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3 Department of Biology, The Pennsylvania State University, Altoona, Pennsylvania 16601-3760 USA; and 4 Department of Biology, The Pennsylvania State University, University Park, Pennsylvania 16802-5301 USA
Received for publication August 10, 1998. Accepted for publication July 8, 1999.
ABSTRACT
The pollen competition hypothesis predicts that when the number of pollen grains deposited onto stigmas exceeds the number of ovules, selection can operate in the time frame between deposition and fertilization. Moreover, because of the overlap in gene expression between the two phases of the life cycle, selection on microgametophytes may alter the resulting sporophytic generation. The extent to which pollen competition occurs in nature has been unclear, because tests of the predictions of the pollen competition hypothesis have used cultivars and/or artificial growth conditions and hand-pollination techniques. In this study we used a wild species, Cucurbita foetidissima, in its natural habitat (southern New Mexico) to determine the amount and timing of the arrival of pollen onto stigmas, the relationship between pollen deposition and seed number, and the effects of the intensity of pollen competition on progeny vigor. We found that ~900 pollen grains are necessary for full seed set and that a single visit by a pollinator results in the deposition of 653.0 ± 101.8 pollen grains. About 29% of the flowers receiving a single pollinator visit had 900 or more pollen grains on its stigma. Moreover, within 2 h of anthesis, >4000 pollen grains were deposited onto a typical stigma, indicating that multiple pollinator visits must have occurred. Fruits produced by multiple visits had greater seed numbers (206 vs. 147) than fruits produced by a single visit. Finally, the progeny produced by multiple pollinator visits were more vigorous than those produced by single visits with respect to five measures of vegetative growth (MANCOVA, Wilks' lambda = 0.96, F6,370 = 2.54, P < 0.02. These data demonstrate that conditions for pollen competition exist in nature and support the prediction that pollen competition enhances offspring vigor.
Key Words: Cucurbita foetidissima • Cucurbitaceae • La Jornada Long-Term Ecological Research Site • pollen • pollen competition
As a consequence of relying on external agents to mediate the transfer of pollen, the flowers on a plant may differ in the size and genetic composition of the pollen loads deposited onto their stigmas (see Stephenson et al., 1995
, for review). When more pollen is deposited onto a stigma than is necessary to fertilize all of the ovules, the arrival schedule of the pollen, the speed of germination, and the growth rates of pollen tubes can determine which microgametophytes fertilize the ovules. To the extent that differences in the germination and growth of the pollen tubes are genetically based, selection can operate in the time frame between pollen deposition and fertilization (Mulcahy, 1979
; Stephenson and Bertin, 1983
). Moreover, because ~90% of the 20 000–23 000 genes expressed by gametophytic phase are also expressed during the sporophytic phase of the life cycle (e.g., Tanksley, Zamir, and Rick, 1981
; Willing and Mascarenhas, 1984
; Willing, Bashe, and Mascarenhas, 1988
), selection on the microgametophyte has the potential to alter the resulting sporophytic generation (see Mulcahy, 1979
; Stephenson et al., 1995
, for reviews).
Many studies in which the sizes of pollen loads have been experimentally manipulated have revealed that the progeny produced by large pollen loads are significantly more vigorous and less variable as seedlings (e.g., Mulcahy and Mulcahy, 1975
; Stephenson, Winsor, and Davis, 1986
; Schlichting et al., 1987
; Winsor, Davis, and Stephenson, 1987
; Bertin, 1990
) and/or have greater reproductive output as adults (Davis, Stephenson, and Winsor, 1987
; Richardson and Stephenson, 1991
; Quesada, Winsor, and Stephenson, 1996a
; Johannsson and Stephenson, 1997
) compared to progeny produced by small pollen loads (but see Snow and Mazer, 1988
; Snow, 1990
). Moreover, recent studies have shown that the progeny resulting from large pollen loads produce pollen grains that outperform the pollen from the progeny of small pollen loads both in vitro (Johannsson and Stephenson, 1997
) and in vivo (Schlichting et al., 1990
; Palmer and Zimmerman, 1994
; Quesada, Winsor, and Stephenson, 1996b
).
