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1 Departamento de Biología Vegetal y Ecología, Universidad de Sevilla, Apdo. 1095, 41080-Sevilla, Spain
Received for publication December 8, 1998. Accepted for publication April 22, 1999.
ABSTRACT
Fruit production and arrangement within the raceme were studied in two dioecious populations of Ceratonia siliqua (Leguminosae: Caesalpinioideae), an arboreal species that produces caulogenous racemes (emerging only from the old branches) with numerous flowers. Fruit production per raceme was low and similar between years and populations and even between individuals. During flowering, there were considerable flower losses from predation and lack of pollination. A mean of nine flowers per raceme began the transformation into fruits, of which 77% aborted. The final fruit production per raceme increased significantly following hand pollination, but was always very much lower than the availability of flowers in the raceme. The results suggest that fruit production of each raceme is limited by both availability of resources and a deficient pollination. In racemes setting fruit arrangement follows a definite pattern that remains constant between years and populations: fruit production was significantly higher in the apical zone of the raceme and lower in the basal zone. The pollinators of C. siliqua (flies and wasps) showed a clear preference for beginning their visits at the apex of a raceme. As a result, the pollen load deposited on the stigmas decreased from apex to base of the raceme. In most of the flowers situated in the central and basal zone of the raceme, the number of pollen grains deposited on their stigmas was lower than the number of their ovules. The high number of seeds in developed fruits suggests that the plant selectively aborts flowers that receive a smaller pollen load. The results indicate that the final pattern of fruit arrangement within the raceme is a direct result of pollinator activity.
Key Words: Ceratonia siliqua • fruit production • Leguminosae • pollinator activity • raceme
The development of a low number of fruits in relation to the number of flowers produced is common in allogamous species (Stephenson, 1981
; Sutherland, 1986
), especially in those with high fruit-production costs (Ehrlén, 1991
; Ramirez, 1993
). This phenomenon has been reported repeatedly in Leguminosae, with fruit-set values being very variable between species (Stephenson, 1981, 1984
; Bawa and Webb, 1984
; Bawa and Buckley, 1989
; Lee, 1989
; Ehrlén, 1992
; Ortiz, Arista, and Talavera, 1999
). A reduced fruit set can be due to various nonexclusive factors: limitation of resources (Willson and Schemske, 1980
; Stephenson, 1981
; Wyatt, 1981
; Lee and Bazzaz, 1982
), pollen limitation (Schemske, 1977
; Waser, 1978
; Willson and Schemske, 1980
; Bierzychudek, 1981
; Guitian, 1995
), predation, adverse environmental factors such as frosts, or genetic load (Stephenson, 1981
; Wiens, 1984
; Lee, 1988
). Surplus flower production is especially marked in hermaphrodite species, although it is also found in species with other reproductive systems (Sutherland, 1986
). It has been suggested that in hermaphrodite species, the flower surplus contributes mainly to improving the male function; that is, many of the flowers are produced only to ensure male fitness (Sutherland and Delph, 1984
). However, other hypotheses postulate the importance of the flower surplus in improving the female function. Such a notion is also applicable to andromonoecious, monoecious, or dioecious species (Sutherland, 1986
), so that flower surplus increases attractiveness for pollinators (Wyatt, 1982
; Ortiz, Arista, and Talavera, 1999
), constitutes an ovary pool against unforeseeable loss of flowers by extrinsic causes (Ehrlén, 1991, 1993
; Guitian, 1993, 1994
; Guitian, Guitian, and Navarro, 1996
), and enables the plant to have a measure of control over the quality of its progeny by selective abortion of developing fruits (Janzen, 1977, 1983
; Lee, 1984
; Stephenson and Winsor, 1986
).
Often, the factors controlling fruiting do not affect the different parts of the plant equally, so that the fruits produced are not distributed uniformly (Bawa and Webb, 1984
; Diggle, 1995
). The availability of resources can vary in both space and time for an individual, due to local competition for the resources (Stephenson, 1981
; Wyatt, 1981
). Hence, within a single plant, resources may be limited for some flowers but not for others. Normally, the order of flower opening determines the order of pollination; pollen is an important source of hormones, and its deposition on the stigma initiates ovary growth (Crane, 1964
; Biale, 1978
). Seeds developing within the ovary produce hormones that mobilize resources into the fruit (Crane, 1964
; Biale, 1978
). Thus, the fruits that begin to develop first are usually those with the highest probability of completing their development (Stephenson, 1980
; Fenster, 1991a
; Ehrlén, 1992
; Guitian, 1993, 1994
). Flowers can also have different reproductive potential, so that the probability of their producing fruits varies from one to another (Berry and Calvo, 1991
; Diggle, 1995
). For example, the number of ovules per flower may be lower in the flowers produced latest (Van Stevenick, 1957
; Thomson, 1989
). It has even been observed that, in some species, the flowers produced at the end of flowering have no fruit-producing capacity because they are functionally masculine (Berry and Calvo, 1991
).
