Increased maleness at flowering-stage and femaleness at
fruiting-stage with size in an andromonoecious perennial,
Veratrum nigrum (Liliaceae)
Wan-Jin Liao*, Da-Yong Zhang
Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering &
Institute of Ecology, Beijing Normal University, Beijing 100875, China
Running heading: Size-dependent sex allocation
* For correspondence. E-mail: liaowj@bnu.edu.cn
Wan-Jin Liao*
College of Life Sciences
Beijing Normal University
Beijing 100875
China
Phone: +86-10-58805081
Fax: +86-10-58807721
E-mail: liaowj@bnu.edu.cn
1
Increased maleness at flowering-stage and femaleness at
fruiting-stage with size in an andromonoecious perennial,
Veratrum nigrum (Liliaceae)
Abstract Theory predicts that cosexual plants should adjust their resource investment in male and
female functions according to their size if female and male fitness are differentially affected by size.
However, few empirical studies have been carried out at both flowering and fruiting stage to
adequately address size-dependent sex allocation in cosexual plants. In this paper, we investigated
resource investment between female and male reproduction, and their size-dependence in a
perennial andromonoecious herb, Veratrum nigrum. We sampled 192 flowering plants, estimated
their standardized phenotypic gender, and assessed the resource investment in male and female
functions in terms of absolute dry biomass. At flowering stage, male investment increased with
plant size more rapidly than female investment, and the standardized phenotypic femaleness
(ranging from 0.267 to 0.776) was negatively correlated with plant size. By contrast, female biased
allocation was found at fruiting-stage, although both flower biomass and fruit biomass were
positively correlated with plant size. We propose that increased maleness with plant size at
flowering-stage may represent an adaptive strategy for andromonoecious plants, because male
flowers promote both male and female fertility by increasing pollinator attraction without
aggravating pollen discounting.
Key words: andromonoecy, phenotypic gender, size-dependent sex allocation, Veratrum nigrum
Financial support: This project was supported by the National Natural Science Foundation of
China (30430160).
2
INTRODUCTION
Resource allocation is central to the life-history and sexual strategies of plants. Optimal
allocation of limiting resources assumes that cosexual plants should allocate resources to various
activities in a way that maximizes individual fitness (Charlesworth and Charlesworth 1981;
Charnov 1982; Bazzaz and Grace 1997; Zhang 2006). Plants are expected to modify their sex
allocation according to their size, if female and male components of fitness are differentially
affected by changes in size or condition (Charnov 1982; Lloyd and Bawa 1984; Klinkhamer et al.
1997; Zhang and Jiang 2002; Sakai and Sakai 2003; Cadet et al. 2004; Sato 2004; Zhang 2006).
Various sex-differential effects have been proposed as selective mechanisms driving sex
allocation adjustments in flowering plants (Ghiselin 1969; Lloyd and Bawa 1984; Charnov and
Bull 1985; Wright and Barrett 1999; Koelewijn and Hunscheid 2000; Sarkissian et al. 2001). In
animal-pollinated species, the male fitness gain curve is expected to decelerate because increased
pollen production leads to more competition for ovules by pollen grains from the same parent
(Lloyd and Bawa 1984), and larger flower numbers result in increased geitonogamy and, in turn,
reduce pollen available for outcrossing (Harder and Barrett 1995). Thus larger plants will often
perform better as females and worse as males and so should benefit by being relatively more female
than smaller plants (Fox 1993; Mendez 1998; Sarkissian et al. 2001). In most of the available
empirical cases with significant size-dependent sex allocation, relative sex investment has often
been estimated using fruitflower ratio, and is more female biased with plant size in
animal-pollinated species (Klinkhamer et al. 1997; de Jong and Klinkhamer 2005).
However, the flowering-stage sex allocation, usually estimated by floral sex ratio and/or
pollenovule ratio, is not always female biased with plant size (Bickel and Freeman 1993; Emms
1996; Wright and Barrett 1999; Koelewijn and Hunscheid 2000; Ishii 2004; Andrieu et al. 2007).
