Song discrimination suggests premating isolation among sympatric indigobird species and host races

Indigobirds (Vidua spp.) are host-specific brood parasites thathave diversified in a recent radiation apparently driven byhost colonization. Behavioral imprinting of both male and femaleindigobirds on host song is thought to promote rapid speciationbecause it results in assortative mating between indigobirdsassociated with a particular host. We conducted a song playbackexperiment to test whether male indigobirds discriminate amongpotential competitors based on song. Of particular interestwas the behavior of two sympatric host races of the Cameroonindigobird Vidua camerunensis that differ only in host songmimicry and other components of their vocal repertoires. Territorialmales of the two V. camerunensis host races and Vidua raricola,a morphologically distinct indigobird species, were tested withplaybacks of each other's songs. Males of all three groups respondedmost aggressively to songs of their own species and/or hostrace, as evidenced by strong and highly significant differencesin a variety of response variables. This differential territorialdefense suggests that an intruding male with different songsdoes not represent a competitive threat and is expected if femalesmate assortatively with respect to song. Thus, our results provideevidence of premating reproductive isolation among recentlyevolved indigobird species and host races.     

A test of host specialization by insect vectors as a mechanism for reproductive isolation among entomophilous fungal species

Epichloë species are self incompatible (heterothallic) fungi that must be fertilized by spermatia from individuals of opposite mating type for successful sexual reproduction to occur. Female flies of the genus Botanophila act as vectors of the fungi by ingesting and defecating spermatia (gametes) onto fungal stromata (fruiting bodies) after oviposition. Larvae feed and develop on the stromata and thus maintain a symbiotic relationship with Epichloë fungi. We hypothesized that sole dependence on fertilized stromata as a food source would promote specialization by flies to single compatible host species and that this specialization would promote reproductive isolation among Epichloë species. Analysis of progeny of ascospores from experimental field plots in Zurich, Switzerland, indicated prevalence of specific matings between stromata of the same host, and thus was consistent with the hypothesis that flies are species-specific in their visitation behaviour. Genetic analyses of spermatia contained in the faeces of individual flies also gave some support for this hypothesis. We recovered spermatia of 4 different Epichloë species from fly faeces. Comparison of spermatia found in fly faeces to those available from stromata showed flies avoided Epichloë clarkii and may have preferred Epichloë typhina . Interestingly, these are the only two Epichloë species known to be interfertile with one another. Individual flies tended to carry spermatia predominantly from one fungal species. Thus, flies may adopt a type of majoring and minoring behaviour when visiting fungi. Yet, Botanophila flies are not monolectic and often visited all hosts that were available within screened cages. In addition to any reproductive isolation flies may provide to some fungal species, differences in competitiveness among spermatia of different species deposited on the same stroma may favor intraspecific matings.     

Speciation via habitat specialization: the evolution of reproductive isolation as a correlated character

Summary A diverse group of theoretical and empirical studies are integrated into a composite model of sympatric speciation via habitat specialization. It is shown that disruptive selection on a continuous distribution of habitat preference can lead to the evolution of prezygotic reproductive isolation as a correlated character. The form of selection eliminates the major theoretical objections to the process of sympatric speciation. The principal difference between this model and the allopatric model of speciation is that the initial barrier to gene flow between subpopulations is produced by the evolution of gaps in the phenotypic distribution of spatial/temporal habitat use, rather than an extrinsic geographical barrier.     

Ring species as bridges between microevolution and speciation

A demonstration of how small changes can lead to species-level differences is provided by ring species, in which two reproductively isolated forms are connected by a chain of intermediate populations. We review proposed cases of ring species and the insights they provide into speciation. Ring species have been viewed both as illustrations of the history of divergence of two species from their common ancestor and as demonstrations that speciation can occur in spite of gene flow between the diverging forms. Theoretical models predict that speciation with gene flow can occur when there is divergent ecological selection, and geographical differentiation increases the likelihood of speciation. Thus ring species are ideal systems for research into the role of both ecological and geographical differentiation in speciation, but few examples have been studied in detail. The Greenish warbler is a ring species in which two northward expansions around the Tibetan plateau have been accompanied by parallel evolution in morphology, ecology, and song length and complexity. However, songs have diverged in structure, resulting in a lack of recognition where the reproductively isolated forms come into contact in Siberia. Our analysis suggests that these differences could have arisen even with gene flow, and that parallel rather than divergent ecological changes have led to divergence in sexually selected traits and subsequent speciation.     

