Olle Pellmyr

Mutualism with pollinating seed parasites amid co-pollinators: constraints on specialization

The genus Greya is closely related to the yucca moths, and Greya species interact with their host plants in many of the same ways as yucca moths. Females both pollinate and oviposit in the flowers of their host. Unlike yucca moths, however, pollinating Greya species share flowers with co—pollinators that do not oviposit on the host. We studied the interaction between Greya politella (Walsingham) (Lepidoptera: Prodoxidae) and Lithophragma parviflorum (Hook.) Torr. & Gray (Saxifragaceae) to evaluate the effect on seed output of a pollinating seed parasite against a background of co—pollinators. Flowers were visited and pollinated mostly by bombyliid flies, solitary bees, and G. politella. Bombyliid flies alone composed 68—88% of the 5522 visits recorded over 2 yr. Although both male and female G. politella visited the flowers and probed for nectar, pollination by this species occurred only as females oviposited through the corolla tube, thereby passively transferring to the stigma pollen adhering to the abdomen. Visitation to flowers by all pollinators averaged 0.3—1.9 visits/h during daylight hours. Consequently, most flowers were visited multiple times during the several days that stigmas were receptive, and 77% of the flowers tagged during the 4 yr had some developing seeds. Pollination did not depend upon visitation by G. politella. Flowers receiving G. politella eggs had the same probability of producing some seed and the same mean number of developing seeds as flowers visited only by other insects. Most flowers received the eggs of only one G. politella female, and the larvae ate 15—27% of the developing seeds. The final number of mature seeds remaining in attacked flowers did not differ from unattacked flowers except at one site in 1 yr, in which the values for attacked plants were marginally lower. Other sources of variation affecting seed output masked the effects of seed consumption by Greya. Overall, G. politella females have the potential to be mutualistic with L. parviflorum: they are effective pollinators, generally visit most plants and about half the flowers in the populations, and impose a fairly small cost on seed output. Nonetheless, the abundant and effective co—pollinators, which do not eat the developing seeds, swamp Greyas mutualistic effects. Under the current conditions at Granite Point, the relationship between G. politella and L. parviflorum may be mostly commensalistic. The evolution of specialization to G. politella as an exclusive pollinator would seem to be possible only in L. parviflorum populations in which effective co—pollinators were either rare or unpredictable. That is, the potential for the evolution of specificity in this mutualism appears to depend upon the community context in which the interaction takes place rather than upon the simple outcome of the pairwise interaction between Greya and Lithophragma.

EVOLUTION OF POLLINATION AND MUTUALISM IN THE YUCCA MOTH LINEAGE

The obligate pollination mutualisms between yucca moths and yuccas are some of the most obvious cases of coevolution, but the phylogenetic origins and extent of coevolution in these interactions are little understood. Ecological and phylogenetic information from the yucca moth family, Prodoxidae, shows that pollination has evolved at least three times from separate moth behaviors. Passive pollination occurs in Greya during nectaring by one species and during oviposition by two other species. Active pollination among prodoxids has evolved only once, in the yucca moths. Several life-history traits necessary for the evolution of obligate mutualism are common traits within the Prodoxidae, and only active pollination and modified mouthparts appear to have been novel traits in the yucca moths. We use moth and host biology in a phylogenetic framework to establish hypotheses for the evolution of active pollination and reciprocal specialization in the form of functional nectarlessness in yuccas.

