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Reproductive Biology |
2Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Colorado 80309 USA; 3Department of Biology, Amherst College, Amherst, Massachusetts 01002 USA
Received for publication January 28, 2004. Accepted for publication August 26, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: andromonoecy • architecture • flower morphology • positional effects • sexual dimorphism • unisexual flowers • Solanum
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Lloyd and Webb, 1977
; Delph, 1996
; Delph et al., 1996
; Eckhart, 1999
). In particular, sexual dimorphism of corolla size is well documented and has played an important role both in the development of theory underlying sex allocation models and in stimulating empirical research in plant evolutionary ecology. The widespread distribution of floral size dimorphism (occurring in 85% of species with unisexual flowers; Delph et al., 1996
) may suggest common underlying explanations for the evolution of sexual size dimorphism, and indeed, a variety of hypotheses have been proposed (summarized in Delph et al., 1996
). These range from "nonfunctional" hypotheses that postulate developmental correlations between stamens and corolla (Darwin, 1877
; Plack, 1957
, 1958
) to "functional" hypotheses that emphasize presumptive biological roles for the perianth in protection (Bawa and Opler, 1975
) and pollinator attraction (Bell, 1985
) or deal with optimizing resource allocation for male or female function (Eckhart, 1992
; Ashman, 1994
; Costich and Meagher, 2001
; Miller and Venable, 2003
).
In a broad survey of monoecious and dioecious species, Delph (1996)
and Delph et al. (1996)
considered several explanations for the evolution of perianth size dimorphism. Although they found little support for the broad applicability of the developmental correlations hypothesis, the pattern of floral size dimorphism was generally consistent with the hypotheses that selection for male function or for protection of developing reproductive organs drives the evolution of floral sexual dimorphism. However, many of the basic assumptions underlying these hypotheses remain only partially supported or unexplored (Wilson et al., 1994
; Conner et al., 1996a
, b
; Campbell, 2000
; Elle and Meagher, 2000
), and there are sufficient deviations from predictions to suggest that other factors also operate on the expression of sexual size dimorphism (summarized in Eckhart, 1999
). Therefore, we believe that additional explanations of floral size dimorphism should be considered. Here, we explore the possibility that two additional and previously overlooked developmental features, architectural effects and developmental plasticity, may contribute to perianth size dimorphism. Because these features can mimic true sexual dimorphism, they can erroneously provide support for functional hypotheses where no causal relationships exist.
An important component of flower size variation is floral position within inflorescences and inflorescence position within the overall architecture of the plant (Diggle, 1995
). Analyses of 65 species in 27 families show significant intra-individual variation in flower size due solely to differences in flower position within inflorescences (Diggle, 2003
). Most of these examples involve taxa that produce only hermaphroditic flowers, however in taxa that bear unisexual flowers, the different flower types are often located in distinct positions within inflorescences or within the overall architecture of individuals (Cox, 1988
; Waller, 1988
). For example, in many monoecious and andromonoecious species, staminate flowers are produced distally whereas female or hermaphroditic flowers are basal within the same inflorescence (Matkze, 1938
; Bell, 1971
; Hopkins, 1984
; Willson and Ruppel, 1984
; Solomon, 1985
; Cox, 1988
; Diggle, 1993
; Hoc et al., 1994
; Ladd, 1994
; Emms et al., 1997
; Elle, 1998
; Krupnick and Weiss, 1998
; Manicacci and Despres, 2001
; Miller and Diggle, 2003
). In other taxa, the flower types may be segregated among different inflorescences (Jones, 1936
; Bell, 1971
; Abul-Fatih and Bazzaz, 1979
; Lovett Doust, 1980
; Pellmyr, 1986
; Spalik and Woodell, 1994
). Another pattern, common among monoecious and dioecious taxa, is the production of solitary female flowers and complex multiflowered staminate inflorescences (Whitaker and Davis, 1962
; Bawa, 1980
; Delasalle, 1992
; Cronquist, 1993
; Costich, 1995
). These examples demonstrate that architectural effects are as likely to occur among species producing unisexual flowers as in taxa with only hermaphroditic flowers. Further, it is conceivable that size differences between the sexual phenotypes (e.g., male and female flowers in monoecious species, or male and hermaphroditic flowers in andromonoecious species) in these species could be a consequence of flower position rather than a direct effect of floral gender.
