Pollinator attraction to volatiles from virgin and pollinated host flowers in a yucca/moth obligate mutualism

The classic obligate pollination–seed consumption mutualism between yuccas and yucca moths has been thought to be mediated by chemical cues, but empirical data on pollinator attraction to host floral volatiles in this association have been lacking. Here we show that the scent from virgin flowers of the host Yucca glauca is sufficient to attract its obligate pollinator Tegeticula yuccasella in Y‐tube olfactometer tests. Interestingly, both sexes of moths were attracted to the scent stimulus. Because yucca moths mate inside host flowers, the attraction of both females and males to host floral volatiles is likely to increase encounter rates. In a second test, female moths did not discriminate between virgin and hand‐pollinated flowers, indicating no post‐pollination change in scent production by the host that would lead to a reduction in pollinator attraction and thereby limit exploitation of the available seeds in host flowers. However, other mechanisms that could stabilise the mutualism between T. yuccasella and its yucca hosts have already been documented, i.e. selective abortion of heavily infested flowers, and a female‐derived host‐marking pheromone. Headspace collection and GC–MS were used to identify the blend of floral volatiles emitted by Y. glauca, which was found to be very similar to those of two other allopatric capsular‐fruited species, Y. elata and Y. filamentosa, revealing strong conservation of this trait within Yucca section Chaenocarpa.     

The evolution of obligate pollination mutualisms: senita cactus and senita moth

We report a new obligate pollination mutualism involving the senita cactus, Lophocereus schottii (Cactaceae, Pachyceereae), and the senita moth, Upiga virescens (Pyralidae, Glaphyriinae) in the Sonoran Desert and discuss the evolution of specialized pollination mutualisms. L. schottii is a night-blooming, self-incompatible columnar cactus. Beginning at sunset, its flowers are visited by U. virescens females, which collect pollen on specialized abdominal scales, actively deposit pollen on flower stigmas, and oviposit a single egg on a flower petal. Larvae spend 6 days eating ovules before exiting the fruit and pupating in a cactus branch. Hand-pollination and pollinator exclusion experiments at our study site near Bahia Kino, Sonora, Mexico, revealed that fruit set in L. schottii is likely to be resource limited. About 50% of hand-outcrossed and open-pollinated senita flowers abort by day 6 after flower opening. Results of exclusion experiments indicated that senita moths accounted for 75% of open-pollinated fruit set in 1995 with two species of halictid bees accounting for the remaining fruit set. In 1996, flowers usually closed before sunrise, and senita moths accounted for at least 90% of open-pollinated fruit set. The net outcome of the senita/senita moth interaction is mutualistic, with senita larvae destroying about 30% of the seeds resulting from pollination by senita moths. Comparison of the senita system with the yucca/yucca moth mutualism reveals many similarities, including reduced nectar production, active pollination, and limited seed destruction. The independent evolution of many of the same features in the two systems suggests that a common pathway exists for the evolution of these highly specialized pollination mutualisms. Nocturnal flower opening, self-incompatible breeding systems, and resource-limited fruit production appear to be important during this evolution. Received: 19 August 1997 / Accepted: 24 November 1997     

Quantifying hummingbird preference for floral trait combinations: The role of selection on trait interactions in the evolution of pollination syndromes

Darwin recognized the flower's importance for the study of adaptation and emphasized that the flower's functionality reflects the coordinated action of multiple traits. Here we use a multitrait manipulative approach to quantify the potential role of selection acting on floral trait combinations underlying the divergence and maintenance of three related North American species of Silene (Caryophyllaceae). We artificially generated 48 plant phenotypes corresponding to all combinations of key attractive traits differing among the three Silene species (color, height, inflorescence architecture, flower orientation, and corolla‐tube width). We quantified main and interaction effects of trait manipulation on hummingbird visitation preference using experimental arrays. The main effects of floral display height and floral orientation strongly influenced hummingbird visitation, with hummingbirds preferring flowers held high above the ground and vertically to the sky. Hummingbirds also prefer traits in a nonadditive manner as multiple two‐way and higher order interaction effects were important predictors of hummingbird visitation. Contemporary trait combinations found in hummingbird pollinated S. virginica are mostly preferred. Our study demonstrates the likelihood of pollination syndromes evolving due to selection on trait combinations and highlights the importance of trait interactions in understanding the evolution of complex adaptations.     