The ecological and evolutionary significance of pollen competition (selection) for natural populations of plants remains controversial, however, because many of the studies of pollen competition (1) use cultivated species; (2) rely on hand-pollinations to create differences in the sizes of pollen loads; and (3) fail to demonstrate that the arrival schedules of pollen actually create the conditions necessary for pollen competition in nature (see Snow, 1982
; Mulcahy, Curtis, and Snow, 1983
; Charlesworth, 1988
; Snow and Mazer, 1988
; Stephenson, Winsor, and Schlichtung, 1988
; Lyons et al., 1989
; Schlichting et al., 1990
; Snow, 1990
; Mitchell, 1997
). In this study, we conducted a series of experiments with natural populations of a noncultivated species, Cucurbita foetidissima, in its natural habitat. First, we examined the arrival and accumulation of pollen on stigmas in our populations. Second, we examined the relationship between the size of the pollen load and seed number, and finally we examined the relationship between the size of the pollen load and progeny vigor. Rather than manipulating the size of the pollen load by hand-pollinations, as has been done in previous studies, we compared the performance of the progeny produced by single pollinator visits to flowers with the performance of progeny produced by open-pollinated flowers (multiple visits possible).
MATERIALS AND METHODS
Plant and study sites
Cucurbita foetidissima (Cucurbitaceae), buffalo gourd, is native to grasslands and deserts of the southwestern United States and northern Mexico (Bailey, 1943
). It is common on disturbed sites such as roadsides and in desert washes. The plant has a perennial root that can weigh up to 70 kg. From this root 1–30 stems, each up to 12 m long, are produced yearly (Kohn, 1989
). It can vegetatively propagate via adventitious roots at the nodes, which produce separate plants after the vines die back. Buffalo gourd stems may also die back to the root during droughts.
Cucurbita foetidissima is gynodioecious, that is, both monoecious and gynoecious (female flowers only) morphs are produced (Dossey, Bemis, and Scheerens, 1981
; Kohn, 1989
). The large, yellow, bell-shaped flowers are similar to those of cultivated squashes; they last a single morning (0500–1000), and they are visited chiefly by solitary bees (i.e., Peponapis and Xenoglossa, "squash bees") and honey bees (Apis). In southern New Mexico, flowering typically begins in June and peaks in July through early September. Spherical fruits, containing as many as 300 seeds (personal observation), mature in 60 d.
The experiments with C. foetidissima were conducted at La Jornada Long-Term Ecological Research Site (LTER) in southern New Mexico. This site consists of lands within New Mexico State University's Chihuahuan Desert Rangeland Research Center and the adjacent lands of the USDA Jornada Experimental Range. Nearly all experimental manipulations were conducted at three locations at the LTER: site 3, an open grass-mesquite area at the edge of a playa, ~0.5 km east of the College Ranch; site 5, an enclosed field ~150 m west of site 3; and site 6, a prominent arroyo on the east slope of Summerford Mountain, in creosotebush (Larrea tridentata) scrub habitat.
Pollinator visitation and pollen deposition
In order to determine the interrelationship among pollinator visitation, pollen deposition, and seed number, we controlled the duration of access of pollinators to female flowers. We selected 134 flowers during August and September of 1994 and 24 flowers in July 1995 (total = 158) as they were available on plants in the study area. The majority of the flowers were obtained from site 5 (site 5—91 flowers; site 6—60 flowers; site 3—7 flowers). A total of 47 plants were sampled, with between one and six flowers obtained from each plant. Both monoecious and gynoecious morphs were sampled. We covered each pistillate flower with a cheesecloth bag one day before anthesis. Just before sunrise, the bags were removed and the flowers exposed to pollinator visitation for controlled lengths of time. The study included four treatments: (1) single visit, in which flowers were uncovered until visited by one pollinator (N = 43); (2) 2 h, in which flowers were uncovered for 2 h immediately after sunrise (N = 37); (3) 4 h, in which flowers were uncovered for 4 h immediately after sunrise (N = 33); and (4) open pollinated, in which flowers were uncovered immediately after sunrise, and left uncovered for the entire morning (N = 45). At noon, the pistils were collected and preserved in 70% ethanol.
To estimate the number of pollen grains deposited onto the stigmatic surfaces of the pistils, we first sliced the stigmas into 8–10 small pieces and placed them in 1.5 mL 8 mol/L NaOH overnight at 50°C. We then added 7 mL distilled water and sonicated the mixture until only fine filaments were visible. In order to capture any pollen grains that had fallen off the stigma in storage, we added the rest of the storage solution to the mixture. The mixture was centrifuged at 1000 rpm and afterwards the liquid was decanted off. The pellet, consisting of the exines of the pollen grains, was reconstituted to 4 mL and thoroughly mixed. We placed ten subsamples of 5 µL each in a hemocytometer and counted the number of pollen grains under a microscope. The total amount sampled consists of ~1.25% of the volume extracted from the stigma. The ten subsamples were pooled for analysis.
