THE EVOLUTION OF HOST-PLANT USE AND SEQUESTRATION IN THE LEAF BEETLE GENUS PHRATORA (COLEOPTERA: CHRYSOMELIDAE)

Leaf beetles in the genus Phratora differ in host plant use and in the chemical composition of their larval defensive secretion. Most species specialize on either poplars or willows (family Salicaceae), but two species feed on birch (family Betulaceae). Phratora vitellinae utilizes salicylates from the host plant to produce its larval secretion, which contains salicylaldehyde, while other Phratora species produce an autogenous secretion. To reconstruct the evolutionary history of host plant use and the larval secretion chemistry in this genus, we sequenced 1383 base pairs of the mt cytochrome oxidase I gene for six European and one North American Phratora species and three outgroup taxa. Bootstrap values of the complete nucleotide sequence were 99-100% for six of eight nodes in the maximum parsimony tree. They were 71% and 77% for the two other nodes. The maximum parsimony tree and the maximum likelihood tree based on nucleotide sequence showed the same relationships as a maximum parsimony tree based on the amino acid sequence. Beetle phylogeny overlapped broadly with host plant taxonomy and chemistry, and it revealed historical constraints influencing host plant use. However, there was one host shift from the willow family (Salicaceae) to the birch family (Betulaceae). The use of host plant phenol glycosides for the larval defensive secretion evolved along the lineage that led to P. vitellinae. Phratora vitellinae feeds on the taxonomically widest range of host plants, which are characterized by moderate to high levels of salicylates. The results support the hypothesis that the use of salicylates for the larval secretion evolved twice independently in chrysomeline leaf beetles.     

GENETIC CONSTRAINTS ON MACROEVOLUTION: THE EVOLUTION OF HOST AFFILIATION IN THE LEAF BEETLE GENUS OPHRAELLA

We hypothesize that the evolution of an ecologically important character, the host associations of specialized phytophagous insects, has been influenced by limitations on genetic variation. Using as a historical framework a phylogenetic reconstruction of the history of host associations in the beetle genus Ophraella (Chrysomelidae), we have employed quantitative-genetic methods to screen four species for genetic variation in larval survival, oviposition (in one species only), and feeding responses to their congeners' host plants, in the Asteraceae. We here report results of studies of one species and evaluate the results from all four. Analysis of half-sib/full-sib families and of progenies of wild females of O. notulata, a specialist on Iva (Ambrosiinae), provided evidence of genetic variation in larval consumption of five of six test plants and in adult consumption of four of six. Larval mortality was complete on five plants; only on Ambrosia, a close relative of the natural host, was there appreciable, and genetically variable, survival. Oviposition on Ambrosia showed marginally significant evidence of genetic variation; a more distantly related plant elicited no oviposition at all. In compiling results from four Ophraella species, reported in this and two other papers, we found no evidence of genetic variation in 18 of 39 tests of feeding responses and 14 of 16 tests of larval survival on congeners' hosts. This result is consistent with the hypothesis that absence or paucity of genetic variation may constrain or at least bias the evolution of host associations. The lower incidence of genetic variation in survival than in feeding behavior may imply, according to recent models, that avoidance is a more common evolutionary response to novel plants than adaptation. The usually great disparity between mean performance on congeners' hosts and the species' natural hosts, and an almost complete lack of evidence for negative genetic correlations, argue against the likelihood that speciation has occurred by sympatric host shift. The presence versus apparent absence of genetic variation in consumption was correlated with the propinquity of relationship between the beetle species tested and the species that normally feeds on the test plant, suggesting that the history of host shifts in Ophraella has been guided in part by restrictions on genetic variation. It was also correlated with the propinquity of relationship between a test plant and the beetle's natural host. The contributions of plant relationships and insect relationships, themselves correlated in part, to the pattern of genetic variation, are not readily distinguishable, but together accord with phylogenetic evidence that these and other phytophagous insects adapt most readily to related plants. In this instance, therefore, the macroevolution of an ecologically important character appears to have been influenced by genetic constraints. We hypothesize that absence of the structural prerequisites for genetic variation in complex characters may affect genetic variation and the trajectory of evolution.     

