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Department of Ecology, Evolution, and Natural Resources, Rutgers University, New Brunswick, New Jersey 08903-0231
Received for publication May 28, 1997. Accepted for publication June 16, 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Bateman's principle • reproductive success • resource allocation • sex allocation • Solanaceae • Solanum carolinense
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Stephenson, 1981
; Primack and Hall, 1990
; Elle, 1996
). Differences among individuals in sex allocation—the allocation of resources to male vs. female function—and resulting fitness differences are believed to be integral to the evolution of different breeding systems such as dioecy and monoecy (Charlesworth and Charlesworth, 1978
; Bawa, 1980
; Bertin, 1982
; Charnov, 1982
; Anderson and Symon, 1989
; Meagher, 1992
). The relative importance of the two levels of allocation (whole organism vs. allocation within reproductive functions) to fitness can often be difficult to distinguish. This is, in part, because division of floral structures into "male" and "female" within hermaphroditic plants is not always straightforward.
The consequences of allocation patterns on success are expected to be sex-specific in hermaphrodite individuals, with female success limited primarily by resources while male success is limited primarily by mating opportunities (Bateman, 1948
; Charnov, 1982
; Bell, 1985
; reviewed and critiqued by Wilson et al., 1994
). Often referred to as Bateman's principle, this idea of sex-specific limiting factors has been integrated into sex allocation theory to predict that female success should be limited by the availability of resources for the production of fruits and seeds, while male success should be limited by the ability to attract pollinators (Charnov, 1982
). Generally speaking, this means that variation among individuals in female success may be a function of differences among them in vegetative size, while male fitness variation may be more intimately related to differences among individuals in allocation to the size or number of floral structures (sex allocation).
Seed production (female success) has repeatedly been shown to be resource limited, with individuals of greater vegetative size having proportionately greater reproductive success (i.e., Law, 1979
; Stephenson, 1981
; Primack and Hall, 1990
; Herrera, 1993
; Mitchell, 1994
; Elle, 1996
; Emms, 1996
). Allocation to reproductive effort (i.e., flower production) has a positive effect on female success in various plant species (Campbell, 1989
, 1991
; Devlin and Ellstrand, 1990
; Dudash, 1991
; Herrera, 1993
; Mitchell, 1994
; Broyles and Wyatt, 1995
; Conner, Rush, and Jennetten, 1996
). Variation in sex allocation within overall reproductive allocation has been studied less frequently. Flower size variation, known to influence male reproductive success (Young and Stanton, 1990
; Campbell et al., 1991
; Morgan and Schoen, 1997
), may affect female success as well (Solomon, 1987
; Stanton and Galloway, 1990
; Stanton et al., 1991
; Wilson et al., 1994
; Conner, Rush, and Jennetten, 1996
; Morgan and Schoen, 1997
). Showy structures such as petals often serve to attract pollinators, leading to increased pollen removal and (presumably) pollen deposition throughout a population (male function), but may also benefit female function by increasing the quantity or genetic diversity of pollen received (Zimmerman and Pyke, 1988
; Anderson and Symon, 1989
). This indicates that petal and flower size should not be assumed to be exclusively male in function (as in Bell, 1985
), and that the relationship of this aspect of sex allocation to female success should be examined. Also important is individual variation in allocation to the production of different flower types in monoecious species. Spatial segregation of the sexes into different flower types leads to more straightforward assignation of a complete flower to one or the other sex, and relative proportions of flower types have been shown to influence sex-specific reproductive gains in several species (Lloyd, 1979
; May and Spears, 1988
; Emms, 1996
).
This paper is the first in a series in which I will examine the relationship between resource allocation, including aspects of sex allocation, and sex-specific reproductive success in the andromonoecious perennial, Solanum carolinense L. Here, I will explore the relationship between allocation and female reproductive success, by addressing the following questions: (1) What are the patterns of resource allocation to reproduction and vegetative size in Solanum carolinense, both within and between years? (2) Is female success primarily determined by total allocation to reproduction (flower number), aspects of sex allocation (proportion male flowers, flower size), or resource availability (vegetative size)?