Plastic responses to variation in resource availability may also explain size differences between sexually dimorphic flowers. Fruit set by early flowers is known to result in the reallocation of resources away from the growth of later initiated flowers (Lloyd, 1980
; Stephenson, 1981
). Often, the result of such resource preemption is decreased size of flowers that are initiated later in development (reviewed in Diggle, 1995
). Obviously, in an analysis of dioecious species, only females will be subject to this plastic response. Consequently, if sampling of female flowers follows fruit initiation, then differences in flower size between females and non-fruit bearing males may be due to plasticity of flower size in females rather than to inherent gender-based differences. Likewise, in monoecious or andromonoecious species, if the sexual phenotypes are produced at different times or at different distances from the resource supply, then one flower type or the other will be more subject to plastic variation due to resource preemption.
In this paper, we address the effects of architecture and plastic responses to resource preemption on the expression of floral sexual dimorphism in andromonoecious Solanum section Lasiocarpa (Solanaceae). Previous work on andromonoecious Solanum has demonstrated that there is striking dimorphism for flower size. Specifically, staminate flowers are smaller than hermaphroditic flowers in a number of species (Smith, 1931
; Martin, 1972
; Reddy and Bahadur, 1977
; Baksh et al., 1979
; Dulberger et al., 1981
; Solomon, 1986
; Whalen and Costich, 1986
; Anderson and Symon, 1989
; Diggle, 1991
). Species of Solanum section Lasiocarpa are especially suited to analyses of the effects of architectural variation and plasticity on flower size dimorphism. Flowers are produced in elongate inflorescences, and staminate flowers, when produced, are initiated distally within each inflorescence, such that floral position has the potential to influence the morphology of those flowers. In addition, the production of staminate flowers in some species is a phenotypically plastic response to the presence of developing fruit (Diggle, 1993
; Miller and Diggle, 2003
; J. S. Miller and P. K. Diggle, unpublished data). Thus, the smaller size of staminate flowers may also be the result of resource preemption rather than an expression of sexual dimorphism.
Differences in morphology between hermaphroditic and staminate flowers for both primary (androecium and gynoecium) and secondary (corolla) sexual traits for six species in Solanum section Lasiocarpa are quantified to address the following questions. Are hermaphroditic flowers larger than staminate flowers, and if so, for which floral traits? To what degree are size differences in floral structures explained by architectural effects (flower position), and does the floral sexual size dimorphism remain after correcting for architectural variation? Does resource preemption by developing fruit (i.e., plasticity) contribute to size variation in floral structures, and do these effects differ between species with fixed vs. phenotypically plastic production of staminate flowers?
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, 1987
; Whalen and Caruso, 1983
; Bernardello et al., 1994
; Bruneau et al., 1995
; Bohs, 2004
). They are sexually reproducing, self-compatible, and andromonoecious. Sexual expression (the proportion of flowers that are staminate) varies considerably among species ranging from near zero (<1%) to over 60% staminate flowers per inflorescence among the seven species studied to date (Diggle, 1993
; Miller and Diggle, 2003
; J. S. Miller and P. K. Diggle, unpublished data). Flowers of Solanum section Lasiocarpa are slightly zygomorphic and five-merous, except for the gynoecium, which is bicarpellate, and are borne in monochasial cymes that resemble racemes at maturity. Within inflorescences, flowers mature acropetally and each inflorescence generally has more than one open flower at any given time. Inflorescences are produced sequentially along branches and multiple inflorescences per branch may bear open flowers simultaneously.
Six species of Solanum section Lasiocarpa are included in the current study: S. candidum, S. ferox var. lasiocarpum, S. stramonifolium var. inerme, S. pectinatum, S. pseudolulo, and S. quitoense. Voucher specimens are housed at the University of Colorado herbarium (COLO; Table 1). There is considerable variation among species in both flower size and the number of flowers per inflorescence. In addition, manipulation of fruit set on clonal replicates of multiple genotypes for each of these species demonstrated interspecific variation for phenotypic plasticity. The strongly andromonoecious species, S. pseudolulo, S. pectinatum, and S. quitoense, were not plastic and produced a large proportion of staminate flowers regardless of fruiting treatment, whereas S. candidum, S. ferox, and S. stramonifolium were phenotypically plastic and produced significantly more staminate flowers in the presence of developing fruit (Table 1; Miller and Diggle, 2003
; J. S. Miller and P. K. Diggle, unpublished data).