PHYLOGENY AND THE EVOLUTION OF HOST PLANT ASSOCIATIONS IN THE LEAF BEETLE GENUS OPHRAELLA (COLEOPTERA,CHRYSOMELIDAE)

Species of Ophraella, a North American genus of leaf beetles (Chrysomelidae), feed variously on eight genera in four tribes of Asteraceae. A phylogenetic analysis, based on morphological features and allozymes, was undertaken to deduce the history of host affiliation within the genus. The two data sets are combined to arrive at a provisional phylogeny of the species, onto which host associations are parsimoniously mapped. Among and within the 12 species studied, at least two shifts are postulated to have occurred among congeneric plant species, five between genera in the same tribe, and four between different tribes of Asteraceae. The phylogeny of Ophraella appears not to be congruent with that of its hosts. This and other evidence indicates that many host shifts in Ophraella postdate the divergence of the host plants, a conclusion that may apply commonly to phytophagous insects. A phenetic analysis of the plants' secondary compounds provides modest support for the hypothesis that host shifts are facilitated by commonalities in plant chemistry. A possible trend in host shifts is evident, from chemically simpler to chemically more forbidding plants. The chemical barriers to host shifts in Ophraella appear to require adaptation in both behavior and in physiological attributes. There is no evidence that the host associations of these insects or the divergence in secondary chemistry of their hosts can be attributed to coevolution.     

Genetic variation in a phylogenetic context: Responses of two specialized leaf beetles (Coleoptera: Chrysomelidae) to host plants of their congeners

This work explores the possibility that constraints on genetic variation guide host shifts and are responsible for the evolutionary conservatism of host affiliation in phytophagous insects. To this end, we used full- and half-sib breeding designs to screen two species of the North American bettle genus Ophraella for genetic variation in larval and adult feeding responses to several host plants of other species of Ophraella. All the plants are in the family Asteraceae. In O. conferta, we observed effectively no feeding response, and hence no genetic variation in response, to three of five test plant species; only those plants related to the species' natural hosts evoked genetically variable responses. In O. artemisiae, adults displayed genetic variation in response to a congener of the natural host, but not to two distantly related plants. However, significant variation among full-sib broods in larval feeding suggests the existence of nonadditive genetic variance in feeding response to all five species of test plants—although survival was very low on most of them. The results suggest that patterns of presence versus apparent absence of detectable genetic variation may be related to the chemical similarity of plants to the insects' natural hosts, but not evidently to the phylogenetic history of host affiliation within the genus. Almost all genetic correlations in responses to host plants were not significantly different from zero; the few significant correlations were positive, and negative correlations that might explain host specificity were not found. Our data do not explain why exclusive shifts to new hosts should occur, but the apparent lack of genetic variation in responses to some plants suggests that the direction of host shifts is genetically constrained.     

MITOCHONDRIAL DNA SEQUENCE-BASED PHYLOGENY AND THE EVOLUTION OF VIVIPARITY IN THE SCELOPORUS SCALARIS GROUP (REPTILIA,SQUAMATA)

The lizard genus Sceloporus contains both oviparous and viviparous species. The scalaris complex is the only monophyletic group within the genus that includes both reproductive modes, thus it is particularly well suited for studies of the evolution of viviparity. Approximately 874 nucleotides of mtDNA sequence data, collected from 38 specimens, comprising 25 populations of all five recognized species within the group, were used in a phylogenetic analysis of the origin of viviparity. Viviparity appears to have evolved twice in this group: once in S. goldmani, included in a clade formed by a northern group consisting of S. scalaris, S. chaneyi, and S. goldmani, and one more time in S. bicanthalis, included in the southern group formed by S. bicanthalis and S. aeneus. An oviparous population of S. bicanthalis nested within that viviparous clade, indicates that reversal from viviparity to oviparity may be possible. Degree of sequence divergence among several S. bicanthalis individuals pertaining to a population in which both parity modes occur, was no larger between oviparous and viviparous lizards than among viviparous lizards. This suggests that this population is a single species, and it may represent a transition from oviparity to viviparity or vice-versa.     

