Laura K. Estep

Evolutionary transitions and mechanisms of matte and iridescent plumage coloration in grackles and allies (Icteridae).

Iridescent structural colour is found in a wide variety of organisms. In birds, the mechanisms that create these colours are diverse, but all are based on ordered arrays of melanin granules within a keratin substrate in barbules. The feathers of the grackles and allies in the family Icteridae range in appearance from matte black to iridescent. In a phylogenetic analysis of this clade, we identified several evolutionary transitions between these colour states. To describe a possible mechanistic explanation for the lability of plumage coloration, we used spectrometry, transmission electron microscopy and thin-film optical modelling of the feathers of 10 icterid species from five genera, including taxa with matte black or iridescent feathers. In matte black species, melanin was densely packed in barbules, while in iridescent species, melanin granules were arranged in ordered layers around the edges of barbules. The structured arrangement of melanin granules in iridescent species created optical interfaces, which are shown by our optical models to be critical for iridescent colour production by coherent scattering. These data imply that rearrangement of melanin granules in barbules is a mechanism for shifts between black and iridescent colours, and that the relative simplicity of this mechanism may explain the lability of plumage colour state within this group.

A Multi-Year Study of Mosquito Feeding Patterns on Avian Hosts in a Southeastern Focus of Eastern Equine Encephalitis Virus

Eastern equine encephalitis virus (EEEV) is a mosquito-borne pathogen that cycles in birds but also causes severe disease in humans and horses. We examined patterns of avian host use by vectors of EEEV in Alabama from 2001 to 2009 using blood-meal analysis of field-collected mosquitoes and avian abundance surveys. The northern cardinal (Cardinalis cardinalis) was the only preferred host (fed on significantly more than expected based on abundance) of Culiseta melanura, the enzootic vector of EEEV. Preferred hosts of Culex erraticus, a putative bridge vector of EEEV, were American robin (Turdus migratorius), Carolina chickadee (Poecile carolinensis), barred owl (Strix varia), and northern mockingbird (Mimus polyglottis). Our results provide insight into the relationships between vectors of EEEV and their avian hosts in the Southeast and suggest that the northern cardinal may be important in the ecology of EEEV in this region.

Estimation of Dispersal Distances of Culex erraticus in a Focus of Eastern Equine Encephalitis Virus in the Southeastern United States

ABSTRACT Patterns of mosquito dispersal are important for predicting the risk of transmission of mosquito-borne pathogens to vertebrate hosts. We studied dispersal behavior of Culex erraticus (Dyar & Knab), a potentially significant vector of eastern equine encephalitis virus (EEEV) that is often associated with foci of this pathogen in the southeastern United States. Using data on the relative density of resting adult female Cx. erraticus around known emergence sites in Tuskegee National Forest, Alabama, we developed a model for the exponential decay of the relative density of adult mosquitoes with distance from larval habitats through parameterization of dispersal kernels. The mean and 99th percentile of dispersal distance for Cx. erraticus estimated from this model were 0.97 and 3.21 km per gonotrophic cycle, respectively. Parameterized dispersal kernels and estimates of the upper percentiles of dispersal distance of this species can potentially be used to predict EEEV infection risk in areas surrounding the Tuskegee National Forest focus in the event of an EEEV outbreak. The model that we develop for estimating the dispersal distance of Cx. erraticus from collections of adult mosquitoes could be applicable to other mosquito species that emerge from discrete larval sites.

HYBRIDIZATION AND BARRIERS TO GENE FLOW IN AN ISLAND BIRD RADIATION

While reinforcement may play a role in all major modes of speciation, relatively little is known about the timescale over which species hybridize without evolving complete reproductive isolation. Birds have high potential for hybridization, and islands provide simple settings for uncovering speciation and hybridization patterns. Here we develop a phylogenetic hypothesis for a phenotypically diverse radiation of finch‐like weaver‐birds (Foudia) endemic to the western Indian Ocean islands. We find that unlike Darwins finches, each island‐endemic Foudia population is a monophyletic entity for which speciation can be considered complete. In explaining the only exceptions—mismatches between taxonomy, mitochondrial, and nuclear data—phylogenetic and coalescent methods support introgressive hybridization rather than incomplete lineage sorting. Human introductions of known timing of one island‐endemic species, to all surrounding archipelagos provide two fortuitous experiments; (1) population sampling at known times in recent evolutionary history, (2) bringing allopatric lineages of an island radiation into secondary contact. Our results put a minimum time bound on introgression (235 years), and support hybridization between species in natural close contact (parapatry), but not between those in natural allopatry brought into contact by human introduction. Time in allopatry, rather than in sympatry, appears key in the reproductive isolation of Foudia species.

Hosts or habitats: What drives the spatial distribution of mosquitoes?