Therefore, in species with indeterminate inflorescences, the production of fruits and seeds often decreases from base to apex of the inflorescence (see Lee, 1988
; Obeso, 1993a, b
). In Leguminosae, this pattern has been found in species such as Caesalpinia eriostachys, Myrospermum frutescens (Bawa and Webb, 1984
), and Lathyrus vernus (Ehrlén, 1992
). The pattern is often attributed to competition of developing fruits for resources (Stephenson, 1981
; Lee, 1988
). However, in other species, such as Myrosmodes cochleares, Epilobium dodonaei, and E. fleischeri, a higher fruit production has been found in the central zone of the raceme (Berry and Calvo, 1991
; Stocklin and Favre, 1994
) and attributed to pollinator activity (Berry and Calvo, 1991
; Diggle, 1995
). Lastly, other species show no consistent fruiting pattern related with position within the inflorescence (Zimmerman and Aide, 1989
; see Diggle, 1995
).
The fact that fruit production is limited by the resources, by a deficient pollination, or by other external factors is important from the evolutionary point of view. If the resources are limited and there is no pollen restriction, sexual selection may operate since the female would exercise mate choice, selectively aborting lower quality fruits and seeds (Janzen, 1977
; Willson, 1979
; Willson and Burley, 1983
). Moreover, an intense, diverse pollination would generate competition between the developing pollen tubes to reach the ovules, resulting in sexual selection via male-male competition (Mazer, 1987
; Silvertown and Lovett-Doust, 1993
). Nevertheless, the opportunity for mate choice can be diminished by pollen limitation (Willson and Burley, 1983
) or the existence of strong maternal effects or environmental constraints causing a particular fruiting pattern (Garwood and Horvitz, 1985
), delaying the evolutionary response (Naylor, 1964
). Ceratonia siliqua (Leguminosae: Caesalpinioideae) is a species that produces a large number of racemes with numerous flowers (Ortiz, Arista, and Talavera, 1996
). Previous studies have shown that this species presents a low percentage of fruiting per raceme in both wild populations (Ortiz, Arista, and Talavera, 1999)
and cultivated ones (Bosh et al., 1996
). The aim of the present work is to determine whether there is a consistent pattern in the arrangement of the fruits within the racemes in this species, and if so, to determine what are its causes.
MATERIALS AND METHODS
Study area and study species
The study was carried out in two populations of the Parque Natural Sierra de Grazalema (southwest Spain, 36°45'–36°47' N and 5°28'–5°22' W, province of Cádiz): one was situated in the Sierra de Zafalgar (called La Camilla) and the other in the Sierra del Pinar (called El Boyar). In both populations, Ceratonia siliqua forms a very open, mixed woodland, together with Quercus faginea Lam., Quercus rotundifolia Lam., Olea europea var. sylvestris Brot., and Abies pinsapo Boiss. The bush stratum is more diverse and abundant, and comprises mainly Juniperus oxycedrus L. subsp. oxycedrus, Juniperus phoenicea L. subsp. phoenicea, Phyllirea angustifolia L., P. media L., Pistacia lentiscus L., P. terebinthus L., Ulex parviflorus Pourret, and Lavandula lanata Boiss. The climate of the zone is typically Mediterranean, with mild winters (the mean temperature of the coldest month is 6.8°C) and warm summers (the mean of the warmest month is 24.6°C); the mean annual precipitation is 2000 mm.