The origin of a mutualism: a morphological trait promoting the evolution of ant-aphid mutualisms

Mutualisms are mutually beneficial interactions between species and are fundamentally important at all levels of biological organization. It is not clear, however, why one species participates in a particular mutualism whereas another does not. Here we show that pre-existing traits can dispose particular species to evolve a mutualistic interaction. Combining morphological, ecological, and behavioral data in a comparative analysis, we show that resource use in Chaitophorus aphids (Hemiptera: Aphididae) modulates the origin of their mutualism with ants. We demonstrate that aphid species that feed on deeper phloem elements have longer mouthparts, that this inhibits their ability to withdraw their mouthparts and escape predators and that, consequently, this increases their need for protection by mutualist ants.     

Herbivore community promotes trait evolution in a leaf beetle via induced plant response

Several recent studies have emphasised that community composition alters species trait evolution. Here, we demonstrate that differences in composition of local herbivore communities lead to divergent trait evolution of the leaf beetle Plagiodera versicolora through plant‐mediated indirect interactions. Our field surveys, genetic analyses and community‐manipulation experiments show that herbivore community composition determines the degree of herbivore‐induced regrowth of willows (Salicaceae), which in turn, promotes the divergent evolution of feeding preference in the leaf beetle from exclusive preference for new leaves to a lack of preference among leaf‐age types. Regrowth intensity depends both on the differential response of willows to different herbivore species and the integration of those herbivore species in the community. Because herbivore‐induced regrowth involves phenological changes in new leaf production, leaf beetle populations develop divergent feeding preferences according to local regrowth intensity. Therefore, herbivore community composition shapes the selection regime for leaf beetle evolution through trait‐mediated indirect interactions.     

The evolutionary ecology of cheating: does superficial oviposition facilitate the evolution of a cheater yucca moth?

Abstract 1. A major question in the study of mutualism is to understand how mutualists may revert to antagonists that exploit the mutualism (i.e. switch to cheating). In the classic pollination mutualism between yuccas and yucca moths, the cheater moth Tegeticula intermedia is sister to the pollinator moth T. cassandra. These moth species have similar ovipositor morphology, but T. intermedia emerges later, oviposits into fruit rather than flowers, and does not pollinate. 2. We tested if the pollinator, T. cassandra, was pre‐adapted to evolve a cheater lineage by comparing its emergence and oviposition behaviour on yucca fruit to a distantly related pollinator, T. yuccasella, that differs in ovipositor morphology and oviposition behaviour. We predicted that if T. cassandra was pre‐adapted to cheat, then these pollinators would emerge later and be able to oviposit into fruit in contrast to T. yuccasella. 3. Contrary to expectations, a common garden‐rearing experiment demonstrated that emergence of T. cassandra was not significantly delayed relative to T. yuccasella. Moth emergence patterns overlapped broadly. 4. No choice oviposition experiments with female moths demonstrated that both pollinator species attempted to oviposit into fruit, but only T. cassandra was successful. Four out of 84 T. cassandra successfully oviposited into older fruit, whereas zero out of 79 T. yuccasella oviposited into older fruit. The rarity of the cheating behaviour in pollinators, however, meant that no significant difference in oviposition ability was detected. 5. The results suggest that a shift in emergence phenology is likely not a pre‐adaptation to the evolution of cheating, but that the ability to successfully lay eggs into fruit may be. The results also demonstrate that cheating attempts are rare in these pollinator species and, hence, the evolutionary transition rate from pollinator to cheater is likely to be low.     

Sexually antagonistic co‐evolution: a model and an empirical test

Models reveal that sexually antagonistic co‐evolution exaggerates female resistance and male persistence traits. Here we adapt an established model by including directional sexual selection acting against persistence. We find similar equilibria to previous models showing that sexually antagonistic co‐evolution can be limited by counteracting sexual, as well as, natural selection. We tested the model using empirical data for the seaweed fly, Coelopa ursina, in which body size acts as a persistence and a resistance trait. Our model can generate continuous co‐evolutionary cycles and stable equilibria, however, all simulations using empirically derived parameter estimates reach stable equilibria. Thus, stable equilibria might be more common in nature than continuous co‐evolutionary cycles, suggesting that sexual conflict is unlikely to promote speciation. The model predicts male biased sexual size dimorphism for C. ursina, comparable with empirically observed values. Male persistence is shown to be more sensitive than female resistance to changes in model parameters.     

