Syst. Biol. 53(1) 2004

Kirchoff et al.
Abstract.—Two studies were conducted to explore the use of complex data in character description and hybrid identification. In order to determine if complex data allow the production of better characters, eight groups of plant systematists were given two classes of drawings of plant parts, and asked to divide them into character states (clusters) in two separate experiments. The first class of drawings consisted only of cotyledons. The second class consisted of triplets of drawings: a cotyledon, seedling leaf, and inflorescence bract. The triplets were used to simulate complex data such as might be garnered by looking at a plant. Each experiment resulted in four characters (groups of clusters), one for each group of systematists. Visual and statistical analysis of the data showed that the systematists were able to produce smaller, more precisely defined character states using the more complex drawings. The character states created with the complex drawings also were more consistent across systematists, and agreed more closely with an independent assessment of phylogeny. To investigate the utility of complex data in an applied task, four observers rated 250 hybrids of Dubautia ciliolata X arborea based on the overall form (Gestalt) of the plants, and took measurements of a number of features of the same plants. A composite score of the measurements was created using principal components analysis. The correlation between the scores on the first principal component and the Gestalt ratings was computed. The Gestalt ratings and PC scores were significantly correlated, demonstrating that assessments of overall similarity can be as useful as more conventional approaches in determining the hybrid status of plants.

Waters and Roy
Abstract.—We used mitochondrial DNA (mtDNA) sequences to test biogeographic hypotheses for Patiriella exigua (Asterinidae), one of the world's most widespread coastal sea stars. This small intertidal species has an entirely benthic life history and yet occurs in southern temperate waters of the Atlantic, Indian, and Pacific oceans. Despite its abundance around southern Africa, southeastern Australia, and several oceanic islands, P. exigua is absent from the shores of Western Australia, New Zealand, and South America. Phylogenetic analysis of mtDNA sequences (cytochrome oxidase I, control region) indicates that South Africa houses an assemblage of P. exigua that is not monophyletic (P=0.04), whereas Australian and Lord Howe Island specimens form an interior monophyletic group. The placement of the root in Africa and small genetic divergences between eastern African and Australian haplotypes strongly suggest Pleistocene dispersal eastward across the Indian Ocean. Dispersal was probably achieved by rafting on wood or macroalgae, which was facilitated by the West Wind Drift. Genetic data also support Pleistocene colonization of oceanic islands (Lord Howe Island, Amsterdam Island, St. Helena). Although many biogeographers have speculated about the role of long-distance rafting, this study is one of the first to provide convincing evidence. The marked phylogeographic structure evident across small geographic scales in Australia and South Africa indicates that gene flow among populations may be generally insufficient to prevent the local evolution of monophyly. We suggest that P. exigua may rely on passive mechanisms of dispersal.

Abstract.—Recent molecular phylogenetic studies of polyploid plants have successfully clarified complex patterns of reticulate evolution. In this study of Elymus repens, an allohexaploid member of the wheat tribe Triticeae, chloroplast and nuclear DNA data reveal an extreme reticulate pattern, revealing at least five distinct gene lineages coexisting within the species, acquired through a possible combination of allohexaploidy and introgression from both within and beyond the Triticeae. Earlier cytogenetic studies of E. repens suggested that Hordeum (genome H) and Pseudoroegneria (St) were genome donors to E. repens. Chloroplast DNA data presented here (from the rpoA gene and from the region between trnT and trnF) identify three potential maternal genome donors (Pseudoroegneria, Thinopyrum, and Dasypyrum), and information from previous molecular work suggests that, of these, Pseudoroegneria is the most likely maternal donor. Nuclear starch synthase gene data indicate that both Hordeum and Pseudoroegneria have contributed to the nuclear genome of E. repens, in agreement with cytogenetic data. However, these data also show unexpected contributions from Taeniatherum, and from two additional donors of unknown identity. One of the sequences of unknown origin falls within the Triticeae, but is not closely associated with any of the sampled diploid genera. The second falls outside of the clade containing Triticeae and its outgroup Bromus, suggesting the acquisition of genetic material from a surprisingly divergent source. Bias toward the amplification of certain starch synthase variants has complicated attempts to thoroughly sample from within individuals, but the data clearly indicate a complex pattern of reticulate evolution, consistent not only with allohexaploidy, but also with introgression from unexpectedly divergent sources.