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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), have greatly reduced, nonfunctional pistils, and are incapable of setting fruit (Solomon, 1986
). Hermaphroditic flowers are produced basally, the male flowers (when present) at the tips of inflorescences. There is no nectar, and the showy anthers are attractive to pollinators, for whom pollen is the only reward (Solomon, 1987
). Flowers are "buzz pollinated" by large-bodied bees, which hang from the flowers and vibrate their flight muscles, causing pollen to be ejected from the anthers (Buchmann, 1983
; Solomon, 1986
). The fruit is a berry, ripening to bright yellow, and containing an average of 160 seeds (Elle, unpublished data). Individuals are self-incompatible, so all seeds produced are through outcrossing. Significant heritabilities for flower number, the proportion of those flowers that are male, and various aspects of flower size were found in a greenhouse experiment conducted on this species (Elle, 1998
), indicating that these traits can respond to selection. Because of the lability in sex expression associated with andromonoecy, lack of nectar production, and obligate outcrossing in this species, it was considered highly appropriate for a study examining the relationship between variation in allocation patterns and sex-specific reproductive success.
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. Selection of plants was based, in part, on their allozyme genotypes, which greatly facilitated the determination of paternity; this portion of the study will be discussed in detail in a forthcoming manuscript. Of the 43 genets eventually chosen, nine originated from a population of plants growing in a sheep pasture at Cook College, Rutgers University, New Brunswick, New Jersey, an additional eight from an old field 
15 yr since abandonment at Hutcheson Memorial Forest, Franklin Township, New Jersey, and the final 26 from a population growing on a closed landfill on Staten Island, New York. These three source populations were from 20 to 70 km distant from one another.
I produced four ramets of each of the 43 genets by taking 2.5 cm root cuttings from mature greenhouse plants. Root cuttings were placed in standard promix, and plants emerged within 2 wk with no further treatment. One month later on 1 June 1994, one ramet of each genet was placed into each of four field arrays, termed plots below. Each plot was a hexagonal array, with 1-m spacing between plants and a different random planting scheme. Spacing of plants and experimental population sizes were based on logistical considerations and were modeled after the size and spacing of the natural populations from which seeds were originally collected. Hexagonal arrays were used to insure uniform density and maximize the number of nearest neighbors for all plants, since distance and density are known to affect pollinator behavior and resulting mating success (for example, Schmitt, 1983
; Van Treuren et al., 1993
; Karron et al., 1995
).
Two plots were placed into each of two sites in opposite corners of 30 x 70 m rectangles. Plots A and B were at Hutcheson Memorial Forest (HMF), Franklin Township, New Jersey, in a 10-yr-old experimental field separated from the original source population by 500 m of woods. The area to be planted was tilled prior to planting, but the surrounding area had various early-successional plant species present. Plots C and D were at the Rutgers Vegetable Research Farm number 3 (VRF), New Brunswick, New Jersey, 25 km away from HMF, and < 2 km from the sheep pasture source population, in a field planted yearly in various crop species. Vegetable crops, including various members of the Solanaceae, were planted in the portion of the field not used for this study in the two years of the experiment. Both experimental fields were bordered by woods, with no known population of S. carolinense within 500 m. In both sites, various annuals grew in the plots. Plants were mulched with hay and watered for 1 mo to aid establishment, after which no manipulation of the plots occurred in either year.