Phylogenetic evidence for the evolution of ecological specialization in Timema walking‐sticks

We used phylogenetic and ecological information to study the evolution of host‐plant specialization and colour polymorphism in the genus Timema, which comprises 14 species of walking‐sticks that are subject to strong selection for cryptic coloration on their host‐plants. Phylogenetic analysis indicated that this genus consists of three main lineages. Two of the lineages include highly generalized basal species and relatively specialized distal species, and one of the lineages comprises four specialized species. We tested for phylogenetic conservatism in the traits studied via randomizing host‐plant use, and the four basic Timema colour patterns, across the tips of the phylogeny, and determining if the observed number of inferred changes was significantly low compared to the distribution of numbers of inferred changes expected under the null model. This analysis showed that (1) host‐plant use has evolved nonrandomly, such that more closely related species tend to use similar sets of hosts and (2) colour pattern evolution exhibits considerable lability. Inference of ancestral states using maximum parsimony, under four models for the relative ease of gain and loss of plant hosts or colour morphs, showed that (1) for all models with gains of host‐plants even marginally more difficult than losses, and for most optimizations with gains and losses equally difficult, the ancestral Timema were generalized, feeding on the chaparral plants Ceanothus and Adenostoma and possibly other taxa, and (2) for all models with gains of colour morphs more difficult than losses, the ancestral Timema were polymorphic for colour pattern. Generation of null distributions of inferred ancestral states showed that the maximum‐parsimony inference of host‐plant generalization was most robust for the most speciose of the three main Timema lineages. Ancestral states were also inferred using maximum likelihood, after recoding host‐plant use and colour polymorphism as dichotomous characters. Likelihood analyses provided some support for inference of generalization in host‐plant use at ancestral nodes of the two lineages exhibiting mixtures of generalists and specialists, although levels of uncertainty were high. By contrast, likelihood analysis did not estimate ancestral colour morph patterns with any confidence, due to inferred rates of change that were high with respect to speciation rates. Information from biogeography, floristic history and the timing of diversification of the genus are compatible with patterns of inferred ancestral host‐plant use. Diversification in the genus Timema appears to engender three main processes: (1) increased specialization via loss of host‐plants, (2) retention of the same, single, host‐plant and (3) shifts to novel hosts to which lineages were ‘preadapted’ in colour pattern. Our evidence suggests that the radiation of this genus has involved multiple evolutionary transitions from individual‐level specialization (multiple‐niche polymorphism) to population‐level and species‐level specialization. Ecological studies of Timema suggest that such transitions are driven by diversifying selection for crypsis. This paper provides the first phylogeny‐based evidence for the macroevolutionary importance of predation by generalist natural enemies in the evolution of specialization.     

A comparison between allozyme data and phenotypic distances from defensive secretion in Oreina leaf-beetles (Chrysomelinae)

The genetic relationships between five Oreina species (Chrysomelidae, Coleoptera) were studied. Of these species, four (O. bifrons, O. gloriosa, O. speciosa, O. variabilis) feed on Apiaceae and secrete mixtures of autogenous cardenolides from defensive glands, whilst the other (O. speciosissima) feeds on Asteraceae and is able both to produce cardenolides and to sequester pyrrolizidines N-oxides (PAs). A dendrogram based on the different mixtures of cardenolides produced by the different species agreed with these genetic relationships. In other words, cardenolide mixtures are good taxonomic markers, since the clustering method based on chemical defense produces a branching pattern similar to that based on genetic relationships.     