Combining taxon‐by‐trait and taxon‐by‐site matrices for analysing trait patterns of macroinvertebrate communities: a rejoinder to Monaghan & Soares ()

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The genetic basis of floral traits associated with mating system evolution in Leptosiphon (Polemoniaceae): an analysis of quantitative trait loci

Mapping of quantitative trait loci (QTL) was used to investigate the genetic architecture of divergence in floral characters associated with the mating system, an important adaptive trait in angiosperms. Two species of Leptosiphon (Polemoniaceae), one strongly self-fertilizing (L. bicolor) and the other partially outcrossing (L. jepsonii), were crossed to produce F2 and both backcross progenies. For each crossing population, a linkage map was created using amplified fragment length polymorphism markers, and QTL were identified for several dimensions of floral size. For each of the five traits examined, three to seven QTL were detected, with independent datasets yielding congruent results in some but not all cases. The phenotypic effect of individual QTL was generally moderate. We estimated that many of the QTL were additive or showed dominance toward L. bicolor, whereas comparison of mean trait values for parental and cross progenies showed apparent overall dominance of L. jepsonii traits. Colocalization of QTL for different dimensions of floral size was consistent with high phenotypic correlations between floral traits. Substantial segregation distortion was observed in marker loci, the majority favoring alleles from the large-flowered parent. A low frequency of male sterility in the F2 population is consistent with the Dobzhansky-Muller model for the evolution of reproductive isolation.     

Connecting thermal performance curve variation to the genotype: a multivariate QTL approach

Thermal performance curves (TPCs) are continuous reaction norms that describe the relationship between organismal performance and temperature and are useful for understanding trade‐offs involved in thermal adaptation. Although thermal trade‐offs such as those between generalists and specialists or between hot‐ and cold‐adapted phenotypes are known to be genetically variable and evolve during thermal adaptation, little is known of the genetic basis to TPCs – specifically, the loci involved and the directionality of their effects across different temperatures. To address this, we took a multivariate approach, mapping quantitative trait loci (QTL) for locomotor activity TPCs in the fly, Drosophila serrata, using a panel of 76 recombinant inbred lines. The distribution of additive genetic (co)variance in the mapping population was remarkably similar to the distribution of mutational (co)variance for these traits. We detected 11 TPC QTL in females and 4 in males. Multivariate QTL effects were closely aligned with the major axes genetic (co)variation between temperatures; most QTL effects corresponded to variation for either overall increases or decreases in activity with a smaller number indicating possible trade‐offs between activity at high and low temperatures. QTL representing changes in curve shape such as the ‘generalist–specialist’ trade‐off, thought key to thermal adaptation, were poorly represented in the data. We discuss these results in the light of genetic constraints on thermal adaptation.     

A tale of two green walls: a functional trait approach to assess vegetation establishment on restored steep slopes

Green walls (GWs) have been increasingly recognized as an important restoration technique for steep slopes resulting from quarrying activities or major infrastructure construction projects. In practice, GW irrigation is considered essential, although studies evaluating vegetation establishment under different irrigation regimes are lacking. Besides taxonomic metrics, functional diversity measures were used to compare the diversity and composition of plant communities of two hydroseeded GWs, with different irrigation regimes (irrigated vs. low‐irrigated). The studied GWs were installed in Peneda‐Gerês National Park (NW Portugal) to minimize the visual impact of shotcrete walls, along a road infrastructure, and promote their ecological restoration. Species' cover was recorded 3 years after installation. Species were classified according to their growth form, life form, and life strategy. Despite hydroseeding approximately 57 species, GW vegetation was dominated by spontaneous species, particularly acrocarpous mosses, regardless of irrigation regime. Species and functional richness were higher under irrigated conditions, while no differences were observed for species diversity and functional dispersion. Functional trait composition did not differ between GWs, indicating that both systems potentially provide the same ecosystem functions and services. Our results also suggest that spontaneous species colonizing GWs are highly adapted to local environmental conditions, given their dominance in both irrigated and low‐irrigated communities. Overall, irrigation did not affect the structure and functioning of GW communities, only their redundancy, since more species exhibiting similar traits were established in the irrigated GW. Therefore, our findings suggest that low‐irrigated GWs, hydroseeded with native species, represent a more cost‐effective solution to reduce the ecological impacts of steep slopes.     

