Zusammenfassung Die Analyse von Artbildungsprozessen erfordert eingehende biologische, ökologische und populationsgenetische Kenntnisse der betreffenden Tiergruppen. Von besonderer Bedeutung ist die jeweilige ‘Nischenstruktur’. Bei bestimmten Tiergruppen, z. B. Parasitoiden oder spezialisierten phytophagen Organismen, sind zahlreiche Nischenelemente (Paarungsplatz, Eiablageplatz, larvale Nahrung, larvale Mortalitätsfaktoren usw.) gemeinsam auf einen Wirt oder eine Wirtspflanze lokalisiert. Solche Arten mit aggregierten Nischenelementen können durch einen Wechsel des Wirts gleichzeitig den Genfluß neu kanalisieren und zahlreiche ökologische Parameter abändern, so daß eine rasche Neu-Einnischung möglich ist. Es wird ein Überblick über Speziationsmodelle und ihre Klassifikation gegeben. Entscheidend für die einzelnen Modelle ist, ob rein exogene, gemischt exogen-endogene oder rein bzw. vorwiegend endogene Barrieren den Genfluß einschränken bzw. unterbinden. Es folgt eine Erörterung parapatrischer und sympatrischer Speziationsmodelle. Dabei wird auf Speziatiogsprozesse durch gleichzeitigen Wechsel von Habitat- und Nahrungsnische besonders eingegangen. Als Indiz für solche Prozesse wird die sympatrische ökologische Divergenz bei Wirtsrassen geschildert. Eine Voraussetzung für die Wirtsrassenbildung ist die bei Arthropoden weit verbreitete Kopplung der Wahl des Paarungs- und Eiablageplatzes (wirtsspezifisches ‘Rendezvous-Verhalten’). Bei Schmetterlingen und Rüsselkäfern bestehen Beziehungen zwischen dem Vorkommen dieses Verhaltens und der Speziationsrate. Die auf dem Rendezvous-Verhalten aufgebauten Modelle einer sympatrischen wirtsbezogenen Speziation von Bush (1975 b) und Bush und Diehl (1982) werden erläutert. Das vorliegende Beobachtungsmaterial erlaubt den Schluß, daß neben geographischen Artbildung und bestimmten Voraussetzungen auch nicht-geographische Speziationsprozesse bei Tieren möglich sind. Wichtiger als die Betonung von geographischen und nicht-geographischen Barrieren erscheint der Umstand, daß einzelne Elemente des Speziationsprozesses kombiniert auftreten können, daß sie mit sehr unterschiedlicher Geschwindigkeit ablaufen können und daß sie entscheidend von biologischen, ökologischen und genetischen Eigenarten der jeweiligen Tiergruppen mitgeprägt werden. Summary Sympatric and parapatric speciation The analysis of speciation processes requires a detailed knowledge of biological, ecological and genetic facts. The niche structure of an animal species is of particular importance. In parasitoids and many specialized phytophagous organisms niche elements such as the mating site, oviposition site, larval food resources and density-controlling factors are coupled and localized at a specific host or host plant. In such cases a transfer to a new host may simultaneously channel the gene flow and completely change the food and habitat niche. The classification of speciation processes as proposed by different authors is discussed. The type of the barriers to gene flow (exclusively extrinsic, extrinsic and intrinsic or predominately intrinsic) characterize the single speciation models. We then describe models of parapatric and sympatric speciation. Emphasis is given to those processes which are due to a simultaneous shift of the habitat and food niche. We discuss host races as a result of a sympatric ecological divergence. A prerequisite for the formation of host races is the coupling of host selection and selection of sites for courthship and mating. Examples are given to demonstrate the wide distribution of this phenomenon among arthropods. Among the Lepidoptera and Curculionidae there exist relationships between speciation rates and the occurrence or non-occurrence of host-specific mating places. The models of sympatric speciation by Bush (1975 b) and Bush and Diehl (1982) are based on this positively assortative mating behaviour. The available data allow the conclusion that in addition to geographic speciation processes of non-geographic speciation are possible in certain groups of animals. However, more important than the contrast originally implied by the terms ‘allopatric’ and ‘sympatric’ appears the possibility of different combinations of the single elements of a speciation process, the different speed of speciation processes, and the influence of biological, ecological and genetic characters on the speciation process.