Ceratonia siliqua is the only wild representative of the subfamily Caesalpinioideae in the Iberian peninsula and presents a basically Mediterranean distribution. It is a species that is mainly dioecious, although there are functional hermaphrodite individuals (Tucker, 1992
). The flowers are small, apetalous, radiate in symmetry, yellowish-green in color and are grouped in erect racemes (Fig. 1). These racemes are caulogenous, that is they emerge only from the old branches of the tree. Female racemes have a mean of 46 flowers, each of which produces a mean accumulated amount of 0.17 mg of sugar per day (Ortiz, Arista, and Talavera, 1996
). The first flowers to become receptive are those situated at the base of the raceme, and the last, those at the apex. However, as the receptive period of each flower is long, all the flowers of the raceme are receptive at the same time during a short period. In the study area, Ceratonia siliqua flowers from the end of September to the end of December, and the fruits are completely ripe in the autumn of the following year.
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To determine whether the reproductive potential of each flower depends on its position in the raceme, flowers were collected from the apex, center, and base of 11 racemes from five trees of La Camilla. These flowers were opened in the laboratory and the number of ovules was counted.
In order to know the causes of flower abscission, traps were placed under four trees to collect fallen flowers during the flowering period of 1997. These flowers were examined under a binocular magnifier to see whether they presented either pollen on the stigma or any external sign of predation.
To discover whether the low fruit production per raceme and the fruit position within the raceme were due to pollen limitation, during the flowering of 1997, all the flowers in 76 marked racemes on four trees were pollinated manually. At the same time, in the same trees, 74 racemes were marked as control (free pollination). In all these racemes, the fruits that began development and those that completed it were counted.
To determine whether pollinator behavior affects the fruiting pattern within a raceme, censuses of flower visitors during the flowering period were made in both populations. The identity of each pollinator and the first-contacted zone of the raceme were noted. As Ceratonia siliqua is a dioecious species, it is to be expected that flower visitors will deposit a greater amount of pollen on the stigmas with which they make first contact. After flowering, 22 racemes were collected (from six trees), from which three flowers from the apex, three from the center, and three from the base were fixed. The stigmas of these flowers were softened with 8 mol/L NaOH, stained with aniline blue, and observed under a fluorescence microscope to count all the pollen grains deposited on them (Martin, 1959
).
Statistical analyses
The data of insect flower-visiting frequency were analyzed using chi-squared tests. The remaining data were analyzed using variance analysis (ANOVA). The possible differences between trees in the production of fruits per raceme, the differences between flowers of the raceme in the production of ovules, or the differences in fruit production between the different zones of the raceme were analyzed using one-way ANOVA. Where the differences were significant, the separation of means was performed using the Tukey HSD test (P < 0.05). To reveal whether the observed patterns of fruit arrangement were due to the characteristics of the trees, to the method of pollination, or to interaction between the two factors, a two-way ANOVA was used. The effects of the population, the individual tree, the position of the fruit in the raceme, or the interactions between them in seed production were analyzed using a MANOVA. A nested ANOVA was used to reveal whether the pattern of pollen deposition on the stigmas was consistent within each raceme or varied between them; for this, the position of the flower was considered the main effect and the raceme a nested effect of position. Prior the statistical analyses, samples with non-normal distributions were normalized by adding 0.01 to each value and taking its logarithm (base 10). Percentage data were arcsin transformed before statistical analysis (Sokal and Rohlf, 1979
).
RESULTS
Fruit production and position pattern
Fruit production in the set racemes ranged between one and 13, and was more or less uniform within each tree. Between trees, the mean fruit production per raceme ranged between 1.65 and 4.6, these differences being significant (F = 7.03574, 19 df, P = 0.000001); however, in most of the trees, the production was similar (some 70% had a mean of two fruits per set raceme), and just two trees, with means of 4.5 and 5, were responsible for the differences observed.
Fruit production per set raceme was markedly similar between years and populations (Table 1). In the population of La Camilla, in both 1996 and 1997, the mean production did not reach three fruits per raceme. In El Boyar, the dioecious trees and the hermaphrodites had similar, low mean fruit productions: 2.71 and 2.1 fruits, respectively.
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Approximately 80% of the flowers of each raceme aborted before or during flowering. A sample of these flowers, collected in the traps during the flowering period of 1997, showed that 42.9% (N = 256) fell before becoming receptive, and 57.1% (N = 338) fell during or immediately after flowering. Of the flowers with early abscission, only 35.2% showed signs of predation, while the rest showed no sign of the cause of their abscission. Of the flowers that aborted during or after flowering, only 21.8% showed signs of predation; in the rest, examination of the stigmas showed that only 4% had been pollinated, but none presented more than three pollen grains.