Dynamic headspace analysis of floral volatiles: A comparison of methods

Lack of standardization of floral scent analysis using dynamic headspace collection and GC-MS has prevented meaningful biological comparisons of results from different studies. We explored technique-related sources of variation by comparing the relative performances of different trapping sorbents, elution solvents and vacuum pump flow rates in the collection of known volatiles from (a) synthetic blends on filter papers and (b) living flowers of Clarkia breweri (Onagraceae). The greatest amounts of volatiles were collected when Porapak Q sorbent and hexane solvent were used, but most analyses were qualitatively similar. Low and high pump flow rates yield a tradeoff, with higher rates trapping low volatility compounds but also raising ambient contamination levels. Significant variation was associated with the use of different sorbents and solvents, but detailed statistical comparisons were precluded by significant interaction effects involving other experimental variables. Given the tightly controlled experimental conditions and use of inbred plant lines, this unexpected result suggests that analyses of small samples carry a high risk of experimental artifact. We recommend that floral scent analyses incorporate experimental and statistical approaches that address as many method-related variables as possible, that samples of 10 or more individuals be used, and that GC-MS parameters be held constant for all comparative analyses.

Reversal of Mutualism as a Mechanism for Adaptive Radiation in Yucca Moths

Species interactions are a major source of adaptive radiation. In mutualisms, such diversification can take the form of evolution of parasites that exploit the resources needed for maintenance of the mutualism. Mutualistic associations often have associated parasitic species, and in some cases, parasitic species have indeed evolved from the mutualists. For example, obligate mutualisms, such as those involving seed‐eating pollinators, have on a few occasions given rise to nonmutualist species. These systems are relatively simple and provide models for identifying factors that facilitate the stable reversal of a mutualistic interaction. We used ecological data in a phylogenetic framework to analyze the origins of two nonmutualist cheater yucca moths. Phylogenetic analysis suggests that the evolution of cheating may be preceded by a change in oviposition behavior. Two different modes of oviposition among moth species cause density‐dependent moth egg mortality through flower abscission in one case (locule‐ovipositing species) but density‐independent mortality in the other (superficially ovipositing species). A mtDNA‐based phylogeny indicated that cheating and superficial oviposition have evolved twice each and that the cheater clades are sisters to the superficially ovipositing species clades. Consideration of the fitness consequences of two trait changes—loss of pollination and phenological delay in oviposition—in which cheaters have diverged from ancestral pollinators suggest that the shift to oviposition into fruit may have occurred first and that loss of pollination behavior was a secondary event. We suggest that secondary coexistence of two pollinator species of opposite oviposition modes may facilitate the shift to fruit oviposition and cheating and that this is applicable in the best documented of the cheater yucca moths. Superficially ovipositing species suffer dual sources of egg mortality when in coexistence with locule‐ovipositing species. Shift to fruit oviposition under this circumstance can be facilitated by access to a seed resource not available to the copollinator, preadaptations in ovipositor morphology, and pollinator phenology late relative to the copollinator. Thus the adaptive radiation of nonmutualists from mutualists may have taken place in specific ecological contexts through few trait changes, and the reversal of mutualism would be a by‐product of a shift to reliance on a previously inaccessible seed resource.

Chemistry and geographic variation of floral scent in Yucca filamentosa (Agavaceae)

We identified volatiles from the floral headspace of Yucca filamentosa using gas chromatography and mass spectrometry and analyzed floral scent composition and variation among populations pollinated by different yucca moth species. Twenty-one scent compounds were repeatedly identified and most could be categorized into two major classes: (1) homoterpenes derived from the sesquiterpene alcohol nerolidol and (2) long chain aliphatic hydrocarbons. Two biosynthetic pathways are thus responsible for the majority of floral volatiles in Y. filamentosa. The homoterpene E-4,8-dimethylnona-1,3,7-triene, which is released systemically by higher plants upon herbivory, was the most abundant compound. Two di-oxygenated compounds not previously reported as floral compounds also were detected. No differentiation in floral scent was observed between populations pollinated by different yucca moths, nor was there any correlation between chemical distance and geographic distance among populations. The total release rate of volatiles differed significantly among populations, but not between populations with different pollinators. The combination of unique compounds and low variation in the fragrance blend may reflect highly selective attraction of obligate pollinators to flowers. The observed lack of differentiation in floral scent can putatively explain high moth-mediated gene flow among sites, but it does not explain conservation of odor composition across populations with different pollinators.

