榕树-传粉者共生体系的协同进化与系统学研究进展及展望

 榕树-传粉者共生体系是目前植物与昆虫协同进化研究中的典型模式之一。国内外已经开展了大量的相关研究,从不同方面探讨了其特殊的一一对应的共生关系。榕树-传粉者的专一性互惠共生关系中蕴含了与系统发育有关的多因子协同进化的机理,因此,进行系统发育研究将有助于更好地揭示榕树-传粉者的协同进化历史和理解二者的专一性互惠共生关系。本文简单地介绍了目前榕树及其传粉者共生体系的研究状况之后,论述了榕树-传粉者协同进化的系统发育分子生物学研究成果。同时针对国际上在榕属植物的传统的系统与分类研究中存在的一些分歧及榕树传粉者亚科分类不匹配等问题,回顾了榕属的分类研究进展及其与传粉者的关系。最后,结合我国榕树与传粉者共生体系的研究状况对我国榕属的重新分类和系统发育研究作了展望。     

The functional implications of active and passive pollination in dioecious figs

Fig-pollinating wasps lay their eggs in fig flowers. Some species of fig-pollinating wasps are active pollinators, while others passively transfer pollen. In dioecious fig species, the ovules of male figs produce wasps but no seeds. By observations and experiments on four dioecious Ficus species we show that (i) passive pollinators distribute pollen haphazardly within figs, but fertilization of female flowers in male figs is inhibited. Consequently, wasp larvae will develop in nonfertilized ovules: they cannot benefit from pollination; (ii) active pollinators efficiently fertilize flowers in which they oviposit. Lack of pollination increases larval mortality. Hence, fig pollinators are not obligate seed eaters but ovule gallers. Active pollination has probably evolved as a way to improve progeny nourishment.
Comparison of pollination and oviposition process in male and female figs, suggests that stigma shape and function have coevolved with pollination behaviour, in relation to constraints linked with dioecy.     

Convergent evolution of agaonine and sycoecine (Agaonidae, Chalcidoidea) head shape in response to the constraints of host fig morphology

Abstract. Similar morphological adaptations have arisen independently across separate lineages within the fig wasps (Agaonidae, Chalcidoidea, Hymenoptera) in response to the extreme selective pressure provided by the morphological constraints of their host fig trees ( Ficus , Moraceae). Evidence is forwarded that supports the convergence of female head shape between two distinct fig wasp lineages, the Agaoninae (pollinators) and Sycoecinae (non-pollinators), utilizing the same host Ficus species (section Gagolychia ). In contrast to the vast majority of the non-pollinating fig wasps, that oviposit from the outside of the fig, the Agaoninae and Sycoecinae must negotiate the fig ostiole for internal oviposition, with the result that these independent lineages are simultaneously exposed to the selective pressure imposed by ostiolar morphology. Selection will favour a head shape that facilitates successful penetration of the fig cavity and this has resulted in the evolution of similar head shapes in the two lineages. Female head shape in both subfamilies was found to correlate with fig size, with elongate heads associated with large fig size. Given that ostiole bract arrangement is uniform within section Galoglychia , it appears that ostiole length may be the main factor contributing to head shape determination. The high degree of co-adaptation of head shape suggests that both the Sycoecinae and Agaoninae have coevolved with their host Ficus species.     

高榕的3类雌花形态及其繁殖特征

榕树(Ficus spp.)及其传粉榕小蜂(Agaonidae)之间经过长期的协同进化, 已形成了高度专一的互惠共生关系。在雌雄同株的榕树中, 同一个榕果内既繁殖种子, 又繁殖榕小蜂, 雌花资源在繁殖种子和榕小蜂之间是怎样分配的?该研究选择广泛分布于西双版纳地区的高榕(F. altissima)及其传粉榕小蜂系统来回答这个问题。高榕果内的雌花柱头形态变异较大, 有3种类型的柱头——球型、弯钩型和火炬型, 它们分别占到雌花总量的54.00%、36.93%和9.07%。3种类型柱头的雌花分别具有不同长度的花柱, 球型柱头雌花的平均花柱长度最短, 火炬型柱头次之, 弯钩型柱头雌花的平均花柱长度最长。在高榕果内, 有传粉者Eupristina altissima和欺骗者Eupristina sp.两类小蜂进入榕果内繁殖, 前者的产卵器长度比96.01%以上的雌花花柱长, 后者的产卵器也要长于85.73%的雌花花柱, 从产卵器长度和雌花花柱长度的匹配情况看, 它们应该可以利用绝大多数雌花产卵繁殖后代。然而, 繁殖榕小蜂的雌花主要是短花柱的雌花, 其中60.64%是球型柱头的雌花; 而繁殖种子的主要是花柱较长的弯钩型柱头和火炬型柱头的雌花。显然, 繁殖榕小蜂和种子的雌花不仅花柱长度有差异, 柱头也分化出了不同的形态, 变异的柱头形状也是调节榕树-榕小蜂繁殖平衡的手段之一。     

