BioIogical Journal of the Linnean Sociely (1991), 42: 325-336. With I figure
O n evolution of andromonoecy and
‘overproduction’ of flowers: a resource
allocation model
KRZYSZTOF SPALIK
Department of Plant Systematics and Geography, University of Warsaw,
Al. Ujazdowskie 4, 00-478 Warszawa, Poland
Received 5 June 1987, revised and accepted f o r publication 26 October 1989
The conditions for the evolution of andromonoecy and male-function-controlled overproduction of
fertile flowers in hermaphrodites are considered using the evolutionarily stable strategy (ESS)
approach. Andromonoecy and male-function-controlled floral display are promoted if further
increase in pollen amount per flower is disadvantageous and/or increase in the number of
polliniferous units is advantageous; the costs of attractive organs and pollen per flower are relatively
low while the cost of ovules per flower and the cost per fruit are relatively high; the probability of
setting a fruit from a pistillate flower is high; male fertility increases with the resources devoted to
flowering; and if selfing is relatively low. Assumptions and predictions of the model are discussed.
KEY WORDS:-Andromonoecy
sexual strategies.
- floral display
-
reproductive functions - resource allocation
-
CONTENTS
Introduction . .
. . .
Model
Discussion. . .
Acknowledgements
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References
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325
326
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INTRODUCTION
The question of male function of flowers has been raised since
‘overproduction’ of flowers or flower and fruit abortion was brought to
prominence (Aker, 1982; Stephenson, 1981, 1984; Sutherland, 1986a, b;
Sutherland & Delph, 1984; Udovic, 1981; Willson & Burley, 1983; Willson &
Price, 1977; Willson & Rathcke, 1974). Although flower and fruit abortion can
be a consequence of resource limitation, low pollination efficiency or of damage
to fruits, some data suggest that the ‘superfluous’ flowers serve pollen dispersal,
i.e. their overproduction is in some respects equivalent to andromonoecy (Bell,
1985; Queller, 1983; Sutherland 1986a, b; Sutherland & Delph, 1984; Willson &
Price, 1977; Willson & Rathcke, 1974).
Male reproductive function has also been studied in terms of sex allocation in
cosexual organisms and several models have already been proposed
0024-4066/91/030325
+ 12 S03.00/0
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0 1991 The Linnean Society of London
326
K.SPALIK
(Charlesworth & Charlesworth, 1981, 1987; Charnov, 1982; Charnov, Maynard
Smith & Bull, 1976; Ross & Gregorius, 1983). However, both in the models and
in observational work concerning comparative sex allocation in plants (Lemen,
1980; Schoen, 1982) only male and female reproductive functions were
separated, usually understood as production of gametes or, sometimes, pollen
and seeds. For some purposes, this approach is unsatisfactory. Pollen and seeds
are produced in discrete packages: flowers and fruits (Lloyd, 1979). Their output
is therefore regulated at two levels: (1) pollen (P) and ovule (0)production in a
flower (and seed production in a fruit) and (2) flower and fruit production. The
models studied so far lack this distinction. They differentiate gender strategies
but not the different pollen and seed packaging strategies represented by
andromonoecy and hermaphroditism. The vast literature on variation in P/O
ratio (starting from Cruden, 1977) underlines the importance of regulation of
pollen and ovule number per flower. However, the evolution of different seed
and pollen packaging strategies suggests that the regulation of total pollen and
seed output can also occur through an increase in reproductive unit number
(flowers and fruits) rather than pollen or seed number per unit. Therefore,
flowers and fruits may sometimes be considered as fixed units of resource
allocation.
In this paper I consider the optimization of reproductive functions as an
optimization of resource allocation to flowering versus fruiting for two different
pollen and seed packaging strategies: hermaphroditism and andromonoecy.
First, I study the ESS fraction of resources devoted to flowers in hermaphrodites
and find conditions when the fraction is greater than that required by female
reproductive function, i.e. when overproduction of hermaphrodite flowers serves
male ends. Then, assuming that the ability to produce male flowers has evolved,
I calculate the fraction of resources that will be devoted to male flowers.
MODEL
Suppose that there is a finite and relatively predictable amount of resources
for sexual reproduction, which is allocated to flowers and fruits. Let h and f
denote, respectively, the fractions of total resource devoted by an hermaphrodite
to flowers and to fruits. Therefore
h+f = 1.