Intrapopulation variability in a functional trait: Susceptibility of Daphnia to limitation by dietary fatty acids

1. Functional traits are measurable characteristics of an organism that have an impact on its fitness. Variation in functional traits between and among species has been suggested to represent the basis for competition and selection, thus allowing for evolution in natural populations.
2. In freshwater ecosystems, the availability of essential polyunsaturated fatty acids (PUFAs), in particular ω3‐ and ω6‐PUFAs, determines the food quality of phytoplankton for the herbivorous zooplankton Daphnia, an unselective filter feeder. The content of such essential PUFAs in the phytoplankton is thus a functional phytoplankton trait affecting the trophic transfer efficiency and dynamics at the pelagic plant–herbivore interface.
3. In turn, the susceptibility of consumers to become limited by the availability of essential PUFAs is a fitness‐determining trait of Daphnia genotypes, and variability of this herbivore trait may thus affect the daphnids’ intrapopulation competition. To estimate the intrapopulation variation in susceptibility, we isolated clonal lines of Daphnia longispina from a natural population and compared the strength of their limitation by dietary PUFA availability via standardised laboratory growth assays. We used a liposome supplementation technique to enrich a PUFA‐poor green alga with essential ω3‐ and ω6‐PUFAs and determined juvenile somatic growth rate of different D. longispina genotypes as a fitness proxy.
4. As expected, D. longispina genotypes that coexisted in a natural population differed markedly in their specific patterns of susceptibility to dietary PUFA availability. On average, the D. longispina population was more strongly susceptible to limitations in the availability of the ω6‐PUFA arachidonic acid (20:4ω6) than to limitations in the availability of ω3‐PUFAs α‐linolenic acid (18:3ω3) and eicosapentaenoic acid (20:5ω3).
5. The ability to cope with PUFA limitation is thus a crucial trait that can probably affect intraspecific competition and Daphnia population structure. Therefore, we suggest that such intrapopulation variation in susceptibility to absence of dietary PUFAs might be one of the driving forces of natural selection and local adaptation among freshwater zooplankton.

     

Living in the city: urban environments shape the evolution of a native annual plant

Urban environments are warmer, have higher levels of atmospheric CO₂ and have altered patterns of disturbance and precipitation than nearby rural areas. These differences can be important for plant growth and are likely to create distinct selective environments. We planted a common garden experiment with seeds collected from natural populations of the native annual plant Lepidium virginicum, growing in five urban and nearby rural areas in the northern United States to determine whether and how urban populations differ from those from surrounding rural areas. When grown in a common environment, plants grown from seeds collected from urban areas bolted sooner, grew larger, had fewer leaves, had an extended time between bolting and flowering, and produced more seeds than plants grown from seeds collected from rural areas. Interestingly, the rural populations exhibited larger phenotypic differences from one another than urban populations. Surprisingly, genomic data revealed that the majority of individuals in each of the urban populations were more closely related to individuals from other urban populations than they were to geographically proximate rural areas – the one exception being urban and rural populations from New York which were nearly identical. Taken together, our results suggest that selection in urban environments favors different traits than selection in rural environments and that these differences can drive adaptation and shape population structure.     

