Research
Pdf files are linked below and can also be found on my CV page.
Parasitic nematodes
Filarial nematodes cause diseases such as river blindness and lymphatic filariasis in humans, or heartworm in dogs. These diseases are typically controlled by the mass administration of drugs (MDA). However, in many communities, the parasite that causes the disease has not been eliminated despite decades of control measures, or they have re-appeared after elimination. We are using genetic data to explore the biology underlying these problems, including defining transmission zones (in Onchocerca volvulus and Wuchereria bancrofti), identifying whether migration has caused the disease to return (in Wuchereria bancrofti), exploring genetic variation in mutualistic, endosymbiotic bacteria (Wolbachia), and identifying genes that may be involved with the evolution of drug resistance (Onchocerca volvulus and Dirofilaria immitis).
Diseases in wild bees
Bees are extremely important pollinators in natural and agricultural settings. Most people know about the importance of the European honey bee in pollination, but there are thousands of bee species that vary considerably in their life history. Bee diseases can be caused by viruses, fungi, and bacteria, and many of them can infect multiple species of bee. Pathogens that cause disease are probably transmitted across species through shared floral resources (nectar and pollen). How are pathogens in bee communities affected by agrochemicals, or the introduction of exotic species? Can we develop generalized models to predict evolution in emerging, multi-host viruses? This work is relevant to evaluating the robustness of wild bee communities to our changing environment.
Hedtke SM, Blitzer EJ, Montgomery GA, Danforth BN. 2015. Introduction of Non-Native Pollinators Can Lead to Trans-Continental Movement of Bee-Associated Fungi. PLoS ONE 10(6): e0130560.
Trees Done Right
Trees - or phylogenies - are graphic representations of historical relationships between organisms. One of the ways we figure out those relationships is by statistical analysis of DNA sequence data. If we have an idea about how DNA mutates and changes over time (i.e., a model of sequence evolution), then differences in DNA sequence that we observe among living species can be used to determine which species share a more recent common ancestor than others. I've published studies examining when these statistical approaches perform poorly, what approaches might perform better, and how we can gather more sequence data. One big problem, called "long-branch attraction," occurs when there has been lots of genetic change since the common ancestor in one species, but not in its closest relative. If this happens in two not-very-related species, the statistical method used to make the phylogeny can erroneously place these two species as sisters, because they share similarities due to random chance. Increasing the number of species analyzed and using better statistical methods are two ways of trying to prevent this problem from happening.
Why do we care? We use phylogenies not just to study relationships between species, but as a tool to answer questions across such disparate fields as evolution, epidemiology, forensics, conservation, and ecology. If the tree is bad... then the conclusions aren't going to be so great either.
Hedtke SM, Morgan MJ, Cannatella DC, Hillis DM. 2013. Targeted enrichment: maximizing orthologous gene comparisons across deep evolutionary time. PLoS ONE: e67908.
Hedtke SM, Patiny S, Danforth BN. 2013. The bee tree of life: a supermatrix approach to apoid phylogeny and biogeography. BMC Evol Biol 13:138.
Brown JE, Hedtke SM, Lemmon AR, Lemmon EM. 2010. When trees grow too long: investigating the causes of highly inaccurate Bayesian branch-length estimates. Syst Biol 59(2):145-161. pdf
Heath TA,* Hedtke SM,* Hillis DM.* 2008. Taxon sampling and the accuracy of phylogenetic estimates. J Syst Evol 46(3): 239-257. *authors contributed equally to this work. pdf
Hedtke, S.M., T.M. Townsend, and D.M. Hillis. 2006. Resolution of phylogenetic conflict in large data sets by increased taxon sampling. Syst Biol 55(3): 522-529. pdf
Attack of the Clam Clones!
Asexual reproduction occurs when offspring have only one parent contributing genes to the next generation: the offspring are clones of (usually) their mother. You may hear "clone" and think of Dolly the sheep, but clones are produced in nature by species that are parthenogenetic (=no males required). There is a lot of variation in how asexual reproduction happens mechanistically, and this affects whether the offspring are genetically identical to their parent. My research on clams in the genus Corbicula (called by such common names as the Asian clam, the heart-shaped clam, or the good-luck clam) shows that there are two species of clonal clams that have invaded North and South America. They reproduce clonally... but the kicker is that it's the genes of Dad, not Mom, that get passed on to offspring. The sperm fertilizes the egg and then kicks out the maternal genome, basically stealing the mom's egg to make a clone of the dad.
EXCEPT...
Mitochondria are the cell's energy powerhouses. Mitochondria evolved from free-living bacteria and have their own small genomes. In most animals and plants (but not all!), the mitochondria are inherited from the mother. In Corbicula, the mitochondrial genome comes from mom while the majority of DNA (the nuclear chromosomes) comes from dad. I used this information to figure out that these clams are able to steal the eggs of another species to reproduce clones of themselves. AND... sometimes, when they steal the egg (and the mitochondria of the egg), they also steal other genes.
SO...
One of the disadvantages to being asexual is the lack of genetic variation. Another is that random mutations that hurt the organism will accumulate in the genome over time. This causes asexual species to go extinct... which is one reason why they are more rare than sexual species. In Corbicula, they may not be having sex regularly, but as a consequence of this rare gene stealing, the species may be less likely to go extinct.
Pigneur L-M, Hedtke SM, Etoundi E, Van Doninck K. 2012. Androgenesis: a review through the study of the selfish shellfish Corbicula spp. Heredity 108:581-591.
Hedtke SM, Glaubrecht M, Hillis DM. 2011. Rare gene capture in a predominantly asexual species. Proc Natl Acad Sci USA 108:9520-9524. pdf
Hedtke SM, Hillis DM. 2011. The potential role of androgenesis in cytoplasmic-nuclear phylogenetic discordance. Syst Biol 60(1):87-96.
Hedtke SM, Stanger-Hall K, Baker RJ, Hillis DM. 2008. All-male asexuality: origin and maintenance of androgenesis in the Asian clam Corbicula. Evolution 65(5): 1119-1136. pdf
Conservation
Life's diversity is astounding, and I use science to inform preservation. Molecular markers can be used to tell us how individuals in a species are moving around the landscape, or who's mating with whom. Conservation managers can use this information when developing a conservation plan. I've developed molecular markers for conservation management in many different vertebrates, from elephants to crows to lizards. One study led to a publication on the Coachella Valley Fringe-toed lizard, a beautiful creature adapted to live in the sand dunes of California; this species is not very genetically diverse and gene flow may occur via movement of dunes rather than of individuals (which complicates conservation). Another collaboration examined correlations between bee life history traits and declines in relative abundance over time; specialist bees and bees with larger body size appear to be more vulnerable to environmental change.
Bartomeus I, Ascher JS, Gibbs J, Danforth BN, Wagner DL, Hedtke SM, Winfree R. 2013. Historical changes in northeastern United States bee pollinators related to shared ecological traits. Proc. Natl. Acad. Sci. USA 110:4656-4660.
Hedtke SM, Zamudio KR, Phillips CA, Losos J, Brylski P. 2007. Conservation genetics of the endangered Coachella Valley Fringe-toed Lizard (Uma inornata). Herpetologica 63(4):411-420. pdf