Patterns of nucleotide variation and reproductive isolation between a Mimulus allotetraploid and its progenitor species

Here we report our characterization of a widespread, highly selfing Mimulus allotetraploid formed by interspecific hybridization between M. nasutus and M. guttatus. Nucleotide variation at two nuclear loci (mCYCA and mAP3) within and among tetraploid populations resolves two haplotype clusters for each locus: one shares near identity with sequences from M. nasutus and the other group shares substantial variation with M. guttatus. With respect to the two loci studied, each allotetraploid individual is a 'fixed heterozygote' carrying sequences from both clusters. Moreover, mCYCA variation is consistent with at least two evolutionary origins for the Mimulus allotetraploid. We show that the allotetraploid is strongly reproductively isolated from M. nasutus and M. guttatus; interploidy crosses produce almost no viable seeds. By extension, we infer strong triploid block and argue that Mimulus allotetraploid formation might proceed in one step via the union of unreduced gametes in an M. nasutus-M. guttatus F(1) hybrid. We also discuss the potential roles of mating system and flowering asynchrony in allotetraploid establishment.     

Hybridization among cryptic species of the cellar fungus Coniophora puteana (Basidiomycota)

In this study we have analysed the genetic variation and phylogeography in a global sample of the cellar fungus Coniophora puteana, which is an important destroyer of wooden materials indoor. Multilocus genealogies of three DNA regions (beta tubulin, nrDNA ITS and translation elongation factor 1alpha) revealed the occurrence of three cryptic species (PS1-3) in the morphotaxon C. puteana. One of the lineages (PS3) is apparently restricted to North America while the other two (PS1-2) have wider distributions on multiple continents. Interspecific hybridization has happened between two of the lineages (PS1 and PS3) in North America. In three dikaryotic isolates, two highly divergent beta tubulin alleles coexisted, one derived from PS1 and one from PS3. Furthermore, one isolate included a recombinant ITS sequence, where ITS1 resembled the ITS1 version of PS3 while ITS2 was identical to a frequent PS1 ITS2 version. This pattern must be due to hybridization succeeded by intralocus recombination in ITS. The results further indicated that introgression has happened between subgroups appearing in PS1. We hypothesize that the observed reticulate evolution is due to previous allopatric separation followed by more recent reoccurrence in sympatry, where barriers to gene flow have not yet evolved. A complex phylogeographical structure is observed in the morphotaxon C. puteana caused by (i) cryptic speciation; (ii) the interplay between natural migration and distribution patterns and probably more recent human mediated dispersal events; and (iii) hybridization and introgression.     

Pollen competition as a reproductive isolating mechanism between two sympatric Hibiscus species

Differences in pollen tube growth rates (certation) between heterospecific (foreign) and conspecific pollen may strongly influence whether hybrid offspring are produced after mixed pollen loads are delivered to a stigma. For both members of a sympatric species pair, Hibiscus moscheutos and H. laevis, pollination by pure loads of foreign pollen resulted in fruit set that was not significantly different from conspecific pollination, indicating that pure loads of foreign pollen could readily result in hybrid offspring. However, the number of seeds per fruit from pure foreign pollinations was significantly less than that of pure conspecific pollination. Simultaneous mixed pollination resulted in a proportion of hybrid seeds (detected by an electrophoretic marker enzyme) that was significantly lower than expected based upon the capacity of foreign pollen to effect fertilization when applied in pure pollinations. After these 50/50% pollen mixtures were applied to stigmas, 8.0 and 7.4% hybrids were produced when H. moscheutos and H. laevis were the ovule parents, respectively. For these Hibiscus species, pollen competition appears to function as a barrier to hybridization that is of moderate intensity compared with similar barriers occurring between other recently studied sympatric species pairs.     

Oncopsis flavicollis (L.) associated with tree birches (Betula): a complex of biological species or a host plant utilization polymorphism?

