficient time to grow into the ovary. Mature fruits produced by marked
flowers were collected and seeds from each fruit were extracted and counted.
Flower stigmas and fruits were collected using 4- to 9-m ladders.
All the styles from flowers that initiated fruits and later aborted and from
flowers that developed into fruits were observed under the microscope. For
each style, pollen grains and tubes were counted under a UV microscope
(Zeiss, Germany) using Martin’s (1959) aniline blue technique. A subsample
of ten flowers that did not develop into fruits (16% of the total sample) were
randomly selected from each tree and observed under the microscope. Pollen
grains and tubes were counted using the same technique described above. A
logistic regression (LOGISTIC; SAS, 1995) was used to determine the effect
of pollen load size on fruit set. The number of pollen grains per stigma and
the number of pollen tubes per style were used as the independent variables.
Fruit set was used as the response variable with the following levels: (1)
flowers that successfully developed into fruit, (2) flowers that initiated fruit
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development but aborted, and (3) flowers that did not develop into fruit. A
regression analysis (REG; SAS, 1995) was performed to determine the relationship between the number of seeds produced per fruit and the number of
pollen grains found on stigmas. A square root transformation was used for
seed number.
To assure that the style excision technique described above did not affect
natural fruit set, we compared the overall fruit set of 17 unmanipulated trees
with the fruit set of the 9 trees with sampled flowers (pollen load experiment).
Total flower and fruit production was recorded for these 17 unmanipulated
trees for 60 consecutive days.
To determine the relationship between the number and size of seeds from
fruits produced from naturally pollinated flowers, we collected a sample of
ten fruits from 20 trees in the same population as above. Seeds from each
mature fruit were counted and classified as either aborted or potentially viable;
aborted seeds are usually wrinkled, hollowed, and black, whereas potentially
viable seeds are well rounded and brown. We selected a sample of five potentially viable seeds per fruit and each seed was individually weighed. Aborted seeds were excluded. A regression analysis (REG; SAS, 1995) was conducted to determine the relationship between the number of seeds produced
per fruit and seed mass.
Breeding system and number of sires—To determine the incompatibility
system of P. quinata, 90 hand pollinations were performed on nine trees.
Outcrossed flowers were obtained by artificially pollinating 45 pristine flowers
using pollen from at least five different donors. Similarly, a total of 45 pristine
flowers were self-pollinated using pollen from the same tree. An equal number
of outcross and self-pollinations were performed on each tree. Flowers were
marked and covered before anthesis with mosquito net bags to prevent pollinator visitation. Both self- and outcross pollen were collected in plastic cups
1 h after anthesis. Hand pollinations were performed by gently saturating the
stigma with pollen (.500 pollen grains per stigma) using a soft paintbrush.
Each marked flower was followed until fruit maturation.
To compare the performance of self- and outcross microgametophytes of
P. quinata, a time series experiment was conducted to determine pollen tube
growth rate of both types of pollen (GLM; SAS, 1995). A total of 36 flowers
from three trees were hand pollinated. Eighteen flowers were self-pollinated
and 18 received pollen from at least four different donors. Six styles (two per
tree) from both self- and outcross flower styles were excised at 2, 4, and 8 h
after pollination. All styles were stained and observed under the UV microscope (Martin, 1959). Pollen tubes were counted in four different regions of
each style: the region near the stigma (region 1), the middle region of the
style near the stigma (region 2), the middle region of the style near the base
of the ovary (region 3), and the region at the base of the style near the ovary
(region 4). A repeated measures two-way ANOVA (analysis of variance) was
used to compare the pollen tube growth rate of outcross vs. self-pollen (GLM;
SAS, 1995). This model considered treatment (outcross vs. self-pollen tubes)
and time intervals of style excision after pollination (2, 4, and 8 h) as the
main effects of the analysis and region within each style (1–4) as the repeated
factor. Region was used as the repeated factor because pollen tube counts
were not independent of each other within the same style.