In all the marked racemes, pollinated either freely or by hand, some fruits were initiated. Hand pollination of all the flowers of the raceme did not increase the mean number of fruits that began development, which was not significantly different between treatments (9.027 of the free pollinated vs. 11.23 of the hand pollinated; Tables 4, 5). The number of fruits beginning development per raceme showed marked, statistically significant differences between trees (Table 5). The number of racemes that completed ripening of any fruit was markedly variable between treatments. Of the 76 racemes pollinated manually, 69.73% produced at least one ripe fruit; this percentage was significantly (P = 0.0004) greater than the 40.54% of those pollinated freely. The mean number of ripe fruits in set racemes after hand pollination was significantly greater than that after free pollination (means of 3.09 and 2.16, respectively; Table 4). The result of a multiple ANOVA shows that this increase was due exclusively to the treatment and not to the differences between trees (Table 5).
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Insect visits and pollen deposition
The main floral visitors of Ceratonia siliqua belonged to the orders Hymenoptera (55.7% of total visits) and Diptera (44.3%). Only two types of Hymenoptera visited the flowers: Vespula sp., which made most of the visits (79%), and Apis mellifera, which was observed only in the population of El Boyar (Fig. 2). The Diptera were more diverse and belonged to the families Muscidae, Calliphoridae, and Syrphidae.
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2 = 14.88, 2 df, P < 0.00058) and El Boyar (2 = 83.218, 2 df, P < 0.000001). On some occasions, the pollinators (particularly wasps) entering the inflorescence by the base did so by walking along the branches of Ceratonia siliqua. In the flowers pollinated freely by the insects, those becoming fruits had at least one pollen grain on their stigma. The number of pollen grains on these stigmas was very variable between flowers and between racemes, ranging from one to a maximum of 162 grains. The amount of pollen deposited depended strongly on the position of the flower in the raceme. The frequency distribution of pollen grains deposited on the stigmas of apical flowers approached a normal one, with a mean of 41.19 pollen grains (Fig. 3). However, the frequency distributions of the central and basal flowers were highly skewed to the right, with means of 19.42 and 10.93 pollen grains, respectively. The results of a nested ANOVA show that the differences in pollen deposition were markedly significant between positions within the raceme (F = 73.85695, 2 df, P < 0.000000) and between racemes (F = 3.63282, 63 df, P < 0.000001), but 94.09% of the variation was due to the situation of the flower in the raceme, and only 4.63% to the difference between them. The number of pollen grains found on most of the stigmas of the central and basal flowers of the raceme was lower than the number of ovules of these flowers, the opposite situation to that in the apical flowers (Fig. 3).
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The production of a low number of fruits per raceme is a constant characteristic in Ceratonia siliqua, as has been shown in previous studies (Bosh et al., 1996
; Ortiz, Arista, and Talavera, 1999
) and in this work. Of the most frequent causes of reduced fruit harvests, predation (above all before anthesis of the flowers), deficient pollination, and limitation of resources operate in Ceratonia siliqua.
The generalized absence of pollen in the open flowers collected in the traps, and the fact that many deficiently pollinated flowers initiated development, could indicate that it is pollination that limits setting. However, manual pollination of all the flowers of the raceme did not increase the number of initiated fruits, suggesting that there is a limit to the number of fruits that can be initiated per raceme. This limitation could be determined by the availability of resources, as shown in different species (Harrold, 1935
; Williamson, 1966
; Stephenson, 1979
).
Although the number of initiated fruits was similar between treatments, both the number of set racemes and the number of ripe fruits per raceme increased after manual pollination. Such increases indicate the existence of pollen limitation in Ceratonia siliqua under natural conditions, as in other species (Willson, Miller, and Rathcke, 1979
; Bierzychudek, 1981
). Various features of the floral biology of Ceratonia siliqua could be responsible for this deficient pollination. Its flowers are apetalous (and hence not very attractive), the racemes are caulogenous (and therefore hardly visible from outside the tree canopy), and the female flowers are much less fragrant than the male ones (and hence less visited) (Ortiz, Arista, and Talavera, 1996
). Moreover, rainfall is frequent during the flowering period, limiting pollinator activity. Although the removal of this pollen limitation by manual pollination increased fruit production, fruit number per raceme continued to be low (a mean of three fruits after manual pollination). This suggests that fruit production in Ceratonia siliqua is also limited by the availability of resources, but that such limitation operates only when there is no pollen limitation.