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Experimental design
Plants were grown from seed and replicated by vegetative cuttings to produce genetically identical plants. The number of clonally replicated genotypes used for each species ranged from six (S. ferox var. lasiocarpum) to 10 (S. stramonifolium var. inerme), and the number of clonal replicates per genotype ranged from four to eight. Plants were maintained in 3-gallon pots containing a 2 : 1 mix of Fafard Growing Mix #2 (Conrad Fafard, Inc., Agawam, Massachusetts, USA) to Persolite (Persolite Products, Inc., Florence, Colorado, USA) plus Osmocote 13–13–13 slow-release fertilizer (Scotts Company, Marysville, Ohio, USA), and were watered daily with 150–200 ppm of Excel Magnitrate fertilizer (Scotts Company, Marysville, Ohio, USA). Plants were grown in neighboring greenhouse rooms, which were maintained at 21°C. For each species, the experiment (described below) was entirely replicated in each greenhouse room.
Replicate plants for each genotype were assigned one of two treatments: plus-fruit or no-fruit (Fig. 1). All flowers on plants in the plus-fruit treatment were pollinated every other day with a mixture of pollen from 
3 conspecific donors. Hermaphroditic flowers remained open for 2–4 d, such that all hermaphroditic flowers were pollinated at least once. In contrast, plants in the no-fruit treatment were not pollinated, and any autogamously produced fruit were removed shortly after their initiation. Total plant resource status is associated with fruit production, and plants with developing fruit have fewer resources available for growth and continued reproduction than plants with no developing fruit (Lloyd, 1980
; Stephenson, 1981
). Thus, experimental manipulation of plant fruiting status is an effective, biologically relevant way to manipulate total plant resource status. Experimental manipulation of plant resource status was necessary to promote staminate flower production in the plastic species and to test for plastic changes in floral phenotypes due to resource effects of developing fruit. Because inflorescences are produced continuously, floral buds, flowers, and developing fruit occur simultaneously on each of multiple branches per plant and resource preemption by early fruit can potentially affect the development of later initiated flowers and fruits.
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48 h old) were collected and fixed in FAA (Berlyn and Miksche, 1976
) before being transferred to 70% EtOH for storage. Flower collections differed for the plastic and nonplastic species. For the plastic species (S. candidum, S. ferox, and S. stramonifolium), the distribution of flower types differs significantly between plants of the no-fruit and plus-fruit treatments. Plants of the no-fruit treatment bear predominantly hermaphroditic flowers at all positions, whereas plus-fruit plants bear hermaphroditic flowers in basal positions and staminate flowers in distal positions within each inflorescence. For these species, hermaphroditic flowers were collected from basal (BH) and distal (DH) positions within inflorescences from plants in the no-fruit treatment and staminate flowers were collected from distal positions of plus-fruit plants (DS; Fig. 1A). For the nonplastic species (S. pectinatum, S. pseudolulo, and S. quitoense), the distribution of flower types is statistically indistinguishable for the two treatments and qualitatively resembles that of the plus-fruit plants of the plastic species (Miller and Diggle, 2003
). Although staminate flowers predominated in distal positions, sufficient numbers of hermaphroditic flowers were produced in distal positions such that we could include them in the sample. Hermaphroditic flowers were collected from basal (BH) and distal (DH) positions in the no-fruit treatment, and staminate flowers from distal positions on both no-fruit (DS-NF) and plus-fruit (DS-Fr) plants (Fig. 1B). Flowers were sampled from all genotypes for each of the species. Flowers were collected from September 2000 to May 2001 for S. candidum, S. pseudolulo, and S. quitoense, from December 2001 to March 2002 for S. ferox, and from July 2002 to September 2002 for S. stramonifolium and S. pectinatum. Flowers were dissected and measured either with digital calipers or under a Zeiss Stemi SV-11 dissecting microscope equipped with an ocular micrometer. For each flower, eight measurements were made: length and width of the dorsal petal, anther length and width (filaments in these species are minute and do not contribute to stamen length), style length, stigma width, and ovary length and width. Although staminate flowers do not produce fruit, they do possess rudimentary gynoecia, which were measured as in hermaphroditic flowers.
Statistical analyses
A principal components analysis was used to summarize variation in the floral characters (JMP 5.0.1a, SAS Institute, 1989– 2002). Measurements of replicate flowers of the same flower identity (a variate that indicates both the position and sex of each flower: BH, DH, or DS for the plastic species; BH, DH, DS-NF, or DS-Fr for the fixed species; see Fig. 1) were averaged for each genotype within species and these means were natural log transformed prior to the principal components analysis. We used a general linear model with species, sex, and their interaction as effects to assess variation in size and shape as defined by the first two principal components. Tukey's honestly significant different (HSD) multiple-comparison procedure was used to compare all species pairwise combinations (JMP 5.0.1a, SAS Institute, 1989–2002).