As an independent test to determine the breeding system of P. quinata, we
conducted an allozyme genetic analysis using enzyme gel electrophoresis. We
collected 20 fruits from each of 15 adult trees and randomly selected and
genotyped four seeds from each fruit. Seeds were germinated in individual
petri dishes. Enzymes were extracted from homogenized hypocotile tissue
obtained 5 d after seed germination using extraction buffers, gel buffers, and
staining protocols followed by Soltis and Soltis (1989) procedures. Six polymorphic enzyme systems were analyzed: leucine aminopeptidase (LAP, EC
3.4.11.1), shikimate dehydrogenase (SKDH, EC 1.1.1.25), phosphoglucoisomerase (PGI, EC 5.3.1.9), aspartate aminotransferase (AAT, EC 2.6.1.1.), esterase (EST, EC 3.1.1.1.), and alcohol dehydrogenase (ADH, EC 1.1.1.1). The
PGI enzyme system showed two polymorphic loci; therefore, seven polymorphic loci were used to determine the selfing rate and genetic relatedness
within P. quinata progenies.
Two genetic parameters were calculated for the progeny produced by trees:
an estimated multilocus outbreeding rate (tm) and paternity correlation (rp)
November 2001]
Fig. 1.
QUESADA
ET AL.—POLLEN LOAD, REPRODUCTIVE SUCCESS, AND KINSHIP
2115
Means and standard errors of the number of pollen grains and the number of pollen tubes at the top and base of style for the fruit set categories.
within and between fruits (i.e., probability of full-sib progeny of seeds within
and between fruits). These parameters were calculated using the genetic models proposed by Ritland (1989) and the MLTR (multilocus mating system
program) computer program (Ritland, 1996). Standard errors were calculated
using the method of bootstrapping with 1000 repetitions (Ritland, 1996).
RESULTS
base of styles (Wald x2 5 12.83; df 5 1; P , 0.0003) (Fig.
1). There is a significant positive relationship between the
number of pollen grains deposited onto a flower’s stigma and
the square root of the number of seeds produced in mature
fruits (Fig. 2). There is also a significant positive relationship
between the number of seeds per fruit and seed size (Fig. 3).
Natural variation in pollen load size and fruit and seed
production—Only 6% of the marked flowers developed into
mature fruits and 10% of the marked flowers initiated fruits
but later aborted them. Similarly, the average fruit set of 17
unmanipulated trees was equivalent to 7.5%, indicating that
the excision of styles 48 h after anthesis did not affect the
expected fruit set under natural conditions. The probability that
a flower will develop into a fruit is significantly affected by
the number of pollen grains on the stigma (Wald x2 5 13.73;
df 5 1; P , 0.0002), the number of pollen tubes at the top
of styles (Wald x2 5 12.99; df 5 1; P , 0.0003), and at the
Breeding system and number of sires—Of the 46 outcross
pollinations performed, only 4% developed into mature fruits.
However, none of the self-pollinations developed into fruit.
The repeated measures two-way ANOVA used to compare
pollen tube growth rate of outcross vs. self-pollen revealed no
overall differences in the number of tubes between self- and
outcross pollen (Fig. 4). We found a significant interaction
between the region of the style where pollen tubes were counted and the time interval when flowers were excised (Wilks’ l
5 0.6279; F 5 2.445; df 5 6,56; P , 0.03). The mean number of pollen tubes is significantly greater at 8 h in region 1
than at 2 h in the other regions of the style (LSMEANS [least
Fig. 2. Relationship between the number of seeds produced per fruit with
respect to the number of pollen grains found on stigmas [Ïseed number 5
0.682 1 0.00029(pollen grains); r2 5 0.34; P , 0.05].
Fig. 3. Relationship between the number of seeds produced per fruit and
seed mass (g) of Pachira quinata (seed number 5 21.1702 1 1510.28(seed
mass); r2 5 0.21; P , 0.01).
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TABLE 1. Multilocus outcrossing frequency (tm) and paternity correlation (rp) within and between fruits for the progeny produced by
trees. Standard errors in parentheses.
Progeny analysis
Seeds within fruits
Seeds between fruits
Fig. 4. Least square means (LSMEANS) 1 1 SE of self and outcross
pollen tubes found in four style regions at three time intervals. Self and outcross flower styles were excised at 2, 4, and 8 h intervals after pollination.
Pollen tubes were counted in four different regions of each style: the region
near the stigma (region 1), the middle region of the style near the stigma
(region 2), the middle region of the style near the base of the ovary (region
3), and the region at the base of the style near the ovary (region 4).
square means]; GLM; SAS, 1995) (Fig. 4). There are significantly more pollen tubes in the region near the stigma (region
1) than the middle regions of the style (regions 2 and 3), and
significantly more in the middle regions than at the base of
the style near the ovary (region 4).
