高榕雌花期传粉榕小蜂和欺骗性小蜂的繁殖特点

榕树及其专一性传粉榕小蜂组成了动植物界最为经典的协同进化关系,传粉榕小蜂演化出欺骗性是非常罕见的。在雌雄同株的高榕隐头果内,共存着一种传粉榕小蜂Eupristina altissima和一种欺骗性的小蜂Eupristina sp.,两种小蜂在雌花期进入隐头果内繁殖,但有不同的繁殖特点。对比研究了两种小蜂从成虫羽化到产卵和传粉这个阶段的雌蜂个体大小、孕卵量及繁殖差异,结果表明:羽化期两种雌蜂的平均个体小,经飞行小个体的雌蜂易死亡,大个体雌蜂到达接受树,但通过苞片通道,一些个体较大的传粉榕小蜂被夹死导致进入果腔的雌蜂相对小,而欺骗性小蜂易通过苞片以至进入果腔的雌蜂个体较大。两种未产卵雌蜂均表现为个体大者孕卵量较多,但两种雌蜂的平均孕卵量没有差异。即使有充足雌花资源产卵,两种雌蜂均未产完所有卵,产卵后两种雌蜂卵巢中的卵量均显著减少,遗留下的卵量两种小蜂间没有差异。传粉榕小蜂只有部分个体传完所携带花粉,并表现为传粉越成功的雌蜂,产卵越多。存在种内竞争时,两种小蜂的产卵量均减少,传粉榕小蜂的传粉效率也降低。在种间竞争背景下,欺骗性小蜂产卵更成功,传粉榕小蜂的产卵和传粉量均受到极大抑制。研究结果说明雌花期隐头果内传粉榕小蜂只适量利用雌花资源产卵繁殖后代,更有效地传粉繁殖榕树种子,这可能是维持榕-蜂互惠系统稳定共存的重要机制之一;欺骗者稳定存在需降低与传粉者的直接竞争,而欺骗者和传粉者分散在不同果内,甚至是不同的树上繁殖是理想的繁殖策略。     

Association of cone thermogenesis and volatiles with pollinator specificity in Macrozamia cycads

Cone traits (volatile components and thermogenesis) of three cycad species in the genus Macrozamia were examined for differences related to their specific insect pollinators, the weevil, Tranes spp., or the thrips, Cycadothrips chadwicki. Linalool (>80% of emissions) dominated cone volatile components of M. machinii (Tranes-pollinated) and beta-myrcene was a minor component (<9% of emissions). Volatiles of M. lucida and M. macleayi cones (Cycadothrips-pollinated) were dominated by beta-myrcene (up to 97% of emissions), but no linalool was detected. Pollinator movement into and out of cones coincided with cone thermogenesis and peak odor emission: around sunset for Tranes, and at mid-day for Cycadothrips. Female cone traits were similar to those of their conspecific male cones. Differences in cone traits between Macrozamia species may thus be responsible for conferring pollinator specificity in areas of sympatry.     

Rapid evolution of host specificity in a parasitic nematode

Summary In eastern North America, the nematodeHowardula aoronymphium parasitizes four species of mushroom-breedingDrosophila:D. falleni andD. recens of the quinaria species group, andD. putrida andD. testacea of the testacea group. One strain ofH. aoronymphium, designated Mendon-87, was initially capable of infecting all four of these host species. After less than 3 years in laboratory culture usingD. falleni as the sole host, this strain had completely lost the ability to infectD. putrida. Two other nematode strains parasitizedD. falleni andD. putrida at equal rates. These results demonstrate the existence of genetic variation for host specificity within this nematode species. More importantly, they show that host specificity can evolve rapidly when only one host is available for parasitization. Ecological conditions are such that natural populations ofH. aoronymphium may comprise numerous host races, lineages incapable of parasitizing the full range of host species. However, I argue that such host races are probably ephemeral and thus unlikely to persist long enough to undergo speciation.     