The number of fruits that ripened in the set racemes hardly varied within each tree and between trees, and even between years and populations. However, the number of flowers per raceme ranged between 15 and 112 (Ortiz, Arista, and Talavera, 1996
), so that there could have been great variations in fruit production. In other species with high fruit-production costs, it has been suggested that each inflorescence could function as an independent unit with regard to resource use (Guitian, 1994
; Guitian, Guitian, and Navarro, 1996
). This may be happening in Ceratonia siliqua, so that the limitation of resources would operate at the inflorescence level, giving rise to very similar maximum fruit productions.
The arrangement of the ripe fruits in the racemes followed a very well-defined pattern that remained constant between years and populations. In all cases, the apical flowers of the raceme had a higher probability of being converted into fruit than those situated closer to the base. This acropetal pattern contrasts strongly with that found in a large number of species (including many Leguminosae), in which fruit production is higher at the base of the inflorescence (Bawa and Webb, 1984
; Lee, 1988
; Berry and Calvo, 1991
; Ehrlén, 1992
; Obeso, 1993a, b
; Diggle, 1995
). The basipetal trend in fruit distribution is usually explained as, on the one hand, earlier pollination of the basal flowers, whose transformation into fruit would limit later fruit set, and on the other, the more favorable position of basal fruits with respect to resources (Stephenson, 1981
; Lee, 1988
; Diggle, 1995
). In a raceme of Ceratonia siliqua, the flowers open from the base to the apex, but at a certain moment, practically all are receptive at the same time, making it possible for the nonbasal flowers to be pollinated first. Even so, once pollinated, those closer to the base will have a greater probability of being transformed into fruit, as happens after manual pollination. However, in Ceratonia siliqua, due to pollinator behavior, not all the flowers of a raceme have the same probability of being pollinated. There are numerous studies on the foraging behavior of pollinating insects in vertical inflorescences. Most of these studies were carried out in protandrous hermaphrodite plants, in which it was shown that bees, bumble bees, and flies tend to move up the inflorescences (Hocking, 1968
; Benham, 1969
; Pyke, 1982
; Waddington and Heinrich, 1979
; Corbet et al., 1981
; Best and Bierzychudek, 1982
; Wyatt, 1982
; Galen and Plowright, 1985
; Kevan, 1990
). Studies on protogynous hermaphrodite species are scarce and show either a visiting pattern opposite to that of the former or the absence of a definite pattern (Wilson, 1978
; McKone et al., 1995
). In dioecious species, there are practically no data on the foraging behavior of the insects. Our results show that the pollinators of Ceratonia siliqua had a very marked tendency to visit the flowers situated closest to the apex of the raceme first, particularly in the case of the Diptera. This pollinator behavior greatly reduces the probability of pollination in the basal flowers, and consequently their transformation into fruit. Thus, the fruit arrangement pattern in the raceme in Ceratonia siliqua seems to be caused directly by pollinator activity. The results of the manual pollinations carried out support this hypothesis, as the fruit arrangement is inverted in racemes so treated—with selective ripening of those at the base of the raceme (those closest to the resource origin).
Three causes have been suggested for determining the foraging pattern of an insect in an inflorescence: (1) the floral reward diminishes with direction along the inflorescence (Pyke, 1978
); (2) the orientation of the flower in the raceme is such that after visiting a flower, the insect has a better vision of the next one in a certain direction (Waddington and Heinrich, 1979
); and (3) the posture adopted by the insect during foraging means that its next visit requires a lower spending of energy in one direction than in another (Corbet et al., 1981
). In Ceratonia siliqua, the racemes are caulogenous; in these, the flowers are arranged helicoidly and their ovaries, curved downwards, overlap the more basal flowers (see Fig. 1). This inflorescence architecture possibly makes it easier for insects to move down the raceme, as movement in the opposite direction would be hindered by the stigmas of more apical flowers; furthermore, access to the nectary, situated at the base of the ovary, would be easier. At the same time, for a flying insect, entrance from the base of a caulogenous raceme appears to be difficult. In fact, some wasps that entered from the base did so by walking along the branches. We do not know whether there is a gradient in the nectar offer along the raceme, but we have no reason to suspect so. It is possible that the arrangement of the flowers in the raceme and of the racemes on the branches in Ceratonia siliqua makes the energy expense to the floral visitor lower when foraging is from the apex towards the base rather than in the opposite direction, causing the pattern observed.