Can. Ent. 104: 97-104 (1972) Comparatively few laboratory-caged apple maggot flies, Rhagoletis pomonella (Walsh), of either sex arrived at the site of assembly for mating (ceresin wax oviposition domes) and no mating pair or ovipositing females were observed until the flies were 7-8 days old. From then on, the level of all three of these activities progressively increased. With respect to the time of day of activity, in the field, assembly of both sexes of flies on the fruit, mating, and oviposition all occurred predominantly during the afternoon on a day when the sun shone brightly throughout the 15-hr daylight period and the ambient temperature was 16°C at dawn, 24.8" in midafternoon, and 17.5" at dusk. Little or no activities of these sorts occurred before mid-morning and toward dusk. In the laboratory, where light intensity was constant at 1000 lux during the 15-hr photophase, the temporal pattern of these activities was the same as in the field when laboratory temperature was programmed to be the same as in the field for the corresponding hour of daylight. However, when laboratory temperature was programmed to be constant a t 24.8", these activities occurred at a high rate throughout the photophase. None of these activities was observed to have been initiated in darkness. It is concluded that sexually mature apple maggot flies may assemble on the fruit, mate, and oviposit whenever temperature is favorable and light intensity is sufficient to permit adequate vision.

In the Great Lakes region, Rhagoletis zephyria Snow (Diptera: Tephritidae) infests snowberries, Symphoricarpos albus variety laevigatus (Fern.) Blake, a western North American native plant that has been introduced widely into eastern North America. These R. zephyria infestations have been hypothesized to be the result of flies that were introduced into eastern North America along with their host plants. In its native range, R. zephyria infests S. albus variety laevigatus, but it infests a related host, Symphoricarpos occidentalis Hook., in the northern Great Plains. Knowledge of the natural geographic and host ranges of R. zephyria is important. R. zephyria is a sibling species of the apple maggot, Rhagoletis pomonella (Walsh), and aspects of R. zephyria biology can be applied to work on speciation mechanisms. Additionally, the possible co-occurrence of morphologically similar R. zephyria and R. pomonella in apple (Malus spp.)-growing regions complicates the positive identification of trap-caught flies in the R. pomonella species group. Here, we examine the hypothesis that R. zephyria in the Great Lakes region has been introduced. Collections of R. zephyria yielded new state/provincial records in Idaho, South Dakota, Nebraska, Wisconsin, Michigan, Pennsylvania, New York, Massachusetts, Maine, and Ontario and Quebec, Canada, and a new host record, S. albus variety albus, which is native to eastern North America. Finding R. zephyria infesting native host plants in the eastern United States leads us to hypothesize that R. zephyria infestations in the Great Lakes region may not be the result of recent (historical) introductions of R. zephyria, but rather they may represent native R. zephyria populations.

The phylogenetic relationships of 43 species in the subtribe Carpomyina (39 Rhagoletis spp., plus Carpomya schineri (Loew), Oedicarena latifrons (Wulp), Rhagoletotrypeta pastranai Aczel, and Zonosemata electa (Say)) are examined using morphological and mitochondrial DNA (mtDNA) characters. The taxon sample includes 5 Palearctic Rhagoletis species (R. almatensis Rohdendorf, R. batava Hering, R. flavicincta (Loew), R. flavigenualis Hering, and R. magniterebra (Rohdendorf)) and 5 Neotropical Rhagoletis species (R. blanchardi Aczel, R. ferruginea Hendel, R. lycopersella Smyth, R. nova (Schiner), and R. psalida Hendel) whose mtDNA relationships have not been previously analyzed. Phylogenetic analysis of 77 morphological features using unweighted parsimony yielded 28,671 most parsimonious reconstructions (MPRs). A strict consensus of these MPRs contained 12 clades, and further analysis using successive approximations improved phylogenetic resolution.

