Topology of syngameons.

Syngameons are sets of species linked by interspecific hybridization. Common observations regarding the structure of syngameons are that hybridization propensity is not uniform across species and that patterns of hybridization are dominated by a few species. I use computer simulations to test these claims in naturally occurring syngameons selected from the literature and from personal observation. Natural syngameons, especially those involving plants, typically exhibit nonrandom structure: The first three order statistics for the number of hybrid partners and the variance in the number of hybrid partners are larger than chance alone would predict. The structure of two insect syngameons examined is not significantly different from random. To test a hypothesis that variation in hybridization propensity across species in natural syngameons is simply an artifact of hybridization opportunity, I examine the structure of four artificial syngameons (fertility relationships) produced by full diallel crosses. Three of four artificial syngameons exhibit nonrandom structure, as the observed variation in number of successful crosses is larger than chance alone would predict. In general, there are no significant results involving the order statistics. Finally, I discuss biogeographic, ecological, and phylogenetic hypotheses for variation in hybridization propensity across species in natural syngameons.

Global patterns of apparent copy number variation in birds revealed by cross-species comparative genomic hybridization.

There is a growing interest in copy number variation (CNV) and the recognition of its importance in phenotype, disease, adaptation and speciation. CNV data is usually ascertained by array-CGH within-species, but similar inter-species comparisons have also been made in primates, mice and domestic mammals. Here, we conducted a broad appraisal of putative cross-species CNVs in birds, 16 species in all, using the standard array-CGH approach. Using a chicken oligonucleotide microarray, we detected 790 apparent CNVs within 135 unique regions and developed a bioinformatic tool 'CNV Analyser' for analysing and visualising cross-species data sets. We successfully addressed four hypotheses as follows: (a) Cross-species CNVs (compared to chicken) are, as suggested from preliminary evidence, smaller and fewer in number than in mammals; this 'dogma' was rejected in the light of the new evidence. (b) CNVs in birds are likely to have a functional effect through an association with genes; a large proportion of detected regions (70 %) were indeed associated with genes (suggesting functional significance), however, not necessarily more so than in mammals. (c) There are more CNVs in birds with more rearranged karyotypes; this hypothesis was rejected. Indeed, Falco species contained fewer than most with relatively standard (chicken-like) karyotypes. (d) There are more CNVs per megabase on micro-chromosomes than macrochromosomes; this hypothesis was accepted. Indeed, in species with rearranged karyotypes characterised by chromosomal fusions, the fused former microchromosomes still 'behaved' as though they were their microchromosomal ancestors. Gene ontology analysis of CNVRs revealed enrichment in immune response and antigen presentation genes and five CNVRs were perfectly correlated with the unique loss of sexual dichromatism in one Galliformes species.

Two-dimensional fractal growth properties of the filamentous fungus Cryphonectria parasitica: the effects of hypovirus infection.

Whole-colony two-dimensional fractal growth patterns produced by hypovirus-infected Cryphonectria parasitica (EP155/CHV1-EP713) were measured and compared with those produced by the isogenic virus-free strain (EP155) on solid medium. We have quantified statistically significant differences in the rates of expansion and spatial dynamics of colony growth between the two strains and concluded that fractal dimension is affected by the presence of the hypovirus. Therefore, fractal dimension measurement is an effective quantitative tool for testing the effects of mycovirus infection on fungal growth parameters.

Nestedness, biogeographic theory, and the design of nature reserves.

I examine the relationship between nested distributional patterns and the degree to which several small reserves will contain more species than would a single reserve of equal total area (SLOSS). Nestedness is a common property of species distributions on real and habitat islands. However, there is considerable variation in nestedness among species distributions, some of which is related to the physical and biological background of the archipelagoes. Nestedness does not vary according to the taxonomic group examined (with the exception of aquatic invertebrates). Nestedness does vary between real and habitat islands (with aquatic invertebrates excluded), but not between oceanic and land-bridge islands. The more a biota is nested, the more likely it is that a single large reserve would preserve more species. However, nestedness is a rather poor predictor of SLOSS, as the vast majority of archipelagoes support a strategy of several small reserves, even though almost all of them are significantly nested. Nestedness says little about optimal reserve design and management, and appears to be a weak conservation tool.

HOW DISCRETE ARE OAK SPECIES? INSIGHTS FROM A HYBRID ZONE BETWEEN QUERCUS GRISEA AND QUERCUS GAMBELII.

The white oaks Quercus gambelii and Q. grisea overlap in distribution in New Mexico and Arizona. Within the region of overlap, there are numerous instances of contact between the two taxa. In some areas of contact morphologically, intermediate trees are common, whereas in others, morphologically intermediate trees are rare or absent. We describe a set of RAPD markers that distinguish between the two species and use these markers to examine patterns of gene exchange in an area of contact in the San Mateo Mountains of New Mexico. The markers are highly coincident with morphology and confirm that hybridization between the two species takes place. Despite the occurrence of hybrids, both species remain distinct, even in areas of sympatry, and marker exchange appears to be limited.