(1)
Flowers and fruits are assumed to be constant units of resource allocation. Let
c, p and o denote, respectively, the costs of auxiliary structures (calyx, corolla
etc.), pollen and ovules in a single flower, while k is the cost of a single fruit with
its seeds. Assume that the number of pollen grains per flower and the number of
seeds per fruit is relatively constant. Although a fruit can initiate from every
flower, only a fraction A can mature due to some extrinsic factors (damage,
pollen availability etc.).
Male fertility is assumed to be proportional to pollen output, i.e. to the
number of flowers:
h
male fertility cc
(2)
c+p+o'
~
EVOLUTION OF ANDROMONOECY
327
If the number of hermaphrodite flowers multiplied by the probability of setting a
fruit exceeds the number of fruits which can be borne, i.e.
>f
k
Ah
c+p+o
(3)
then female fertility is assumed to be proportional to the number of fruits:
female fertility K f
-.
(4)
k
If the number of hermaphrodite flowers which can develop into fruits is smaller
than the number of fruits which can be borne, then female fertility is limited by
flower production and probability of setting a fruit:
Ah
female fertility K
(5)
c+p+o'
Clearly, female fertility takes its maximum at
~
Ah - f
c+p+o
k'
Figure 1 shows female fertility as a function of h.
The formula for fitness is a particular case of that in Charlesworth &
Charlesworth ( 1978) and it includes partial selfing and inbreeding depression.
The sum of the male and female components of fitness is given by
W K [2s( 1 - 6) + 1 -s] x female fertility
+ male fertility x ( 1 -s)
female fertility*
male fertility* '
h'
PI
Figure I . Female fertility as a function of h, the fraction of resources devoted to hermaphrodite
flowers. h' denotes the fraction maximizing female fertility (condition 6 in the text). Note that h+ f
equals I for hermaphrodites and I -m for andromonoecious individuals.
(7)
K. SPALIK
328
where s denotes the fraction of selfed flowers, 6 is inbreeding depression and *
indicates values for the population into which a mutant is introduced at a low
frequency. Values without * are for the mutant. The ESS for any parameter p is
that p* which satisfies the condition W(p*,p*) > W(p,p*) for all p # p* where
W(a,b ) denotes fitness of a mutant a in a population of b strategists (Maynard
Smith, 1982).
The question is what is the ESS for h, the fraction of resources devoted to
hermaphrodite flowers? First, consider the case when female fertility depends on
the number of flowers (equation 5 ) . Replacing male and female firtihies with
equations (2) and ( 5 ) we get
WOC [ 2 S ( 1-6)
or
+ 1 -s]
Ah
Ah( 1 - s )
c+p+o+
c+p+o
~
w OC 2[s( 1-6) + 1 -s] c + pA+ o
~
h*
h*'
x h.
Since the fitness is a linear function of h, the ESS for h is the highest value the
parameter can take (equation 6). Bearing in mind equation ( l ) , we can write the
ESS for h as
h=
c+p+o
c + p + o + Ak'
In other words, the ESS for h is that resource allocation which maximizes female
fertility. Note that F, fruit/flower ratio or fruit set reflects only the external
limitation of fruit production, i.e.
F = A.
(9)
Now, consider the case when female fertility is limited by resources devoted to
fruiting (equation 4).Applying a similar approach and substituting 1 -h forf
(side condition 1) we can write the fitness of a mutant introduced at a low
frequency into a population as a function of h:
w CC [2s( 1 -6) + 1-4
1 -h
+ (1 -h*)h(
h*k
-
k
1 -s)
By differentiating with respect to h and setting the derivative equal to zero we
can get the ESS for h (Maynard Smith, 1982):
h=
1 -s
2[s( 1-6)
+ 1 -s]'
Note that the ESS does not depend on the costs of flower and fruit or on the
probability of setting a fruit but on the selfing rate and inbreeding depression.
For total out-crossing it becomes equal to 0.5, the value usually obtained for
male function in other models (for instance, Maynard Smith, 1971). The fruit/
flower ratio is
F=
[2s(1-6)+1-s](c+p+o)
k(1-s)
EVOLUTION OF ANDROMONOECY
329
We can now specify the condition (3), i.e. when the overproduction of
hermaphrodite flowers serves male ends. Substituting 1 -h for J and the ESS
value from (10) for h in the expression (3) we get the condition
<
c+p+0
c+p+o+Ak
or
Ak >
241 -6)
1 --s
1 --s
2 [ ~ ( 1 - 6 ) + l-s]’
+ 1 --s
(C
+p + 0 ) .