Abstract.—The influence of nonindependence among sites on phylogenetic reconstructions and bootstrap scores was investigated both analytically and empirically. First, the sampling properties of the bootstrap support in the four-species case was derived for the maximum-parsimony method, assuming either independently or nonindependently evolving sites. The influence of various models of departure from the independence assumption was quantified. Second, trees and bootstrap scores estimated from subsets of consecutive (potentially coevolving) versus dispersed (presumably independent) sites of a ribosomal RNA data set were contrasted. The two approaches consistently suggest that a departure from the assumption of independent sites tends to reduce the amount of phylogenetic information contained in the data but to increase the apparent statistical support for reconstructed trees, as measured by the bootstrap. In particular, nonindependence can lead to strongly supported wrong internal branches.

Nylander et al.
Abstract.—The recent development of Bayesian phylogenetic inference using Markovchain Monte Carlo (MCMC) techniques has facilitated the exploration of parameter-rich evolutionary models. At the same time, stochastic models have become more realistic (and complex) and have been extended to new types of data, such as morphology. Based on this foundation, we developed a Bayesian MCMC approach to the analysis of combined data sets and explored its utility in inferring relationships among gall wasps based on data from morphology and four genes (nuclear and mitochondrial, ribosomal and protein coding). Examined models range in complexity from those recognizing only a morphological and a molecular partition to those having complex substitution models with independent parameters for each gene. Bayesian MCMC analysis deals efficiently with complex models: convergence occurs faster and more predictably for complex models, mixing is adequate for all parameters even under very complex models, and the parameter update cycle is virtually unaffected by model partitioning across sites. Morphology contributed only 5% of the characters in the data set but nevertheless influenced the combined-data tree, supporting the utility of morphological data in multigene analyses. We used Bayesian criteria (Bayes factors) to show that process heterogeneity across data partitions is a significant model component, although not as important as among-site rate variation. More complex evolutionary models are associated with more topological uncertainty and less conflict between morphology and molecules. Bayes factors sometimes favor simpler models over considerably more parameter-rich models, but the best model overall is also the most complex and Bayes factors do not support exclusion of apparently weak parameters from this model. Thus, Bayes factors appear to be useful for selecting among complex models, but it is still unclear whether their use strikes a reasonable balance between model complexity and error in parameter estimates.

Degnan et al.
Abstract.—Bacterial endosymbionts are widespread across several insect orders and are involved in interactions ranging from obligate mutualism to reproductive parasitism. Candidatus Blochmannia gen. nov. (Blochmannia) is an obligate bacterial associate of Camponotus and related ant genera (Hymenoptera: Formicidae). The occurrence of Blochmannia in all Camponotus species sampled from field populations and its maternal transmission to host offspring suggest that this bacterium is engaged in a long-term, stable association with its ant hosts. However, evidence for cospeciation in this system is equivocal because previous phylogenetic studies were based on limited gene sampling, lacked statistical analysis of congruence, and have even suggested host switching. We compared phylogenies of host genes (the nuclear EF-1aF2 and mitochondrial COI/II) and Blochmannia genes (16S ribosomal DNA[rDNA], groEL, gidA, and rpsB), totaling more than 7 kilobases for each of 16 Camponotus species. Each data set was analyzed using maximum likelihood and Bayesian phylogenetic reconstruction methods. We found minimal conflict among host and symbiont phylogenies, and the few areas of discordance occurred at deep nodes that were poorly supported by individual data sets. Concatenated protein-coding genes produced a very well-resolved tree that, based on the Shimodaira–Hasegawa test, did not conflict with any host or symbiont data set. Correlated rates of synonymous substitution (ds) along corresponding branches of host and symbiont phylogenies further supported the hypothesis of cospeciation. These findings indicate that Blochmannia–Camponotus symbiosis has been evolutionarily stable throughout tens of millions of years. Based on inferred divergence times among the ant hosts, we estimated rates of sequence evolution of Blochmannia to be ~0.0024 substitutions per site per million years (s/s/MY) for the 16S rDNA gene and ~0.1094 s/s/MY at synonymous positions of the genes sampled. These rates are several-fold higher than those for related bacteria Buchnera aphidicola and Escherichia coli. Phylogenetic congruence among Blochmannia genes indicates genome stability that typifies primary endosymbionts of insects.