Insect Diversity, Body Size, Specialization, and Speciation Insects are among the most abundant multicellular organisms on earth. Sampling in the tropics and subtropics suggests that there are 10 to 30 million species of insects (Erwin 1982). As a group, invertebrate insects, therefore, represent an inordinately large percentage of the world's fauna, far outnumbering their larger and more conspicuous vertebrate cousins, such as fish (about 24000), amphibians (about 4000), reptiles (7907), birds (9808), and mammals (4629). In fact, the number of species in many families of Heteroptera, Diptera, Coleoptera, and Hymenoptera outnumber the species in most classes of vertebrates. Why are there so many insect species? One apparent reason is body size (Bush 1993). Their small size allows insects to subdivide habitats and specialize on resources that large animals are unable to exploit. Frequently, much of the life cycle occurs exclusively on the resource, which is particularly important since it induces assortative mating. Specialization permits a habitat occupied by a single vertebrate species to support a much greater number of small, often closely related, invertebrate species that can coexist in close sympatry with minimal competition. Vertebrate sister species usually have similar ecological needs, and are seldom sympatric because they competitively exclude one another. In contrast, it is estimated that the majority of insects, over 70% of which either feed on plants or are parasites and parasitoids, are highly host specific (Jaenike 1990). Sister species may sometimes even feed on different parts of the same host.

Here we draw on phylogenies of figs and fig wasps to suggest how modes of speciation may be affected by interspecific interactions. Mutualists appear to have cospeciated with their hosts to a greater extent than parasites, which showed evidence of host shifting. However, we also repeatedly encountered a pattern not explained by either cospeciation or host switching. Sister species of fig parasites often attack the same host in sympatry, and differences in ovipositor length suggest that parasite speciation could result from divergence in the timing of oviposition with respect to fig development. These observations on fig parasites are consistent with a neglected model of sympatric speciation.

True fruit flies belonging to the Rhagoletis pomonella (Diptera:Tephritidae) sibling species complex possess several attributes consistent with a history of sympatric divergence via host plant shifts. Hen, we investigate whether hybridization and genetic introgression is occurring between two members of the group, Rhagoletis pomonella (Walsh), whose primary hosts are domestic apples (Malus pumila) and hawthorns (Crataegus spp., and R, zephyria (Snow) whose host is snowberries (Symphoricarpos spp.). These flies are important because they appear to be at a transition stage between taxa reproductively isolated solely on the basis of host plant-related adaptations and those that have evolved additional non-host dependent sterility and inviability. Observing extensive hybridization and introgression between R. pomonella and R. zephyria would have major repercussions for current models of sympatric speciation.In a survey of allozyme and mitochondrial DNA (mtDNA) variation for 1105 flies collected throughout the northern United States we found two results suggesting that low level hybridization occurs between R. pomonella and R. zephyria. (1) Two flies reared from snowberries and one fly reared from hawthoms had genotypes indicative of them being R, pomonella and R. zephyria, respectively. Rhagoletis pomonella and R. zephyria adults map therefore occasionally frequent each others host plant, providing the opportunity for hybridization, (2) Four flies collected from hawthorns and one from snowberries had genotypes that made them likely to be F1 hybrids.Likelihood analysis revealed the data were also consistent with an hypothesis of shared ancestral alleles (i,e., the pattern of genetic variation could also be explained by R. pomonella and R, zephyria sharing alleles/haplotypes whose origins date to a common ancestor). We estimated that, in the absence of interspecific mating, random assortment of genes within R. pomonella and R. zephyria populations would produce an average of 5.4 flies with genotypes suggesting they were F1 hybrids - a number equivalent to the 5 putative F1 hybrids observed in the study. Our results therefore underscore the difficulty in distinguishing between hypotheses of low level introgression and shared ancestral polymorphism. But even if hybridization is occurring, the data suggest that it is happening at a very low and probably evolutionarily insignificant level (perhaps 0.09% per generation), consistent with sympatric speciation theory, Future tests are discussed that could help resolve the hybridization issue for R, pomonella and R. zephyria.