PHYLOGENY OF THE TRIBE CERATAPHIDINI (HOMOPTERA) AND THE EVOLUTION OF THE HORNED SOLDIER APHIDS

The horned soldier aphids of the Cerataphidini, unlike most social insects that reside in nests, live on the open surface of plants. The lack of a nest and other obvious ecological correlates makes it unclear why secondary-host soldiers might have evolved. Here I present a molecular phylogenetic analysis of 32 species of the Cerataphidini, including 10 species from the genera Ceratovacuna and Pseudoregma that produce horned soldiers. The phylogeny suggests that horned soldiers evolved once and were lost once or twice. Most horned soldiers are a morphologically specialized caste and two species that have unspecialized soldiers are independently derived from species with specialized castes. The genus Ceratovacuna appears to have undergone a relatively rapid radiation. Mapping secondary-host plants and geographic ranges onto the phylogeny suggests that bamboos were the ancestral secondary-host plants and that the Asian tropics and subtropics were the ancestral geographic regions for the genera Astegopteryx, Ceratoglyphina, Ceratovacuna Chaitoregma, and Pseudoregma and possibly for the entire tribe. There is evidence for vicariant events that separate the tropical and subtropical lineages in all of the major lineages of the tribe and for dispersal of some lineages. Based on these results, I present hypotheses for the causes and consequences of horned-soldier evolution.     

A genome-wide phylogeny and the diversification of genus Liriomyza (Diptera: Agromyzidae) inferred from anchored phylogenomics

The genus Liriomyza Mik (Diptera: Agromyzidae) is a diverse and globally distributed group of acalyptrate flies. Phylogenetic relationships among Liriomyza species have remained incompletely investigated and have never been fully addressed using molecular data. Here, we reconstruct the phylogeny of the genus Liriomyza using various phylogenetic methods (maximum likelihood, Bayesian inference, and gene tree coalescence) on target-capture-based phylogenomic datasets (nucleotides and amino acids) obtained from anchored hybrid enrichment (AHE). We have recovered tree topologies that are nearly congruent across all data types and methods, and individual clade support is strong across all phylogenetic analyses. Moreover, defined morphological species groups and clades are well-supported in our best estimates of the molecular phylogeny. Liriomyza violivora (Spencer) is a sister group to all remaining sampled Liriomyza species, and the well-known polyphagous vegetable pests [L. huidobrensis (Blanchard), L. langei Frick, L. bryoniae. (Kaltenbach), L. trifolii (Burgess), L. sativae Blanchard, and L. brassicae (Riley)]. belong to multiple clades that are not particularly closely related on the trees. Often, closely related Liriomyza species feed on distantly related host plants. We reject the hypothesis that cophylogenetic processes between Liriomyza species and their host plants drive diversification in this genus. Instead, Liriomyza exhibits a widespread pattern of major host shifts across plant taxa. Our new phylogenetic estimate for Liriomyza species provides considerable new information on the evolution of host-use patterns in this genus. In addition, it provides a framework for further study of the morphology, ecology, and diversification of these important flies.     

Angiosperm to Gymnosperm host‐plant switch entails shifts in microbiota of the Welwitschia bug,Probergrothius angolensis (Distant, 1902)

The adaptation of herbivorous insects to new host plants is key to their evolutionary success in diverse environments. Many insects are associated with mutualistic gut bacteria that contribute to the host's nutrition and can thereby facilitate dietary switching in polyphagous insects. However, how gut microbial communities differ between populations of the same species that feed on different host plants remains poorly understood. Most species of Pyrrhocoridae (Hemiptera: Heteroptera) are specialist seed‐feeders on plants in the family Malvaceae, although populations of one species, Probergrothius angolensis, have switched to the very distantly related Welwitschia mirabilis plant in the Namib Desert. We first compared the development and survival of laboratory populations of Pr. angolensis with two other pyrrhocorids on seeds of Welwitschia and found only Pr. angolensis was capable of successfully completing its development. We then collected Pr. angolensis in Namibia from Malvaceae and Welwitschia host plants, respectively, to assess their bacterial and fungal community profiles using high‐throughput amplicon sequencing. Comparison with long‐term laboratory‐reared insects indicated stable associations of Pr. angolensis with core bacteria (Commensalibacter, Enterococcus, Bartonella and Klebsiella), but not with fungi or yeasts. Phylogenetic analyses of core bacteria revealed relationships to other insect‐associated bacteria, but also found new taxa indicating potential host‐specialized nutritional roles. Importantly, the microbial community profiles of bugs feeding on Welwitschia versus Malvaceae revealed stark and consistent differences in the relative abundance of core bacterial taxa that correlate with the host‐plant switch; we were able to reproduce this result through feeding experiments. Thus, a dynamic gut microbiota may provide a means for insect adaptation to new host plants in new environments when food plants are extremely divergent.     