Phylogenetic relationships of fig wasps pollinating functionally dioecious Ficus based on mitochondrial DNA sequences and morphology

The obligate mutualism between pollinating fig wasps in the family Agaonidae (Hymenoptera: Chalcidoidea) and Ficus species (Moraceae) is often regarded as an example of co-evolution but little is known about the history of the interaction, and understanding the origin of functionally dioecious fig pollination has been especially difficult. The phylogenetic relationships of fig wasps pollinating functionally dioecious Ficus were inferred from mitochondrial cytochrome oxidase gene sequences (mtDNA) and morphology. Separate and combined analyses indicated that the pollinators of functionally dioecious figs are not monophyletic. However, pollinator relationships were generally congruent with host phylogeny and support a revised classification of Ficus. Ancestral changes in pollinator ovipositor length also correlated with changes in fig breeding systems. In particular, the relative elongation of the ovipositor was associated with the repeated loss of functionally dioecious pollination. The concerted evolution of interacting morphologies may bias estimates of phylogeny based on female head characters, but homoplasy is not so strong in other morphological traits. The lesser phylogenetic utility of morphology than of mtDNA is not due to rampant convergence in morphology but rather to the greater number of potentially informative characters in DNA sequence data; patterns of nucleotide substitution also limit the utility of mtDNA findings. Nonetheless, inferring the ancestral associations of fig pollinators from the best-supported phylogeny provided strong evidence of host conservatism in this highly specialized mutualism.     

Molecular phylogeny of the Ceratosolen species pollinating Ficus of the subgenus Sycomorus sensu stricto: biogeographical history and origins of the species-specificity breakdown cases

The 14 species of Ficus of the subgenus Sycomorus (Moraceae) are invariably pollinated by Ceratosolen species (Hym. Chalcidoidea), which in turn reproduce in the fig florets. They are distributed mostly in continental Africa, Madagascar, and the Mascarene and Comoro Islands, but 1 species extends its geographical range all over the Oriental region. Fig-pollinator relationships are usually strictly species specific, but exceptions to the 'one-to-one' rule occur within the group we studied. In order to understand both the biogeographical history of the Ceratosolen species associated with Ficus of the subgenus Sycomorus and the origins of the specificity breakdown cases, we have used cytochrome b sequences to reconstruct a phylogeny of the fig wasps. The results show that the pollinators from the Malagasy region and those from continental Africa form two distinct clades, which probably diverged after the crossing of the Mozambique Channel by an ancestral population. The Oriental wasp species show strong affinities with the African species. The two species-specificity exceptions are due to different evolutionary events. The occurrence of the two West African pollinators associated with F. sur can be explained by successive speciation events of the mutualistic partner without plant radiation. In contrast, we hypothesize that C. galili shifted by horizontal transfer from an unknown, presumably extinct, Ficus species to F. sycomorus after this native Malagasy fig species colonized Africa.     

Exceptions to the one:one relationship between African fig trees and their fig wasp pollinators: possible evolutionary scenarios

Abstract. The mutualistic breeding system involving fig trees (Moraceae, Ficus ) and fig wasps (Hymenoptera, Chalcidoidea, Agaoninae) would appear so specialized that one may wonder at the evolutionary processes that could be responsible for the existence of about 750 species-specific associations. In this paper we present data concerning two cases of species specificity breakdown between African fig trees and fig wasps. We then analyse the possible evolutionary scenarios which could be responsible, as well as the possible evolutionary outcomes of the observed situations. Our analyses range in scale from continental to habitats, and fig crops to individual figs (syconia). Habitat shifts and ecological barriers seem to be the more likely explanations for the evolution of novel fig tree-fig wasp associations, but sympatric and parapatric scenarios cannot be ruled out.     