Flowers usually functioned as a male organ (Bell 1985; Queller 1997), and hence plants would
maximize its fitness by increasing maleness at flowering-stage (Emms 1996; Ishii 2004).
Nevertheless, most of the theoretical work on size-dependent sex allocation did not distinguish sex
allocation at flowering-stage from fruiting-stage. Therefore, it is essential to examine sex allocation
at both flowering and fruiting-stage for a better understanding of reproductive success through both
male and female functions. A good example of such joint study was the animal-pollinated
3
hermaphrodite Narthecium asiaticum in which maleness increased with raceme size at both
flowering- and fruiting-stage (Ishii 2004). In fact, increased femaleness could occur at
fruiting-stage provided increased fruit set with size, even though maleness increased with plant size
at flowering-stage.
In this paper, we examine the size dependence of sex allocation at flowering- and
fruiting-stage in an andromonoecious perennial herb Veratrum nigrum L. (Liliaceae), based on
floral sex ratio, standard phenotypic gender, and biomass allocation. The specific questions are: (1)
are the patterns different in size-dependent sex allocation between at flowering- and fruiting-stage?
(2) And if the patterns are different, does the fruit set increase with plant size?
RESULTS
Effects of plant size on fruit set and flowering-stage sex allocation
The height of 192 sampled individuals ranged from 63 cm to 188 cm, with a mean of 115.65 ±
1.83cm. The coefficient of variation (CV) of plant height, which was the standard deviation
expressed as a percentage of the mean, was 22.0. The fruit set averaged 0.36 with a range between
0.01 and 0.89, which increased significantly with plant size (Figure 1).
The number of hermaphroditic flowers per plant was 104.06 ± 5.20, ranging from 18 to 628,
while the number of male flowers was 261.54 ± 11.774, ranging from 0 to 1063. Both of them
increased with plant height, and the male flowers increased more rapidly than the hermaphroditic
flowers (Figure 2A). Similar pattern was observed in biomass investment in both male and female
reproductive components. The flowering-stage female allocation, ratio of female investment to
total flower biomass, was negatively correlated with plant height (Figure 2B).
Standardized phenotypic femaleness (G i) ranged from 0.267 to 0.776 in the studied population
(Figure 3A). The coefficient of variation of Gi was 17.8. Standardized phenotypic femaleness was
negatively correlated with plant height (Figure 3B), also indicating maleness increased with plant
size at flowering-stage in V. nigrum.
4
Fruiting-stage sex allocation
Total flower biomass and fruits biomass were treated as male and female investment,
respectively. In the regression analysis of male and female investment against plant height, all
regression coefficients were larger than 0. That is, both male and female investment increased with
plant size at fruiting-stage (Figure 4A). And the increase of female investment with plant size was
faster than that of male investment. The fruiting-stage female allocation, ratio of fruits biomass to
the sum of flowers and fruits biomass, was positively correlated with plant size, which implied
larger plants were more female biased allocation (Figure 4B).
DISCUSSION
We have found size-dependent sex allocation and gender modification in andromonoecious V.
nigrum. In the studied population, all the flowering individuals were cosexual. The standardized
phenotypic femaleness was negatively correlated with plant size. At flowering-stage, maleness
increased with plant size, while femaleness increased with size at fruiting-stage.
Increased maleness with size at flowering-stage
Size-dependent sex allocation has received considerable attention because the adjustment of
sex allocation is directly linked with evolution of sexual system in plants. Flowering-stage sex
allocation, estimated by standard phenotypic gender, floral numbers and biomass, indicated an
increased maleness with plant size in V. nigrum (Figure 2 and 3). In the sampled 192 flowering
plants, all produce male flowers and hermaphroditic flowers simultaneously except for only one
plant that does not produce male flowers. Furthermore, the number of male and hermaphroditic
flowers increase with plant size (Figure 2A), and the male flowers increase more rapidly. As a
result, the relative maleness increases with increasing plant size (Figure 2B), inconsistent with
general expectations (e.g. Sarkissian et al. 2001).