Does ant predation favour leaf beetle specialization on toxic host plants?

Generalist predators are frequently seen as evolutionary forces that narrow the host range in herbivorous insects. Predators may favour specialization of herbivores on host plants containing toxic chemicals (which can be used by herbivores for their own defence) if host plant‐derived defences provide better protection from enemies than do autogenously produced defences. We compared the effectiveness of these two defensive strategies in the larvae of six species of leaf beetle (Chrysomelidae) against wood ants (Formica rufa group) in field experiments. Ants were more strongly repelled by larvae with host plant‐derived, salicylaldehyde‐containing secretions than by larvae with various autogenous secretions, but collectively foraging ants ultimately overcame any type of chemical defence by social interactions, chemical signalling, and olfactory learning. As a result, ants killed all larvae of Chrysomela lapponica defended by salicylaldehyde‐containing secretions within 2 days of their introduction to willows within 15 m of ant nests. We conclude that in the field neither type of chemical defence provides complete protection against wood ants in the vicinity of their nests, and that evolutionary shifts from autogenous production of secretion to sequestration of plant allelochemicals in leaf beetles may be favoured mostly at low ant densities on the periphery of ant foraging areas.     

Coping with an antagonist: the impact of a phytopathogenic fungus on the development and behaviour of two species of alpine leaf beetle

Herbivorous insects and phytopathogenic fungi often share their host plants. This creates a network of direct and indirect interactions, with far‐reaching consequences for the ecology and evolution of all three parties. In the Alps, the leaf beetles Oreina elongata and Oreina cacaliae (Coleoptera: Chrysomelidae), and the rust fungus Uromyces cacaliae (Uredinales: Pucciniaceae) are found on the same host plant, Adenostyles alliariae (Asterales: Asteraceae). We compare the impact of rust infection on these two closely‐related beetle species, one of which, O. cacaliae, is a specialist on A. alliariae, while the other, O. elongata, moves repeatedly between Adenostyles and an alternative host, Cirsium spinosissimum. Larval performance, feeding preference, oviposition choice and dispersal behaviour were studied in field and laboratory experiments. When reared on rust‐infected leaves, larvae of both beetle species had lower growth rates, lower maximum weights and longer development times. Larvae and adults discriminated among diets in feeding trials, showing a preference for discs cut from healthy leaves over those bearing a patch of sporulating rust, those from elsewhere on an infected leaf, and those from an upper leaf on an infected plant. Females of the two species differed in behaviour: in O. cacaliae they favoured healthy leaves for larviposition, while in O. elongata they showed no significant preference during oviposition. In the field, larvae and adults of both species dispersed more rapidly when placed on infected host plants. The results demonstrate that rust infection reduces the quality of the plant as a host for both Oreina species, and they combine the ability to detect systemic infection with the evolution of evasive behaviours. For these beetles, competition with a rust clearly increases the difficulty of survival in the harsh conditions of alpine environments, and may have a profound impact on the evolution of their life history traits and host plant use.     