Understanding the factors that influence the distribution of vectors is critical to assess the risk of disease transmission across landscapes. For mosquitoes, existing spatial models use only habitat elements (hydrologic and floristic) to predict the distribution of disease vectors within the landscape, ignoring the potential importance of the distribution of hosts. We tested the hypothesis that the distribution of hosts would better explain the spatial distribution of mosquitoes than habitat variables using Information Theory and Poisson Regression. We analyzed the abundance of ectotherm-biting mosquitoes and their hosts from 37 points over a 28-km2 area in Tuskegee National Forest, Alabama, USA. We compared models built using information regarding habitat, hosts, or both for their ability to explain the abundance of Culex peccator and Culex territans, two mosquito species that specialize on reptiles and amphibians. Models built using both host and habitat information were the best models for both specie...

Developing GIS-based eastern equine encephalitis vector-host models in Tuskegee, Alabama

BackgroundA site near Tuskegee, Alabama was examined for vector-host activities of eastern equine encephalomyelitis virus (EEEV). Land cover maps of the study site were created in ArcInfo 9.2® from QuickBird data encompassing visible and near-infrared (NIR) band information (0.45 to 0.72 μm) acquired July 15, 2008. Georeferenced mosquito and bird sampling sites, and their associated land cover attributes from the study site, were overlaid onto the satellite data. SAS 9.1.4® was used to explore univariate statistics and to generate regression models using the field and remote-sampled mosquito and bird data. Regression models indicated that Culex erracticus and Northern Cardinals were the most abundant mosquito and bird species, respectively. Spatial linear prediction models were then generated in Geostatistical Analyst Extension of ArcGIS 9.2®. Additionally, a model of the study site was generated, based on a Digital Elevation Model (DEM), using ArcScene extension of ArcGIS 9.2®.ResultsFor total mosquito count data, a first-order trend ordinary kriging process was fitted to the semivariogram at a partial sill of 5.041 km, nugget of 6.325 km, lag size of 7.076 km, and range of 31.43 km, using 12 lags. For total adult Cx. erracticus count, a first-order trend ordinary kriging process was fitted to the semivariogram at a partial sill of 5.764 km, nugget of 6.114 km, lag size of 7.472 km, and range of 32.62 km, using 12 lags. For the total bird count data, a first-order trend ordinary kriging process was fitted to the semivariogram at a partial sill of 4.998 km, nugget of 5.413 km, lag size of 7.549 km and range of 35.27 km, using 12 lags. For the Northern Cardinal count data, a first-order trend ordinary kriging process was fitted to the semivariogram at a partial sill of 6.387 km, nugget of 5.935 km, lag size of 8.549 km and a range of 41.38 km, using 12 lags. Results of the DEM analyses indicated a statistically significant inverse linear relationship between total sampled mosquito data and elevation (R2 = -.4262; p < .0001), with a standard deviation (SD) of 10.46, and total sampled bird data and elevation (R2 = -.5111; p < .0001), with a SD of 22.97. DEM statistics also indicated a significant inverse linear relationship between total sampled Cx. erracticus data and elevation (R2 = -.4711; p < .0001), with a SD of 11.16, and the total sampled Northern Cardinal data and elevation (R2 = -.5831; p < .0001), SD of 11.42.ConclusionThese data demonstrate that GIS/remote sensing models and spatial statistics can capture space-varying functional relationships between field-sampled mosquito and bird parameters for determining risk for EEEV transmission.

Influential disease foci in epidemics and underlying mechanisms: a field experiment and simulations

Pathogen invasions pose a growing threat to ecosystem stability and public health. Guidelines for the timing and spatial extent of control measures for pathogen invasions are currently limited, however. We conducted a field experiment using wheat (Triticum aestivum) stripe rust, caused by the wind-dispersed fungus Puccinia striiformis, to study the extent to which host heterogeneity in an initial outbreak focus influences subsequent disease spread. We varied the frequency of susceptible host plants in an initial outbreak focus and in the non-focus of experimental plots, and observed the progress of epidemics produced by artificial inoculation. The frequency of susceptible hosts in the initial outbreak focus increased the spread of stripe rust in the experimental plots, while frequency of susceptible hosts outside the initial outbreak focus did not. This suggests that factors influencing pathogen reproduction in the initial outbreak focus are key to the control of epidemics of stripe rust. Two mechanisms may underlie the field results. The first is the continuing, direct infection of susceptible hosts in areas outside the initial outbreak focus by disease propagules arriving from the initial outbreak focus. The second is highly local proliferation of disease caused by direct descendants of colonizing individuals originating from the initial outbreak focus. We considered these two alternatives in simulations of a generalized pathogen exhibiting fat-tailed dispersal, similar to P. striiformis. Simulations showed a dominant effect of conditions in the initial outbreak focus, in agreement with the field experiment, but indicated that, over time, this dominance may erode. Analysis of the duration of focal dominance led to the conclusion that both mechanisms contribute to the phenomenon of focal dominance, and that the frequency of susceptible hosts in the initial outbreak focus had a stronger influence when the proportion of propagules that remained local during dispersal was higher. Overall, our results suggest that targeting pathogen reproduction in the initial outbreak focus will have a disproportionately large impact on subsequent epidemic spread.