Pattern and timing of diversification in Yucca (Agavaceae): specialized pollination does not escalate rates of diversification

The yucca–yucca moth interaction is one of the most well-known and remarkable obligate pollination mutualisms, and is an important study system for understanding coevolution. Previous research suggests that specialist pollinators can promote rapid diversification in plants, and theoretical work has predicted that obligate pollination mutualism promotes cospeciation between plants and their pollinators, resulting in contemporaneous, parallel diversification. However, a lack of information about the age of Yucca has impeded efforts to test these hypotheses. We used analyses of 4322 AFLP markers and cpDNA sequence data representing six non-protein-coding regions (trnT–trnL, trnL, trnL intron, trnL–trnF, rps16 and clpP intron 2) from all 34 species to recover a consensus organismal phylogeny, and used penalized likelihood to estimate divergence times and speciation rates in Yucca. The results indicate that the pollination mutualism did not accelerate diversification, as Yucca diversity (34 species) is not significantly greater than that of its non-moth-pollinated sister group, Agave sensu latissimus (240 species). The new phylogenetic estimates also corroborate the suggestion that the plant–moth pollination mutualism has at least two origins within the Agavaceae. Finally, age estimates show significant discord between the age of Yucca (ca 6–10 Myr) and the current best estimates for the age of their pollinators (32–40 Myr).

Systematic revision of the yucca moths in the Tegeticula yuccasella complex (Lepidoptera: Prodoxidae) north of Mexico

The yucca moths (Tegeticula and Parategeticula) are of great importance in studies of coevolution because of their obligate mutualism with their yucca hosts. Historically, three species of Tegeticula have been recognized. One of them, T. yuccasella, has been regarded as the pollinator of all but two yucca species, but morphological, molecular and biological data show that this is a large complex of monophagous and oligophagous species that differ greatly in their biology. It also includes derived ‘cheater’ species that do not pollinate their hosts and oviposit into fruits rather than flowers. Here the yuccasella complex north of Mexico is revised. The nominotypic pollinator species yuccasella is redescribed, and ten new pollinator species described: altiplanella, baccatella, carnerosanella, cassandra, elatella, maderae, mojavella, rostratella, superficiella and treculeanella. Two non‐pollinating cheater species are recognized. One such species originally misidentified as a Prodoxus species, then synonymised with yuccasella, is re‐erected as the non‐pollinating intermedia. In addition, the new non‐pollinator species corruptrix is described.

POLLINATING SEED EATERS: WHY IS ACTIVE POLLINATION SO RARE?

Insects whose larvae depend on developing seed are very common, but active pollination, meaning that the insect possesses specific structures and behaviors for the purpose of assuring pollination, is only known to have evolved twice in such insects, namely in yucca moths and in fig wasps. This rarity could be due to high cost of pollination, phylogenetic constraints, alternative life history shifts to reduce or avoid risk of seed nondevelopment, or ecological factors such as co-pollinators that can satiate pollen requirements and mask variation in pollinator effectiveness among ovipositing seed eaters. Ecological costs of being a pollinator were measured for a yucca moth species and were found to be low: active time allocated to pollen pickup and deposit was on average 4.1%, an average of 0.42% of female body mass was allocated to specific structures for pollen manipulation, and the average pollen load weighed <4% of moth body mass. These estimates suggest that ecological costs need not be a major obstacle to evolution of active pollination. In contrast, recent combined ecological–phylogenetic analyses for the yucca moth family suggest that the evolution of active pollination and transition to mutualism depended largely on preadaptations, and that few traits were truly novel. If general, active pollination would be predicted to be likely to evolve only in lineages with life histories that facilitate mutualism. Alternative outcomes to evolution of active pollination include delayed oviposition, detection of floral pollination status, egg placement that allows the larva to select a fruit, and modified egg dispersion strategies to balance the cost of pollination. The historical significance of these factors can be assessed only when mechanisms are documented in many lineages and analyzed in a phylogenetic framework. Active fungal inoculation among arthropods is ecologically analogous to active pollination, and offers a complement for comparative analyses. Specific structures for spore transport have evolved many times in at least three orders of insects, and several times in mites. The large number of independent lineages of active pollen and fungus dispersers jointly provide a platform for testing hypotheses about, e.g., the role of preadaptations in evolution of mutualism, reversals of mutualism, and the role of mutualism in diversification.