Reproductive effort
Allocation to reproduction was measured as total flower number for each individual, and variation within reproductive allocation, or sex allocation, was measured as flower size and the proportion of flowers produced that were male (and so incapable of setting fruit). Flower size is considered an aspect of sex allocation, rather than reproductive allocation, since variation in flower size is predicted to have sex-specific effects on reproductive success (Charnov, 1982
; Bell, 1985
). I counted the number of flowers of each type (hermaphrodite and male) produced by each individual, from which I calculated (1) total flower number and (2) the proportion of male flowers. I also determined flower size by measuring corolla width, pistil length, and anther length and width of four hermaphrodite flowers and (when available) four male flowers. Production of male flowers was uneven among individuals, with some individuals producing no male flowers at all. Consequently, flower size estimates for individuals use only hermaphrodite flowers, with male flower production represented as the proportion of male flowers in the analysis. In 1994, I measured flowers on three dates in August; in 1995, to ensure measurement of flowers on all individuals, I measured flower size on the first four hermaphrodite flowers produced by each genet. Mean size of flower parts was then calculated for each individual that flowered. Very few flowers were measured in plot C in 1994, due to high levels of herbivory by rodents facilitated by a thriving population of annual grass in the area; flower size data for this plot are therefore not presented, and this plot is dropped from all multivariate analyses (see below).
Reproductive success
Female reproductive success was estimated as total seed production by an individual plant within each plot and year. Fruit set was scored for each flower on each inflorescence produced. Fruit were harvested within 1 wk of the first killing frost, which was earlier at HMF than VRF in both years. All fruit were collected by 31 October in each year. Seeds were removed from the fruit and counted for each fruit produced in 1994, and for a random sample of five fruits per genet per plot in 1995. Seed production was therefore an absolute measure in 1994, but estimated in 1995 as the product of mean seed number for the five sampled fruits and the total fruit number produced. Relative female success was then determined for each plant by dividing total individual seed production by the mean seed production for that plot in that year (Lande and Arnold, 1983
).
Vegetative vigor
To estimate allocation to vegetative size, I measured plant height, amount of branching, and leaf size for every experimental individual. Final height was the height of the main (longest) stem for each genet at the end of the growing season, and branch number was the total number of branches that arose from it. In 1995, most genets had more than one ramet (see below), so number of ramets is included in the estimate of plant size for this year; height and branch number were determined for each ramet. I also measured the length and width of the two largest leaves in 1994 for each genet and of one leaf per ramet in 1995. When multiple measurements were taken for a character (such as leaf length) on an individual, the mean of that character was calculated for each individual for use in analyses.
In 1995, the amount of vegetative growth and increase in ramet number necessitated the removal of two plots from the design due to logistical constraints; with a three- to fourfold increase in ramet number in each plot, it was not possible to measure all variables adequately for each plant. Plants were therefore studied for 2 yr in plots A and D and 1 yr in plots B and C. Plots A and D were chosen since 1994 data were most complete for these plots (see above) and to keep one plot per site. For all plots, ramet production was determined in the spring of 1995; afterwards, plants in plots B and C were removed.
Statistical analysis
Patterns of allocation
All analyses were facilitated by the SAS program package (SAS, 1985). Analysis of variance (ANOVA) was employed to determine whether aspects of plant allocation differed by genotype, site, plot within site, and year, including all aspects of reproductive effort, reproductive success, and vegetative size. Ramet production was tested with a different model, including only genotype, site, and plot-within-site effects, since this variable could be measured in 1995 only. Since plots are nested within sites, the site term was tested against the plot-within-site term in the model. Total flower production and ramet production were log transformed for all plots to increase normality, and branch number was log transformed for 1995 plots only. Proportion male was arcsine square root transformed, the suggested transformation for proportional data (Sokal and Rohlf, 1981
, p. 427); all other variables were acceptably normal.
To elucidate the relationships among variables, phenotypic correlations were calculated for each plot/year combination, including the variables flower number, proportion male, flower size, vegetative size, and ramet production. Flower size and vegetative size were estimated by performing a principal components analysis within each plot/year combination on the four variables measured for each group and using the first principal component to represent overall size in both this analysis and the selection gradient analysis (below). That is, "flower size" refers to the first principal component calculated from individual means of corolla diameter, pistil length, and anther length and width within each plot and year. In all cases, the first principal component (PC) explained > 40% of the total variance in the data, and in all cases but one (plot A, 1994, flower size) all four variables were positively loaded on the first PC (Table 1).
