A pinboard by
Sophia Carryl

PhD candidate, University of Chicago

 A microbiome is an ecological community of microorganisms that includes bacteria, archaea, fungi, and viruses. Once considered solely disease-causing agents, we now know that microorganisms can have both pathogenic and beneficial relationships with their hosts. Beneficial relationships include: mediating nutrient uptake, training the immune and endocrine systems, and fighting against pathogens. Microorganisms can also modulate host behaviors by inducing changes in gene expression in key brain regions, and by producing and responding to shared biomediators (e.g. stress hormones and neurotransmitters) in their hosts. This interkingdom signaling is considered a bidirectional relationship in microbial endocrinology, where hosts control their microorganisms, and microorganisms control their hosts. For example, a strain of bacteria, Lactobacillus rhamnosus, can dampen stress responses in rodents by producing gamma aminobutyric acid (GABA). GABA is an inhibitory neurotransmitter that decreases the release of stress hormones from pituitary glands in the brain of rodents. This decrease in stress hormones is correlated with a decrease in stress-like behaviors that can increase survival. 

 The goal of my research is to investigate similar relationships between microorganisms and their avian hosts using house sparrows (Passer domestics) as a study organism. As of right now, ~90% of microbial research is conducted in mammals, but birds have unique adaptations of their own that may contribute to a different host-microbe relationship. Moreover, it is not known what role microorganisms may play in avian behaviors, particularly stress behaviors that are associated with high costs (e.g. decreased survival, neural damage, immunosuppression, reduced reproductive output).

 My work employs molecular and physiological techniques, along with experimental ecology to study interactions between microorganisms and birds, and how such interactions may lead to changes in avian stress responses (stress hormone release and stress behaviors). My work may demonstrate convergent evolution of microorganisms modulating stress responses in birds and mammals via shared biochemical pathways of interkingdom signaling. Or it will demonstrate that avian stress responses operate under a pathway independent from microbial influences, and that avian stress responses do not support interkingdom bidirectional signaling, which will then beg the question why? And how?          

Density-dependent habitat selection by brown-headed cowbirds ( Molothrus ater) in tallgrass prairie.

Abstract: Local distributions of avian brood parasites among their host habitats may depend upon conspecific parasite density. We used isodar analysis to test for density-dependent habitat selection in brown-headed cowbirds ( Molothrus ater) among tallgrass prairie adjacent to wooded edges, and prairie interior habitat (>100 m from wooded edges) with and without experimental perches. Eight study sites containing these three habitat treatments were established along a geographical gradient in cowbird abundance within the Flint Hills region of Eastern Kansas and Oklahoma, USA. The focal host species of our study, the dickcissel ( Spiza americana), is the most abundant and preferred cowbird host in the prairie of this region. Cowbird relative abundance and cowbird:host abundance ratios were used as estimates of female cowbird density, whereas cowbird egg density was measured as parasitism frequency (percent of dickcissel nests parasitized), and parasitism intensity (number of cowbird eggs per parasitized nest). Geographical variation in cowbird abundance was independent of host abundance. Within study sites, host abundance was highest in wooded edge plots, intermediate in the experimental perch plots, and lowest in prairie interior. Cowbirds exhibited a pattern of density-dependent selection of prairie edge versus experimental perch and interior habitats. On sites where measures of cowbird density were lowest, all cowbird density estimates (female cowbirds and their eggs) were highest near (< or =100 m) wooded edges, where host and perch availability are highest. However, as overall cowbird density increased geographically, these density estimates increased more rapidly in experimental perch plots and prairie interiors. Variation in cowbird abundance and cowbird:host ratios suggested density-dependent cowbird selection of experimental perch over prairie interior habitat, but parasitism levels on dickcissel nests were similar among these two habitats at all levels of local cowbird parasitism. The density-dependent pattern of cowbird distribution among prairie edge and interior suggested that density effects on perceived cowbird fitness are greatest at wooded edges. A positive relationship between daily nest mortality rates of parasitized nests during the nestling period with parasitism intensity levels per nest suggested a density-dependent effect on cowbird reproductive success. However, this relationship was similar among habitats, such that all habitats should have been perceived as being equally suitable to cowbirds at all densities. Other unmeasured effects on cowbird habitat suitability (e.g., reduced cowbird success in edge-dwelling host nests, cowbird despotism at edges) might have affected cowbird habitat selection. Managers attempting to minimize cowbird parasitism on sensitive cowbird hosts should consider that hosts in otherwise less-preferred cowbird habitats (e.g., habitat interiors) are at greater risk of being parasitized where cowbirds become particularly abundant.

Pub.: 18 Sep '04, Pinned: 21 Jun '17

Sampling locality is more detectable than taxonomy or ecology in the gut microbiota of the brood-parasitic Brown-headed Cowbird (Molothrus ater).

Abstract: Brown-headed Cowbirds (Molothrus ater) are the most widespread avian brood parasite in North America, laying their eggs in the nests of approximately 250 host species that raise the cowbird nestlings as their own. It is currently unknown how these heterospecific hosts influence the cowbird gut microbiota relative to other factors, such as the local environment and genetics. We test a Nature Hypothesis (positing the importance of cowbird genetics) and a Nurture Hypothesis (where the host parents are most influential to cowbird gut microbiota) using the V6 region of 16S rRNA as a microbial fingerprint of the gut from 32 cowbird samples and 16 potential hosts from nine species. We test additional hypotheses regarding the influence of the local environment and age of the birds. We found no evidence for the Nature Hypothesis and little support for the Nurture Hypothesis. Cowbird gut microbiota did not form a clade, but neither did members of the host species. Rather, the physical location, diet and age of the bird, whether cowbird or host, were the most significant categorical variables. Thus, passerine gut microbiota may be most strongly influenced by environmental factors. To put this variation in a broader context, we compared the bird data to a fecal microbiota dataset of 38 mammal species and 22 insect species. Insects were always the most variable; on some axes, we found more variation within cowbirds than across all mammals. Taken together, passerine gut microbiota may be more variable and environmentally determined than other taxonomic groups examined to date.

Pub.: 09 Apr '14, Pinned: 21 Jun '17