Larger plants must benefit from producing more male flowers if the increased maleness with
size is adaptive. According to Bertin (1982) and Spalik (1991), such increased maleness seems
likely in andromonoecious plant species, because female reproductive success is limited by
5
resources rather than pollen grains. And hence individual fitness would be enhanced by male
flowers once female reproductive success is maximized (Bertin 1982). If the numbers of
hermaphroditic flowers produced are just enough to use all the resources available for fruit
maturation (Spalik 1991), the large plants will benefit from the additional resources spending on
extra male flowers. First, male flower is less costly than hermaphroditic flower, as is the case in V.
nigrum (Liao et al. 2003). Indeed, several studies in andromonoecious Leptospermum scoparium
(Primack and Lloyd 1980), Lloydia serotina (Manicacci and Despres 2001), Sagittaria guyanensis
ssp lappula (Huang 2003) and Solanum carolinense (Solomon 1986) have reported that male
flower is less costly. Second, the addition of male flowers would promote both male and female
fertility by increasing pollinator attraction without aggravating pollen discounting (Harder and
Barrett 1996). In Solanum carolinense, male success is enhanced by an increase in the proportion of
male flowers produced but not by an increase in total flower numbers, even though all flowers
contain male parts (Elle and Meagher 2000). Moreover, the female success is often increased by the
production of male flowers (Vallejo-Marin and Rausher 2007). Consequently, andromonoecious
plants may invest much more resources in male flowers, rather than hermaphroditic flowers as
plants grow, to maximize fitness gain.
Several other studies of andromonoecious species also indicated increased maleness with size
at flowering-stage (Emms 1996). However, the percentage of male flowers is negatively correlated
with the number of flowers in andromonoecious Pseudocymopterus montanus (Schlessman and
Graceffa 2002), and does not change with plant size in andromonoecious Solanum carolinense and
Leptospermum scoparium (Primack and Lloyd 1980; Solomon 1985).
Increased femaleness with size at fruiting-stage
Based on the models of Klinkhamer et al. (1997) and Zhang and Jiang (2002), larger plants
within a population will allocate more resources to female function than smaller plants if the female
gain curve saturates less rapidly than the male gain curve. In the extreme case when the female gain
curve is linear, individuals above a certain threshold size will allocate a fixed amount of resources
to male function, whereas absolute female allocation increases linearly. In fact, female biased
allocation is very common for plants. For example, Klinkhamer et al. (1997) reviewed the results
and estimated that 25 out of 26 plants observed to have size-dependent sex allocation allocate more
6
to female function when large. In our present investigation, both female and male investment
increase with plant size, and larger plants allocate more resources to female function than smaller
plants at fruiting-stage (Figure 4).
Such female biased sex allocation with size occurs under the condition that female gain curve
saturates less rapidly than the male gain curve, which seems likely for such an andromonoecious
herb with hundreds of flowers. In animal-pollinated plants, various factors such as pollinator
saturation, pollinators’ grooming, geitonogamy or local mate competition could cause diminishing
returns on investment in male function (Klinkhamer and de Jong 1997; Klinkhamer et al. 1997;
Campbell 2000; Sato 2002). The female gain curve is generally considered to be linear, but likely
affected by the mode of seed dispersal. Especially for plants with wind-dispersed seeds, female
fitness return may increase with plant size (de Jong and Klinkhamer 2005). The seeds of V. nigrum
are winged which disperse by wind and could carry over greater distance on larger plants. These
factors indicate that the female gain curve may saturate less rapidly than the male gain curve, and
hence favored increased female allocation with size.
Plant size has also a direct effect on sex allocation through pollinator attraction (Klinkhamer
and de Jong 1997). It is suggested that larger plants are better pollinated and therefore produce
more seeds per flower. If so, seed set in small plants should be limited by compatible pollen gains
deposited on stigma, rather than by resources available for seeds maturation. However, this
hypothesis is unlikely since the female reproductive success is not pollen limited, but resource
limited in V. nigrum (Liao et al. 2006).