Pollen load and seed number
The relationship between pollen intensity and seed production in C. foetidissima was determined in two ways. First, we harvested and counted the seeds in mature fruits produced by the single-pollinator visits and open pollinations. This section of the study was restricted to single pollination and open-pollinated fruits produced in the pollinator visitation experiment described above. In C. foetidissima, pollen tubes require 18–22 h to reach the ovary; so flowers whose pistils were excised before 18 h post-pollination do not produce fruits. Further, drought and mammal herbivory in sites 3 and 6 greatly reduced the yield of fruits from the single visit and open pollination fruits. Forty-six fruits were available for this analysis and all were collected from plants on site 5: 24 single visit fruits and 22 open pollinated fruits. Between two and six fruits were collected per plant.
Second, we performed a series of controlled pollinations in which small, medium, and large (stigma saturated with pollen) pollen loads were deposited onto stigmas. Hand-pollinations were made in the following manner. To exclude pollinators, each pistillate flower was covered with a cheesecloth bag one day before anthesis and removed one day after anthesis. Each day, staminate flowers were collected from a minimum of five plants from at least two locations; the pollen from these flowers was removed with a small brush, thoroughly mixed in a plastic container, and then applied to the stigmas using a stainless steel rod having a diameter of 2.5 mm. A thin uniform distribution of pollen on the end of a rod constituted one application. Direct counts of 40 such samples yielded an average of 462 ± 48 grains per application (Quesada, Winsor, and Stephenson, 1993
). In this study, a small pollen load consists of one application, and a medium load consists of two applications. Large pollen loads were accomplished by saturating the stigmatic surface with pollen (>10 000 grains). In all pollinations, care was taken to ensure an even distribution of pollen over all the stigmatic lobes. Pistillate flowers were selected as available from populations throughout the study area. In all, we made 54 small pollinations, 43 medium pollinations, and 45 large pollinations distributed over 45 plants. After 60 d, we harvested and counted the seeds in the mature fruits.
Pollen load and progeny vigor
In order to assess the effects of the size of the pollen load on offspring vigor, we performed two greenhouse trials using seeds from fruits produced by natural pollinations (single visits vs. open pollinations) of C. foetidissima. The fruits were selected from those produced on nine plants from five populations: one on site 3, two on site 5, and two on site 6. Wherever possible, a single-visit fruit was paired with an open-pollinated fruit from the same plant. In the first greenhouse trial we selected 17 seeds per fruit from each of two fruits from nine plants, a total of 305 seeds. In the second trial we selected 22 seeds per fruit from each of two fruits (one open-pollinated fruit and one single-visit fruit) from seven plants (selected from the same plants used in the first trial) for a total of 308 seeds. All seeds were weighed and only seeds ranging from 0.03 to 0.50 g were selected. Mean seed mass was significantly greater for the sample of seeds drawn from the open-pollinated fruits (mean ± SD = 0.043 ± 0.010 g vs. 0.039 ± 0.013 g) than from the single-visit fruits. The seeds were sown at a depth of 0.75 cm in a soilless potting mix (Control-Gro, Hyde Park Products, Inc., Mamaroneck, New York, USA) in 12-cm plastic pots (2000 cm3), placed in a greenhouse, and watered as needed. Supplementary light was provided by 200-W halogen lamps on a 16 h light–8 h dark cycle.
Seedlings were scored as emerged when any part of the plant broke through the soil surface. After emergence, the seedlings were watered as needed and fertilized at 10 and 20 d post-emergence (2% Peter's 20–20–20 NPK, Robert Peters Co., Allentown, Pennsylvania, USA). We recorded the number of days from sowing to emergence, the number of days from emergence to the production of the first leaf, and the number of leaves on the 15th day post-emergence. Leaves were counted as present if the leaf blade had expanded to a fully planar orientation. At the termination of the growth period (30 d), the plants were harvested, cleared of potting soil mix, dried to constant mass at 60°C, and weighed.
RESULTS
Pollinator visitation and pollen deposition
At La Jornada LTER site, pollinator visits occurred in the 4–5 hour period from first light until ~1000. During the first hour of anthesis most of the visitors were the squash bees, Xenoglossa spp., but these bees rarely appeared after the first hour of anthesis (mean arrival time = 0701; N = 15). After the first hour Apis spp. were the most common visitors. A fixed-effect model ANOVA revealed that treatment (duration of pollinator access; single visit, 2 h, 4 h, and open-pollination) had a significant effect on the number of pollen grains deposited onto stigmas (F3,113 = 14.05, P < 0.0001). Flowers exposed for a single pollinator visit received significantly fewer pollen grains (mean ± 1 SE = 653.0 ± 100.8 grains) than flowers exposed for 2 h (4285.1 ± 695.1 grains), 4 h (4117.5 ± 739.5 grains), or for the entire morning (3672.7 ± 560.1 grains), (Tukey test; P < 0.001; Table 1).