Domestic apple (Malus pumila)- and hawthorn (Crataegus sp.)-infesting races of Rhagoletis pomonella, Walsh (Diptera: Tephritidae) provide an excellent model to examine the role that host plant specificity plays during sympatric speciation (i.e., divergence in the absence of geographic isolation). Previous work has shown that these races differ in their propensities to accept apple and hawthorn fruits in behavioral choice assays, and that this discrimination translates into "host fidelity" in the field (i.e., apple flies tend to mate on and oviposit into apples and hawthorn flies on hawthorns). We present the results of a study examining possible physiological factors contributing to host choice differences in R. pomonella. We tested whether apple and hawthorn flies differ in their electroantennogram (EAG) responses to biologically relevant volatile compounds emitted from apples and hawthorns. Significant differences were found in the relative EAG responses of apple and hawthorn flies to host fruit compounds at five of six paired study sites across the eastern United States. The geographic pattern of EAG variation was complex, however, with local populations of apple and hawthorn flies tending to be more similar to one another than to flies of the same race at distant sites. This pattern was largely due to EAG responses for several compounds showing longitudinal or latitudinal clines, the latitudinal clines being similar to those observed for allozyme loci in the host races. We also found evidence for sex-related differences, as males tended to have higher mean EAG responses to compounds than females. Host-associated differences were therefore nested within geographic and sex-related differentiation in R. pomonella.Further behavioral studies are needed to distinguish whether the EAG differences are responsible for, as opposed to being a consequence of, host-plant fidelity and adaptation. Crosses are also required to establish a genetic basis for the EAG responses, although we did find significant correlations between EAG scores for several compounds and the allozymes NADH-Diaphorase-2 and Hydroxyacid dehydrogenase at one of the study sites. Questions therefore remain concerning the evolutionary significance of the EAG response differences between apple and hawthorn fly races. Nevertheless, these differences raise the possibility that antennal responses to fruit-related volatile compounds contribute to host plant discrimination in R. pomonella. Regardless, the EAG responses represent another set of traits, in addition to diapause/eclosion time phenotypes and allozyme frequencies, differing between apple and hawthorn host races of R. pomonella.

Mating occurs on the larval host plant in all Rhagoletis species (Diptera: Tephritidae). We show how this attribute, when coupled with certain differences in other biological traits, strongly influences the mode of speciation. In species of the suavis species group, host shifts have never occurred during speciation, and larvae feed in the husks of any walnut species (Juglans spp.), which are highly toxic. Taxa are allopatric or parapatric and exhibit deep phylogenetic nodes suggesting relatively ancient speciation events. Traits responsible for species and mate recognition, particularly in parapatric species, are morphologically distinct and strongly sexually dimorphic. All aspects of their biology, genetics and distribution are consistent with a slow rate of allopatric speciation followed by morphological divergence in secondary contact. In contrast, speciation in the pomonella species group has always involved a shift to a new, usually unrelated, non-toxic host, and all taxa within these groups are sympatric, monophagous and morphologically indistinguishable from one another. Phylogenetic nodes are very shallow, indicating recent sympatric speciation. Sympatric divergence is promoted by genetic variation which allows a portion of the original species to shift to a new habitat or host. Evidence suggests that changes in a few key loci responsible for host selection and fitness on a new host may initiate host shifts. By exploiting different habitats, competition for resources between diverging populations is reduced or avoided. We provide evidence that in phytophagous and parasitic insects sufficient intrinsic barriers to gene flow can evolve between sister populations as they adapt to different habitats or hosts to allow each population to establish independent evolutionary lineages in sympatry.

Despite a considerable literature describing the biology of flies in the tephritid genus Rhagoletis, the phylogenetic relationships of the more than 60 species classified within the genus are not well resolved. Knowledge of these relationships is important, not only in terms of obtaining information that will be useful for the control of Rhagoletis where it is an agricultural pest, but also in determining what role host plant shifts have played in the generation of Rhagoletis species diversity, a focal point in debate over the role of sympatric speciation in the evolutionary process. In this paper, the phylogenetic history of 28 Rhagoletis species and 6 species in related tephritid genera is inferred from nucleotide sequences of subunit II of the mitochondrial cytochrome oxidase (COII) gene and the adjacent tRNALeu/COII intergenic region. Analyses of the data using distance and character-state approaches suggest the following: (i) the genus Rhagoletis as currently defined is not monophyletic; (ii) the 5 predominantly North American Rhagoletis species groups do constitute a monophyletic assemblage; (iii) the North American species groups form 2 clades, 1 consisting of taxa in the R. pomonella and R. tabellaria species groups (plus R. fausta), and the other consisting of taxa in the R. ribicola, R. cingulata, and R. suavis species groups; and (iv) the origin(s) of the North American species groups is obscure, as evidenced by the recovery of clades containing both Palearctic and Neotropical taxa. Areas of congruence and conflict with published phylogenies of Rhagoletis are examined and, while many areas of conflict may be due to an insufficient number of characters or incomplete taxon sampling, we cannot discount the possibility that real differences exist between the Rhagoletis mitochondrial DNA gene tree and the Rhagoletis species tree.