Day and Aarssen (1997) focused on mortality during the reproductive season. Female
reproduction includes seed ripening and lasts longer. Small plants with a female bias, may run a
high risk of not surviving before reproduction is completed. However, such hypothesis applies
especially to short-lived annuals that live in an unpredictable environment with a high death rate.
The focal species in the present study, V. nigrum, is a long-lived perennial (Hess et al. 1967)
distributed in deciduous forest, although exact data are not available. Thereforethis hypothesis
does not appear to apply.
Joint study of size-dependent sex allocation at flowering- and fruiting-stage
Joint studies of size-dependent sex allocation at both flowering- and fruiting-stage are helpful
7
to understand reproductive success through both male and female functions. It is well known that
flowers usually function as a male organ (Bell 1985; Queller 1997). Plants would increase male
investment to transfer more pollen gains. While at fruiting-stage, plants would maximize its fitness
by producing more seeds. Hence, a reasonable expectation is male biased allocation at
flowering-stage and female biased allocation at fruiting-stage. Unfortunately, most of the available
empirical studies on size-dependent sex allocation only focused on flowering-stage allocation or
fruiting-stage allocation, except one joint study on Narthecium asiaticum (Ishii 2004). Our present
study on size-dependent sex allocation in V. nigrum witnessed the increased maleness at
flowering-stage and femaleness at fruiting-stage.
It seemed conflicting that sex allocation was male biased at flowering-stage and female biased
at fruiting-stage. In fact, increased femaleness could occur at fruiting-stage provided increased fruit
set with size, even though maleness increased with plant size at flowering-stage. Our results
represented in Figure 1 provide evidence for such increased fruit set with size.
MATERIALS AND METHODS
Study organism and sites
Veratrum nigrum L. (Liliaceae) is a non-clonal and long-lived herbaceous perennial of
Asia-Temperate and Europe. The species flowers from the end of July to the end of August. It is an
insect-pollinated, andromonoecious and predominantly outcrossing species (Liao et al. unpubl.).
Hermaphroditic flowers are formed at the tip raceme of the panicle, and male flowers usually at the
lateral racemes. Fruits mature in the end of September and beginning of October. The field study
was conducted in the Xiaolongmen National Forest Park (39°57' N, 115°25' E). The studied
population is located in a temperate deciduous forest dominated by Quercus liaotungensis Koidz.
and Juglans mandshurica Maxim., covering ca. 2.0 km2.
Flowering-stage allocation
Standardized phenotypic gender
The standardized phenotypic gender (Gi) was calculated to investigate size-dependent gender
8
modification (Lloyd 1980; Lloyd and Bawa 1984). This index depicted the standardized
phenotypic femaleness of plant i in a population as
Gi = d i (d i + l i E )
where di was the maternal expenditure, li was the paternal investment, and the equivalence factor E
was the ratio of investments in maternal and paternal functions in the population as a whole:
E=
∑ d ∑l .
i
i
In the studied population, 192 flowering individuals were marked in early July before the
flowering period. The height of plants was measured. The number of hermaphroditic flowers and
male flowers of each plant were recorded and used to calculate Gi. Since the number of pollen
grains (Liao et al. 2003) and pollen viability (Liao et al. unpubl.) did not differ between
hermaphroditic and male flowers, the paternal investment was estimated by the number of
polleniferous flowers (the sum of hermaphroditic flowers and male flowers). And the maternal
investment was estimated by the number of ovule-bearing flowers (hermaphroditic flowers).