Estimation of the contribution of quantitative trait loci (QTL) to the variance of a quantitative trait by means of genetic markers

The estimation of the contribution of an individual quantitative trait locus (QTL) to the variance of a quantitative trait is considered in the framework of an analysis of variance (ANOVA). ANOVA mean squares expectations which are appropriate to the specific case of QTL mapping experiments are derived. These expectations allow the specificities associated with the limited number of genotypes at a given locus to be taken into account. Discrepancies with classical expectations are particularly important for two-class experiments (backcross, recombinant inbred lines, doubled haploid populations) and F₂ populations. The result allows us firstly to reconsider the power of experiments (i.e. the probability of detecting a QTL with a given contribution to the variance of the trait). It illustrates that the use of classical formulae for mean squares expectations leads to a strong underestimation of the power of the experiments. Secondly, from the observed mean squares it is possible to estimate directly the variance associated with a locus and the fraction of the total variance associated to this locus (r _l ² ). When compared to other methods, the values estimated using this method are unbiased. Considering unbiased estimators increases in importance when (1) the experimental size is limited; (2) the number of genotypes at the locus of interest is large; and (3) the fraction of the variation associated with this locus is small. Finally, specific mean squares expectations allows us to propose a simple analytical method by which to estimate the confidence interval of r _l ² . This point is particularly important since results indicate that 95% confidence intervals for r _l ² can be rather wide:2–23% for a 10% estimate and 8–34% for a 20% estimate if 100 individuals are considered.     

Taking a big bite: Working together to better understand the evolution of feeding in primates

The study of adaptation requires the integration of an array of different types of data. A single individual can find such integration daunting, if not impossible. In an effort to clarify the role of diet in the evolution of the primate craniofacial and dental apparatus, we assembled a team of researchers that have various types and degrees of expertise. This interaction has provided a range of insights for all contributors, and this has helped to refine questions, clarify the possibilities and limitations that laboratory and field settings offer, and further explore the ways in which laboratory and field data can be suitably integrated. A complete and accurate picture of dietary adaptation cannot be gained in isolation. Collaboration provides the bridge to a more holistic view of primate biology and evolution.     

Natural selection on extrafloral nectar production in Chamaecrista fasciculata: the costs and benefits of a mutualism trait

Cost-benefit models of the evolution of mutualism predict that the current state of mutualism results from trade-offs between fitness costs of mutualist traits and the fitness benefits of association. We test the assumptions of such models by measuring patterns of natural selection on a mutualist trait, extrafloral nectar production in Chamaecrista fasciculata. Selection was measured on plants from which ants had been excluded (removing the mutualist benefit of the trait), from which all insects had been excluded (removing costs of herbivory in addition to mutualist benefits), and unmanipulated plants (where both costs and benefits were present). Selection analysis based on half-sibling-mean regressions of fitness on the trait revealed no evidence of costs of extrafloral nectar production in the absence of all insects or in the absence of ants. However, examination of the selective surfaces for these treatments suggest that costs of nectar production may exist and are exacerbated by the presence of herbivory. In the presence of ants, natural selection favors high extrafloral nectar production, consistent with a fitness benefit to this mutualist trait in the presence of the mutualist partner. In this study, the interaction of costs and benefits did not produce an evolutionary optimum for the trait within the range of variation observed, suggesting that application of a cost-benefit framework to this trait will benefit from considering the influence of temporal and spatial variation on the quality of costs and benefits.     

Identification of quantitative trait loci affecting resistance to gastrointestinal parasites in a double backcross population of Red Maasai and Dorper sheep

A genome‐wide scan for quantitative trait loci (QTL) affecting gastrointestinal nematode resistance in sheep was completed using a double backcross population derived from Red Maasai and Dorper ewes bred to F₁ rams. This design provided an opportunity to map potentially unique genetic variation associated with a parasite‐tolerant breed like Red Maasai, a breed developed to survive East African grazing conditions. Parasite indicator phenotypes (blood packed cell volume – PCV and faecal egg count – FEC) were collected on a weekly basis from 1064 lambs during a single 3‐month post‐weaning grazing challenge on infected pastures. The averages of last measurements for FEC (AVFEC) and PCV (AVPCV), along with decline in PCV from challenge start to end (PCVD), were used to select lambs (N = 371) for genotyping that represented the tails (10% threshold) of the phenotypic distributions. Marker genotypes for 172 microsatellite loci covering 25 of 26 autosomes (1560.7 cm ) were scored and corrected by Genoprob prior to qxpak analysis that included Box–Cox transformed AVFEC and arcsine transformed PCV statistics. Significant QTL for AVFEC and AVPCV were detected on four chromosomes, and this included a novel AVFEC QTL on chromosome 6 that would have remained undetected without Box–Cox transformation methods. The most significant P‐values for AVFEC, AVPCV and PCVD overlapped the same marker interval on chromosome 22, suggesting the potential for a single causative mutation, which remains unknown. In all cases, the favourable QTL allele was always contributed from Red Maasai, providing support for the idea that future marker‐assisted selection for genetic improvement of production in East Africa will rely on markers in linkage disequilibrium with these QTL.     