Ricklefs et al.
Abstract.—We used phylogenetic analyses of cytochrome b sequences of malaria parasites and their avian hosts to assess the coevolutionary relationships between host and parasite lineages. Many lineages of avian malaria parasites have broad host distributions, which tend to obscure cospeciation events. The hosts of a single parasite or of closely related parasites were nonetheless most frequently recovered from members of the same host taxonomic family, more so than expected by chance. However, global assessments of the relationship between parasite and host phylogenetic trees, using Component and ParaFit, failed to detect significant cospeciation. The event-based approach employed by TreeFitter revealed significant cospeciation and duplication with certain cost assignments for these events, but host switching was consistently more prominent in matching the parasite tree to the host tree. The absence of a global cospeciation signal despite conservative host distribution most likely reflects relatively frequent acquisition of new hosts by individual parasite lineages. Understanding these processes will require a more refined species concept for malaria parasites and more extensive sampling of parasite distributions across hosts. If parasites can disperse between allopatric host populations through alternative hosts, cospeciation may not have a strong influence on the architecture of host-parasite relationships. Rather, parasite speciation may happen more often in conjunction with the acquisition of new hosts followed by divergent selection between host lineages in sympatry. Detailed studies of the phylogeographic distributions of hosts and parasites are needed to characterize these events.

Percy et al.
Abstract.—An increasing number of plant-insect studies using phylogenetic analysis suggest that cospeciation events are rare in plant-insect systems. Instead, nonrandom patterns of phylogenetic congruence are produced by phylogenetically conserved host switching (to related plants) or tracking of particular resources or traits (e.g., chemical). The dominance of host switching in many phytophagous insect groups may make the detection of genuine cospeciation events difficult. One important test of putative cospeciation events is to verify whether reciprocal speciation is temporally plausible. We explored techniques for double-dating of both plant and insect phylogenies. We use dated molecular phylogenies of a psyllid (Hemiptera)-Genisteae (Fabaceae) system, a predominantly monophagous insect-plant association widespread on the Atlantic Macaronesian islands. Phylogenetic reconciliation analysis suggests high levels of parallel cladogenesis between legumes and psyllids. However, dating using molecular clocks calibrated on known geological ages of the Macaronesian islands revealed that the legume and psyllid radiations were not contemporaneous but sequential. Whereas the main plant radiation occurred some 8 million years ago, the insect radiation occurred about 3 million years ago. We estimated that >60% of the psyllid speciation has resulted from host switching between related hosts. The only evidence for true cospeciation is in the much more recent and localized radiation of genistoid legumes in the Canary Islands, where the psyllid and legume radiations have been partially contemporaneous. The identification of specific cospeciation events over this time period, however, is hindered by the phylogenetic uncertainty in both legume and psyllid phylogenies due to the apparent rapidity of the species radiations.

Abstract.—This is the first comparative study of correlated evolution between figs (Ficus species, Moraceae) and their pollinators (Hymenoptera: Agaoninae) based on molecular phylogenies of both lineages. Fig relationships based on the internal transcribed spacer region (ITS) of nuclear ribosomal DNA and pollinator relationships inferred from mitochondrial cytochrome oxidase I (COI) sequences enabled the study of correlated evolution based on molecular phylogenies for the largest set of interacting species ever compared. Comparative methods have been applied to tests of adaptation, but the application of these methods in tests of coadaptation, defined as reciprocal evolutionary change in interacting lineages, has received less attention. I have extended tests of correlated evolution between two traits along a phylogeny to the case of interacting lineages, where two traits may or may not share a common phylogenetic history. Independent contrasts and phylogenetic autocorrelation rejected the null hypothesis that trait correlations within lineages are stronger than trait correlations between interacting lineages. Fig style lengths and pollinator ovipositor lengths, for example, were more highly correlated than were pollinator body size and ovipositor length. Mutualistic interactions between figs and their pollinators illustrate the novel ways in which phylogenies and comparative methods can detect patterns of correlated evolution. The most outstanding evidence of correlated evolution between these obligate mutualists is that interacting trait correlations are stronger than within-lineage allometric relationships.