Global estimates of the number of insect species now range from 10 to 30 million and the tally keeps growing. This means that roughly 75–95% of all living eukaryotic organisms are insects. No matter which figure you care to choose, the numbers are impressively large. What is it about insects that accounts for this inordinately large number of species? An assessment of their biological attributes provides at least three important clues. The most important concerns their relatively high degree of resource specialization. Approximately 70% of British insects, which are probably representative of the world's insect fauna, are parasitoids or parasites on animals and plants (Price 1980). Of these about half feed on plants, with the majority infesting one or a few closely related hosts (Strong et al. 1984). A second important clue is that when sister species of these host specialists are found coexisting sympatrically or parapatrically they are almost always feeding on different host plant species. Finally, a third important characteristic shared by many of these host specialists is that they use their host plant or their host plant's habitat as a rendezvous site for locating a mate (Bush 1975b; Zwölfer 1975).

Three types of genes have been proposed to promote sympatric speciation: habitat preference genes, assortative mating genes and habitat-based fitness genes. Previous computer models have analysed these genes separately or in pairs. In this paper we describe a multilocus model in which genes of all three types are considered simultaneously. Our computer simulations show that speciation occurs in complete sympatry under a broad range of conditions. The process includes an initial diversification phase during which a slight amount of divergence occurs, a quasi-equilibrium phase of stasis during which little or no detectable divergence occurs and a completion phase during which divergence is dramatic and gene flow between diverging habitat morphs is rapidly eliminated. Habitat preference genes and habitat-specific fitness genes become associated when assortative mating occurs due to habitat preference, but interbreeding between individuals adapted to different habitats occurs unless habitat preference is almost error free. However, 'non-habitat assortative mating', when coupled with habitat preference can eliminate this interbreeding. Even when several loci contribute to the probability of expression of non-habitat assortative mating and the contributions of individual loci are small, gene flow between diverging portions of the population can terminate within less than 1000 generations.

Ssa1/2p, members of one of the yeast cytosolic hsp70 subfamilies, have been implicated in the translocation of secretory proteins into the lumen of the ER. The involvement of these hsp70s in translocation was tested directly by examining the effect of immunodepleting Ssa1/2p from yeast cytosol and subsequently testing the cytosol for its ability to support co- and post-translational translocation of prepro-alpha- factor. Depletion of Ssa1/2p had no effect on the efficiency of translocation in this in vitro assay. The system was used to examine the effect of the absence of Ssa1/2p on two other putative hsp70 functions: cotranslational folding of nascent luciferase and refolding of denatured luciferase. Depletion of Ssa1/2p had no effect on the ability of the yeast lysate to synthesize enzymatically active luciferase, but had a dramatic effect on the ability of the lysate to refold chemically denatured luciferase. These results demonstrate, for the first time, the refolding activity of Ssa1/2p in the context of the yeast cytosol, and define refolding activity as a chaperone function specific to Ssa1/2p, aprt from other cytosolic hsp70s. They also suggest that Ssa1/2p do not play a significant role in chaperoning the folding of nascent polypeptides. The implications of these findings for Ssa1/2p activity on their proposed role in the process of translocation are discussed.

Rhagoletis pomonella, living on apple, and R. mendax, infesting blueberry, represent a closely related sibling species pair with very limited morphological differentiation. Allozyme studies indicate that these species coexist with no detectable gene flow in areas where both hosts occur in close sympatry (Feder et al., 1989). However, viable interspecific F1, F2 and back-cross progeny can be easily produced in the laboratory (Bush, 1966). Therefore, premating isolating barriers must exist that restrict hybridization between these sibling species in nature.