Biogeography of the grasses (Poaceae): a phylogenetic approach to reveal evolutionary history in geographical space and geological time

The grasses (Poaceae) are the fifth most diverse family of angiosperms, including 800 genera and more than 10 000 species. Few phylogenetic studies have tried to investigate palaeo‐biogeographical and palaeo‐ecological scenarios that may have led to present‐day distribution and diversity of grasses at the family level. We produced a dated phylogenetic tree based on combined plastid DNA sequences and a comprehensive sample of Poaceae. Furthermore, we produced an additional tree using a supermatrix of morphological and molecular data that included all 800 grass genera so that ancestral biogeography and ecological habitats could be inferred. We used a likelihood‐based method, which allows the estimation of ancestral polymorphism in both biogeographical and ecological analyses for large data sets. The origin of Poaceae was retrieved as African and shade adapted. The crown node of the BEP + PACCMAD clade was dated at 57 Mya, in the early Eocene. Grasses dispersed to all continents by approximately 60 million years after their Gondwanan origin in the late Cretaceous. PACCMAD taxa adapted to open habitats as early as the late Eocene, a date consistent with recent phytolith fossil data for North America. C₄ photosynthesis first originated in Africa, at least for Chloridoideae in the Eocene at c. 30 Mya. The BEP clade members adapted to open habitats later than PACCMAD members; this was inferred to occur in Eurasia in the Oligocene. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 162 , 543–557.     

Convergence to a novel environment: comparative method versus experimental evolution

Abstract Laboratory adaptation allows researchers to contrast temporal studies of experimental evolution with comparative studies. The comparative method is here taken to mean the inference of microevolutionary processes from comparisons among contemporaneous populations of diverse origins, from one or multiple species. The data contrasted here come from Drosophila subobscura populations that were introduced to the laboratory at several different times and from two different locations. Two questions were addressed. First, can we correctly infer evolutionary dynamics from comparative data collected simultaneously from disparate populations? In most cases, we could, except for the character of starvation resistance. Second, are the evolutionary dynamics inferred from the comparative approach similar to those revealed by temporal studies of experimental evolution? For fecundity characters, they were. Overall the results show that both comparative and temporal studies are useful, though the former can be uninformative for characters with complex evolutionary trajectories.     

Trade‐offs in the evolution of bumblebee colony and body size: a comparative analysis

Trade‐offs between life‐history traits – such as fecundity and survival – have been demonstrated in several studies. In eusocial insects, the number of organisms and their body sizes can affect the fitness of the colony. Large‐than‐average body sizes as well as more individuals can improve a colony's thermoregulation, foraging efficiency, and fecundity. However, in bumblebees, large colonies and large body sizes depend largely on high temperatures and a large amount of food resources. Bumblebee taxa can be found in temperate and tropical regions of the world and differ markedly in their colony sizes and body sizes. Variation in colony size and body size may be explained by the costs and benefits associated with the evolutionary history of each species in a particular environment. In this study, we explored the effect of temperature and precipitation (the latter was used as an indirect indicator of food availability) on the colony and body size of twenty‐one bumblebee taxa. A comparative analysis controlling for phylogenetic effects as well as for the body size of queens, workers, and males in bumblebee taxa from temperate and tropical regions indicated that both temperature and precipitation affect colony and body size. We found a negative association between colony size and the rainiest trimester, and a positive association between the colony size and the warmest month of the year. In addition, male bumblebees tend to evolve larger body sizes in places where the rain occurs mostly in the summer and the overall temperature is warmer. Moreover, we found a negative relationship between colony size and body sizes of queens, workers, and males, suggesting potential trade‐offs in the evolution of bumblebee colony and body size.