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Individual analyses of variance for each measure of plant vigor revealed trends in the direction predicted by the pollen competition hypothesis. Progeny from open pollinations outperformed progeny from single-visit pollinations, although the differences between treatments were only significant (at P < 0.05) for days to seedling emergence and days to first leaf (Table 3). Seedlings produced by open pollination emerged slightly sooner (9.9 d vs. 10.1 d after planting), produced their first leaves significantly sooner (9.4 d vs. 9.8 d), produced similar numbers of leaves after 15 d (4.0 leaves); produced more leaves after 30 d (9.0 vs. 8.8 leaves), and had greater dry masses after 30 d (3.4 vs. 3.3 g) compared to seedlings produced by single visits of pollinators (Table 3). The individual ANCOVAs also revealed significant effects of seed mass, trial, and parent for all measures. Overall, the model predicted 90% of the variance in emergence time, 83% in time to produce the first leaf, 53% in leaf production at 15 d, 55% in leaf production at 30 d, and 41% in dry mass at 30 d.
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In Cucurbita foetidissima, as in other Cucurbita species, anthesis is of relatively short duration. In the populations at the Jornada LTER, flowers open just before daylight and close by ~1000. Pollinator visitation occurs throughout this period with squash bees the predominant visitors when the flowers first open and honey bees the predominant visitors later in the morning. Our data indicate that pollen accumulates rapidly on the stigmas of flowers following anthesis and that within 2 h of anthesis the vast majority of stigmas have pollen loads well in excess of the minimum necessary to achieve full seed set. These data, as well as observation that fruits rarely abort except as a result of herbivory or drought stress, indicate that on these sites and in these years, pollen is not limiting to fruit set. A single visit by a specialized "squash bee" results in the deposition of ~650 ± 100 pollen grains (Table 1), which is sufficient to produce ~150 seeds (~4.3 pollen grains per mature seed) (Table 2). Moreover, visitation within the first 2 h after anthesis results in the deposition of a sufficiently large pollen load (4000 pollen grains) to produce full seed set (when compared to hand-pollinations in which the stigmas were saturated with pollen), and pollen loads on open-pollinated flowers do not differ significantly from those measured on flowers exposed to pollinators for only 2 h.
Our finding that ~4.3 pollen grains are necessary to produce a mature seed is rather typical of the 20 or so species that have been examined in this regard (see Snow, 1986
; Richardson and Stephenson, 1991
; Stephenson et al., 1995
). For a full complement of seeds (~200 seeds), it would be necessary for ~900 pollen grains to be deposited onto a stigma of C. foetidissima. For pollen competition to occur, it is necessary for >900 pollen grains to be deposited in a single visit or it is necessary for sequentially deposited pollen loads to arrive in close temporal proximity. In this study, only 12 of the 42 stigmas (29%) that received a single visit had >900 grains. In contrast, 97 of the 112 stigmas (87%) that were exposed to pollinators for 
2 h received >900 grains. Moreover, because pollen rapidly accumulates on stigmas in the first 2 h after anthesis, it is likely that the arrival schedule for the first two pollen loads is sufficiently close, at least in some flowers, to permit pollen competition to occur.
