Functional Andromonoecy in Euphorbia (Euphorbiaceae)

Abstract

The occurrence of cyathia containing staminate flowers but lacking a pistillate flower was studied in 17 species of Euphorbia. Male cyathia were found in the majority of species studied (88·2 %) giving functional andromonoecy. In the male cyathia, the pistillate flower is generally totally absent, but sometimes a vestigial pistillate flower with a non‐functional ovary is present. The proportion of male cyathia varied at both the population and species level. The position of male cyathia within the inflorescence showed a constant pattern among species: the proportion of male cyathia decreased from the first to the last levels of the pleiochasia. In general, perennial species had significantly higher proportions of male cyathia than annual species (mean 20 and 2·3 %, respectively). In annual species there was a trend for production of male cyathia only in the first level of the inflorescence, whereas in perennials production up to the fourth level of the inflorescence was usual. Functional andromonoecy is common in Euphorbia and represents a new sex segregation in the genus. The selective forces causing this secondary sex segregation in Euphorbia, such as improved pollination or increased outcrossing, are discussed.

INTRODUCTION

The genus Euphorbia comprises approx. 1600 species with a cosmopolitan distribution, in both temperate and tropical regions (Mabberley, 1990; Benedí et al., 1997). The genus has reduced unisexual flowers grouped into characteristic pseudanthia called cyathia. The cyathium resembles a hermaphrodite flower to such an extent that Linnaeus and other authors interpreted the cyathium as a perfect flower; however, Lamarck interpreted the cyathium as an inflorescence and this is now unquestioned (Gilbert, 1994). The cyathium consists of a central pistillate flower surrounded by five groups of staminate flowers. All flowers are enclosed within an involucre with four marginal glands. The pistillate flower of the cyathium develops before the male ones; thus each cyathium is functionally a protogynous hermaphrodite ‘flower’. The glands of the cyathium usually produce nectar, and pollination is mainly zoophilous (Ehrenfeld, 1979; Webster, 1994; Traveset and Sáez, 1997).

The genus is considered as basically monoecious with a few dioecious species such as Euphorbia hedytoides N. E. Br. (Rauh, 1992) and E. curtisii Engelm. ex Chapm. (Park, 1998). The monoecious species studied to date have been reported to be self‐compatible (Ehrenfeld, 1979; Traveset and Sáez, 1997; Narbona unpubl. res.). Recently, the presence of male cyathia (those lacking a pistillate flower) has been described in the monoecious E. burgei M. Gilbert from Ethiopia (Gilbert, 1992), in E. boetica Boiss. and E. nicaeensis All. from Spain (Narbona et al., 2000) and in E. xanti Engelm. ex Boiss. from North America (Kiflawi, 2000). In these species, male and hermaphrodite cyathia co‐occur in the same individuals, leading to a situation that can be considered as functional andromonoecy.

Andromonoecy is a sexual system in which individuals bear both hermaphrodite and staminate flowers. Frequently, sex expression (the proportions of hermaphrodite and staminate flowers per plant) is variable among individuals, populations and years (Benseler, 1975; Stephenson, 1979; Primack and Lloyd, 1980; May and Spears, 1988; Andersen, 1990; Diggle, 1991, 1993, 1994). This variability can be a consequence of ecological conditions (Solomon, 1985) or may reflect changes in the availability of resources for reproduction (Stephenson, 1979; Diggle, 1991; Gibbs et al., 1999). Within a single plant, resources may be limited for some flowers but not for others, and inflorescence architecture influences resource availability (Diggle, 1995).

Mating systems are known to vary with life history. Outcrossing has been associated with a perennial habit because it has been found that self‐fertilization is much more common in annual than in perennial plants (Stebbins, 1950). Long‐lived species tend to have higher genetic loads that result in strong inbreeding depression when selfing occurs (Morgan et al., 1997). This may favour the evolution of mechanisms to promote outcrossing (Barrett et al., 1997).

The aims of this study were to investigate the occurrence of functional andromonoecy in diverse species of Euphorbia, to study variation in the proportions of hermaphrodite and male cyathia among populations and species, both annual and perennial, and to analyse the distributional patterns of both kinds of cyathia within the plant.

MATERIALS AND METHODS

The presence of male cyathia was investigated in 17 species of Euphorbia during several field expeditions. The only criterion used to determine which species were included in the study was the presence of flowering individuals. No intentional bias was exhibited toward species with any particular phenotypic attributes. Most species were collected in Andalusia (southern Spain), where the climate is typically Mediterranean. One species, E. caecorum, was collected in the neotropical savannas near Uberlândia in central Brazil (locally named cerrado), E. graminea, E. heterophylla, E. hyssopifolia and E. pulcherrima were sampled in the margin of the tropical cloud forest near Caracas in Venezuela, and E. latazi and E. pulcherrima were obtained from the Botanic Garden of Santa Fe de Bogotá in Colombia (Table 1).