(12)
Otherwise, the production of hermaphrodite flowers does not exceed that
required by female reproductive function. In other words, the ESS for h is always
the greater of the two values.
Since a fraction of the hermaphrodite flowers produced may serve male
function only, substantial savings could be made if superfluous ovaries were not
produced. Let m denote the fraction of resources devoted to male flowers, with
m+h+f=
(13)
1.
Assume that a hermaphroditic flower produces the same number of pollen grains
as a male flower. Male fertility can therefore be written as
+ -.c +mp
h
male fertility a c+p+0
(14)
The question is what is the ESS for m, the fraction of resources devoted to male
flowers? Consider three cases:
1. Female fertility is limited by the number of flowers (equation 5 ) .
Substituting female and male fertilities with the expressions ( 5 ) and (14) in
formula (7) we get the fitness of the mutant as a function of h and m:
wa [2S(l - 6 ) +
1 --s]
Ah
c+p+0
~
+ (5+ -) h
c+p
(1 -s)
c+p+0
+
Ah*/(c+p 0 )
[m*/(c+p)l+ [h*/(C+P+O)l*
By differentiating with respect to h and n we get
aw- h*A( 1 -s)
dm m*(c+p+o) +h*(c+p))
aw
[2s( 1-6)
h*A(1 - s ) ( c + ~ )
+ 1 -s) + m* (c+p
+ +h* (c+p)
0)
The first expression is greater than zero for h > 0; the second one is always
greater than zero; hence, there is no solution (and no ESS for h and m ) .
K. SPALIK
330
2. Female fertility is limited by resources devoted to fruiting (equation 4).
Applying a similar approach to that used before and substituting 1 - h - m forf
(from side condition 13) we get the following formula:
woc [2s(l-6)+1-s]
1-h-m
k
+
h
(A
c+p
c+p+o
(1-h*-m*)/X
(1 -s) m * / ( c + p ) + h * / ( c + p + o ) '
+ -)
The derivatives are:
aw--ah
[2s(i-6)+i-s~
k
aw- -- [2s(l-6)+1-s]
k
am
(1 -h* - m * ) / k
m*/(c+p) +h*/(c+p+o)'
+-c + p + o
+-(I-s)
c+p
X
(1-h*-m*)/k
m*/(c+p) +h*/(c+p+o)'
If the derivatives are set as zero the solution can only be obtained if c + p =
c + p + o . However, if o = 0 then the distinction between h and m does not make
sense and the solution in equation (10) is reached but for h + m instead of h.
Hence, there is no ESS for h and m.
3. The number of hermaphrodite flowers produced is just enough to use all the
resources available for fruit maturation (equation 6). This case seems to be
intuitively reasonable: if both male and hermaphrodite flowers are produced
then the number of pistillate flowers should reflect female reproductive function
only.
Using formula (7), equation (14) for male fertility, either of the two equations
(4or 5) for female fertility and side conditions (6) and (13) we can express the
'fitness of a mutant as a function of a single parameter only, namely m:
A ( 1- m )
w OC [2s( 1 -6) + 1 -s] c+p+
o+ Ak
A(l -m*)/c+p+o+Ak
m * / ( c + p ) + (1 - m * ) / ( c + p + o + A k ) '
By differentiating with respect to m we get
dW
-dm
c+p+o+Ak
-[2s(l-6)+
1-s]+
+
( 1 - m*) ( 0 A k ) ( 1 -s)
m*(o+Ak) + c + p
Setting the derivative equal to zero we get the ESS for m:
m=
(c+P + O + A k ) (1 -s) - 2 ( ~ + p[s(
) 1- 6 )
+ 1 -s]
2 ( 0 + A k ) [ s ( 1 - 6 ) + 1-J]
(15)
T o make clearer the relationship between m and the costs of reproductive
structures, selfing and inbreeding depression we can write expression (15) as
follows:
m=
1 -s
2[s(l -a)+ 1 -J]
--p + c
o+Ak
X
2s(l-6)+1-s
2[~(1-6) 1 -s]'
+
EVOLUTION OF ANDROMONOECY
33 1
A high cost per ovule and fruit, high probability of setting a fruit and high
inbreeding depression increase the fraction of resources devoted to male flowers.
Allocation to male flowers decreases if the costs of pollen production and
auxiliary stuctures as well as selfing are high.
Now, we can answer the question: when will we expect andromonoecy? The
strategy is andromonoecious when m > 0, i.e.
