Demographic projection of high-elevation white pines infected with white pine blister rust: a nonlinear disease model.

Matrix population models have long been used to examine and predict the fate of threatened populations. However, the majority of these efforts concentrate on long-term equilibrium dynamics of linear systems and their underlying assumptions and, therefore, omit the analysis of transience. Since management decisions are typically concerned with the short-term (< 100 years), asymptotic analyses could lead to inaccurate conclusions or, worse yet, critical parameters or processes of ecological concern may go undetected altogether. We present a stage-structured, deterministic, nonlinear, disease model which is parameterized for the population dynamics of high-elevation white pines in the face of infection with white pine blister rust (WPBR). We evaluate the model using newly developed software to calculate sensitivity and elasticity for nonlinear population models at any projected time step. We concentrate on two points in time, during transience and at equilibrium, and under two scenarios: a regenerating pine stand following environmental disturbance and a stand perturbed by the introduction of WPBR. The model includes strong density-dependent effects on population dynamics, particularly on seedling recruitment, and results in a structure favoring large trees. However, the introduction of WPBR and its associated disease-induced mortality alters stand structure in favor of smaller stages. Populations with infection probability (beta) > or = 0.1 do not reach a stable coexisting equilibrium and deterministically approach extinction. The model enables field observations of low infection prevalence among pine seedlings to be reinterpreted as resulting from disease-induced mortality and short residence time in the seedling stage. Sensitivities and elasticities, combined with model output, suggest that future efforts should focus on improving estimates of within-stand competition, infection probability, and infection cost to survivorship. Mitigating these effects where intervention is possible is expected to produce the greatest effect on population dynamics over a typical management timeframe.

Phylogeography of a parasitoid wasp (Diaeretiella rapae): no evidence of host-associated lineages.

The exceptional diversity of insects is often attributed to the effects of specialized relationships between insects and their hosts. Parasite-host interactions are influenced by current natural selection and dispersal, in addition to historical effects that may include past selection, vicariance, and random genetic drift. Both current and historical events can lead to reduced fitness on some hosts. If trade-offs in fitness on alternate hosts are common, adaptation to one host can prevent adaptation to another, giving rise to genetic differentiation among host-associated lineages. Previous studies of Diaeretiella rapae (Hymenoptera: Aphidiidae), a parasitoid of aphids, have revealed additive genetic differences in performance between populations that parasitize different aphid host species. To determine whether D. rapae populations collected from different aphid hosts have diverged into genetically independent lineages, we constructed a haplotype network based on sequence variation in mitochondrial DNA (mtDNA). We used single strand conformation polymorphism (SSCP) analysis to examine 2041 base pairs of mtDNA and to identify nucleotide sequences of 42 unique SSCP haplotypes. We found no association between mtDNA haplotypes and host species in either the ancestral range (Europe, Mediterranean region, Middle East, Asia) or part of the introduced range (western North America). Haplotypes likely to be ancestral were geographically widespread and found on both hosts, suggesting that the ability to use both hosts evolved prior to the diversification of the mtDNA. Ongoing gene flow appears to prevent the formation of host races.

Population structure, mating system, and sex-determining allele diversity of the parasitoid wasp Habrobracon hebetor.

Besides haplo-diploid sex determination, where females develop from fertilized diploid eggs and males from unfertilized haploid eggs, some Hymenoptera have a secondary system called complementary sex determination (CSD). This depends on genotypes of a 'sex locus' with numerous sex-determining alleles. Diploid heterozygotes develop as females, but diploid homozygotes become sterile or nonviable diploid males. Thus, when females share sex-determining alleles with their mates and produce low fitness diploid males, CSD creates a genetic load. The parasitoid wasp Habrobracon hebetor has CSD and displays mating behaviours that lessen CSD load, including mating at aggregations of males and inbreeding avoidance by females. To examine the influence of population structure and the mating system on CSD load, we conducted genetic analyses of an H. hebetor population in Wisconsin. Given the frequency of diploid males, we estimated that the population harboured 10-16 sex-determining alleles. Overall, marker allele frequencies did not differ between subpopulations, but frequencies changed dramatically between years. This reduced estimates of effective size of subpopulations to only N3 approximately 20-50, which probably reflected annual fluctuations of abundance of H. hebetor. We also determined that the mating system is effectively monogamous. Models relating sex-determining allele diversity and the mating system to female productivity showed that inbreeding avoidance always decreased CSD loads, but multiple mating only reduced loads in populations with fewer than five sex-determining alleles. Populations with N3 less than 100 should have fewer sex-determining alleles than we found, but high diversity could be maintained by a combination of frequency-dependent selection and gene flow between populations.

Population genomics: genome-wide sampling of insect populations.