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).
Relationships among groups of variables
To determine the relationship between resource allocation patterns and reproductive success within years, I performed a standardized selection gradient analysis as described in Lande and Arnold (1983)
. This analysis is used to determine the strength of directional selection acting on phenotypic characters by regressing relative fitness on the phenotypic traits of interest, after the traits are standardized to mean = 0 and variance = 1. This standardization allows the magnitude of the selection gradients for the characters to be compared. Here, the characters of interest were total flower number, the proportion of male flowers produced, and flower and vegetative size (summarized by a principal components analysis; see above). Only flowering individuals were included in this analysis. Prior to standardization, flower number and proportion male were transformed as noted above to meet the assumption of normality; ramet production was included in the selection gradient analysis for 1995, and was log transformed as noted above. Plot C was dropped from all multivariate analyses because of missing values for flower size (see above). The selection gradient analysis was performed separately for three 1994 plots and two 1995 plots. Both the full model incorporating all four variables and a reduced model including just flower number and proportion male were tested.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Relative female success, flower number, the proportion of male flowers, corolla diameter, anther length, branch number, and leaf length and width all had significant genotype effects, indicating a genetic component to the allocation patterns of identical genets grown in different locations. Even when such genetic effects on allocation are accounted for, however, there are effects of local environment on allocation: there were significant plot effects on flower number, proportion male, anther length, final height, leaf length and width, and ramet production, and significant year effects on all variables except relative female success, flower number, and anther width. There were no significant site effects, however, so variation among plots was not due to large differences between the two sites used for this experiment, but rather was due to smaller scale variation among the plots themselves.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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principle, predicts that male reproductive success in a hermaphrodite should be limited by access to mates (pollinator attraction), while female reproductive success should be limited by the resources required for the production of fruit and/or seeds (Charnov, 1982
). In S. carolinense, female success is not directly affected by resource availability as determined by vegetative size, but, as in other studies, it is determined by flower production (Campbell, 1989
, 1991
; Devlin and Ellstrand, 1990
; Dudash, 1991
; Herrera, 1993
; Mitchell, 1994
; Broyles and Wyatt, 1995
; Conner, Rush, and Jennetten, 1996
). This indicates that total reproductive effort is the most important predictor of female success in this species.
Many of the variables measured in this experiment differed with the year or plot in which they were measured, indicating that the environment had a strong influence on the expression of floral and vegetative characters. Also important was the effect of genotype, which was significant for eight out of the 12 variables measured. A greenhouse study of S. carolinense indicated a genetic basis for floral traits as well. Using plants from the same three populations included in the experimental arrays of the current study, small but significant amounts of heritable genetic variation were detected for most floral characters (Elle, 1998
). Flower number was found to have a heritability of between 15 and 20% and proportion male flowers a heritability of 28–42%. This indicates that the strong selection observed in the current experiment to increase flower number and decrease the proportion of male flowers has the potential to lead to evolutionary change, if selection is acting similarly in natural populations.
One of the benefits of using multiple regression for the analysis of data such as these is that all variables can be considered simultaneously for their effect on the response variable. Differences among individuals in vegetative size, for example, are effectively held constant in the multiple regression when the importance of flower number variation on female success is examined, and vice versa (Lande and Arnold, 1983
; Emms, 1993
). The multiple regression indicates that, in S. carolinense, variation in vegetative size does not affect the resulting variation in female success, but that when vegetative size is held constant, variation in flower number and proportion male flowers does.