Phylogenetic relationships of functionally dioecious FICUS (Moraceae) based on ribosomal DNA sequences and morphology

Figs (Ficus, Moraceae) are either monoecious or gynodioecious depending on the arrangement of unisexual florets within the specialized inflorescence or syconium. The gynodioecious species are functionally dioecious due to the impact of pollinating fig wasps (Hymenoptera: Agaonidae) on the maturation of fig seeds. The evolutionary relationships of functionally dioecious figs (Ficus subg. Ficus) were examined through phylogenetic analyses based on the internal transcribed spacer (ITS) region of nuclear ribosomal DNA and morphology. Forty-six species representing each monoecious subgenus and each section of functionally dioecious subg. Ficus were included in parsimony analyses based on 180 molecular characters and 61 morphological characters that were potentially informative. Separate and combined analyses of molecular and morphological data sets suggested that functionally dioecious figs are not monophyletic and that monoecious subg. Sycomorus is derived within a dioecious clade. The combined analysis indicated one or two origins of functional dioecy in the genus and at least two reversals to monoecy within a functionally dioecious lineage. The exclusion of breeding system and related characters from the analysis also indicated two shifts from monoecy to functional dioecy and two reversals. The associations of pollinating fig wasps were congruent with host fig phylogeny and further supported a revised classification of Ficus.     

Pollen waste and unrelated traits in a fig-fig wasp symbiosis: a new behaviour suggesting a host shift

In a fig-fig wasp symbiosis, we have discovered that male fig pollinators (Alfonsiella fimbriata Waterston) bite into the dehiscent anthers of Ficus natalensis leprieuri Miq., thus scattering the pollen grains throughout the syconium. Female pollinators are the only ones to transfer pollen to conspecific trees, and collect pollen actively from the anthers only. Thus, this male behaviour appears to be antagonistic to the pollination process. We compare different wasp pollinating behaviours between fig species exhibiting dehiscent and non-dehiscent anthers and conclude that this male behaviour is new and not required with spontaneously dehiscent anthers. These findings could suggest a host shift of Alfonsiella fimbriata.     

Complementary fruiting phenologies facilitate sharing of one pollinator fig wasp by two fig trees

Aims Most pollinator fig wasps are host plant specific, with each species only breeding in the figs of one fig tree species, but increasing numbers of species are known to be pollinated by more than one fig wasp, and in rare instances host switching can result in Ficus species sharing pollinators. In this study, we examined factors facilitating observed host switching at Xishuangbanna in Southwestern (SW) China, where Ficus squamosa is at the northern edge of its range and lacks the fig wasps that pollinate it elsewhere, and its figs are colonized by a Ceratosolen pollinator that routinely breeds in figs of F. heterostyla .Methods We recorded the habitat preferences of F. squamosa and F. heterostyla at Xishuangbanna, and compared characteristics such as fig size, location and colour at receptive phase. Furthermore, the vegetative and reproductive phenologies in the populations of F. squamosa and F. heterostyla were recorded weekly at Xishuangbanna Tropical Botanical Garden for 1 year.Important findings Ficus squamosa is a shrub found near fast-flowing rivers, F. heterostyla is a small tree of disturbed forest edges. Although preferring different habitats, they can be found growing close together. Both species have figs located at or near ground level, but they differ in size when pollinated. Fig production in F. squamosa was concentrated in the colder months. F. heterostyla produced more figs in summer but had some throughout the year. The absence of its normal pollinators, in combination with similarly located figs and partially complementary fruiting patterns appear to have facilitated colonization of F. squamosa by the routine pollinator of F. heterostyla. The figs probably also share similar attractant volatiles. This host switching suggests one mechanism whereby fig trees can acquire new pollinators and emphasizes the likely significance of edges of ranges in the genesis of novel fig tree–fig wasp relationships.     