Sources of variation in pollinator contribution within a guild: the effects of plant and pollinator factors

Among plants visited by many pollinator species, the relative contribution of each pollinator to plant reproduction is determined by variation in both pollinator and plant traits. Here we evaluate how pollinator movement among plants, apparent pollen carryover, ovule number, resource limitation of seed set, and pollen output affect variation in contribution of individual pollinator species to seed set in Lithophragma parviflorum (Saxifragaceae), a species visited by a broad spectrum of visitors, including beeflies, bees and a moth species. A previous study demonstrated differences among visitor species in their single-visit pollination efficacy but did not evaluate how differences in visitation patterns and pollen carryover affect pollinator efficacy. Incorporation of differential visitation patterns and pollen carryover effects —commonly cited as potentially important in evaluating pollinator guilds — had minor effects (0–0.6% change) on the estimates of relative contribution based on visit frequency and single-visit efficacy alone. Beeflies visited significantly more flowers per inflorescence than the bees and the moth. Seed set remained virtually constant during the first three visited flowers for beeflies and larger bees, indicating that apparent pollen carryover did not reduce per-visit efficacy of these taxa. In contrast, Greya moth visits showed a decrease in seed set by 55.4% and the smaller bees by 45.4% from first to second flower. The larger carryover effects in smaller bees and Greya were diminished in importance by their small overall contribution to seed set. Three variable plant traits may affect seed set: ovule number, resource limitation on seed maturation, and pollen output. Ovule number per flower declined strongly with later position within inflorescences. Numbers were much higher in first-year greenhouse-grown plants than in field populations, and differences increased during 3 years of study. Mean pollen count by position varied 7-fold among flowers; it paralleled ovule number variation, resulting in a relatively stable pollen:ovule ratio. Resource limitation of seed set increased strongly with later flowering, with seed set in hand-pollinated flowers ranging from 66% in early flowers to 0% in the last two flowers of all plants. Variation in ovule number and resource limitation of seed maturation jointly had a strong effect on the number of seeds per flower. Visitation to early flowers had the potential to cause more seed set than visitation to later flowers. Overall, the most important sources of variation to seed production contribution were differences among pollinators in abundance and absolute efficacy (ovules fertilized on a single visit) and potentially differential phenology among visitor species. These effects are likely to vary among populations and years.

EXAMINING GENETIC STRUCTURE IN A BOGUS YUCCA MOTH: A SEQUENTIAL APPROACH TO PHYLOGEOGRAPHY

Abstract Understanding the phylogeography of a species requires not only elucidating patterns of genetic structure among populations, but also identifying the possible evolutionary events creating that structure. The use of a single phylogeographic test or analysis, however, usually provides a picture of genetic structure without revealing the possible underlying evolutionary causes. We used current analytical techniques in a sequential approach to examine genetic structure and its underlying causes in the bogus yucca moth Prodoxus decipiens (Lepidoptera: Prodoxidae). Both historical biogeography and recent human transplantations of the moths host plants provided a priori expectations of the pattern of genetic structure and its underlying causes. We evaluated these expectations by using a progression of phylogenetic, demographic, and population genetic analyses of mtDNA sequence data from 476 individuals distributed across 25 populations that encompassed the range of P. decipiens. The combination of these analyses revealed that much of the genetic structure has evolved more recently than suggested by historical biogeography, has been influenced by changes in demography, and can be best explained by long distance dispersal and isolation by distance. We suggest that performing a suite of analyses that focus on different temporal scales may be an effective approach to investigating the patterns and causes of genetic structure within species.