GENETIC CONSTRAINTS AND THE PHYLOGENY OF INSECT-PLANT ASSOCIATIONS: RESPONSES OF OPHRAELLA COMMUNA (COLEOPTERA: CHRYSOMELIDAE) TO HOST PLANTS OF ITS CONGENERS

We ask whether patterns of genetic variation in a phytophagous insect's responses to potential host plants shed light on the phylogenetic history of host association. Ophraella communa feeds chiefly, and in eastern North America exclusively, on Ambrosia (Asteraceae: Ambrosiinae). Using mostly half-sib breeding designs, we screened for genetic variation in feeding responses to and larval survival on its own host and on seven other plants that are hosts (or, on one case, closely related to the host) of other species of Ophraella. We found evidence for genetic variation in feeding responses to five of the seven test plants, other than the natural host. We found no evidence of genetic variation in feeding responses to two plant species, nor in capacity for larval survival on six. These results imply constraints on the availability of genetic variation; however, little evidence for constraints in the form of negative genetic correlations was found. These results are interpreted in the context of a provisional phylogeny of, and a history of host shifts within, the genus. Ophraella communa does not present evidence of genetic variation in its ability to feed and/or survive on Solidago, even though it is probably descended from a lineage that fed on Solidago or related plants, possibly as recently as 1.9 million years ago. Genetic variation in performance on this plant may have been lost. Based on evidence for genetic variation and on mean performance, by far the greatest potentiality for adaptation to a congener's host was evinced in responses to Iva frutescens, which not only is related and chemically similar to Ambrosia, but also is the host of a closely related species of Ophraella that may have been derived from an Ambrosia-associated ancestor. Genetic variation in O. communa's capacity to feed and/or survive on its congeners' hosts is less evident for plants that do not represent historically realized host shifts (with one exception) than for those that may (but see Note Added in Proof). The results offer some support for the hypothesis that the evolution of host shifts has been guided in part by constrained genetic variation.     

Evolution and diversity of Arsenophonus endosymbionts in aphids

Endosymbiotic bacteria are important drivers of insect evolutionary ecology, acting both as partners that contribute to host adaptation and as subtle parasites that manipulate host reproduction. Among them, the genus Arsenophonus is emerging as one of the most widespread lineages. Its biology is, however, entirely unknown in most cases, and it is therefore unclear how infections spread through insect populations. Here we examine the incidence and evolutionary history of Arsenophonus in aphid populations from 86 species, characterizing the processes that shape their diversity. We identify aphids as harbouring an important diversity of Arsenophonus strains. Present in 7% of the sampled species, incidence was especially high in the Aphis genus with more than 31% of the infected species. Phylogenetic investigations revealed that these Arseno‐phonus strains do not cluster within an aphid‐specific clade but rather exhibit distinct evolutionary origins showing that they undergo repeated horizontal transfers (HT) between distantly related host species. Their diversity pattern strongly suggests that ecological interactions, such as plant mediation and parasitism, are major drivers for Arsenophonus dispersal, dictating global incidence across insect communities. Notably, plants hosting aphids may be important ecological arenas for global exchange of Arsenophonus, serving as reservoirs for HT.     

Can plant resistance to specialist herbivores be explained by plant chemistry or resource use strategy?

At both a macro- and micro-evolutionary level, selection of and performance on host plants by specialist herbivores are thought to be governed partially by host plant chemistry. Thus far, there is little evidence to suggest that specialists can detect small structural differences in secondary metabolites of their hosts, or that such differences affect host choice or performance of specialists. We tested whether phytochemical differences between closely related plant species are correlated with specialist host choice. We conducted no-choice feeding trials using 17 plant species of three genera of tribe Senecioneae (Jacobaea, Packera, and Senecio; Asteraceae) and a more distantly related species (Cynoglossum officinale; Boraginaceae) containing pyrrolizidine alkaloids (PAs), and four PA-sequestering specialist herbivores of the genus Longitarsus (Chrysomelidae). We also assessed whether variation in feeding by specialist herbivores is attributable to different resource use strategies of the tested plant species. Plant resource use strategy was quantified by measuring leaf dry matter content, which is related to both plant nutritive value and to plant investment in quantitative defences. We found no evidence that intra-generic differences in PA profiles affect feeding by specialist herbivores. Instead, our results indicate that decisions to begin feeding are related to plant resource use strategy, while decisions to continue feeding are not based on any plant characteristics measured in this study. These findings imply that PA composition does not significantly affect host choice by these specialist herbivores. Leaf dry matter content is somewhat phylogenetically conserved, indicating that plants may have difficulty altering resource use strategy in response to selection pressure by herbivores and other environmental factors on an evolutionary time scale.     