2.4 1-6)
+
1 --s
(C+P).
1 --s
Otherwise, m = 0 and one simply comes to the solution in equation (8). The
conditions for andromonoecy ( 16) and male-function-controlled floral display
( 12) are generally similar.
o+Ak >
DISCUSSION
The basic assumption of this model is that there is a finite and relatively fixed
pool of resources for sexual reproduction and that the only difference between
competing strategies is the allocation of these resources to flowers and fruits. This
is likely to be true for semelparous plants, for which it is unprofitable not to make
the most of resource gained. Iteroparous plants are able to regulate reproductive
effort by changing resource allocation to sexual and vegetative reproduction and
growth (Bostock & Benton, 1979; Colosi & Cavers, 1984; Harper, 1977; Kawano
& Masuda, 1980; Willson & Burley, 1983). Extra or unused resources can be
stored or used to increase the probability of survival until the next season. If
flowering and fruit-bearing are separated by a growing season, hermaphrodites
and females have more total resources for reproduction than males. The
temporal displacement of male and female reproductive effort has been
suggested as a factor supporting hermaphroditism in plants (Charnov, 1982;
Charnov et al., 1976; Maynard Smith, 1978) though this factor has sometimes
been neglected (Lloyd, 1982). Obviously, in iteroparous plants, males may use
resources gathered in the last growing season. The present model does not
require that all the resources devoted to reproduction are available at the
beginning of flowering, only a certain fraction of expected resources.
In species with indeterminate flowering, competition between flowers and
fruits resulting in inhibition of flower growth by developing fruits has been found
(Bierzychudek, 1981; Colosi & Cavers, 1984; Hickman, 1975; Stephenson,
1981) . For many plants, however, the assumption that flowering and fruiting are
limited by the same pool of resources may prove an oversimplification.
In the present model, the following conditions promote andromonoecy:
(i) further increase in the amount of pollen per hermaphroditic flower is
disadvantageous and the regulation of total pollen output occurs rather through
the differential production of polliniferous units than pollen per unit;
(ii) the cost of attractive organs and pollen per flower is relatively low.
(iii) the cost per ovule and probability of setting a fruit are relatively high (i.e.
possible savings on reduction of ovaries are high);
(iu) the cost per fruit is relatively high;
( u ) male fertility increases with the resources devoted to flowering;
(ui) selfing is relatively low.
Note that the conditions for the evolution of male-function-controlled floral
display are similar except for the condition (iii) which is ‘probability of setting a
332
K. SPALIK
fruit is relatively high’. In that case, clearly there are no savings from reduction
of ovaries. Only the first three conditions particularly favour andromonoecy or
increased floral display: the rest are standard conditions increasing allocation to
male function (Bertin, 1982; Charlesworth & Charlesworth, 1981; Lloyd, 1979;
Primack & Lloyd, 1980).
The data on variation in pollen production per flower are rather poor. I n
hermaphroditic Armeria maritima (Mill.) Willd. (Plumbaginaceae) , pollen grain
number per flower rose two-fold during the growing season (Woodell, Mattsson
& Philipp, 1977), contrary to andromonoecious Anthriscus sylvestris L. (Apiaceae) ,
where no temporal variation in pollen amount per flower was found; however,
the proportion of male flowers significantly increased (Spalik & Woodell,
unpublished). This seems to confirm the hypothesis that, in andromonoecious
species, total pollen output is regulated through an increase in flower number
rather than pollen amount per flower, but certainly more data are needed.
Pollen grain number per male flower does not differ from that per
hermaphroditic flower of Solanum carolinense L. (Solanaceae, Solomon, 1986) and
is actually lower for A . sylvestris (Spalik & Woodell, unpublished).
There is a certain amount of evidence that flowers and fruits may be
considered as relatively fixed units of resource allocation. Entomogamy may
select for fixed flower size: for 16 species of angiosperms, Berg ( 1956, 1958) found
that only the size of ‘flat’ parts of flowers (free petals) was correlated with plant
size while the size of ‘tubular’ parts (corolla tube, spur) was relatively constant,
In Heloniopsis orientalis (Thunb.) Tanaka (Liliaceae), a response to altitude and
decrease in biomass occurs only in the number of flowers (Kawano & Masuda,
1980). Constant seed size is very common among angiosperms (Harper, Love11 &
Moore, 1970 and reference therein). In some plants, the variation in fruit size is
also limited. For instant, in Yucca whipplei Torr. (Agavaceae), seed number per
fruit is relatively constant and independent of plant size (Aker, 1982).