The lack of a relationship between resource availability (vegetative size) and female success in S. carolinense appears somewhat surprising, contrasting with expectations of a strong link between these two characters. Strong correlations between vegetative size and flower production have been documented in several species (i.e., Samson and Werk, 1986
; Primack and Hall, 1990
; Herrera, 1993
; Mitchell, 1994
; Mendez, 1998
), and larger plants often have greater female success than smaller plants (i.e., Stephenson, 1981
; Mitchell, 1994
; Elle, 1996
; reviewed in Klinkhamer, de Jong, and Metz, 1997
). Plant dry mass was related to both total flower and total seed production in Physalis longifolia, a hermaphroditic member of the Solanaceae (Lawrence, 1993
). Emms (1996)
found that increased vegetative size and increased flower number independently increased seed production in the andromonoecious lily Zigadenus paniculatus. However, Bierzychudek (1984)
found no relationship between leaf area and seed production in the monoecious Arisaema triphyllum. Similarly, Primack and Lloyd (1980)
found no relationship between vegetative size and the proportion of hermaphroditic flowers produced in an andromonoecious shrub, and Delesalle (1989)
found no relationship between vegetative size and the proportion of female flowers in a monoecious cucurbit. Thus it appears that a strong link between vegetative vigor and female function should not be assumed.
In the current study, there were no trade-offs between seed production in the two years or seed production (sexual reproduction) and ramet production (asexual reproduction). This is in contrast to the expectation that reproduction engenders a cost in terms of future growth and fecundity (Abrahamson and Caswell, 1982
; Meagher and Antonovics, 1982
; Reekie and Bazzaz, 1987
; Ackerman and Montalvo, 1990
; Primack and Hall, 1990
; Elle, 1996
; Emms, 1996
). It has been hypothesized that clonal growth or vegetative size would be enhanced in andromonoecious species by the production of an increased number of male flowers, due to resource savings from the production of smaller pistils and fewer fruits and seeds (Solomon, 1986
; Anderson and Symon, 1989
). Yet there was no relationship between the proportion of male flowers produced and vegetative size in S. carolinense.
Lability in the proportion of male flowers produced may instead be important as a way to regulate fruit set (May and Spears, 1988
; Diggle, 1993
, 1994
; O’Brien, 1994
), which may be one of the major selective factors leading to the evolution of andromonoecy (Bertin, 1982
; Whalen and Costich, 1986
; Diggle, 1993
; Emms, 1996
). Plants producing a higher proportion of male flowers in the current study did, indeed, produce fewer total seeds than other plants. The question remains as to why produce male flowers at all, once the optimum level of fruit set has been achieved. One hypothesis is that extra male flowers increase the attractiveness of a plant to pollinators, and so enhance male function (Lloyd, 1979
; Bertin, 1982
; Solomon, 1987
). An increased proportion of male flowers does lead to increased male success in S. carolinense (Elle and Meagher, unpublished data).
In conclusion, this study indicated that total reproductive effort was the most important level of allocation affecting female reproductive success in S. carolinense and that at least one aspect of sex allocation, the proportion of male flowers produced, was also important. Neither the size of individual flowers, expected to influence pollinator behavior, nor the vegetative size of individuals, a measure of resource availability, was an important determinant of female success in this species.
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FOOTNOTES |
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2 Current address: Department of Entomology, University of California, Riverside, CA 92521 (elle{at}citrus.ucr.edu
).
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J. M. Snyder and J. H. Richards Floral phenology and compatibility of sawgrass, Cladium jamaicense (Cyperaceae) Am. J. Botany, April 1, 2005; 92(4): 736 - 743. [Abstract] [Full Text] [PDF]
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A. L. Parachnowitsch and E. Elle Variation in sex allocation and male-female trade-offs in six populations of Collinsia parviflora (Scrophulariaceae s.l.) Am. J. Botany, August 1, 2004; 91(8): 1200 - 1207. [Abstract] [Full Text] [PDF]
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C. M. Caruso, S. B. Peterson, and C. E. Ridley Natural selection on floral traits of Lobelia (Lobeliaceae): spatial and temporal variation Am. J. Botany, September 1, 2003; 90(9): 1333 - 1340. [Abstract] [Full Text] [PDF]
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