Wendy L. Clement

An Extreme Case of Plant-Insect Codiversification: Figs and Fig-Pollinating Wasps

It is thought that speciation in phytophagous insects is often due to colonization of novel host plants, because radiations of plant and insect lineages are typically asynchronous. Recent phylogenetic comparisons have supported this model of diversification for both insect herbivores and specialized pollinators. An exceptional case where contemporaneous plant-insect diversification might be expected is the obligate mutualism between fig trees (Ficus species, Moraceae) and their pollinating wasps (Agaonidae, Hymenoptera). The ubiquity and ecological significance of this mutualism in tropical and subtropical ecosystems has long intrigued biologists, but the systematic challenge posed by >750 interacting species pairs has hindered progress toward understanding its evolutionary history. In particular, taxon sampling and analytical tools have been insufficient for large-scale cophylogenetic analyses. Here, we sampled nearly 200 interacting pairs of fig and wasp species from across the globe. Two supermatrices were assembled: on an average, wasps had sequences from 77% of 6 genes (5.6 kb), figs had sequences from 60% of 5 genes (5.5 kb), and overall 850 new DNA sequences were generated for this study. We also developed a new analytical tool, Jane 2, for event-based phylogenetic reconciliation analysis of very large data sets. Separate Bayesian phylogenetic analyses for figs and fig wasps under relaxed molecular clock assumptions indicate Cretaceous diversification of crown groups and contemporaneous divergence for nearly half of all fig and pollinator lineages. Event-based cophylogenetic analyses further support the codiversification hypothesis. Biogeographic analyses indicate that the present-day distribution of fig and pollinator lineages is consistent with a Eurasian origin and subsequent dispersal, rather than with Gondwanan vicariance. Overall, our findings indicate that the fig-pollinator mutualism represents an extreme case among plant-insect interactions of coordinated dispersal and long-term codiversification. [Biogeography; coevolution; cospeciation; host switching; long-branch attraction; phylogeny.].

Phylogenetic position and biogeography of Hillebrandia sandwicensis (Begoniaceae): a rare Hawaiian relict

The Begoniaceae consist of two genera, Begonia, with approximately 1400 species that are widely distributed in the tropics, and Hillebrandia, with one species that is endemic to the Hawaiian Islands and the only member of the family native to those islands. To help explain the history of Hillebrandia on the Hawaiian Archipelago, phylogenetic relationships of the Begoniaceae and the Cucurbitales were inferred using sequence data from 18S, rbcL, and ITS, and the minimal age of both Begonia and the Begoniaceae were indirectly estimated. The analyses strongly support the placement of Hillebrandia as the sister group to the rest of the Begoniaceae and indicate that the Hillebrandia lineage is at least 51-65 million years old, an age that predates the current Hawaiian Islands by about 20 million years. Evidence that Hillebrandia sandwicensis has survived on the Hawaiian Archipelago by island hopping from older, now denuded islands to younger, more mountainous islands is presented. Various scenarios for the origin of ancestor to Hillebrandia are considered. The geographic origin of source populations unfortunately remains obscure; however, we suggest a boreotropic or a Malesian-Pacific origin is most likely. Hillebrandia represents the first example in the well-studied Hawaiian flora of a relict genus.

Evolutionary history predicts plant defense against an invasive pest

It has long been hypothesized that invasive pests may be facilitated by the evolutionary naïveté of their new hosts, but this prediction has never been examined in a phylogenetic framework. To address the hypothesis, we have been studying the invasive viburnum leaf beetle (Pyrrhalta viburni), which is decimating North American native species of Viburnum, a clade of worldwide importance as understory shrubs and ornamentals. In a phylogenetic field experiment using 16 species of Viburnum, we show that old-world Viburnum species that evolved in the presence of Pyrrhalta beetles mount a massive defensive wound response that crushes eggs of the pest insect; in contrast, naïve North American species that share no evolutionary history with Pyrrhalta beetles show a markedly lower response. This convergent continental difference in the defensive response of Viburnum spp. against insect oviposition contrasts with little difference in the quality of leaves for beetle larvae. Females show strong oviposition preferences that correspond with larval performance regardless of continental origin, which has facilitated colonization of susceptible North American species. Thus, although much attention has been paid to escape from enemies as a factor in the establishment and spread of nonnative organisms, the colonization of undefended resources seems to play a major role in the success of invasive species such as the viburnum leaf beetle.

Barcoding success as a function of phylogenetic relatedness in Viburnum, a clade of woody angiosperms.