Individuals of Lotus corniculatus L. (Leguminosae) fertilized with high levels of
nutrients had significantly lower average seed number per fruit than unfertilized
ones but self-thinning of fruits with the lowest seed number occurred as well
(Stephenson, 1984; Stephenson & Winsor, 1986). Abortion of fruits with lower
seed number is common (Stephenson, 1981).
It seems that andromonoecy should always be favoured: fruit production
usually consumes most of biomass devoted to reproduction (Cruden & Lyon,
1985; Stephenson & Bertin, 1983) and this would tend to increase the fraction of
resources devoted to male function (equation 15; Charlesworth & Charlesworth,
1981 ). Andromonoecy is usually associated with expensive fruits (Bertin, 1982;
Lloyd, 1979; Symon, 1979; Whalen & Costich, 1986). For 39 species of Solanum,
the fraction of male flowers has been found to increase with the fruit size
(Whalen & Costich, 1986). Biomass allocation seems to be quite a good
estimator of costs of reproduction (Colosi & Cavers, 1984) or at least the most
popular one (although nutrient allocation may be better for some cases (Lovett
Doust, 1980b; Lovett Doust & Harper, 1980); however, measurements of
allocation of dry weight may substantially underestimate the metabolic costs of
flowering. Nectar production is usually neglected although its total energy
content may be twice as great as that allocated to seeds (Southwick, 1984).
Nectar production does not always serve both reproductive functions equally. In
many dichogamous species more nectar is provided in the female stage of
EVOLUTION OF ANDROMONOECY
333
flowering (Cruden, Hermann & Peterson, 1983) but the contrary has also been
found (Bell el al., 1984). In addition, flowering may involve some indirect costs,
especially in species with big inflorescences (Kay, 1987). Therefore, commonly
observed fruiting-biased dry mass allocation does not necessarily mean that the
fruiting is more costly; hence, the conditions for the evolution of andromonoecy
are not satisfied.
It is generally accepted that in andromonoecious species, male flowers are
smaller than hermaphroditic ones (Solomon, 1986; Symon, 1979; Whalen &
Costich, 1986); however, it is not obvious whether this is achieved only through
reduction of ovaries and styles (Symon, 1979) or whether secondary structures
are also reduced (Whalen & Costich, 1986). In Anlhriscus sylvestris, disc male
flowers have smaller petals and stylopodia and their biomass is on average half
that of hermaphroditic flowers (personal observations).
Male fertility is usually assumed to follow a law of diminishing returns and
saturation of male reproductive effort seems to limit resource allocation to male
function (Maynard Smith, 1978; Charnov, 1982), though the question ‘to what
extent’ is still open. Charlesworth (1984), in her review of androdioecy, assumed
that male function is saturated at a very low level of resource allocation and the
only gain for a male is in survival. In the present model, male fertility was
assumed to increase proportionally to invested resources. It is not obvious
whether male fertility always follows a law of diminishing returns: a high
correlation between pollinia removed and inflorescence size has been observed in
Asclepias syriaca L. (Asclepiadaceae; Willson & Rathcke, 1974), however, it has
not been proved that higher pollinium removal results in an increase of male
fertility. Similarly, in andromonoecious species, additional male flowers increase
the attractiveness of the plant for pollinators (Whalen & Costich, 1986) but it is
not known whether this increases pollen donation.
Andromonoecy has been suggested to restrict rather than enhance outcrossing (Berth, 1982; Primack & Lloyd, 1980; Whalen & Costich, 1986) and
this would tend to promote transfer of resources from male to female function
(equation 15; also Charlesworth & Charlesworth, 1981; Charnov, 1982). One
may notice that andromonoecy is usually associated with a certain pattern of
inflorescence organization: in Solanurn, hermaphroditic flowers are borne at a
base of an inflorescence (Symon, 1979; Whalen & Costich, 1986) and, in
Apiaceae, the proportion of hermaphroditic flowers decreases towards the centre
of an umbellet and the centre of an umbel (Lovett Doust, 1980a). If pollinators
start foraging in the hermaphroditic part of an inflorescence and terminate it in
the male part then andromonoecy would enhance pollen dispersal without an
increase in selfing. Such behaviour of pollinators may be promoted through
differential nectar secretion, as occurs in many dichogamous plants (Cruden
et al., 1983).