The closely related species of leafhoppers, Oncopsis flavicollis (L.) and 0. subangulata (Sahl.), are restricted to birches, Betula pendula Roth and B. pubescens Ehrh., as host plants. Morphometric discriminant analyses of adult insects from S Wales showed O. flavicollis populations from the two Betula species to be significantly different. The best discrimination was provided by characters of the male dorsal abdominal apodemes. Analyses of such apodeme morphology in populations more widely from S Britain showed three distinct types: in western localities type 1 dominantly on B. pubescens , type 2 dominantly on B. pendula and type 3 absent; in eastern localities type 3 only on B. pendula , type 2 on both species of Betula and type 1 only on B. pubescens. Acoustic calling and courtship signals of males showed clear differences between the three apodeme types of 0. flavicollis. It is concluded that the three 0. flavicollis types, together with 0. subangulata , are distinct but very closely related biological species. Contrary to earlier suggestions, this example provides no evidence for host plant utilization polymorphisms.     

Demographic history,geographical distribution and reproductive isolation of distinct lineages of blue rockfish (Sebastes mystinus), a marine fish with a high dispersal potential

Understanding the barriers to genetic exchange in taxonomic groups that have a high dispersal potential will provide critical information on speciation in general. Blue rockfish (Sebastes mystinus) are good taxa to examine speciation because they are nonmigratory inhabitants of shallow rocky reef habitats along the eastern North Pacific with a pelagic larval stage lasting 3–5 months. The goal of this study was to analyse the evolutionary history and distribution patterns of different lineages within S. mystinus described previously and use this information to understand the speciation process in this group of high dispersal fish. The molecular data derived from specimens sampled over approximately 1650 km of the S. mystinus range revealed a northerly and southerly distribution for the two lineages. Almost equal frequencies of both lineages occurred at centrally located sample locations, with evidence of reproductive isolation between the lineages. A demographic analysis showed that the two lineages diverged and experienced sudden expansion prior to the last glacial maximum, which affected the observed pattern of genetic structure. The spatial distribution, demographic history and degree of genetic distinctiveness found from the genetic analysis, despite the high potential for dispersal in S. mystinus, suggest both lineages diverged in allopatry.     

Temperature-induced complementarity as a mechanism for biomolecular assembly

Recent advances in the measurement and theory of “hydration” interactions between biomolecules provide a basis on which to formulate mechanisms of biomolecular recognition. In this paper we have developed a mathematical formalism for analyzing specificity encoded in dynamic distributions of surface polar groups, a formalism that incorporates newly recognized properties of directly measured “hydration” forces. As expected, attraction between surfaces requires complementary patterns of surface polar groups. In contrast to usual expectations, thermal motion can create these complementary surface configurations. We have demonstrated that assembly can occur with an increase in conformational entropy of polar residues. Elevated temperature then facilitates recognition rather than hinders it. This mechanism might underlie some temperature-favored assembly reactions common in biological systems that are usually associated with the “hydrophobic effect” only. © 1994 Wiley-Liss, Inc.     

Variation in hostplant relations and courtship signals of weed-associated populations of the brown planthopper, Nilaparvata lugens (Stål), from Australia and Asia: a test of the recognition species concept

Populations of the morphological species, Nilaparvata lugens (Stål), were found to breed and feed on the grass, Leersia hexandra Schwartz, at six sites in Queensland, Australia. They differ from sympatric rice-feeding populations in characters of pulse repetition frequencies of male and female acoustic courtship signals. The two host-derived populations hybridize freely in the laboratory, but in mate choice experiments show very significant preferences for homogametic matings. No indication of field hybridization has been found, so that the two morphologically inseparable populations represent sympatric biological species in Australia.
Populations from L. hexandra are also reported from four localities in Sri Lanka and one in Orissa, India. These resemble previously studied populations from the Philippines. They differ significantly in courtship call characters, both from sympatric rice-associated populations and from allopatric Leersia -associated populations from Australia.
The geographical variation reported for acoustic signals is not consistent with Paterson's recognition concept of species, but may be interpreted in terms of theories of allopatric speciation involving sexual selection for mate recognition signals.     

Evidence for autotetraploidy associated with reproductive isolation in Saccharomyces cerevisiae: towards a new domesticated species

Partial or whole‐genome duplications have played a major role in the evolution of new species. We have investigated the variation of ploidy level in a panel of domesticated strains of Saccharomyces cerevisiae coming from different geographical origins. Segregation studies and crosses with tester strains of different ploidy levels showed that part of the strains were well‐balanced autotetraploids displaying tetrasomic inheritance. The presence of up to four different alleles for various loci is consistent with a polyploidization mechanism relying on the fusion of two nonreduced meiospores coming from two S. cerevisiae strains. Autotetraploidy was also in accordance with karyotype and flow cytometry analyses. Interestingly, most bakery strains were tetraploids, suggesting a link between ploidy level and human use. The null or drastically reduced fertility of the hybrids between tetraploid and diploid strains indicated that domesticated S. cerevisiae strains are composed of two groups isolated by post‐zygotic reproductive barriers.     