For each species, univariate analyses were also carried out for each floral character to assess the contributions of architecture, plasticity, and flower sex to size variation. Specifically, data for each floral character were analyzed using separate general linear models that included the fixed effect of flower identity (as defined above; JMP 5.0.1a, SAS Institute, 1989–2002). For each trait, we used a linear contrast to test whether hermaphroditic flowers (pooled over all flower positions) were larger than staminate flowers (pooled over all treatments). Tukey's HSD multiple-comparison procedure was used in separate analyses to compare all flower identity categories (JMP 5.0.1a, SAS Institute, 1989–2002).
Subsequent data analyses and interpretation differed slightly for the plastic and nonplastic species. For all species, significant differences between BH and DH flowers of nonfruiting plants is a measure of architectural variation (Fig. 1). In the plastic species, comparison of DH of nonfruiting and DS of fruiting plants controls for position and measures both plastic responses to the fruiting treatment (i.e., resource preemption) and floral sexual dimorphism (Fig. 1A). In the nonplastic species, because both hermaphroditic and staminate flowers occurred in distal positions of the nonfruiting treatment, plasticity can be separated from sexual dimorphism. Comparison of DS flowers from the two treatments holds position and gender constant and is a measure of plasticity, whereas comparison of DS and DH flowers from nonfruiting plants holds position and treatment constant and is a measure of floral sexual dimorphism (Fig. 1B).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Univariate analyses
Analyses of individual traits revealed a pattern similar to the principal components analysis. Hermaphroditic flowers were significantly larger than staminate flowers for all floral traits measured, with the single exception of anther width in S. stramonifolium (Table 3). However, there were differences in the magnitude of sexual dimorphism for specific floral structures. Style length ranged from 2.8 times larger in hermaphroditic compared to staminate flowers of S. stramonifolium to 5.9 times larger in S. quitoense (Table 3). Similarly, on average for all species, ovaries were 1.6 times longer and 1.4 times wider, and stigmas were 1.7 times wider in hermaphroditic than staminate flowers. In contrast, anther size, although statistically different between hermaphroditic and staminate flowers, showed considerably less dimorphism. Anthers in hermaphroditic flowers were on average only 1.07 times longer and wider than those in staminate flowers. Petal size was 1.1 times longer and 1.2 times wider in hermaphroditic flowers averaged across the six species.
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In the nonplastic species, it was possible to control for both architectural variation and fruiting treatment. Comparison of distal hermaphroditic and staminate flowers in the no-fruit treatment (DH vs. DS-NF, Table 4) identifies those character differences associated with either primary sexual function or sexual dimorphism. For all three of the fixed species, the four gynoecial characters were significantly smaller in staminate compared to hermaphroditic flowers (Table 4). In addition to the gynoecium, anthers of staminate flowers were smaller than those of hermaphroditic flowers for S. pectinatum, but not for S. pseudolulo or S. quitoense. In contrast to the primary sexual organs, there were no significant differences between distal hermaphroditic flowers and distal staminate flowers for either petal length or width for any of the nonplastic species (Table 4).
Plastic effects on floral morphology
In the nonplastic species, comparison of distal staminate flowers between the fruiting and nonfruiting treatments identifies the effects of resource preemption by developing fruit (DS-NF vs. DS-Fr, Table 4). These effects were negligible for most floral characters in the nonplastic species. With the exception of anther length and width in S. quitoense and ovary width in S. pectinatum, distal staminate flowers in the no-fruit and plus-fruit treatments were indistinguishable (Table 4). Although fruiting treatment did not appear to affect allocation to the sizes of floral structures within flowers, this treatment did have a significant effect on the number of flowers produced per inflorescence or total number of inflorescences for several species (data not shown).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Greater hermaphroditic flower size has been documented for numerous other andromonoecious Solanum (e.g., S. hirtum, Diggle, 1991
; S. carolinense, Solomon, 1986
, Elle, 1998
; S. campanulatum, S. chippendalei, S. clarkiae, S. diversiflorum, S. eburneum, S. melanospermum, Anderson and Symon, 1989
; S. melongena, Smith, 1931
; and S. marginatum, Dulberger et al., 1981
). Similarly, among andromonoecious species from other taxa, hermaphroditic flowers typically are larger (or heavier) than staminate flowers (Leptospermum, Primack and Lloyd, 1980
; Aquilegia, Brunet, 1990
; Zigadenus, Emms, 1993
; Anthriscus, Spalik and Woodell, 1994
; Gagea, Wolfe, 1998
). Though not universal (exceptions include Solanum torvum, Hossain, 1973
, Besleria triflora, Podolsky, 1993
; and Sagittaria guyanensis, Huang, 2003
), there appears to be a trend for larger hermaphroditic (compared to staminate) flower size among andromonoecious species.