Host specificity,phenotype matching and the evolution of reproductive isolation in a coevolved plant–pollinator mutualism

Coevolutionary interactions between plants and their associated pollinators and seed dispersers are thought to have promoted the diversification of flowering plants ( Raven 1977 ; Regal 1977 ; Stebbins 1981 ). The actual mechanisms by which pollinators could drive species diversification in plants are not fully understood. However, it is thought that pollinator host specialization can influence the evolution of reproductive isolation among plant populations because the pollinator’s choice of host is what determines patterns of gene flow in its host plant, and host choice may also have important consequences on pollinator and host fitness ( Grant 1949 ; Bawa 1992 ). In this issue of Molecular Ecology, Smith et al. (2009) present a very interesting study that addresses how host specialization affects pollinator fitness and patterns of gene flow in a plant host. Several aspects of this study match elements of a seminal mathematical model of plant–pollinator codivergence ( Kiester et al. 1984 ) suggesting that reciprocal selection for matched plant and pollinator reproductive traits may lead to speciation in the host and its pollinator when there is strong host specialization and a pattern of geographic subdivision. Smith et al.’s study represents an important step to fill the gap in our understanding of how reciprocal selection may lead to speciation in coevolved plant–pollinator mutualisms.     

Floral volatiles, pollinator sharing and diversification in the fig-wasp mutualism: insights from Ficus natalensis, and its two wasp pollinators (South Africa)

Combining biogeographic, ecological, morphological, molecular and chemical data, we document departure from strict specialization in the fig-pollinating wasp mutualism. We show that the pollinating wasps Elisabethiella stuckenbergi and Elisabethiella socotrensis form a species complex of five lineages in East and Southern Africa. Up to two morphologically distinct lineages were found to co-occur locally in the southern African region. Wasps belonging to a single lineage were frequently the main regional pollinators of several Ficus species. In South Africa, two sister lineages, E. stuckenbergi and E. socotrensis, pollinate Ficus natalensis but only E. stuckenbergi also regularly pollinates Ficus burkei. The two wasp species co-occur in individual trees of F. natalensis throughout KwaZulu-Natal. Floral volatile blends emitted by F. natalensis in KwaZulu-Natal were similar to those emitted by F. burkei and different from those produced by other African Ficus species. The fig odour similarity suggests evolutionary convergence to attract particular wasp species. The observed pattern may result from selection for pollinator sharing among Ficus species. Such a process, with one wasp species regionally pollinating several hosts, but several wasp species pollinating a given Ficus species across its geographical range could play an important role in the evolutionary dynamics of the Ficus-pollinating wasp association.     

Why resource history matters: age and oviposition history affect oviposition behaviour in exploiters of a mutualism

1. Acceptance of hosts for oviposition is often hardwired in short‐lived insects, but can be dynamic at the individual level due to variation in physiological state determinants such as ageing and prior oviposition. However, the effect of the oviposition history of resources together with time taken to accept less preferred hosts in ageing insects has rarely been investigated. 2. The time taken by parasitic fig wasps to accept resources with different oviposition histories was recorded in order to investigate the effect of wasp physiological state and resource oviposition history on oviposition behaviour. These wasps, which differ in life‐history traits, oviposit at specific developmental stages of enclosed fig inflorescences called syconia. 3. Behavioural assays were performed with naive wasps and wasps aged with and without prior oviposition experience. Syconia at the same developmental stage but differing in oviposition history were offered in no‐choice assays and the time taken to first oviposition attempt was recorded. 4. One short‐lived pro‐ovigenic galler species exhibited a decline with age in time taken to accept a syconium for oviposition. The exact timing of the transition from non‐acceptance to acceptance of less preferred syconia was determined in terms of the proportion of elapsed life span at the transition; this occurred at 25% of elapsed life span. 5. Longer‐lived parasitoids did not show any decline in specificity despite being aged for 50% of their life span. Therefore, host quality, trophic position, egg load and age may individually affect oviposition decisions or have interaction effects.     

The mechanism of pollinator specificity between two sympatric fig varieties: a combination of olfactory signals and contact cues

Background and Aims

Pollinator specificity facilitates reproductive isolation among plants, and mechanisms that generate specificity influence species boundaries. Long-range volatile attractants, in combination with morphological co-adaptations, are generally regarded as being responsible for maintaining extreme host specificity among the fig wasps that pollinate fig trees, but increasing evidence for breakdowns in specificity is accumulating. The basis of host specificity was examined among two host-specific Ceratosolen fig wasps that pollinate two sympatric varieties of Ficus semicordata, together with the consequences for the plants when pollinators entered the alternative host variety.

Methods

The compositions of floral scents from receptive figs of the two varieties and responses of their pollinators to these volatiles were compared. The behaviour of the wasps once on the surface of the figs was also recorded, together with the reproductive success of figs entered by the two Ceratosolen species.