Hedgerows as corridors for woodland plants: a test on the Po Plain,northern Italy

The use of hedgerows as corridors for forest vascular species has been widely studied, but only in humid oceanic and continental climates; no replicated trials have ever been performed on corridor function. Given these premises, a study was done on the eastern Po Plain, in a transition area between the Temperate (Eurosiberian) and Mediterranean climatic zones, adopting the same sample shape and dimensions as a North-American study [Corbit et al. (1999) J Ecol 87:220–232]. The following research questions were posed: (1) how common are forest species in hedgerows? (2) do origin, isolation, distance from source, width and adjacent land-use factors correlate with the frequency of forest species? (3) are hedgerows corridors for forest species? To address these, three functional types of hedgerows, identified by comparing old aerial-photos, were sampled: remnant attached (n = 12) and remnant isolated (n = 6) with respect to the nearest woodland and regenerated attached (n = 4). If wooded patches were a source for hedgerows, then regenerated hedgerows should be more similar to an adjacent woodland than an isolated remnant. A 900 m² circular plot in woodlands and an adjacent 90-m transect along hedgerows were sampled for the presence and cover of all plant species, then 39 woodland taxa were selected. Significant differences between the three hedgerow types emerged in forest species richness, but not in cover. The forest species composition in both remnant and regenerated attached hedgerows showed a strong affinity with the adjacent stand, implying a dispersal process from woodland (source) to regenerated hedgerows (sink). A distance effect on forest species distribution clearly linked to a corridor function was found only in regenerated hedgerows, while in the remnant attached ones, even with a composition similar to that of the nearest woodland source, other additional factors cannot be ignored to explain the fine scale distribution of forest species. The cover of the most common ant-dispersed species showed a similar distance effect while vertebrate-dispersed ones did not show any significant trend with distance from woodlands. Habitat suitability for forest species was affected by width, especially in hedges wider than 12 m, but not by adjacent land use.     

Genetic isolation and divergence in sexual traits: evidence for the northern rockhopper penguin Eudyptes moseleyi being a sibling species

The taxonomic status of populations of rockhopper penguins (Eudyptes chrysocome) is still enigmatic. Northern populations differ from southern ones in breeding phenology, song characteristics and head ornaments used as mating signals. We conducted a molecular analysis using mitochondrial DNA sequencing to test if there is a gene flow barrier between northern (subtropical) populations and southern (subantarctic) populations in relation to the Subtropical Convergence, a major ecological boundary for marine organisms. Sequences of the control region and the ND2 gene were analysed in rockhopper penguins and in the macaroni penguin (Eudyptes chrysolophus), a closely related species. Genetic distances and phylogenetic analyses showed a clear split into three clades, two rockhopper clades and the macaroni penguin. Moreover, Theta(ST) and gene flow estimates also suggested genetic structuring within the northern rockhoppers. Our results add further support to the notion that the two rockhopper penguin taxa, often considered as two subspecies, can be recognized as two species E. chrysocome and E. moseleyi. The divergence in mating signals found between these two taxa seems to have occurred recently and relatively rapidly. Thus, the behavioural changes may have been enough to isolate these taxa without the need for morphological differentiation. The findings have important conservational implications, since E. moseleyi is far less abundant than E. chrysocome, but more populations may warrant an uplisting to endangered status if full species status should be recognized for more subpopulations.     

Natural transformation as a mechanism of horizontal gene transfer among environmental Aeromonas species