Table 1 shows the estimated multilocus outbreeding rate (tm)
and paternity correlation (rp) within and between fruits for the
progeny produced by trees. Ritland (1989) states that the paternity correlation is inversely related to the number of outcross parents (n), where rp 5 1/n. A paternity correlation value
of 1 indicates that the progeny are full-sibs and a value of 0
indicates that the progeny do not share sires. These results
indicate that P. quinata is predominantly an outcrossing species. In general, the proportion of full-sib progeny was greater
within fruits than between fruits. Seeds within fruits are sired
by one or two donors (Table 1). The probability of common
paternity is significantly greater within fruits than between
fruits. The mean correlation of paternity (rp 5 0.252, SE 5
0.0814) shows that the genetic diversity within trees of P.
quinata could be explained by three to five sires with equal
contribution to the pollen pool.
DISCUSSION
Our study shows that P. quinata produces far more flowers
than mature fruits. This relationship has been shown for many
other species of plants but few studies have evaluated the role
of the size of the pollen load per pistil in determining the
probability of fruit maturation under natural pollination conditions (Lloyd, 1980; Stephenson, 1981; Sutherland and
Delph, 1984; Lee, 1988; Stephenson et al., 1995). The size of
the pollen load deposited onto the stigma of flowers is an
important factor in the production of fruit and seeds in P.
quinata. The only flowers that successfully developed fruit
received a mean load of 422 pollen grains (2.6 : 1 pollen grain
: ovule) and an average of 23 pollen grains were deposited for
each mature seed. Flowers that received ,200 pollen grains
on the stigma did not develop a fruit or aborted. In P. quinata,
about half of the pollen grains transferred to a flower stigma
germinated and developed pollen tubes to the base of the style.
Only the flowers that set fruit received a sufficient number of
tm
0.918
(0.06144)
0.918
(0.051)
rp
0.6384
(0.096)
0.252
(0.0814)
microgametophytes at the base of the style to fertilize all the
ovules in an ovary (Fig. 1). However, in these fruits only a
mean of 14% of the ovules matured into seeds. One possible
explanation for this result is that all the ovules are fertilized
but only a few develop to maturity due to the limited resources
of the maternal plant. Our study indicates that the number of
seeds per fruit increases with the size of the pollen load (Fig.
2) and that seed mass increases with seed number (Fig. 3).
Apparently, most fruits produced under natural conditions do
not reach the maximum number and size of seeds a fruit of P.
quinata could potentially produce. These results suggest that
flowers that set fruit are primarily affected by the number of
microgametophytes in the pistil, rather than the amount of resources provided by the plant. An alternative explanation is
that pollen tube attrition occurs at the base of the style or at
the ovary and fruit maturation occurs only after a large number
of compatible microgametophytes achieve fertilization. Our results indicate that as the number of compatible microgametophytes increases, seed set increases (Fig. 2).
Few studies have established a direct quantitative relationship between the microgametophyte load (i.e., both pollen
grains and tubes) received by a flower style and the production
of fruit and seed under natural conditions. A review of the
literature shows that most studies have analyzed this phenomenon by either establishing a relationship between the number
of pollen grains deposited onto a flower’s stigma and the number of ovules in the ovary (Snow and Roubik, 1987; Uma
Shaanker and Ganeshaiah, 1990) or by establishing a relationship between the number of pollen grains (but not pollen
tubes) deposited on a flower’s stigma with respect to the production of fruits and seeds (Ornduff, 1971; Bertin, 1982;
Snow, 1982, 1986; Casper, 1983; McDade, 1983; McDade and
Dadivar, 1984; Galen, Zimmer, and Newport, 1987; Snow and
Roubik, 1987; Spira et al., 1992; Winsor, Peretz, and Stephenson, 2000; but see Mulcahy, Curtis, and Snow, 1983; Levin,
1990; Niesenbaum and Casper, 1994). In partially self-compatible or incompatible plants, it is difficult to interpret the
significance of the size of the pollen load deposited under natural conditions by measuring only the number of pollen grains
present on a stigma. In many tropical trees with self-incompatibility, including several species in the family Bombacaceae
(Bawa, 1979; Hamrick and Murawski, 1990; Murawski et al.,
1990; Murawski and Hamrick, 1992; Baum, 1995), the incompatibility reaction occurs at the base of the flower style or at
the ovary. The results of the hand pollination experiments and