Key Results

The receptive-phase floral scents of the two varieties had different chemical compositions, but only one Ceratosolen species displayed a preference between them in Y-tube trials. Specificity was reinforced at a later stage, once pollinators were walking on the figs, because both species preferred to enter figs of their normal hosts. Both pollinators could enter figs of both varieties and pollinate them, but figs with extra-varietal pollen were more likely to abort and contained fewer seeds. Hybrid seeds germinated at normal rates.

Conclusions

Contact cues on the surface of figs have been largely ignored in previous studies of fig wasp host preferences, but together with floral scents they maintain host specificity among the pollinators of sympatric F. semicordata varieties. When pollinators enter atypical hosts, post-zygotic factors reduce but do not prevent the production of hybrid offspring, suggesting there may be gene flow between these varieties.     

Chemosensory attraction of fig wasps to substances produced by receptive figs

In the mutualism between figs (Ficus spp., Moraceae) and their species-specific fig wasp pollinators (Hymenoptera: Agaonidae), location of a receptive host tree by the adult insect is a critical step. The adult female wasp lives only a few days, and must usually fly to a different tree than her natal tree to locate receptive figs. Trees in receptive phase often occur at very low densities. Reproductive success of both fig and wasp depends on trasmission of a very strong signal by the plant. Some evidence exists for the role of olfaction in location of receptive hosts by fig wasps, but very little work has been done on the chemical ecology of host location and host specificity. Here the first experimental evidence is presented for long-distance olfactory attraction of wasps by volatile substances produced by receptive figs, and for short-distance or contact chemostimulation by host volatiles that elicit entry of the wasp into the fig. In studies usingFicus carica L., pentane extracts of receptive-phase figs attract the pollinatorBlastophaga psenes L. from distances of at least 5 m in the field. Short-distance chemostimulation was demonstrated in laboratory bioassays. Pentane extracts of receptive figs, when painted onto the ostiole of non-receptive figs, elicit entry of pollinator wasps. Figs emit volatile compounds attractive to pollinating wasps only during the period of receptivity; pentane extracts of non-receptive figs are not attractive. A simple reliable procedure is described to compare the attractivity of different types of extracts (total, internal, and external extracts) and of different fractions, in the first step towards identifying attractant substances.     

Geographical matching of volatile signals and pollinator olfactory responses in a cycad brood-site mutualism

Brood-site mutualisms represent extreme levels of reciprocal specialization between plants and insect pollinators, raising questions about whether these mutualisms are mediated by volatile signals and whether these signals and insect responses to them covary geographically in a manner expected from coevolution. Cycads are an ancient plant lineage in which almost all extant species are pollinated through brood-site mutualisms with insects. We investigated whether volatile emissions and insect olfactory responses are matched across the distribution range of the African cycad Encephalartos villosus. This cycad species is pollinated by the same beetle species across its distribution, but cone volatile emissions are dominated by alkenes in northern populations, and by monoterpenes and a pyrazine compound in southern populations. In reciprocal choice experiments, insects chose the scent of cones from the local region over that of cones from the other region. Antennae of beetles from northern populations responded mainly to alkenes, while those of beetles from southern populations responded mainly to pyrazine. In bioassay experiments, beetles were most strongly attracted to alkenes in northern populations and to the pyrazine compound in southern populations. Geographical matching of cone volatiles and pollinator olfactory preference is consistent with coevolution in this specialized mutualism.     

Host specificity and reproductive success of yucca moths (Tegeticula spp. Lepidoptera: Prodoxidae) mirror patterns of gene flow between host plant varieties of the Joshua tree (Yucca brevifolia: Agavaceae)

Coevolution between flowering plants and their pollinators is thought to have generated much of the diversity of life on Earth, but the population processes that may have produced these macroevolutionary patterns remain unclear. Mathematical models of coevolution in obligate pollination mutualisms suggest that phenotype matching between plants and their pollinators can generate reproductive isolation. Here, we test this hypothesis using a natural experiment that examines the role of natural selection on phenotype matching between yuccas and yucca moths (Tegeticula spp.) in mediating reproductive isolation between two varieties of Joshua tree (Yucca brevifolia var. brevifolia and Y. brevifolia var. jaegeriana). Using passive monitoring techniques, DNA barcoding, microsatellite DNA genotyping, and sibship reconstruction, we track host specificity and the fitness consequences of host choice in a zone of sympatry. We show that the two moth species differ in their degree of host specificity and that oviposition on a foreign host plant results in the production of fewer offspring. This difference in host specificity between the two moth species mirrors patterns of chloroplast introgression from west to east between host varieties, suggesting that natural selection acting on pollinator phenotypes mediates gene flow and reproductive isolation between Joshua‐tree varieties.     