Aeromonas species are common inhabitants of aquatic environments and relevant as human pathogens. Their potential as pathogens may be related in part to lateral transfer of genes associated with toxin production, biofilm formation, antibiotic resistance, and other virulence determinants. Natural transformation has not been characterized in aeromonads. DNA from wild-type, prototrophic strains that had been isolated from environmental sources was used as donor DNA in transformation assays with auxotrophs as the recipients. Competence was induced in 20% nutrient broth during the stationary phase of growth. Optimal transformation assay conditions for one chosen isolate were in Tris buffer with magnesium or calcium, pH 5–8, and a saturating concentration of 0.5 μg of DNA per assay (3.3 ng of DNA μl⁻¹) at 30 °C. Sodium was also required and could not be replaced with ammonium, potassium, or lithium. The maximal transformation frequency observed was 1.95 × 10⁻³ transformants (recipient cell)⁻¹. A survey of environmental Aeromonas auxotrophic recipients (n = 37), assayed with donor DNA from other wild-type environmental aeromonads under optimal assay conditions, demonstrated that 73% were able to act as recipients, and 100% were able to act as donors to at least some other aeromonads. Three different transformation groups were identified based on each isolates’ ability to transform other strains with its DNA. The transformation groups roughly corresponded to phylogenetic groups. These results demonstrate that natural transformation is a general property of Aeromonas environmental isolates with implications for the genetic structures of coincident Aeromonas populations.     

Distance to suburban/wildland border interacts with habitat type for structuring exotic plant communities in a natural area surrounding a metropolitan area in central Chile

Background: The explosive growth of urbanisation in Mediterranean ecosystems in Chile has favoured the rapid expansion of exotic plant species, yet factors driving these invasion patterns in adjacent natural areas remain poorly assessed.

Aims: To assess how distance to a suburban/wildland border, habitat type, site-scale disturbance and woody plant cover of native species influences the diversity of exotic species in a natural area surrounding the city of Santiago, Chile.

Methods: Three watersheds were chosen, and the diversity of exotic species was assessed in 36 100-m-long transects, equally distributed over two distance categories and three habitats. For each transect, we measured woody plant cover of native species and frequency of rabbit faeces as a measure of competitive exclusion and site-scale disturbance, respectively.

Results: Species diversity decreased as the distance from the suburban/wildland border increased, and it was found to be higher in north-facing habitats compared to south-facing and alluvial habitats. Neither native woody plant cover nor frequency of rabbit faeces had an effect on species diversity.

Conclusions: The current pattern of exotic plant species in this natural area is mainly influenced by the distance to suburban border and habitat type. An adequate management of conditions favouring exotic species in suburban/wildland border may prevent the spread of these into natural areas next to urban settings.     

Pollen tube competition as a mechanism of prezygotic reproductive isolation between Mimulus nasutus and its presumed progenitor M. guttatus

This paper considers the extent to which differences in pollen tube growth rates can provide prezygotic reproductive isolation between Mimulus nasutus and its presumed progenitor, Mimulus guttatus . Mimulus nasutus is partially cleistogamous, but its larger chasmogamous flowers offer appreciable opportunity for outcrossing. Mimulus nasutus was found to have smaller pollen grains and shorter styles than M. guttatus . No differences were observed in pollen grain germination on conspecific and heterospecific stigmas. However, pollen tube growth rates of M. nasutus were found to be much slower than those of M. guttatus in the styles of that species. Consequently, any M. nasutus pollen transferred to an M. guttatus stigma was found to be competitively disadvantaged in an M. gutattus style. By contrast, no difference in pollen tube growth rate was detected between the species when growing in M. nasutus styles, possibly because M. nasutus styles are unable to support fast pollen tube growth. We tested the prediction from the pollen tube studies that a 50:50 mix of M. guttatus and M. nasutus pollen would produce 50% hybrid seeds when M. nasutus was the maternal parent, and near to 0% hybrid seed when M. guttatus was the maternal parent. The results were found to support this prediction. We conclude that pollen–pistil interactions can effect strong reproductive isolation between these species, as M. guttatus pollen tubes have a competitive advantage over those of M. nasutus in an M. guttatus style, but not in an M. nasutus style.     

Choice of host plant as a factor in reproductive isolation of the aphid genus Cryptomyzus (Homoptera, Aphididae)

Abstract. 1. Host plant preference experiments were conducted with closely related taxa of the aphid genus Cryptomyzus. Males, and presexual morphs (sexuparae and gynoparae), were used to determine the impact of host plant choice on reproductive isolation. In the case of host-alternating species these morphs are migratory and so will select the host plant.
2. Host plant preference of two closely related taxa of C. alboapicalis (Theobald) was found to promote their reproductive isolation. The preference of sexuparae of these monoecious taxa was more pronounced than that of the males.
3. Host plant preference and subsequent production of oviparae showed that C. galeopsidis (Kaltenbach) consists of two host races restricted to Ribes rubrum L. and R. nigrum L., respectively. The existence of clones, intermediate in their preference and reproductive performance on these plants, suggests that hybridization occurs.