Sorenson et al.
Abstract.—The African brood parasitic finches (Vidua spp.) are host specialists that mimic the songs and nestling mouth markings of their finch hosts (family Estrildidae). Although recent molecular analyses suggest rapid speciation associated with host switches in some members of this group, the association of different Vidua lineages with particular host genera suggests the possibility of cospeciation at higher levels in the host and parasite phylogenies. We compared a phylogeny of all Vidua species with a phylogeny of their estrildid finch hosts and compared divergence time estimates for the two groups. Basal divergences among extant members of the Vidulidae and among Vidua species are more recent than those among host genera and species, respectively, allowing a model of cospeciation to be rejected at most or all levels of the Vidua phylogeny. Nonetheless, some tests for cospeciation indicated significant congruence between host and parasite tree topologies. This result may be an artifact of clade-limited colonization. Host switches in parasitic finches have most often involved new hosts in the same or a closely related genus, an effect that increases the apparent congruence of host and parasites trees.

Abstract.—Historically, comparisons of host and parasite phylogenies have concentrated on cospeciation. However, many of these comparisons have demonstrated that the phylogenies of hosts and parasites are seldom completely congruent, suggesting that phenomena other than cospeciation play an important role in the evolution of host-parasite assemblages. Other coevolutionary phenomena, such as host switching, parasite duplication (speciation on the host), sorting (extinction), and failure to speciate can also influence host-parasite assemblages. Using mitochondrial and nuclear protein-coding DNA sequences, I reconstructed the phylogeny of ectoparasitic toucan chewing lice in the Austrophilopterus cancellosus subspecies complex and compared this phylogeny with the phylogeny of the hosts, the Ramphastos toucans, to reconstruct the history of coevolutionary events in this host-parasite assemblage. Three salient findings emerged. First, reconstructions of host and louse phylogenies indicate that they do not branch in parallel, and their cophylogenetic history shows little or no significant cospeciation. Second, members of monophyletic Austrophilopterus toucan louse lineages are not necessarily restricted to monophyletic host lineages. Often, closely related lice are found on more distantly related but sympatric toucan hosts. Third, the geographic distribution of the hosts apparently plays a role in the speciation of these lice. These results suggest that for some louse lineages biogeography may be more important than host associations in structuring louse populations and species, particularly when host life history (e.g., hole nesting) or parasite life history (e.g., phoresis) might promote frequent host switching events between syntopic host species. These findings highlight the importance of integrating biogeographic information into cophylogenetic studies.

Clayton et al.
Abstract.—Phylogenetic congruence is governed by various macroevolutionary events, including cospeciation, host switching, sorting, duplication, and failure to speciate. The relative frequency of these events may be influenced by factors that govern the distribution and abundance of the interacting groups; i.e., ecological factors. If so, it may be possible to predict the degree of phylogenetic congruence between two groups from information about their ecology. Unfortunately, adequate comparative ecological data are not available for many of the systems that have been subjected to cophylogenetic analysis. An exception is provided by chewing lice (Insecta: Phthiraptera), which parasitize birds and mammals. For a few genera of these lice, enough data have now been published to begin exploring the relationship between ecology and congruence. In general, there is a correspondence between important ecological factors and the degree of phylogenetic congruence. Careful comparison of these genera suggests that dispersal is a more fundamental barrier to host switching among related hosts than is establishment. Transfer experiments show that host-specific lice can survive and reproduce on novel hosts that are similar in size to the native host as long as the lice can disperse to these hosts. To date, studies of parasite dispersal have been mainly inferential. A better understanding of the role of dispersal will require more direct data on dispersal frequency and distances.