The foraging behavior of insects in long inflorescences can have important implications for the reproductive success of the plant. In hermaphrodite species with dichogamy, insect visits upwards on protandrous plants and downwards on protogynous ones would favor allogamous pollination (Pyke, 1978
; Wilson, 1978
). In dioecious plants, pollination is necessarily allogamous, and insect foraging pattern has no effect in this matter. However, the pattern does affect the size of the pollen load received by each flower. If several female flowers are visited consecutively, those visited first will receive a greater amount of pollen, since the effect of pollen carry-over will presumably decrease in each visit. In Ceratonia siliqua, between 42.6 and 55.4% (depending on population) of times that an insect arrived at an inflorescence, it visited the apical flowers first. Therefore, it is probable that pollinators deposit more pollen in these flowers. The number of pollen grains per stigma was higher in apical flowers of the raceme than in basal ones, confirming this notion. Thus, the pattern of pollen deposition decreases from the top downwards along the raceme, reflecting pollinator activity.
The amount of pollen in the flowers situated in the central or basal zones of the raceme was, in many cases, insufficient to fertilize all the ovules of those flowers. In free pollination each raceme set a mean of nine fruits, of which only two ripened. Thus, 77.7% of the fruits set aborted during their development, possibly because of a lack of resources (see Stephenson, 1981
; Lee and Bazzaz, 1982
). The fact that the ripe fruits, independently of their position in the raceme, presented a high number of seeds suggests that in Ceratonia siliqua there is selective abortion of fruits. Differences in the amount of pollen deposited on the stigmas result in variations in the number of seeds per fruit, in turn affecting the probability of ripening. A positive relationship between the number of seeds and the probability of fruit ripening has been found previously in Leguminosae (Lee and Bazzaz, 1982
; Stephenson and Winsor, 1986
), and in many other species (Stephenson, 1981
; Winsor, Davis and Stephenson, 1987
; Falque et al., 1995
).
Evolutionary consequences
The overall results obtained in Ceratonia siliqua suggest that the enormous surplus of flowers counteracts the loss of ovaries for extrinsic causes, ensures a certain level of pollinator visits, and enables selective ripening of the better pollinated fruits. Despite this, the low fruit production per raceme is the consequence of a limitation of resources and of an insufficient pollination: the number of flowers effectively pollinated is slightly lower than the number of fruits that the resources permit to develop. Without this pollen limitation, fruit production of each raceme would not have changed much in amount, but qualitatively, the difference could have been considerable. Because the number of sufficiently pollinated flowers is so close to the number of fruits that the inflorescence can develop, the margin for sexual selection via mate choice is very limited. The small amount of pollen present on the stigmas of many of the pollinated flowers would result in a selection of fruits with a higher number of seeds, independently of their quality. Not even the best-pollinated flowers had large pollen loads, considerably restricting the possibility of competition between pollen grains of differing quality. Nevertheless competition between males may occur prior to pollen dispersal, when plants may compete to attract pollinators (Mazer, 1987
; Silvertown and Lovett-Doust, 1993
). The raceme architecture and the caulogenous situation led to a pollinator foraging behavior leaving the basal flowers deficiently pollinated; hence, the resources were allocated to the apical fruits. Constraint of fruiting pattern by pollinator behavior has also been reported in other Leguminosae (Fenster, 1991b
). This kind of environmental constraint might limit the possibility of sexual selection (Garwood and Horvitz, 1985
), delaying the evolutionary response (Naylor, 1964
). However, after hand pollination of all the flowers of the raceme, the arrangement pattern of the fruits was inverted, showing the existence of a strong maternal effect in the distribution of resources. Thus, even in the absence of pollen limitation or environmental constraints, in Ceratonia siliqua there will be little opportunity for sexual selection via mate choice.
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1 The authors thank Dr. R. Niesenbaum and an anonymous reviewer for their useful comments, which have improved the manuscript, and R. Tavera for drawing the Ceratonia raceme. This research was supported by grant 4086 of the Programa de Ayuda a los grupos de Investigación (Junta de Andalucía) and by the Proyecto Pinsapar (A.M.A.). The A.M.A. provided the work in the Nature Reserve (Parque Natural Sierra de Grazalema). 
2 Author for correspondence (e-mail: MPALMERO{at}CICA.ES
FAX: + 954557059).
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