Ancient fig wasps indicate at least 34 Myr of stasis in their mutualism with fig trees

Fig wasps and fig trees are mutually dependent, with each of the 800 or so species of fig trees (Ficus, Moraceae) typically pollinated by a single species of fig wasp (Hymenoptera: Agaonidae). Molecular evidence suggests that the relationship existed over 65 Ma, during the Cretaceous. Here, we record the discovery of the oldest known fossil fig wasps, from England, dated at 34 Ma. They possess pollen pockets that contain fossil Ficus pollen. The length of their ovipositors indicates that their host trees had a dioecious breeding system. Confocal microscopy and scanning electron microscopy reveal that the fossil female fig wasps, and more recent species from Miocene Dominican amber, display the same suite of anatomical characters associated with fig entry and pollen-carrying as modern species. The pollen is also typical of modern Ficus. No innovations in the relationship are discernible for the last tens of millions of years.     

Nuclear DNA contents in the genusFicus (Moraceae)

Nuclear DNA contents in 15 species of large tropical hardwood genusFicus have been determined by cytophotometry. The 2 C-values are rather low and uniform, suggesting no appreciable changes during speciation. The small genome size is discussed in relation to woody habit.     

Floral sexual dimorphism and flower choice by pollinators in a nectarless monoecious vine Akebia quinata (Lardizabalaceae)

We investigated the pollination system and movement patterns of pollinators among flowers of the nectarless, monoecious vine Akebia quinata in natural populations and experimental floral arrays. Female flowers did not offer any rewards for pollinators and were larger than male flowers. Pollinators of A.quinata , such as small solitary bees and hoverflies, clearly discriminated between male and female flowers. Hoverflies always visited male flowers and rarely visited female flowers. In contrast, solitary bees tended to visit female flowers first when entering the array, but then switched to male flowers within the same foraging bout. This tendency disappeared when the sepal size of female flowers was experimentally reduced to the size of male flowers. Thus, observed non-randomness in flower choice by solitary bees may be caused by female>male sexual dimorphism and may increase the chance of cross-pollination and lower the probability of geitonogamous pollination in a single visit to a plant. Therefore, floral sexual dimorphism in A.quinata is considered to be an adaptation for deceptive pollination associated with discriminating pollinators.     

Physiological determinants of Ficus fruit temperature and implications for survival of pollinator wasp species: comparative physiology through an energy budget approach

Figs are completely dependent for pollen dispersal on species-specific fig-pollinating wasps that develop within developing fig fruits. These wasps are very sensitive to heat and die at temperatures only a few degrees above ambient. Such temperatures are expected and observed in objects exposed to full sunlight, as fig fruits frequently are. In detailed field and experimental studies of 11 species of Panamanian figs with fruit ranging in size from 5 mm to 50 mm in diameter, we found that both the relative and absolute contribution of transpiration to maintaining non-lethal fruit temperatures increased with fruit size. Small and large fruits reached temperatures of 3 and 8°C, respectively, above air temperature in full sunlight when transpiration was prevented by grease. The temperature reached by large, nontranspiring fruits was sufficient to kill their pollinators. Control fruits which transpired reached temperatures of 2–3°C above air temperature in sunlight, regardless of size. An analysis of the solar energy budget of fruit revealed that large fruits must transpire to maintain tolerable temperatures for the wasps because heat diffusion from fruit to air was too low to balance net radiation in sunlight. By contrast, small fruits do not need to transpire to maintain tolerable temperatures for the pollinators.     

Thread-forming structures in angiosperm anthers: Their diverse role in pollination ecology

This paper reviews the origin, nature, systematic distribution, and the respective function of the highly variable and diverse thread-forming structures in angiosperm anthers (including somewhat similar, rare features in ferns and gymnosperms). On one hand, such threads may function as pollen-connecting vectors in forming pollen dispersal units, as sporopollenin threads (viscin threads), e.g. in Onagraceae, or sporopollenin-less threads in surprisingly many other angiosperm families. On the other hand, as is known from theImpatiens — pollen basket, threads or ropes may be involved in pollen presentation. In addition, for the first time two new examples of pollen baskets in Boraginaceae and Scrophulariaceae are reported. InEchium the basket is formed by cellular elements from the modified septal regions, whereas inEsterhazya a similar effect is achieved in an analogous manner by trichomes of the epidermal layer of the thecal wall. There is obviously a different function of these seemingly very similar baskets: inEchium the feature acts preferably as a pollen presentation agent, whereas inEsterhazya the primary function is to prevent all the pollen from being dispersed too soon.     