中国榕小蜂触角感受器形态特征及进化适应性分析

【目的】榕树与传粉榕小蜂体系是协同进化、专性传粉的经典系统,每种榕树上一般还生活有种类丰富、数目众多的非传粉榕小蜂。在选择压力下,榕小蜂为在榕果内生存产生了明显的适应性形态。触角感受器是昆虫通讯系统的单元,其形态是反映生态和进化适应性的最佳特征之一。本文旨在对中国部分传粉和非传粉榕小蜂的触角感受器的形态多样性和进化适应性进行系统研究。【方法】对来源于海南和云南15种榕树上24种榕小蜂54个型的触角感受器形态进行了扫描电镜观察,基于现有的分子系统发育树,对传粉榕小蜂触角感受器的形态特征进行了性状演化分析,并对榕小蜂的形态特征进行了进化适应性分析。【结果】榕小蜂触角感受器普遍存在雌雄二型现象。雌蜂触角感受器种类有毛状、锥状、板状、刺状、钟形、腔锥状和栓锥状等,数目丰富,并且进果产卵的传粉雌蜂和果外产卵的非传粉雌蜂之间、进果产卵的传粉雌蜂和进果产卵的非传粉雌蜂之间在形态上存在差异。传粉雄蜂和不具有雄性多型现象的非传粉雄蜂触角感受器极为退化,具有雄性多型的非传粉雄蜂触角感受器形态在种内不具有显著差异。性状演化分析表明进化路径相当复杂,可能存在多次独立进化过程。触角感受器的形态类型与其进化适应性相关。【结论】榕小蜂触角感受器类型多样,形态变化丰富,并为适应榕果内的生存而产生了进化适应性特点。雌蜂和雄蜂在榕果内受到了完全不同的选择压力,行使不同的生态功能,从而产生了不同的适应性形态。不同的适应性形态可能与雌蜂不同的产卵行为、雄蜂不同的交配策略具有一定联系。该文是首次对中国榕小蜂触角感受器形态进行系统研究的报道,有助于更好地理解榕小蜂的形态特征、进化路线、行为策略和生态关系。     

Spatial heterogeneity and host repression in fig-fig wasp mutualism

It is generally believed that physical heterogeneity in common resource or evolutionary restraint can sufficiently prevent direct conflict between host and symbionts in mutualism systems. Our data on fig/fig wasp reciprocal mutualism(Ficus racemosa), however, show that structural barriers of female flowers or genetic constraints of pollinators previously hypothesized exist, but cannot sufficiently maintain the mutualism stability. The results show that a positive relationship between seed and wasp production could be maintained in warm season, which might be because of density dependence restraint among foundresses and their low oviposition and pollination efficiency, keeping common resource(female flowers) utilization unsaturated. Whilst, a negative correlation between wasp offspring and viable seed production was also observed in cold season, which might be that the increased oviposition and pollination efficiency maximized the common resource utilization. The fitness trade-off between fig and pollinator wasps is greatly affected by environmental or ecological variations. The local stability might result from temporal low exploitation efficiency of pollinators together with interference competition among pollinators. We suggest that host repression through the active regulation of bract closure, which can create interference competition among the foundresses and prevent extra more foundresses sequential entry in fruit cavities, would help the figs avoiding the cost of over-exploitation. This essentially takes the same role as sanctioning of cheating or competitive behaviors.     

BREEDING SYSTEM IN FICUS CARICA,THE COMMON FIG. I. FLORAL DIVERSITY

Coevolution in Ficus carica (Moraceae) and the fig wasp (Blastophaga psenes, Agaonidae, Chalcidoidea) has resulted in a complex breeding system involving two tree morphs (Caprifig and Edible fig), three floral forms (long-styled female, short-styled female, and male flowers) and the insect pollinator. The two female floral forms have been reported to differ only in style length and stigma shape. In the present study, we demonstrate that the two female flowers differ from inception—short-styled flower primordia are smaller and exhibit significantly greater individual variation than do those of the long-styled flower, and the relative growth rate of each flower type differs. Mature forms exhibit disparity in style length, in stigma characteristics, and in degree of fusion of stylar lobes. Female flowers of both tree morphs are unisexual from inception. Male flowers of the Caprifig tree morph are initiated as hermaphrodites and gynoecium abortion occurs before megaspore mother cell stage. A single inflorescence therefore expresses two pathways to unisexuality. Hermaphrodite flower primordia were repeatedly found in the supposedly unisexual female syconium of the Edible fig tree morph. Based on its developmental morphology, Ficus carica appears to be of gynomonoecious ancestry.     