Table 1.

Proportion of male cyathia of the species of Euphorbia studied

Taxa	Life form	Male cyathia (%) mean ± s.e.	Populations studied (n)	Populations with male cyathia
Subg. Esula
E. characias L.	P	28·2 ± 10·4	3	3
E. exigua L.	A	5·3 ± 5·3	2	1
E. helioscopia L.	A	2·9 ± 1·0	5	4
E. hirsuta L.	P	3·0 ± 3·0	2	1
E. nicaeensis All.	P	29·0 ± 1·2	2	2
E. paralias L.	P	–	2	2
E. peplus L.	A	5·3 ± 4·0	3	2
E. pterococa Brot.	A	1·1 ± 1·1	3	1
E. segetalis L.	A/P	7·2 ± 3·5	5	3
E. serrata L.	P	36·6 ± 5·9	3	3
E. terracina L.	A/P	18·5 ± 7·7	5	4
Subg. Chamaesyce
E. caecorum DC.	P	7·0	1	1
E. graminea Jacq.	A	0	1	0
E. hyssopifolia L.	A	0	1	0
Subg. Poinsettia
E. heterophylla L.	A	6·1	1	1
E. latazi H. B. & K.	P	–	1	1
E. pulcherrima Willd  ex Klotzsch	P	–	2	2

Life form: P, perennial; A, annual.

Populations collected from Spain (E. characias, E. exigua, E. helioscopia, E. hirsuta, E. nicaeensis, E. paralias, E. peplus, E. pterococa, E. segetalis, E. serrata, E. terracina), Brazil (E. caecorum), Venezuela (E. graminea, E. hyssopifolia, E. heterophylla, E. pulcherrima) and Colombia (E. latazi, E. pulcherrima).

All the species studied have cyathia arranged in pleiochasia and each pleiochasial branch forms several pleiochasia or dichasia; the degree of branching in the inflorescence is variable. The 11 Andalusian species studied belong to the subgenus Esula; Euphorbia heterophylla, E. latazi and E. pulcherrima are included in the subgenus Poinsettia, and the three remaining species (E. caecorum, E. graminea and E. hyssopifolia) belong to the subgenus Chamaesyce (Table 1).

For each species between one and five populations were studied (see Table 1). In each population, ten to 30 plants were randomly sampled in the middle of the flowering period. In the laboratory the number of branching levels in the inflorescence was recorded, assigning level 1 to the terminal cyathium (i.e. the first to be produced; see Fig. 1) in each inflorescence. Using a dissecting microscope, the sex (male or hermaphrodite) of each cyathium was determined, such that the total number of cyathia and the proportion of male cyathia at each level of the inflorescence were estimated for each plant.

Fig. 1. Diagrammatic representation of the inflorescence of Euphorbia showing the flowering order of the cyathia in the inflorescence (levels).

We tested for differences in male cyathia production between annuals and perennials. It is possible that the species studied are not independent data points because of phylogenetic relatedness. To avoid this, an analysis of phylogenetically independent contrast may be necessary. Unfortunately, the species studied here contained only four phylogenetically independent taxonomic groups in which both annuals and perennials are found, and this sample size is too small to analyse in this way. Nevertheless, differences between annuals and perennials in male cyathia production were tested using a two‐way ANOVA considering life form and taxonomic group as fixed effects. The results obtained in this analysis should therefore be treated with caution. Differences between male cyathia production at different inflorescence levels in each species were tested using one‐way ANOVA.

RESULTS

In all species studied, the cyathia were arranged in pleiochasia or dichasia (see Fig. 1) while the extent of branching in the inflorescences was variable. Male cyathia were found in most species (88·2 %; Fig. 2) and in all three subgenera (Table 1), with only two species, Euphorbia graminea and E. hyssopifolia, lacking male cyathia. In most male cyathia, the pistillate flower was totally absent, but in some a vestigial pistillate flower was present (Fig. 2B and G); this vestigial flower has a non‐functional ovary which never protrudes from the cyathium receptacle.

Fig. 2. Cyathia of four Euphorbia species at flowering. A, Male cyathium of E. paralias. B, Male cyathium of E. paralias showing vestigial pistillate flower (arrow). C, Hermaphrodite cyathium of E. paralias during male phase. D, Male cyathium of E. terracina. E, Hermaphrodite cyathium of E. terracina during male phase. F, Male cyathium of E. characias. G, Functional (left) and vestigial (right) pistillate flowers of E. characias. H, Hermaphrodite cyathium of E. characias during female phase. I, Male cyathium of E. segetalis. J, Hermaphrodite cyathium of E. segetalis during male phase.