The near ubiquity of this apparent size dimorphism among andromonoecious taxa has generated speculation about its functional and perhaps adaptive significance. For example, Anderson and Symon (1989)
suggest that the larger corollas of hermaphroditic flowers promote cross-pollination of self-compatible species of Solanum. Conversely, smaller staminate flowers may represent a substantial resource savings that can be invested in subsequent flower production, or vegetative growth and maintenance (Bertin, 1982
; Solomon, 1986
).
In many Solanum, as well as other andromonoecious taxa, however, hermaphroditic and staminate flowers are produced in different positions within inflorescences. We show for six species of Solanum that when variation due to architecture is controlled, no differences in corolla characters remain (with one exception petal width in S. candidum). Therefore, before hypotheses for the potential functional significance of floral sexual dimorphism can be considered, the confounding effects of flower position should be determined.
Architectural effects and sexual dimorphism in Solanum
Comparison of hermaphroditic flowers from basal and distal positions within inflorescences of unpollinated plants reveals the presence of architectural effects for all of the morphological traits measured. These effects were more common in some species (S. candidum, S. stramonifolium, S. ferox, and S. pseudolulo), but in general all six species showed architectural variation in size of most floral structures and the pattern was consistently one of basal to distal decrease (Table 4). Thus, in the absence of the confounding effects of gender and resource competition from developing fruit (see Fig. 1), distal hermaphroditic flowers are predictably and significantly smaller than basal hermaphroditic flowers within the same inflorescence (Table 4; Fig. 3). Given that hermaphroditic and staminate flowers tend to be spatially segregated within inflorescences of Solanum (Solomon, 1985
; Whalen and Costich, 1986
; Anderson and Symon, 1989
; Diggle, 1993
; Miller and Diggle, 2003
) the positional variation documented among hermaphroditic flowers within inflorescences suggests that much of the difference in size between hermaphroditic and staminate flowers may be due, not to the sexual phenotype of the flower, but to its location within an inflorescence.
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; J. S. Miller and P. K. Diggle, unpublished data). In the plastic species, as in the fixed species, differences between hermaphroditic and staminate flowers for the majority of nongynoecial characters disappear after correcting for architectural effects (compare Tables 3 and 4). Thus, there is no dimorphism of these characters in the plastic species. For the remaining characters, petal width in S. candidum, anther length in S. candidum and S. stramonifolium, and gynoecial dimensions for all species, differences between the flower types remain. For these species, however, we cannot unequivocally separate the effects of plasticity from true sexual dimorphism (see Fig. 1A).
Plastic effects of fruiting history on floral morphology
For Solanum species with fixed expression of andromonoecy, plasticity of floral characters could be isolated (Fig. 1). In contrast to the dramatic effects of architecture, the effects of the fruiting treatment on flower size were minimal. Although fruit set is a cue that directly or indirectly elicits staminate flower production in the plastic species (Miller and Diggle, 2003
) and clearly has an effect on other aspects of allocation such as flower or inflorescence number in these species, the sizes of most floral organs were surprisingly invariant with respect to fruit set treatment. Plasticity does not appear to contribute to size dimorphism in these species. Floral organ plasticity has been demonstrated in numerous hermaphroditic taxa, however (reviewed in Diggle, 1995
; additional references in Cresswell et al., 2001
), and should be examined further in taxa with unisexual flowers.
Architectural effects and the evaluation of hypotheses for the evolution of andromonoecy in Solanum
In all six species of Solanum, architectural effects on flower size mimic sexual dimorphism. Analyses of flower morphology made without regard to the confounding effects of architecture (and plasticity) showed profound size dimorphism between hermaphroditic and staminate flowers for both primary and secondary sexual characteristics. Elimination of these effects shows that it is primarily the gynoecium that differs between the flower types. Thus, analyses of the driving force for the evolution of andromonoecy in this group should concentrate specifically on the advantages to be gained by reducing gynoecial size and eliminating female function, rather than issues related to flower size such as resource costs or functional aspects of the corolla such as attraction or protection.