THE PLATYCARYA PERPLEX AND THE EVOLUTION OF THE JUGLANDACEAE

We report on the leaves, fruits, inflorescences, and pollen of two fossil species in the genus Platycarya. The association of these dispersed organs has been established by their repeated co-occurrence at a large number of localities, and for two of the organs (fruit and pistillate inflorescence, and pollen and staminate inflorescence) by apparent organic attachment of compression fossils. Each of the two species can be distinguished by characteristics of all the known megafossil organs. We also review the fossil record of dispersed platycaryoid fruits and inflorescences, recognizing three additional species of Platycarya and two of Hooleya. Two of the fossil Platycarya species are morphologically very different from the living Platycarya strobilacea Sieb. et Zucc., but they show the diagnostic features of the genus. Hooleya is a generalized member of the Platycaryeae that is probably close to the ancestry of Platycarya. The two Platycarya species known from multiple organs provide a remarkable example of mosaic evolution in which fertile and foliar structures have attained different levels of morphological specialization. The leaves, often considered the most plastic of plant organs, retain several features that are otherwise seen only in the Engelhardieae. These similarities in leaf architecture between the fossil Platycarya species and Engelhardieae are advanced features for the Juglandaceae, and thus indicate a sister-group relationship between the two lines. In contrast to the leaves, the fruits, inflorescences, and pollen of the fossil Platycarya species are almost as specialized as those of the extant P. strobilacea and bear little resemblance to the same structures in other genera of the family. The morphology, taphonomy, sedimentary setting, and geographic and stratigraphic distribution of three of the fossil platycaryoid species suggest that they were wind-dispersed, early successional plants that grew in thickets. This habit is retained by Platycarya strobilacea and is typical of many of the amentiferae (e.g. Myricaceae, Betulaceae). The r-selected life-history pattern of the Platycarya line may well have contributed to its low diversity through geologic time.     

Evolution of ant-cultivar specialization and cultivar switching in Apterostigma fungus-growing ants

Almost all of the more than 200 species of fungus-growing ants (Formicidae: Attini) cultivate litter-decomposing fungi in the family Lepiotaceae (Basidiomycota: Agaricales). The single exception to this rule is a subgroup of ant species within the lower attine genus Apterostigma, which cultivate pterulaceous fungi distantly related to the Lepiotaceae. Comparison of cultivar and ant phylogenies suggests that a switch from lepiotaceous to pterulaceous fungiculture occurred only once in the history of the fungus-growing ants. This unique switch occurred after the origin of the genus Apterostigma, such that the basal Apterostigma lineages retained the ancestral attine condition of lepiotaceous fungiculture, and none of the Apterostigma lineages in the monophyletic group of pterulaceous fungiculturists are known to have reverted back to lepiotaceous fungiculture. The origin of pterulaceous fungiculture in attine ants may have involved a unique transition from the ancestral cultivation of litter-decomposing lepiotaceous fungi to the cultivation of wood-decomposing pterulaceous fungi. Phylogenetic analyses further indicate that distantly related Apterostigma ant species sometimes cultivate the same cultivar lineage, indicating evolutionarily frequent, and possibly ongoing, exchanges of fungal cultivars between Apterostigma ant species. The pterulaceous cultivars form two sister clades, and different Apterostigma ant lineages are invariably associated with, and thus specialized on, only one of the two cultivar clades. However, within clades Apterostigma ant species are able to switch between fungi. This pattern of broad specialization by attine ants on defined cultivar clades, coupled with flexible switching between fungi within cultivar clades, is also found in other attine lineages and appears to be a general phenomenon of fungicultural evolution in all fungus-growing ants.     