As part of our research to determine phylogenetic relationships of organisms within the phytobacterial species Xanthomonas campestris, we have examined the use of the random amplified polymorphic DNA (RAPD) technique. The objective of this aspect of our research was to determine if a valid cladistic character analysis could be carried out by direct comparison of RAPD products separated on ethidium bromide-stained agarose gels. RAPD products were amplified from 47 Xanthomonas campestris DNA templates using a single oligonucleotide primer. These RAPD products were compared and variation was characterized by Southern analysis of both RAPD products and genomic DNA of the 47 bacterial strains using two cloned RAPD products as probes. Analysis of the data set revealed that the RAPD products were not necessarily homologous or independent, crucial prerequisites for characters to be analyzed in a cladistic phylogenetic analysis. It has been commonly assumed that RAPD variation occurs due to insertion/deletion events or alterations in the primer binding site. Within our data set, we demonstrate absence phenotypes arising from the apparent absence of corresponding loci and also due to the preferred synthesis of alternative RAPD products from unrelated loci. These different types of variation are a reflection of different types of genotypic variation, and direct examination of RAPD products did not allow us to distinguish by which mechanism a particular absence phenotype arose. Although this may not be important for phenetic analyses, for analyses of homologous characters using a cladistic approach it is critical. We also detected unrelated, co-migrating RAPD products and multiple related RAPD products within reaction mixtures. These could both contribute to errors in estimates of similarity, important in any phylogenetic analysis. All of these characteristics of RAPD products should be taken into consideration when RAPD products are used for phylogenetic comparisons.

True fruit flies belonging to the Rhagoletis pomonella (Walsh) sibling species complex have been proposed to speciate sympatrically by shifting and adapting to new host plants. Here, we report the results from a series of ecological and genetic experiments conducted at a study site near Grant, Michigan, U.S.A., aimed at clarifying the relationship between host specialization and reproductive isolation for these flies. Our findings indicate that apple (Malus pumila) and hawthorn (Crataegus mollis) infesting populations of R. pomonella are partially allochronically isolated. Differences in the timing of adult eclosion account for part of the allochronic divergence, as apple adults emerge approximately ten days earlier than hawthorn flies in the field. Genetic analyses across different life history stages of the fly show that adults do not randomly move between apple and hawthorn trees, but trend to attack the same species of plant that they infested as larvae. Estimates of interhost migration from the allozyme data suggest that from 2.8 to 10% of the apple population is of hawthorn origin and that over 20% of the hawthorn population is of apple origin. The length and quality of the growing season appear to affect the genetic composition of the host races, as allele frequencies in the hawthorn population are correlated with ambient temperature and rainfall during the spring of the preceding year. Finally, allele frequencies for six allozyme loci displaying host associated differentiation also show significant linear regressions with the timing of adult eclosion within both races. These regressions establish a link between allozyme loci displaying inter-host differentiation and a developmental trait (adult eclosion) responsible for partially isolating the races. The slopes of the regressions are paradoxical, however, as they suggest that apple adults should eclose later, not earlier, than hawthorn flies. We conclude by discussing potential resolutions to the eclosion time paradox.

New allozyme frequency data on the Rhagoletis pomonella (Walsh) species group, together with previously published data, are summarized to aid in identification of the species and in estimating the phylogeny of the group. Seven taxa were included in the study: the four described species, one host race or undescribed species infesting Cornus florida L. (flowering dogwood), and two outgroup species, each from a different species group. The phylogenetic tree was estimated by direct parsimony analysis of allele frequencies at 29 enzyme loci. To facilitate comparison with earlier electrophoretic studies, a UPGMA clustering of Nei unbiased genetic distance was also performed. The phylogenetic analysis suggests that two lineages exist in the R. pomonella species group, one consisting solely of R. cornivora Bush, and the other, the remainder of the group, consisting of a complex of extremely similar taxa. Support for the most parsimonious phylogeny is weak. The systematic status of the Cornus florida host-associated population and the evolution of fruit size preference in light of the allozyme phylogeny are discussed.