Modern population genetics underwent a major paradigm shift during the last decade of the 20th century with the discovery that thousands of genes of known function and position in a genome can be analyzed simultaneously in a single individual. The impact of this technology on insect population genetics is potentially profound. Sampling distributions of genetic statistics can now be derived from many individual loci or among many segregating sites within a gene. Inferences regarding random mating, gene flow, effective population sizes, disequilibrium, and relatedness among populations can now be based on patterns of variation at many loci. More importantly, genome-wide sampling enables population geneticists to distinguish effects that act on the whole genome from those that act on individual loci or nucleotides. We introduce the term "population genomics" to describe the process of simultaneous sampling of numerous variable loci within a genome and the inference of locus-specific effects from the sample distributions. The four critical assumptions implicit in the population genomics approach are explained in detail. Studies adopting this paradigm are reviewed, and the steps necessary to complete a population genomics study are outlined.

Perspective: evolution's struggle for existence in America's public schools.

The ongoing creation-evolution controversy in North America thrives on the widespread special creationist beliefs of a significant portion of the public. Creation science supports a literal interpretation of the Judeo-Christian Bible, an earth that is no more than 10.000 years old and created ex nihilo in six days by a monotheistic God, with no new kinds arising since the period of creation, and with a single flood of staggering force shaping layers of rocks and trapping the organisms fossilized within them. Despite decisions in numerous court cases that specifically exclude creationism and creation science from primary and secondary biology classes in America's public schools, creationists now work locally to minimize or remove evolution from science teaching standards. The nationally organized movement to resist the teaching of evolution has proven highly effective, influencing state and district school boards in addition to individual teachers and schools. Thus, if teaching about evolution and the nature of science is to survive in America's primary and secondary schools, scientists must likewise work with teachers and reach out to state and local school boards. In this perspective we outline the typical creationist arguments we encounter from students, teachers, school board members, and neighbors. We explain briefly how knowledge of both microevolution and macroevolution is important in medicine, agriculture, and biotechnology. We describe a science education controversy that arose within our own school district, how we responded, and what we learned from it. Finally, we argue that even modest outreach efforts to science teachers will be richly repaid.

Linkage analysis of sex determination in Bracon sp. near hebetor (Hymenoptera: Braconidae).

To test whether sex determination in the parasitic wasp Bracon sp. near hebetor (Hymenoptera: Braconidae) is based upon a single locus or multiple loci, a linkage map was constructed using random amplified polymorphic DNA (RAPD) markers. The map includes 71 RAPD markers and one phenotypic marker, blonde. Sex was scored in a manner consistent with segregation of a single "sex locus" under complementary sex determination (CSD), which is common in haplodiploid Hymenoptera. Under haplodiploidy, males arise from unfertilized haploid eggs and females develop from fertilized diploid eggs. With CSD, females are heterozygous at the sex locus; diploids that are homozygous at the sex locus become diploid males, which are usually inviable or sterile. Ten linkage groups were formed at a minimum LOD of 3.0, with one small linkage group that included the sex locus. To locate other putative quantitative trait loci (QTL) for sex determination, sex was also treated as a binary threshold character. Several QTL were found after conducting permutation tests on the data, including one on linkage group I that corresponds to the major sex locus. One other QTL of smaller effect had a segregation pattern opposite to that expected under CSD, while another putative QTL showed a female-specific pattern consistent with either a sex-differentiating gene or a sex-specific deleterious mutation. Comparisons are made between this study and the in-depth studies on sex determination and sex differentiation in the closely related B. hebetor.

Evolutionary relationships among the Braconidae (Hymenoptera: Ichneumonoidea) inferred from partial 16S rDNA gene sequences.

Phylogenetic relationships among the Braconidae were examined using homologous 16S rDNA gene sequence data. Analyses recovered the few well-supported relationships evident in this family from morphological analyses, viz the monophyly of the microgastroid complex of subfamilies, the monophyly of the cyclostome complex of subfamilies (= braconoids), a sister-group relationship between the Alysiinae and Opiinae, and a close relationship between the Helconinae and Blacinae. With respect to the braconoid complex of subfamilies, a sister-group relationship was recovered between Aphidiinae and Mesostoinae, and a clade composed of Gnamptodontinae + Histeromerinae + Rhyssalinae + Aphidiinae + Mesostoinae was also recovered. The Doryctinae and Rogadinae sensu lato (s.l.) were generally not resolved as monophyletic. With respect to the helconoid complex of subfamilies, a sister-group relationship was recovered between Sigalphinae and Agathidinae, whereas Neoneurinae fell out among other helconoid subfamilies. Other relationships among the helconoid subfamilies were unclear from these analyses. With respect to the microgastroid complex of subfamilies, our data conform to morphological estimates, recovering ((Microgastrinae + Miracinae) + Cardiochilinae) + Cheloninae. The topology of our trees suggests that the cyclostome subfamilies are a natural derived group, inferring that endoparasitism (not ectoparasitism) is the ancestral state for the Braconidae, unless all of the ectoparasitic ancestors of the helconoid + microgastroid subfamilies are now extinct.