The proportion of male cyathia was very variable among species, with a mean ranging from 1·09 % in Euphorbia pterococa to 37·5 % in E. serrata (Table 1). Euphorbia paralias, E. latazi and E. pulcherrima produced male cyathia (Fig. 2); however, very few blooming individuals were observed and thus these species were excluded from the analyses. Life form was correlated with the proportion of male cyathia: in general, perennial species produced a significantly greater proportion of male cyathia than annual species (F = 7·06, 1 d.f., P = 0·03). Neither the effect of taxonomic group nor the life form × taxonomic group interaction was significant (P > 0·4). In annuals, the mean proportion of male cyathia was 2·3 ± 0·9 % and in perennials it was 20 ± 4·8 % (medians 1 and 23·1, respectively). This difference according to life form was also present at an intraspecific level: both Euphorbia segetalis and E. terracina have perennial and annual varieties, and only the former had male cyathia (Table 2). At the population level, male cyathia were absent in only one population of a perennial species, whereas nearly 50 % of annual populations did not produce male cyathia (Table 1).

Table 2.

Proportion of male cyathia in two Euphorbia species with annual and perennial varieties

Taxa	Life form	n (Plants)	Mean ± s.e.	Median
E. terracina var. retusa	A	1 (24)	0	0
E. terracina var. terracina	P	4 (69)	23·1 ± 7·9	18·2
E. segetalis var. segetalis	A	2 (30)	0 ± 0	0
E. segetalis var. pinea	P	3 (45)	12·0 ± 3·3	10·4

The position of the male cyathia within the inflorescence showed a definite pattern that was constant between species: in general, the proportion of male cyathia decreased from the first to the last levels of the inflorescence. These differences between levels were highly significant in all species (P < 0·01) except Euphorbia heterophylla (F = 2·0, 4 d.f., P = 0·09) and E. pterococa (F = 1·2, 4 d.f., P = 0·3). In annuals there was a trend to produce male cyathia only in the first level of the inflorescence or, at most, in the second one (Fig. 3). Perennial species showed more variability in the production of male cyathia. Euphorbia hirsuta and E. caecorum followed a pattern similar to that of the annual species (Fig. 4). However, in general, perennials produced male cyathia in more levels of the inflorescence, with male cyathia usually being produced up to the fourth level of the inflorescence.

Fig. 3. Mean proportions of male cyathia in annual and perennial Euphorbia species in each inflorescence level (level 1 is the first formed during flowering). Boxes represent s.e. and the bars are 1·96 s.e.

Fig. 4. Mean proportions of male cyathia in each inflorescence level (level 1 is the first formed during flowering) in the perennial species of Euphorbia studied. In E. segetalis and E. terracina only perennial varieties were included. Boxes represent s.e. and the bars are 1·96 s.e.

DISCUSSION

Male cyathia were found in the majority of species studied with only two exceptions: Euphorbia graminea and E. hyssopifolia. These two species are annuals and for each of these species only one population was studied. Given that male cyathia production in annuals was very variable among populations, it is likely that male cyathia may occur in other populations of these species. Male cyathia were found in all the three subgenera represented in this study: Esula, Chamaesyce and Poinsettia. The presence of male cyathia in monoecious Euphorbia was described previously in the Ethiopian E. burgei of the subgenus Euphorbia (Gilbert, 1992), in E. nicaeensis and E. boetica of the subgenus Esula (Narbona et al., 2000) and in E. xanti of the subgenus Agaloma (Kiflawi, 2000). These studies indicate the widespread presence of male cyathia in the genus Euphorbia.

Euphorbia have very reduced flowers comprising either staminate flowers consisting of just one stamen, or pistillate flowers comprising a three‐locular ovary (Weberling, 1986). The aggregation of such flowers into specialized inflorescences is important for pollination (Armbruster and Steiner, 1992; Armbruster et al., 1992; Webster, 1994; Vieira and Carvalho‐Okano, 1996). In fact, the cyathial inflorescences of the genus Euphorbia ‘mimic’ the hermaphrodite zoophilous flowers of other plants to a remarkable degree. Moreover, the cyathium can undergo further transformations as in Euphorbia caecorum, for example, where the glands develop flat, petaloid appendices; in this way, the superficial likeness of this cyathium to a hermaphrodite zoophilous flower is remarkable. In some cases, the cyathium even becomes zygomorphic and some of the glands are fused to form a large nectar reservoir, as in Euphorbia pulcherrima (see Weberling, 1986). Thus, the cyathial inflorescence units of many species of Euphorbia function as pseudoflowers. In addition, our results show that in some Euphorbia species the central pistillate flower of the cyathium may be aborted, thus giving functional andromonoecy at the level of the pseudoflower. However, the question remains as to what selective forces operate to cause this secondary sex segregation in Euphorbia.