Sugar secretion and ant protection in Ficus benguetensis: Toward a general trend of fig–ant interactions

The relationship between plants and ants is often mediated by the presence of extrafloral nectaries (EFNs) that attract ants and provide rewards by protecting plants from herbivores or parasites. Ficus trees (Moraceae) and their pollinators (Hymenoptera: Agaonidae) are parasitized by many nonpollinating fig wasp species (Hymenoptera: Chalcidoidea) that decrease the reproductive output of the mutualistic partners. Previous studies have shown that ants living on and patrolling Ficus species can efficiently deter parasitic wasps. The aim of this study was to verify the presence of EFNs on figs of Ficus benguetensis and test the hypothetical protection service provided by ants. Figs in different developmental stages were collected from Fu-Yang Eco Park, Taipei, Taiwan. Sugars on the fig surface were collected and analyzed through high-performance anion-exchange chromatography. Moreover, ants were excluded from the figs to determine the effect of ants on the nonpollinating fig wasps. We identified three oligosaccharides whose relative proportions varied with the fig developmental phase. In addition, results showed that the ant-excluded figs were heavily parasitized and produced three times less pollinators than did the control figs. Finally, the specific interactions of Ficus benguetensis with ants and the relationship between figs and ants in general are discussed.     

鸡嗉果榕榕小蜂产卵时序与种群数量分析

榕树与传粉榕小蜂的互惠共生体系被多种非传粉榕小蜂利用。在同一个榕果中,具有较高种群密度的传粉榕小蜂和多种非传粉榕小蜂生存在一起并相互作用。通过野外观察和采样分析的方法,对鸡嗉果榕内5种榕小蜂（传粉榕小蜂Ceratosolen gravelyi;非传粉榕小蜂Apocrypta sp., Philotrypesis dunia, Platyneura cunia和Sycoscaptertr ifemmensis）的产卵行为进行了研究。结果证明,鸡嗉果榕内生活的各种榕小蜂通过比较严格的产卵时序在榕果内产卵以占领不同的生态位,这反映了这些榕小蜂各自的生物学特性。自然种群数量分析表明,单个非传粉榕小蜂物种的种群数量几乎不与传粉榕小蜂种群数量呈负相关,而所有非传粉榕小蜂物种的种群个体总量与传粉榕小蜂的种群个体数呈正相关,这可能是多个物种共存于单个榕果内的进化适应。     

One fig to bind them all: host conservatism in a fig wasp community unraveled by cospeciation analyses among pollinating and nonpollinating fig wasps

The study of chalcid wasps that live within syconia of fig trees (Moraceae, Ficus ), provides a unique opportunity to investigate the evolution of specialized communities of insects. By conducting cospeciation analyses between figs of section Galoglychia and some of their associated fig wasps, we show that, although host switches and duplication have evidently played a role in the construction of the current associations, the global picture is one of significant cospeciation throughout the evolution of these communities. Contrary to common belief, nonpollinating wasps are at least as constrained as pollinators by their host association in their diversification in this section. By adapting a randomization test in a supertree context, we further confirm that wasp phylogenies are significantly congruent with each other, and build a "wasp community" supertree that retrieves Galoglychia taxonomic subdivisions. Altogether, these results probably reflect wasp host specialization but also, to some extent, they might indicate that niche saturation within the fig prevents recurrent intrahost speciation and host switching. Finally, a comparison of ITS2 sequence divergence of cospeciating pairs of wasps suggests that the diversification of some pollinating and nonpollinating wasps of Galoglychia figs has been synchronous but that pollinating wasps exhibit a higher rate of molecular evolution.     

Why do fig wasps actively pollinate monoecious figs?