Absolute measurement of biomass allocation
Ecological and evolutionary analyses of gender plasticity usually consider absolute measures
of female and male reproductions (Venable 1992; Sarkissian et al. 2001). Dry biomass was chosen
as allocation currency because it can be considered as an integral measure of allocation. For each
flowering individual in the studied population, plant height was measured, and the number of male
flowers (Nm) and hermaphroditic flowers (Nh) were counted. In late July, we sampled 1
hermaphroditic flower and 1 male flower from the same position for each of 75 randomly chosen
flowering plants, soon after the flowers opened. As a total, 75 hermaphroditic flowers and 75 male
flowers were sampled, and then oven dried at 80 for 48 hr to constant weight and weighted to the
nearest 0.0001g. Then dry mass per hermaphroditic flower (Wh) and male flower (Wm) were
determined. Flowering-stage male investment (MIfl), female investment (FIfl), and female
allocation (FAfl) were calculated, respectively, as follows:
MI fl = Wm × (N h + N m )
FI fl = (Wh − Wm ) × N h
FA fl = FI fl (MI fl + FI fl )
9
Fruiting-stage allocation
Flowers were commonly regarded as a male organ because seed set is generally not limited by
the level of pollination, but pollen export is (Queller 1997). Therefore, we used hermaphroditic and
male flowers as an estimate of male allocation and fruits as an estimate of female allocation at
fruiting-stage for V. nigrum.
All the reproductive plants were harvested at maturation and oven dried at 80 for 48 hr to
constant weight and weighted to the nearest 0.0001g. Fruiting-stage male investment (MIfr = MIfl +
FIfl) and female investment were measured by the dry mass of flowers and fruits, respectively. The
sum of male and female investment at fruiting-stage represented the reproductive biomass.
Fruiting-stage female allocation was the ratio of female investment to total reproductive
investment.
Data analyses
Plant height, rather than above-ground biomass, was used to estimate the plant size in order to
avoid artificial autocorrelation in the regressions. Linear models were used to measure relationships
between plant size and standardized phenotypic gender, fruit set, sex investment and allocation.
The regression coefficient measured the rate of linear increase of investment with size. All the
statistical analysis was performed using SPSS (version 10.0).
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Figure legend
Fig. 1 The increase of fruit set with plant height (N = 82, R2 = 0.10, P < 0.01).
Fig. 2A. The relationships of hermaphroditic and male flower production to the height of plants of
Veratrum nigrum. Open cycles represent hermaphroditic flower production, and the dotted line
illustrates the corresponding regression predictions on plant size (N = 192, R2 = 0.31, P < 0.01).
Closed symbols represent male flower production, and the solid line also illustrates the
corresponding regression predictions (N = 192, R2 = 0.48, P < 0.01). B. The relationship between
flowering-stage female allocation (the ratio of female investment to total flower biomass) and plant
height (N = 192, R2 = 0.02, P < 0.05).
Fig. 3A. Variation in standardized phenotypic gender within the studied population of
andromonoecious Veratrum nigrum sampled in west Beijing, China. The line represents the
observed cumulative frequency distribution of standardized phenotypic femaleness for 192
sampled plants. B. The observed relationship between standardized phenotypic femaleness and the
plant height (N = 192, R2 = 0.05, P < 0.01).
Fig. 4A. The relationships of flowers and fruits biomass to the height of plants of Veratrum nigrum.
Open cycles represent fruits biomass, and the dotted line illustrates the corresponding regression
predictions on plant size (N = 82, R2 = 0.27, P < 0.01). Closed symbols represent flowers biomass,
and the solid line also illustrates the corresponding regression predictions (N = 82, R2 = 0.67, P <
0.01). B. The relationship between fruiting-stage female allocation with plant height (N = 82, R2 =
0.05, P < 0.05).
13
Figure 1
1.0
0.8
Fruit set
0.6
0.4
0.2
0.0
60
90
120
150
180
Plant height (cm)
14
Figure 2
A.
1200
Flower number
900
600
300
0
60
90
120
150
180
Plant height (cm)
B.
Female allocation at flowering-stage
0.3
0.2
0.1
0.0
60
90
120
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180
Plant height (cm)
15
Figure 3
A.
1.0
Cumulative proportion
0.8
0.6
0.4
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
Standarized phenotypic femaleness, Gi
Standardized phenotypic femaleness, Gi
B.
1.0
0.8
0.6
0.4
0.2
0.0
60
90
120
150
180
Plant height (cm)
16
Figure 4
A.
12
Biomass (g)
9
6
3
0
60
90
120
150
180
Plant height (cm)
Female allocation at fruiting-stage
B.
0.8
0.6
0.4
0.2
0.0
60
90
120
150
180
Plant height (cm)
17