BackgroundThe chloroplast genes matK and rbcL have been proposed as a “core” DNA barcode for identifying plant species. Published estimates of successful species identification using these loci (70-80%) may be inflated because they may have involved comparisons among distantly related species within target genera. To assess the ability of the proposed two-locus barcode to discriminate closely related species, we carried out a hierarchically structured set of comparisons within Viburnum, a clade of woody angiosperms containing ca. 170 species (some 70 of which are currently used in horticulture). For 112 Viburnum species, we evaluated rbcL + matK, as well as the chloroplast regions rpl32-trnL, trnH-psbA, trnK, and the nuclear ribosomal internal transcribed spacer region (nrITS).ResultsAt most, rbcL + matK could discriminate 53% of all Viburnum species, with only 18% of the comparisons having genetic distances >1%. When comparisons were progressively restricted to species within major Viburnum subclades, there was a significant decrease in both the discriminatory power and the genetic distances. trnH-psbA and nrITS show much higher levels of variation and potential discriminatory power, and their use in plant barcoding should be reconsidered. As barcoding has often been used to discriminate species within local areas, we also compared Viburnum species within two regions, Japan and Mexico and Central America. Greater success in discriminating among the Japanese species reflects the deeper evolutionary history of Viburnum in that area, as compared to the recent radiation of a single clade into the mountains of Latin America.ConclusionsWe found very low levels of discrimination among closely related species of Viburnum, and low levels of variation in the proposed barcoding loci may limit success within other clades of long-lived woody plants. Inclusion of the supplementary barcodes trnH-psbA and nrITS increased discrimination rates but were often more effective alone rather than in combination with rbcL + matK. We surmise that the efficacy of barcoding in plants has often been overestimated because of the lack of comparisons among closely related species. Phylogenetic information must be incorporated to properly evaluate relatedness in assessing the utility of barcoding loci.

Morphological Evolution in the Mulberry Family (Moraceae)

Abstract The mulberry family Moraceae comprises 37 genera and approximately 1,100 species distributed throughout tropical and temperate regions worldwide. Moraceae exhibit a complex array of inflorescence architectures, breeding systems, and pollination syndromes, which forms the basis of traditional taxonomic classification. However, morphologically based classification conflicts with evolutionary relationships proposed by molecular phylogenetics. In this study we assembled a morphological character matrix for analysis separately and in combination with existing molecular data. We evaluated 81 vegetative, reproductive, and wood anatomical characters for 94 species representing nearly all Moraceae genera. Using parsimony and Bayesian methods, these characters were evaluated with respect to ndhF chloroplast and 26S nuclear DNA sequences. Topological comparisons tested whether opposing classification schemes are more or less supported by the data. Results did not support any classification of Moraceae based on morphology. We present a revised tribal classification and describe a new tribe, Maclureae, revise the membership of tribe Castilleae and describe two subtribes, Castillineae and Antiaropsineae, and reinstate the genera Malaisia (including Malaisia scandens) and Sloetia (including Sloetia elongata). Lastly we discuss the evolution of inflorescence architecture in relation to other floral features.

Evolution of leaf form correlates with tropical-temperate transitions in Viburnum (Adoxaceae)

Strong latitudinal patterns in leaf form are well documented in floristic comparisons and palaeobotanical studies. However, there is little agreement about their functional significance; in fact, it is still unknown to what degree these patterns were generated by repeated evolutionary adaptation. We analysed leaf form in the woody angiosperm clade Viburnum (Adoxaceae) and document evolutionarily correlated shifts in leafing habit, leaf margin morphology, leaf shape and climate. Multiple independent shifts between tropical and temperate forest habitats have repeatedly been accompanied by a change between evergreen, elliptical leaves with entire margins and deciduous, more rounded leaves with toothed or lobed margins. These consistent shifts in Viburnum support repeated evolutionary adaptation as a major determinant of the global correlation between leaf form and mean annual temperature. Our results provide a new theoretical grounding for the inference of past climates using fossil leaf assemblages.

Phylogenetic and Experimental Tests of Interactions among Mutualistic Plant Defense Traits in Viburnum (Adoxaceae)

Plant traits that mediate mutualistic interactions are widespread, yet few studies have linked their macroevolutionary patterns with the ecological interactions they mediate. Here we merged phylogenetic and experimental approaches to investigate the evolution of two common mutualistic plant traits, extrafloral nectaries (EFNs) and leaf domatia. By using the flowering plant clade Viburnum, we tested whether macroevolutionary patterns support adaptive hypotheses and conducted field surveys and manipulative experiments to examine whether ecological interactions are concordant with evolutionary predictions. Phylogenetic reconstructions suggested that EFN-bearing species are monophyletic, whereas the evolution of domatia correlated with leaf production strategy (deciduous or evergreen) and climate. Domatia were also more common in the EFN clade, suggesting that the two traits may jointly mediate ecological interactions. This result was further investigated in a common-garden survey, where plants with domatia and EFNs on the leaf blade had more mutualistic mites than plants with other trait combinations, and in manipulative field experiments, where the traits additively increased mutualist abundance. Taken together, our results suggest that mutualistic traits in Viburnum are not ecologically independent, as they work in concert to attract and retain mutualists, and their long-term evolution may be influenced by complex interactions among multiple traits, mutualists, and geography.