According to the model, if the fruit/flower ratio is controlled by female
reproductive function (equation 9) then it reflects only the external limitations of
fruit set (i.e. pollen or resource availability or damage) and is therefore
independent on flower and fruit cost and selfing. However, if pollen availability
limits fruit set then selfing may substantially increase fruit/flower ratio. The
above also concerns fertile flower production in andromonoecious plants. If
overproduction of Rowers serves male ends (equation 1 1) then fruit/flower ratio
decreases with the cost per fruit and increases with the cost per flower and with
334
K. SPALIK
selfing. Selfing may therefore increase the fruit/flower ratio in both cases and selfcompatible species have actually significantly lower fruitlflower ratios than selfincompatible plants with hermaphrodites showing much greater variation than
andromonoecious species (Sutherland & Delph, 1984; Sutherland, 1986b).
Fruit/flower ratio is lower in self-compatible species bearing ‘expensive’ fruits
than in those with ‘cheap’ fruits: for self-incompatible plants no difference has
been found (Sutherland, 1986a). Perhaps, therefore, flower production is
controlled by female reproductive function in self-incompatible species, whereas
in self-compatible plants it is driven by male function. A high probability of a
flower’s setting a fruit favours both andromonoecy and male-function-controlled
overproduction of flowers (see conditions 12 and 16); hence, if pollen availability
limits fruit set then selfing may select for either andromonoecy or male-functioncontrolled floral display in spite of its opposite effect on allocation to male
function.
Lower fruit set in self-incompatible hermaphrodities than andromonoecious
plants has been suggested as an important evidence of male function of flowers in
hermaphrodites (Sutherland & Delph, 1984; Sutherland, 1986a, b). According
to the model, the data alone may also reflect female function of ‘superfluous’
flowers: low probability of setting a fruit stabilizes ‘normal’ hermaphroditism
both us. andromonoecy and male-function-driven overproduction of flowers. I n
other words, retention of ovules is favoured and andromonoecy has not evolved.
However, fruit set in hermaphrodities is also lower than that in monoecious and
dioecious plants and this supports the male-function hypothesis (Sutherland,
1986a).
Low fruit set or high flower display, apart from male function of flowers, has
been also explained in terms of selective abortion and female mate choice,
resource and pollination limitation and pollinator attraction (Stephenson, 1981;
Sutherland, 1986a, b; Sutherland & Delph, 1984; Willson & Burley, 1983). The
hypotheses are not mutually exclusive. The parameter A in the model fits only
some of the cases. If fruit set is resource limited then extra resources may raise it
up to 100%; in such a case the pool of resources for reproduction is not fixed and
the model does not hold. The same concerns those cases of pollination limitation
when additional pollination increases fruit set. The parameter A may reflect
pollinator activity, i.e. relatively constant probability of pollination (note that
additional pollination would not increase fruit set in this case since there are no
resources provided for maturation of extra fruits). One can argue that A may
therefore be an increasing function of flower number or allocation to organs of
attraction rather than a constant (Bell, 1985; Charlesworth & Charlesworth,
1987) since a high number of flowers attracts more pollinators. In that case,
andromonoecy is promoted: the higher the A value the more resource is allocated
to male flowers (equation 15). However, if A reflects selective abortion or female
mate choice rather than probability of pollination then there is no reason to
assume that it is dependent on flower number. Moreover, if abortion is caused by
flower and fruit predation then A may be a decreasing function of a number of
flowers: hence, hermaphroditism is stabilized.
One may ask why, given that adequate conditions are satisfied, andromonoecy
is not always selected over the male-function-controlled floral display? If the cost
of producing ovules is small as compared with the whole cost of flowering then
the gain of an andromonoecious individual is negligible (Bertin, 1982).
EVOLUTION O F ANDROMONOECY
335
Furthermore, for andromonoecy to evolve there must have been a mutation
removing pistils from some hermaphroditic flowers and the mechanism of
regulation of male flower number must have appeared. The floral display
requires only a change of resource allocation towards greater allocation to
flowering; therefore, it may be easier to evolve than andromonoecy. One cannot
also exclude the possibility that floral display, although driven by male
reproductive function, increases female fertility as well (Stephenson, 1981, 1984).
ACKNOWLEDGEMENTS
I am highly indebted to Deborah Charlesworth for her valuable suggestions
and comments on the model. I also thank Alan Grafen, David Mabberley, Ewa
Symonides and Tomek Wyszomirski for their comments on earlier drafts of this
paper.
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