Intensive linkage mapping in a wasp (Bracon hebetor) and a mosquito (Aedes aegypti) with single-strand conformation polymorphism analysis of random amplified polymorphic DNA markers.

The use of random amplified polymorphic DNA from the polymerase chain reaction (RAPD-PCR) allows efficient construction of saturated linkage maps. However, when analyzed by agarose gel electrophoresis, most RAPD-PCR markers segregate as dominant alleles, reducing the amount of linkage information obtained. We describe the use of single strand conformation polymorphism (SSCP) analysis of RAPD markers to generate linkage maps in a haplodiploid parasitic wasp Bracon (Habrobracon) hebetor and a diploid mosquito. Aedes aegypti. RAPD-SSCP analysis revealed segregation of codominant alleles at markers that appeared to segregate as dominant (band presence/band absence) markers or appeared invariant on agarose gels. Our SSCP protocol uses silver staining to detect DNA fractionated on large thin polyacrylamide gels and reveals more polymorphic markers than agarose gel electrophoresis. In B. hebetor, 79 markers were mapped with 12 RAPD primers in six weeks; in A aygpti, 94 markers were mapped with 10 RAPD primers in five weeks. Forty-five percent of markers segregated as codominant loci in B. hebetor, while 11% segregated as codominant loci in A. aegypti. SSCP analysis of RAPD-PCR markers offers a rapid and inexpensive means of constructing intensive linkage maps of many species.

Mate finding via a trail sex pheromone by a parasitoid wasp.

In field observations and laboratory experiments, we found that virgin females of the solitary parasitoid Aphelinus asychis did not emit a volatile sex pheromone to attract males, contrary to what has been reported in many other parasitoid species. Instead, we found that virgin females deposited a sex pheromone on the substrate to which males responded by intensively searching on and near the marked area. Males did not respond to leaves exposed to mated females or to other males. In patches of 64 wheat leaves, males were dispersed from a central release point, and more males were subsequently observed on leaves exposed to virgin females than on unexposed leaves. The pheromone faded to inactivity in less than 24 h. To examine whether the trail pheromone would be sufficient for mate finding by males in the field, we modeled random movement of males among plant stems where the trail pheromone was the only cue males used to find females. The probability that females encountered at least one male in their lifetime increased with male density and time after female emergence. Given the range of densities of A. asychis in barley and wheat fields near Montpellier, France, the model generated an encounter probability sufficient to explain the survival of established populations. The model also suggested that difficulty in finding mates at low density might be a problem for invading populations.

Molecular taxonomy using single-strand conformation polymorphism (SSCP) analysis of mitochondrial ribosomal DNA genes.

Single-strand conformation polymorphism (SSCP) analysis detects single point mutations in DNA molecules. We demonstrate that SSCP analysis of mitochondrial ribosomal DNA (rDNA) genes is a sensitive taxonomic tool because these genes often differ at numerous sites among closely related species. Using conserved primers, portions of the 12S or 16S rDNA genes were amplified using the polymerase chain reaction (PCR) in congeneric species of ticks, leafhoppers, mosquitoes, and closely related endoparasitic wasps. SSCP was performed and products were visualized with silver staining. Species-specific patterns were observed in all taxa. Intraspecific variation at the level of single nucleotide substitutions was detected. SSCP diagnostics are less expensive and time consuming to develop than PCR with species-specific primers, and, unlike PCR with arbitrary primers, there is minimal concern with DNA contamination from non-target organisms.

Long-distance dispersal by a parasitoid (Anagrus delicatus, Mymaridae) and its host.

We demonstrate that an egg parasitoid, Anagrus delicatus (Mymaridae, Hymenoptera) and its host, Prokelesia marginata (Delphacidae, Homoptera) regularly disperse 1 km or more in a north Florida saltmarsh. Anagrus delicatus were caught on yellow sticky traps on offshore islets and oyster bars throughout the spring, summer, and fall, whereas P. marginata were caught during one pulse in the spring. Parasitism rates were higher on offshore islets than at mainland sites, even though egg densities were higher at the mainland sites. The majority of parasitoids caught offshore were females. Long-distance dispersal by A. delicatus may be a cause of inverse density-dependent or density-independent spatial patterns of parasitism and may represent a risk-spreading strategy.