In flowering plants, the availability of resources for an individual can vary in both space and time, due to local competition (Stephenson, 1981; Wyatt, 1981). Hence, within a single plant, resources may be limited for some flowers but not for others. Male function requires fewer resources than female function (Bateman, 1948; Lloyd and Webb, 1977; Charnov, 1979; Delph, 1999), and spatial segregation may permit placement of females in positions that are physiologically more advantageous from a nutritional point of view (Bawa et al., 1982). In some hermaphrodite species, it has been observed that the hermaphrodite flowers produced at the top of the inflorescence are functionally staminate (Berry and Calvo, 1991). In many andromonoecious plants the staminate flowers are situated at the apex of the inflorescence with the hermaphrodite flowers towards the base (Solomon, 1985; Anderson and Symon, 1989). This type of distribution is found within the unisexual flowers of some genera of Euphorbiaceae, e.g. in Cnidoscolus urens pistillate flowers are borne at lower levels of the inflorescence (Bawa et al., 1982). Likewise, in the genus Euphorbia, with its very reduced unisexual flowers grouped in cyathia, pistillate flowers are located in the central, more favourable position (Gilbert, 1994).

However, this study reveals that Euphorbia species with some male cyathia depart from this general rule of female‐function flowers being located in the lower, more favourable positions of the inflorescence, since in all cases the male cyathia were found in the lowest levels of the pleiochasia. This departure from the general rule for pistillate flower placement may be due to aspects of the pollination biology of these species. The cyathial pseudoflowers of Euphorbia are protogynous. Dichogamy and pollinator movements on inflorescences can create different transfer probabilities for pollen at different positions and thus can influence sex allocation (Brunet and Charlesworth, 1995). With protogyny, the ratio of potential mates to ovule competitors is lowest in first‐position flowers and increases in flowers that develop later. As a result, fewer pollen grains are available when stigmas of first‐position flowers are receptive. Therefore, the reproductive success of first‐position flowers is expected to be greater through male function (relative to female), whereas the reverse pattern is expected in flowers that open later (Brunet and Charlesworth, 1995). This could operate to reduce allocation to female function in early‐opening flowers and could be the force behind the pistillate flower of the lowest cyathia in Euphorbia aborting. Moreover, in Euphorbia, the number of cyathia increases as successive levels of pleiochasia begin to flower. Floral display generally increases with the number of flowers per plant or inflorescence (Willson and Price, 1977; Schemske, 1980; Eckhart, 1991; Andersson, 1996). Thus, the phenological sequence observed (male cyathia opening before the hermaphrodites) could represent a strategy to enhance pollen receipt of pistillate flowers, which are produced when pollinator attendance is higher. Similar phenological sequences have been found in other andromonoecious species (Anderson and Symon, 1989).

Finally, in Euphorbia, the proportion of male cyathia varied according to life form, with perennial species producing a significantly greater proportion of male cyathia compared with annuals. Long‐lived species tend to have higher genetic loads that result in strong inbreeding depression when selfing occurs; in addition, long‐lived plants are often large and therefore susceptible to geitonogamous selfing. This may favour the evolution of mechanisms to promote outcrossing in long‐lived plants (Barrett et al., 1997). In fact, it has been reported that woody perennials generally self less often than herbs (Lloyd, 1980; Barrett and Eckert, 1990) and that annual herbs tend to self more than perennial herbs (Barrett et al., 1997). Thus, increased production of male cyathia in perennial species of Euphobia can be seen as a mechanism to promote outcrossing, but this work does not indicate the cause of this phenomenon. Further investigations are needed to clarify the selective scenarios we have proposed.

ACKNOWLEDGEMENTS

We are grateful to Drs P. E. Gibbs, L. Delph and V. M. Eckhart for helpful comments on the manuscript. This work was supported by the Natural Park Sierra de Grazalema (Proyecto Pinsapar), by the Proyecto de Colaboración con Iberoamérica del Ministerio de Ecucación y Cultura and by grant 4086 of the Programa de Ayuda a los grupos de Investigación (Junta de Andalucía).

Received: 17 October 2001; Returned for revision: 15 January 2002; Accepted: 4 February 2002.