If competition among pollen tubes is greater in the open-pollinated flowers than in the flowers receiving a single visit and if there is a genetic basis for pollen performance (speed of germination, pollen tube growth rate, and ability to fertilize ovules), then the average vigor of the offspring produced by the open pollinations should be greater than that produced by the single pollinator visits. We found that the progeny produced by open pollinations exhibited enhanced vigor in greenhouse tests. Compared to progeny produced by a single visit of a pollinator, progeny produced by open pollinations (pollen competition) emerged earlier, produced their first leaves sooner after emergence, produced more leaves at 15 and 30 d after emergence, and attained a greater final plant dry mass. Taken singly, the differences were subtle, but always in favor of the open pollinations, and a MANCOVA test revealed a highly significant effect of treatment on overall progeny vigor. It should be noted, moreover, that ~29% of the single visit fruits received enough pollen to create slight to moderate levels of pollen competition, while ~13% of the fruits from pollination were produced under conditions of little or no pollen competition. Nevertheless, the superior performance of the progeny produced by open pollinations indicates that not only do conditions for pollen tube competition exist on the pistils of C. foetidissima growing in natural population, but that it affects progeny performance. Alternatively, the superior performance of the progeny produced by open pollinations could be due to a greater diversity of pollen parents of the seeds within a fruit (see Marshall and Folsom, 1991
)
Since the late 1970s a number of studies have shown an effect on progeny vigor (increased percentage germination, speed of germination, and/or seedling growth rate) when the number of pollen grains deposited onto a stigma was varied or when the location of pollen deposition on a stigma was varied (e.g., Mulcahy, Mulcahy, and Ottaviano, 1975, 1978
; Fingerett, 1979
; Lee and Hartgerinck, 1986
; McKenna, 1986
; Ramstetter and Mulcahy, 1988
; Bertin, 1990
; Richardson and Stephenson, 1992
; but see Snow and Mazer, 1988
). The causes of the effects of pollen load size on progeny vigor have been greatly debated (Mulcahy, 1979
; Charlesworth, 1988
; Stephenson et al., 1988
; Snow and Mazer, 1988
; Lyons et al., 1989
; Schlichting et al., 1990
). To explain these data, Mulcahy (1979)
hypothesized that under conditions of pollen competition only the fastest growing pollen tubes achieve fertilization whereas both fast and slow growing pollen tubes achieve fertilization, when there are fewer pollen tubes than ovules. The correlation between vigorous pollen tubes and vigorous progeny must result from numerous genes being expressed during both stages of the life cycle or from linkage between loci influencing pollen and seedling performance.
Studies with cultivated cucurbits (varieties of C. pepo) were designed to test the pollen competition hypothesis by experimentally varying the number of pollen grains deposited onto stigmas (Stephenson, Winsor, and Davis, 1986
; Schlichting et al., 1987
; Winsor, Davis, and Stephenson, 1987
; Stephenson et al., 1988
). Fruits produced by small and medium pollen loads contained less than a full complement of seeds (large pollen loads) and, consequently, were produced under conditions of little or no pollen competition. Greenhouse and field trials revealed that progeny produced by large pollen loads were significantly more vigorous and less variable as seedlings (Stephenson, Winsor, and Davis, 1986
; Schlichting et al., 1987
; Winsor, Davis, and Stephenson, 1987
) and had greater reproductive output as adults (Davis, Stephenson, and Winsor, 1987
) compared to progeny from low pollen loads. Subsequent experiments (Schlichting et al., 1990
) were designed to examine the transmission of the pollen load effect to the next generation through the ovules (female role) and through the pollen (male role). The results revealed a significant effect of pollen load on pollen performance (the ability to sire seeds), and a significant overall effect of pollen load on the vigor of the next generation, as reflected in the response of 35 traits of the progeny.
Studies of similar design with F1 hybrids of C. pepo and its wild progenitor C. texana (Quesada, Winsor, and Stephenon, 1993
) and with C. texana (Johannsson and Stephenson, 1997
) demonstrated significant effects of pollen load on progeny vigor in taxa having greater genetic variation than cultivars of C. pepo. These studies controlled for the alternative hypotheses that had been advanced to explain the effects of pollen load on progeny. Moreover, these studies demonstrated that the progeny produced by large pollen loads produce pollen that significantly outperforms the pollen produced by the progeny of small pollen loads both in vivo and in vitro (Quesada, Winsor, and Stephenson, 1996a, b
; Johannsson and Stephenson, 1997
).
This study extends previous experiments with cucurbits in two ways. First, by using natural pollinations rather than experimental manipulations of pollen load, it demonstrates that deposition of pollen on plants in a natural population frequently exceeds the 900 or so grains necessary for complete seed set (i.e., not pollen limited). Indeed, simultaneous deposition of such pollen loads is observed in 29% of flowers receiving only a single visit from a pollinator. Thus, conditions for pollen competition are common in the field. Second, by comparing the vigor in progeny produced by open pollinations with those produced by a single visit, this study confirms, for natural populations, one of the central predictions of the pollen competition hypothesis: selection among microgametophytes has consequences for the fitness of the sporophytic progeny. In Cucurbita foetidissima, these consequences of microgametophytic competition are manifested in the enhanced vigor of progeny produced under conditions of pollen competition.
FOOTNOTES
1 The authors thank J. Anderson, the Biology Department of New Mexico State University, and the United States Department of Agriculture for use of research sites and facilities, and G. Avila and A. Freeman for assistance in the field at the Jornada, V. Davis, T. Diehl, and M. Jones for valued assistance in the greenhouse. This research was supported by National Science Foundation grant BSR-93-18224 to A.G. S. and J.A.W. and a Research Experience for Undergraduate (REU) supplement to support S.P. 
5 Current address: Department of Biology, Yale University, New Haven, Connecticut 06511 USA.
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