Interactions between host plants, endophytic fungi, and a phytophagous insect in an oak (Quercus grisea x Q. gambelii) hybrid zone.

We examined interactions between host plants, endophytic fungi, and leaf-mining moths (Phyllonorycter sp.) in an oak (Quercus grisea x Q. gambelii) hybrid zone. The community of endophytic fungi and two common endophyte species examined responded to host plant hybridization. Total fungal frequency (TFF) and frequency of Gnomonia cerastis were lowest on hosts resembling Q. grisea, and increased linearly towards those resembling Q. gambelii. In contrast, Coccochorella quercicola was most frequently isolated from Q. grisea-like hosts and decreased in frequency across hybrids towards Q. gambelii. Frequency of G. cerastis and TFF covaried with Phyllonorycter density across the hybrid zone, but direct effects of endophytes on Phyllonorycter density were not detected. Associations between endophytes and unexplained mortality of Phyllonorycter varied according to endophyte species and state of Phyllonorycter development. In the sap-feeding stage, unexplained mortality was negatively associated with TFF and frequencies of Hormonema sp. and Preussia funiculata; whereas, in the tissue-feeding stage, unexplained Phyllonorycter mortality was positively associated with G. cerastis frequency. Three-way interactions between plant hybridization, endophytic fungi, and the insect herbivore were not significant.

Reduced parasitism of a leaf-mining moth on trees with high infection frequencies of an endophytic fungus.

We investigated relationships between endophytic fungi and a leaf-mining moth, Phyllonorycter sp., along an elevational gradient from 2255 to 2895 m. The fungi and moth larvae inhabit leaves of Quercus gambelii. Fungal frequencies and larval densities varied with elevation. However, larval densities were not associated with the frequencies of infection by endophytic fungi. Survival of larvae was positively associated with the most dominant fungus, Gnomonia cerastis, owing to reduced parasitism of moth larvae on trees with high frequencies of Gnomonia.

A three-trophic-level analysis of the effects of plant hybridization on a leaf-mining moth.

We investigated relationships between host plant hybridization in the Quercus grisea x Q. gambelii species complex and the distribution and performance of a leaf-mining moth in the genus Phyllonorycter. In 2 years at two sites Phyllonorycter densities were lowest on Q. grisea and increased through the categories of hybrid host plants to Q. gambelii. Direct host plant effects on Phyllonorycter performance were consistent with Phyllonorycter distribution; unexplained mortality of larvae in the mines, which is often associated with direct plant effects, decreased from Q. grisea through the hybrids to Q. gambelii. Plant hybridization influenced parasitism of Phyllonorycter. Parasitism was density dependent, and across all densities was higher on Q. grisea and Q. gambelii than on hybrid host plants.

Sex-biased herbivory in Ephedra trifurca: the importance of sex-by-environment interactions.

We examined patterns of herbivory in the dioecious desert shrub, Ephedra trifurca (Ephedraceae), along a small watershed in the northern Chihuahuan Desert. We documented significant (p=0.002) sexual dimorphism in densities of Lasioptera ephedrae and L. ephedricola (Diptera: Cecidomyiidae), with male E. trifurca supporting higher densities than did females. Densities of gall midges varied significantly (p<0.001) among four sample populations of E. trifurca arrayed along the watershed and exhibited an inverse gradient with elevation. There was also a significant (p=0.006) sex-by-site interaction such that the degree of intersexual variation in gall densities decreased down the watershed. Variation in gall densities and intersexual variation in gall densities exhibited inverse trends with variation in soil water and inorganic nitrogen concentrations along the watershed.

Nonequilibrial community structure of sawflies on arroyo willow.

We use Wiens' (1984) nonequilibrium-equilibrium continuum concept to evaluate aspects of community structure for sawfly herbivores (Hymenoptera: Tenthredinidae) that attack arroyo willow, Salix lasiolepis. The sawfly community on arroyo willow shares many characteristics of nonequilibrium communities: unsaturation, biotic decoupling, lack of density dependence, and loose emergent patterns. Species abundances exhibit highly significant differences among clones (P<0.001) and among years (P<0.001) and exhibit significant and complex clone-by-year interactions both for absolute and relative abundances. Organizing forces, such as interspecific competition and regulation by natural enemies, are largely absent from the arroyo willow system. Sawfly densities exhibit significant positive covariation among willow clones and appear to be affected by largely stochastic abiotic forces, primarily winter precipitation.

Structure of herbivore communities in two oak (Quercus spp.) hybrid zones.

We examined patterns of density and species diversity for leaf-mining Lepidopterans and gall-forming Hymenopterans in two oak (Quercus spp.) hybrid zones: Quercus depressipes x Q. rugosa and Q. emoryi x Q. coccolobifolia. In both species complexes, hybrid hosts typically supported significantly lower densities and species diversity of parasites than did parental types. This contradicts the findings of Whitham (1989) that suggested that hybrid hosts may act as parasite sinks both in ecological and evolutionary time. We discuss features of hybrid zones that are likely to influence patterns of herbivory.