Risk of Exposure to Eastern Equine Encephalomyelitis Virus Increases with the Density of Northern Cardinals

For a variety of infectious diseases, the richness of the community of potential host species has emerged as an important factor in pathogen transmission, whereby a higher richness of host species is associated with a lowered disease risk. The proposed mechanism driving this pattern is an increased likelihood in species-rich communities that infectious individuals will encounter dead-end hosts. Mosquito-borne pathogen systems potentially are exceptions to such “dilution effects” because mosquitoes vary their rates of use of vertebrate host species as bloodmeal sources relative to host availabilities. Such preferences may violate basic assumptions underlying the hypothesis of a dilution effect in pathogen systems. Here, we describe development of a model to predict exposure risk of sentinel chickens to eastern equine encephalitis virus (EEEV) in Walton County, Florida between 2009 and 2010 using avian species richness as well as densities of individual host species potentially important to EEEV transmission as candidate predictor variables. We found the highest support for the model that included the density of northern cardinals, a highly preferred host of mosquito vectors of EEEV, as a predictor variable. The highest-ranking model also included Culiseta melanura abundance as a predictor variable. These results suggest that mosquito preferences for vertebrate hosts influence pathogen transmission.

Degree of host susceptibility in the initial disease outbreak influences subsequent epidemic spread

Disease epidemics typically begin as an outbreak of a relatively small, spatially explicit population of infected individuals (focus), in which disease prevalence increases and rapidly spreads into the uninfected, at-risk population. Studies of epidemic spread typically address factors influencing disease spread through the at-risk population, but the initial outbreak may strongly influence spread of the subsequent epidemic.We initiated wheat stripe rust Puccinia striiformis f. sp. tritici epidemics to assess the influence of the focus on final disease prevalence when the degree of disease susceptibility differed between the at-risk and focus populations.When the focus/at-risk plantings consisted of partially genetic resistant and susceptible cultivars, final disease prevalence was statistically indistinguishable from epidemics produced by the focus cultivar in monoculture. In these experimental epidemics, disease prevalence was not influenced by the transition into an at-risk population that differed in disease susceptibility. Instead, the focus appeared to exert a dominant influence on the subsequent epidemic.Final disease prevalence was not consistently attributable to either the focus or the at-risk population when focus/at-risk populations were planted in a factorial set-up with a mixture (~28% susceptible and 72% resistant) and susceptible individuals. In these experimental epidemics, spatial heterogeneity in disease susceptibility within the at-risk population appeared to counter the dominant influence of the focus.Cessation of spore production from the focus (through fungicide/glyphosate application) after 1.3 generations of stripe rust spread did not reduce final disease prevalence, indicating that the focus influence on disease spread is established early in the epidemic.Synthesis and applications. Our experiments indicated that outbreak conditions can be highly influential on epidemic spread, even when disease resistance in the at-risk population is greater than that of the focus. Disease control treatments administered shortly after the initial outbreak within the focus may either prevent an epidemic from occurring or reduce its severity.

A Multi-Scale Analysis of Competition between the House Finch and House Sparrow in the Southeastern United States

Abstract. The decline of House Sparrow (Passer domesticus) populations in eastern North America has been proposed to be a consequence of competition between the House Sparrow and the House Finch (Carpodacus mexicanus), both introduced species. Previous research testing the hypothesis that House Sparrow declines are due to competition with House Finches focused on populations in the northeastern U.S., excluding other regions where the species coexist. We tested for effects of competition between these two species in the southeastern U.S. at two scales of analysis. First, we looked for evidence of competition at a local scale during the breeding season by examining patterns of co-occurrence of House Finches and House Sparrows at count points within a 28.3-km2 study area centered on Auburn, Alabama. Second, we tested for the effects of competition at a regional scale across seven southeastern states by analyzing trends in Christmas Bird Count data. Using null-model analysis, we found no evidence for competitive exclusion in our assessment of local co-occurrence, suggesting that if there is competition between these species it is not important enough to affect their spatial distribution during the breeding season. At the regional scale, contrary to findings in the northeastern U.S., the decline of House Sparrow populations within the southeastern U.S. was not significantly correlated with House Finch abundance. Our results suggest that competition with House Finches is not a major cause of decline of the House Sparrow population in the southeastern U.S.