Host specificity ofPareuchaetes insulata (Lep.: Arctiidae), a biological control agent forChromolaena odorata (Compositae)

Larvae of the arctiid moth,Pareuchaetes insulata, from Florida fed on the leaves ofChromolaena odorata, a serious composite, alien weed in Natal, South Africa. In starvation test trials using 48 plant species,P. insulata completed its development onC. odorata andAgeratum houstonianum. Subsequent attempts to cultureP. insulata on these two plants was only successful onC. odorata. The biology ofP. insulata is similar to that of two other well studiedPareuchaetes species namelyP. pseudoinsulata andP. aurata aurata. Repeated defoliation ofC. odorata byP. insulata could contribute to its control as has been found withP. pseudoinsulata in Guam.Pareuchaetes insulata has been approved for release as a biological control agent ofC. odorata in South Africa.     

Evaluation ofHadena perplexa [Lepidoptera: Phalaenidae] as a biocontrol agent of bladder campionSilene vulgaris [Caryophyllaceae] in Canada: Rearing and host specificity

Rearing techniques and results of preliminary host range tests are reported forHadena perplexa (Denis & Schiffermuller) (Lep.: Phalaenidae) a candidate biocontrol agent against the weed bladder campion,Silene vulgaris (Moench) Garcke, in Manitoba, Canada. In the laboratory, it was necessary to pipette a 15% honey solution in water into the flowers as food for the adult moths. When reared singly to avoid cannibalism, 56% of the 1^st instar larvae developed to pupae. Larvae fed on a natural diet for 10 days can then be reared on either one of 2 artificial diets. Choice oviposition tests and no-choice larval feeding tests were conducted with plant species closely related toS. vulgaris in the generaSilene, Dianthus, Gypsophila, Lychnis, Saponaria. Species in 4 of 5 of these genera were accepted for oviposition, and species in all 5 genera supported the development of 1^st instar larvae to the pupal stage.H. perplexa should not be introduced into Canada.      

Host specificity and biology ofHeteropsylla spinulosa [Hom.: Psyllidae] introduced into Australia and Western Samoa for the biological control ofMimosa invisa

The biology and host specificity of the psyllidHeteropsylla spinulosa Muddiman, Hodkinson & Hollis collected fromMimosa invisa Von Martius, was studied in Brazil and in Australia. Oviposition was recorded on 19 of 100 plant species tested but nymphs died without feeding, except onM. invisa. H. spinulosa was shown to be specific toM. invisa and was subsequently released and established in Australia and Western Samoa.      

Biology and host specificity ofWeiseana barkeri (Col.: Chrysomelidae): A biological control agent forAcacia nilotica (mimosaceae)

The leaf-feeding beetleWeiseana barkeri Jacoby feeds onAcacia nilotica (L.) Willdenow ex Delile in Kenya. Host specificity tests show it is host specific toA. nilotica and approval has been given for its field release in northwest Queensland. A novel rearing and host specificity-testing technique is reported whereA. nilotica foliage stimulates oviposition into strips of corrugated cardboard.     

Tropical pollinators in the canopy and understory: Field data and theory for stratum “preferences”

Claims have been made for a canopy preference by large bees pollinating tropical flowers—without data or tests that support or refute this opinion. The working hypothesis for bee foraging behavior in three dimensions is that forager experience can produce stratum fidelity, just as rewarding foraging produces floral fidelity. Wideranging search behavior should allow bees to track spatiotemporal distribution of resources. A systematic study of 20 bee species and 10 genera: Apis, Trigona, Eulaema, Centris, Euglossa, Scaptotrigona, Partamona, Megalopta, Rhinetula,and Oxytrigonawas made in two forests in Panama. Two traps were operated simultaneously at canopy height and in the understory to test whether there were consistent stratum associations. Studies were continuous for 1 and 8 years. The only high-canopy foragers were two nocturnal bees, all the rest flew at both heights with similar probability or consistently came to lower traps. Large euglossines showed a tendency to forage high, which was directly related to their capacity for heat loss during flight, compared to smaller euglossines. They are also more conspicuous in warning coloration, another expected correlate of foraging more often in the open. Although large variance in stratum association predominates, some medium-sized diurnal forest bees avoid the exposed upper canopy, while some nocturnal bees tend to forage there.