A long-standing hypothesis for the evolution of andromonoecy suggests that the production of hermaphroditic flowers is costly and the production of smaller staminate flowers conserves resources (Bertin, 1982
; Solomon, 1986
; Wolfe, 1998
). However, because staminate flowers are not intrinsically smaller than hermaphroditic flowers, further development or investigation of this hypothesis for Solanum should focus exclusively on resource savings from producing nonfunctional gynoecia. Given the relatively large size of the ovary of staminate flowers (e.g., ovary length is 62–74% and width is 70– 84% that of hermaphroditic flowers), the resource recovery may be minimal. An advantage may accrue, rather, from precluding post-anthesis female function of these flowers (Lloyd, 1980
; Whalen and Costich, 1986
). That is, termination of gynoecial development prior to anthesis will prevent investment in fruit that ultimately cannot be matured. When attention is refocused away from the confounding issues of flower size, hypotheses regarding resource allocation can be formulated with greater precision.
Another potential explanation for the presence of reduced gynoecia in staminate flowers may be interference of pollen export (male function) by female sexual structures (Solomon, 1985
; Elle and Meagher, 2000
). The general importance of interference between gender functions has received recent attention (Fetscher, 2001
; Barrett, 2002
) and its relevance to the evolution of andromonoecy is highlighted by focusing specifically on gynoecial differences between the flower types. In hermaphroditic flowers of Solanum, the style exceeds the stamens in length and protrudes from the central cone formed by five poricidal anthers. Solanum species are buzz-pollinated, a syndrome in which a pollen-collecting bee curls its body over the androecium and vibrates the stamens, causing the release of pollen onto the insect (Whalen, 1979
, 1984
). It is possible that the protruding style interferes with this process and hence, with pollen export. The production of staminate flowers with shorter styles could enhance male function via increased efficiency of pollen removal. Geitonogamous pollen discounting (Barrett, 2002
) might also be reduced because the stigmas of staminate flowers would not receive pollen from previously visited flowers. Consistent with an hypothesis of interference is the occurrence of enantiostyly in the closely related Solanum section Androceras (Whalen, 1984
). In enantiostylous species, such as Solanum rostratum, all flowers are morphologically hermaphrodite, but the style is curved to one side, does not protrude through the cone of anthers (Bowers, 1975
) and does not interfere with pollen collection (Jesson and Barrett, 2002
, 2003). This interference hypothesis could be experimentally tested in andromonoecious Solanum.
Summary
The discovery of significant architectural variation that mimics floral sexual dimorphism across several species of Solanum leads us to question how many other reports of size dimorphism reported in the literature may, in fact, be due to architectural differences. This is an empirical question and requires analysis of the extent to which unisexual flowers are produced in distinct locations within the architecture of inflorescences or individuals. Information about the spatial distribution of unisexual flower in diverse taxa is not readily available but consideration of a few examples suggests that further analysis is warranted. A survey of andromonoecious taxa shows that hermaphroditic and staminate flowers are almost universally segregated within or among inflorescences (Bell, 1971
; Hopkins, 1984
; Solomon, 1985
; Diggle, 1993
; Hoc et al., 1994
; Ladd, 1994
; Emms et al., 1997
; Elle, 1998
; Krupnick and Weiss, 1998
; Manicacci and Despres, 2001
; Miller and Diggle, 2003
). Monoecious species may have female and male flowers segregated within inflorescences (Matzke, 1938
; Willson and Ruppel, 1984
; Cox, 1988
), as in andromonoecy, or the two flower types may occur on inflorescences that differ dramatically in location or morphology (Jones, 1936
; Bell, 1971
; Abul-Fatih and Bazzaz, 1979
; Lovett Doust, 1980
; Condon and Gilbert, 1988
; Delasalle, 1992
; Costich, 1995
). Dioecious taxa also may have gender specific inflorescence morphology (Whitiker and Davis, 1962
; Bawa, 1980
; Costich, 1995
). If patterns of spatial segregation of flower types such as these are common among taxa with unisexual flowers, then the confounding effects of architecture may be creating patterns of sexual dimorphism where no inherent difference in the flower types exist. Thus, we propose that consideration of architectural effects on flower size is a critical component of analyses of unisexual flowers and should precede the evaluation of evolutionary hypotheses regarding form and function.
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FOOTNOTES |
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4 Author for correspondence (pamela.diggle{at}colorado.edu
)
5 Authors contributed equally to this research (jsmiller{at}amherst.edu
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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