Phylogenetic Relationships of Asian Begonia, with an Emphasis on the Evolution of Rain-ballist and Animal Dispersal Mechanisms in Sections Platycentrum, Sphenanthera and Leprosae

Abstract While most Begonia species have a similar fruit morphology that shows adaptations to wind dispersal, a few species have atypical fruits and are adapted to either animal or rain dispersal. Such differences in fruit morphology have traditionally been emphasized in sectional classifications of Begonia and some of the currently recognized sections can only be distinguished using ovary and fruit characteristics. We evaluated the monophyly and evolution of three Asian sections with atypical fruit morphologies: Platycentrum, Sphenanthera, and Leprosae, along with members of nine other Asian sections with fruit morphologies typical of wind dispersed Begonia. A parsimony analysis of nrDNA ITS/ 5.8S sequence data of 46 Asian Begonia species suggests that the members of section Platycentrum, which have fruit morphologies indicative of rain dispersal, evolved from wind dispersed Asian taxa following the colonization of wetter habitats. From within this rain dispersed group, species of section Sphenanthera with fleshy, animal dispersed fruits subsequently evolved on multiple occasions. Members of section Leprosae, which have fleshy fruit, evolved on two separate occasions, in one case independently of the members of the sections Platycentrum and Sphenanthera. As currently recognized, sections Platycentrum, Sphenanthera and Leprosae are polyphyletic.

DISSOLUTION OF VIBURNUM SECTION MEGALOTINUS (ADOXACEAE) OF SOUTHEAST ASIA AND ITS IMPLICATIONS FOR MORPHOLOGICAL EVOLUTION AND BIOGEOGRAPHY

This study marks a significant increase in the number of species and genetic loci used in reconstructing the phylogeny of Viburnum. In particular, we expanded sampling of the morphologically heterogeneous section Megalotinus of Southeast Asia, which to date has been represented by only one species. Our results provide increased support for the monophyly of most of the previously named clades and for relationships within them. However, the four subsections of Megalotinus are placed with confidence in widely separate places in the phylogeny, and their disparate relationships are supported by morphological characters including branching patterns, inflorescence types, and trichomes. These findings, along with the phylogenetic placement of several additional Southeast Asian species, are critical in assessing the ancestral condition of Viburnum inflorescence architecture and endocarp shape. Our results also highlight a new biogeographic possibility, namely that Viburnum may have originated and initially diversified in montane subtropical forests in Southeast Asia and later moved into northern temperate forests, which most of them are associated with today. This study provides a clear-cut example of the importance of including in phylogenetic studies rare and difficult-to-obtain species from outside the main centers of diversity and of the value of dismantling nonmonophyletic taxonomic groups.

THE EVOLUTION OF PHOTOSYNTHETIC ANATOMY IN VIBURNUM (ADOXACEAE)

Premise of research. Leaf mesophyll is often differentiated into a palisade layer with tightly packed, elongated cells (I-cells) and a spongy layer with loosely packed, complex shaped cells. An alternative palisade type, composed of branched H-cells, has evolved in a number of plant lineages. Viburnum (Adoxaceae) possesses both types of palisade, providing an opportunity to assess the significance of evolutionary switches between these forms. Methodology. An anatomical survey of 80 species spanning the Viburnum phylogeny permitted an analysis of palisade differences in relation to other characters. A geometric model of leaf mesophyll surface area for CO2 absorption correlated well with measured photosynthetic capacity in a subset of species, allowing us to infer shifts in photosynthetic function. Pivotal results. Ancestrally, viburnums probably produced a palisade with one layer of H-cells. Multiple transitions to two layers of H-cells (H2) and to one or two layers of I-cells (I1, I2) occurred. These shifts were correlated with increases in photosynthetic capacity, and H2 appear functionally equivalent to I1 with respect to CO2 absorption. Conclusions. Photosynthetic anatomy H2 and I1 palisade may represent alternative evolutionary solutions for increasing leaf CO2 absorption. Additionally, H-cells and I-cells might perform differently with respect to light absorption and/or drought tolerance. The evolution of I-palisade cells may thus have tracked movements into open environments, while H2 could increase photosynthetic capacity in the forest understory.