Mitochondrial DNA Phylogeny and the Evolution of Host-Plant Use in Palearctic Chrysolina (Coleoptera, Chrysomelidae) Leaf Beetles

The genus Chrysolina consists of specialized phytophagous leaf-beetles (Coleoptera, Chrysomelidae) with feed on several plant families. There is no explicit phylogenetic hypothesis available for this genus, which includes 65 subgenera and more than 400 species with a wide distribution. We obtained 839-bp sequence data from the 16S rDNA and cytochrome oxidase subunit I (COI) mitochondrial genes. Thirty Chrysolina taxa representing eight host–plant affiliations, two species of the closely related genus Oreina, and two outgroups were sampled. These data sets were used separately and combined to obtain the mitochondrial cladogram of the group using maximum-parsimony and maximum-likelihood criteria. The results were compared to current proposals for Chrysolina systematics that are based on morphological, ecological, and karyological data. The trees obtained were in the most part congruent with the proposed ancestral association of Chrysolina to Lamiaceae based on chromosome number in several lineages. A minimum of five host-plant switches from the ancestral state inferred at the family level and two at the subclass level suggests the absence of parallel evolution of beetles and their host plants. Another switch leading to oligophagy at the family level was deduced to have occurred in the lineage of the subgenus Chrysolina s.str. Received: 22 May 1998 / Accepted: 16 September 1998     

Phylogenetic test of speciation by host shift in leaf cone moths (Caloptilia) feeding on maples (Acer)

The traditional explanation for the exceptional diversity of herbivorous insects emphasizes host shift as the major driver of speciation. However, phylogenetic studies have often demonstrated widespread host plant conservatism by insect herbivores, calling into question the prevalence of speciation by host shift to distantly related plants. A limitation of previous phylogenetic studies is that host plants were defined at the family or genus level; thus, it was unclear whether host shifts predominate at a finer taxonomic scale. The lack of a statistical approach to test the hypothesis of host‐shift‐driven speciation also hindered studies at the species level. Here, we analyze the radiation of leaf cone moths (Caloptilia) associated with maples (Acer) using a newly developed, phylogeny‐based method that tests the role of host shift in speciation. This method has the advantage of not requiring complete taxon sampling from an entire radiation. Based on 254 host plant records for 14 Caloptilia species collected at 73 sites in Japan, we show that major dietary changes are more concentrated toward the root of the phylogeny, with host shift playing a minor role in recent speciation. We suggest that there may be other roles for host shift in promoting herbivorous insect diversification rather than facilitating speciation per se.     

Convergent Evolution of Viviparity, Matrotrophy, and Specializations for Fetal Nutrition in Reptiles and Other Vertebrates

Quantitative analyses based upon the superimposition of phylogeneticand reproductive data have revealed that viviparity has originatedon at least 132 independent occasions among vertebrates, with98 of these origins having occurred among reptiles. The viviparouslineages have given rise to at least 24 matrotrophic clades,all but four of which are anamniotes. Traditional scenariosassume progressive, gradualistic evolution from oviparity tolecithotrophic viviparity to matrotrophic viviparity. However,mammalian evidence indicates that matrotrophy can precede theevolution of viviparity. Moreover, data on reptiles seem tobe consistent with a punctuated equilibrium model for viviparityand a saltatory model for incipient matrotrophy and placentation. Among the specializations for fetal nutrition, strong convergenceis evident at organismal, organological, and cytological levels.Examples include yolk sac placentation, trophotaeniae, and adaptationsfor embryonic cannibalism. Certain lizards of the genera Mabuyaand Chalcides have converged strongly on eutherian mammals withrespect to morphology of the chorioallantoic placenta. Placentalspecializations that have evolved independently in some eutheriansand matrotrophic lizards include placentomes, giant binucleatecells, deciduate maternal tissue, and chorionic areolae.