Active pollination, although rare, has been documented in a few pollination mutualisms. Such behaviour can only evolve if it benefits the pollinator in some way. The wasps that pollinate Ficus inflorescences can be active or passive pollinators. They lay their eggs in fig flowers, so that a proportion of flowers will host a wasp larva instead of a seed. We show in an actively pollinated monoecious fig that lack of pollination does not induce fig abortion or affect wasp offspring size but results in smaller numbers of offspring. Hence, conversely to other active pollination systems, seed formation is not obligatory to sustain developing pollinator larvae; however there is a direct fitness cost to active pollinators not to pollinate. We then compared the locations of eggs and fertilised flowers of three actively pollinated Ficus species and one passively pollinated species. We found that more flowers containing wasp eggs were fertilised in the actively pollinated species relative to those of the passively pollinated one. These results along with comparison with similar studies on dioecious figs, support the hypothesis that active pollination has evolved in fig wasps to ensure that more flowers containing wasp eggs are fertilised as this may increase the chances of successful gall development. The stigmatic platform characterising actively pollinated figs is probably an adaptation to increase pollen dispersion within the fig.     

对叶榕传粉榕小蜂在时间格局上的种群动态

在西双版纳热带地区,研究了不同时间里对叶榕传粉榕小蜂的自然种群数量变化规律,并结合控制性放蜂实验获得了实验种群的变化规律。结果表明,在不同年的同月份和同年不同月份,对叶榕传粉榕小蜂自然种群数量变化主要受气候因素的影响,低温的1月和干热的4月对传粉榕小蜂种群繁殖和生长均不利,导致种群数量最低。在人为控制情况下放蜂,不同月放蜂对后代产生雌蜂数量影响非常明显,而对雄蜂数量则无明显差异。在同月相近的不同日放蜂时,对叶榕传粉榕小蜂繁殖的后代种群数量无显著差异。     

Extremely high proportions of male flowers and geographic variation in floral ratios within male figs of Ficus tikoua despite pollinators displaying active pollen collection

Most plants are pollinated passively, but active pollination has evolved among insects that depend on ovule fertilization for larval development. Anther‐to‐ovule ratios (A/O ratios, a coarse indicator of pollen‐to‐ovule ratios) are strong indicators of pollination mode in fig trees and are consistent within most species. However, unusually high values and high variation of A/O ratios (0.096–10.0) were detected among male plants from 41 natural populations of Ficus tikoua in China. Higher proportions of male (staminate) flowers were associated with a change in their distribution within the figs, from circum‐ostiolar to scattered. Plants bearing figs with ostiolar or scattered male flowers were geographically separated, with scattered male flowers found mainly on the Yungui Plateau in the southwest of our sample area. The A/O ratios of most F. tikoua figs were indicative of passive pollination, but its Ceratosolen fig wasp pollinator actively loads pollen into its pollen pockets. Additional pollen was also carried on their body surface and pollinators emerging from scattered‐flower figs had more surface pollen. Large amounts of pollen grains on the insects' body surface are usually indicative of a passive pollinator. This is the first recorded case of an actively pollinated Ficus species producing large amounts of pollen. Overall high A/O ratios, particularly in some populations, in combination with actively pollinating pollinators, may reflect a response by the plant to insufficient quantities of pollen transported in the wasps’ pollen pockets, together with geographic variation in this pollen limitation. This suggests an unstable scenario that could lead to eventual loss of wasp active pollination behavior.     

Seasonal change in the structure of fig-wasp community and its implication for conservation

Figs (Moraceae) and their pollinating wasps (Agaonidae) constitute a famous reciprocal mutualism in which figs provide some female flowers for the development of fig wasp offspring while the fig wasps pollinate fig flowers. However, figs also host many non-pollinating wasps which are either parasitoids or resource competitors of pollinators, and bring no benefit for figs and are detrimental to fig’ fitness. Our data onFicus racemosa in Xishuangbanna showed that the numbers of non-pollinators and the mature syconia without pollinator wasps increase in rainy season, especially in the highly fragmented forest. This might be because of the longer developing time of the syconia and thereby longer oviposition time to non-pollinators in the dry season. The galled flower and the viable seed percentages in dry seasons are also larger than in rainy seasons in both primary forest and fragmented forest, and the development of non-pollinators is mainly at the expense of pollinator wasps. Our results showed that there exists a discriminative seasonal impact of non-pollinators and fragmentation effects on population size of fig’s pollinators. This implies that fig/fig wasp mutualism is more fragile in dry season, and that the critical population size and breeding units of figs in seasonal area might be larger than previously estimated without considering the seasonal change of pollinator population.