the genetic analysis (Table 1; Fig. 4), revealed that this population of P. quinata is predominantly outcrossing (tm 5
0.918, SE 5 0.0514), but that both self- and outcross pollen
tubes can grow through flower styles. Therefore, the interaction of compatible and incompatible microgametophytes can
potentially affect the performance of compatible pollen and its
ability to sire seeds in P. quinata. Pollen–pollen and pollen–
November 2001]
QUESADA
ET AL.—POLLEN LOAD, REPRODUCTIVE SUCCESS, AND KINSHIP
pistil interactions have been reported to affect pollen performance and the siring success of donors, due to factors associated with the maternal sporophyte and the microgametophyte
(Cruzan, 1990; Schlichting et al., 1990; Quesada et al., 1991;
Mulcahy, Mulcahy, and Searcy, 1992; Plitman, 1993; Cruzan
and Barrett, 1996). Although our study could not detect the
frequency in which self- or cross microgametophytes occur in
the pistils, it is possible that the flowers that never set fruit are
the result of mainly self-pollinations. In contrast, in the flowers
that set fruit, both incompatible and compatible microgametophytes could have been found in similar proportions in the
pistils but the cross-compatible pollen outcompeted the selfpollen, successfully siring seeds in the fruits that developed to
maturity. An alternative explanation is that the pollen tube
growth of compatible pollen is in fact not affected or even
facilitated by incompatible pollen tubes. Flowers that set fruit
received similar loads of compatible and incompatible pollen
but the incompatible tubes were arrested at the base of the
style without affecting the performance of cross-compatible
microgametophytes. It is also possible that because our hand
pollinations (Fig. 4) tested the performance of self- and cross
microgametophytes separately, when mixed together only
cross-pollen tubes grow through the style. However, this type
of pollen–pollen interaction is rare, and it has not been reported for other Bombacaceous trees (Sukhada and Jayachandra, 1981; Baum, 1995). Even so, it is not clear whether the
competitive ability of compatible pollen or pollen tube attrition
(or both) are important mechanisms operating in the pistils of
P. quinata; the only fruits that developed to maturity are the
result of pollinations performed with large cross-compatible
pollen loads.
The identity of pollen grains contained in naturally deposited pollen loads may also affect the probability of fruit development. Our study indicates that in P. quinata, seeds within
fruits are sired mainly by one or two cross-compatible donors
and that the total seed crop within trees is sired by three to
five cross-compatible donors (Table 1). Outcross single-sired
fruits are expected when pollinators deliver pollen to a flower
stigma after previously visiting one pollen donor or when pollinators deliver pollen from many donors but only one donor
successfully sires seeds. Because the P. quinata individuals
sampled produced an average of 60 flowers per day (SE 5
9.86) throughout the reproductive season, it is possible that
the main pollinators, the long-tongued bat (Glossophaga soricina) and sphingid moths, visited and collected pollen from
many flowers on one tree or only a few trees each night. It
has been documented for some plant species that feeding rates
of G. soricina may be as high as 1000 visits/h to a single plant
and that individual bats showed feeding fidelity to individual
plants over their flowering season (Lemke, 1984). Glossophaga soricina may fly up to 2 km between foraging sties (Fleming, Hooper, and Wilson, 1972) and sphingid moths up to several kilometers (Miller, 1981; Haber and Frankie, 1989); therefore, they have the capacity to move pollen relatively long
distances and promote outcrossing (Heithaus, Fleming, and
Opler, 1975). However, if resources are abundant within a few
individuals of P. quinata during peak flowering season, it
might not be necessary for pollinators to visit many trees in
one night, which could explain the relatively low number of
sires we observed in the genetic analysis.
In conclusion, our study demonstrates, using natural pollinations rather than controlled hand crosses, that fruit and seed
production in natural populations of P. quinata are directly
2117
related to size of the pollen load and that only a few crosspollen donors sire seeds within fruits. Future studies on natural
plant populations need to determine if the variation of microgametophyte performance is associated to the competitive ability of sires. These studies should also determine how pollen
dispersal (via pollinator activity) and pollen–pistil interactions
affect the ability of donors to sire seeds.
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