Research

I also plan to investigate the microevolutionary processes of both parasites and their hosts because understanding these processes may help us determine how ecological interactions can result in long-term evolutionary processes. Using coalescent and other population genetic approaches to study host-parasite population dynamics (codemography) may clarify how processes such as cospeciation are initiated as well as how these processes differ among host-parasite assemblages. The host-parasite systems that I study have profound differences in population size and structure. For example, pocket gophers are normally parasitized by hundreds of chewing lice whereas heteromyids usually harbor few sucking lice and some host populations may lack lice altogether. Despite these differences, both louse groups show significant patterns of cophylogeny with their hosts. Examining the population dynamics of both hosts and parasites may help us understand how demographic factors influence interactions between associated taxa. Research in the Reed lab at the University of Florida is currently focused on isolating fast-evolving molecular markers (microsatellites) from primate lice to undertake population-level studies. Collaborating in this research as well as forming new collaborations to examine the population genetics of rodents will provide me with the skills to thoroughly assess microevolutionary processes underlying associations of distantly related taxa.

Host-Parasite Systems – I am studying three host-parasite systems that involve organisms with fundamentally different evolutionary backgrounds and life histories (mammals, insects, and bacteria) in an effort to determine the prevalence of cospeciation in mammal-parasite systems and to understand how symbiotic organisms have been associated through time. Part of my dissertation research examined cophylogenetic relationships between three lineages of Mexican pocket gophers and their lice. Two congeneric and sympatric lineages of chewing lice parasitize each pocket gopher lineage, thus, one louse lineage serves as a natural replicate of the other providing the first opportunity to investigate potential differences in the way mammal lice respond to a common environment. While the gopher-louse system has been instrumental in developing the field of cophylogenetics, these chewing lice are mostly benign to their hosts and are actually commensals rather than parasites. Therefore, I also explored an additional symbiotic assemblage to examine the coevolutionary interactions between a true parasite and its mammal host: heteromyid rodents and sucking lice (Phthiraptera: Anoplura). Because heteromyid rodents are sister to pocket gophers, the heteromyid-sucking louse system is ideal for additional studies of coevolution and comparisons to the gopher-chewing louse assemblage. As part of my postdoctoral research, I am examining new aspects of cophylogenetic analysis in the three-tiered assemblage of mammals (specifically primates), lice, and endosymbiotic bacteria. Sucking lice cannot live without their endosymbionts and it has been presumed that these taxa have coevolved together as the lice radiated along with their mammalian hosts over the last 60 to 80 million years. This assemblage therefore provides a rare opportunity to study three disparate associates at one time.

Additional Research - Cophylogenetic studies require well resolved and well supported host and parasite phylogenies. I therefore maintain an active interest in the taxonomy, systematics, and biology of mammals and invertebrates. I have analyzed relationships among various pocket gopher species including the Cratogeomys fumosus species complex (Hafner et al., 2004), the C. merriami species complex (Hafner et al., 2005), and Pappogeomys alcorni (Demastes et al., 2003) using molecular, chromosomal, and morphological data and explored relationships using rigorous phylogenetic and morphometric techniques. For each of these studies, I relied heavily upon recent field collections and museum specimens to obtain molecular and morphological data, respectively. Although heteromyid rodents are the subject of numerous and active research programs, relationships among heteromyid subfamilies (represented by kangaroo rats, pocket mice, and spiny pocket mice) were only recently resolved (Hafner et al., 2007).

My research with invertebrates is a byproduct of my dissertation and undergraduate research. To properly undertake the cophylogenetic analyses, it was necessary to hypothesize relationships among louse species. I resolved relationships in two different louse lineages (chewing lice and sucking lice; Light and Hafner, 2007; Light and Hafner, in press). Furthermore, I trained myself to identify both chewing and sucking lice using morphological characters, facilitating future cophylogenetic studies involving lice. As an undergraduate at the University of Michigan, I investigated relationships among leeches (family Glossiphoniidae; Light and Siddall, 1999) and this research has led to several recent publications (Burreson et al., 2005; Light et al., 2005).

My research program has primarily focused on the coevolutionary associations between distantly related organisms, specifically mammals and their parasites. Many parasites have complex ecological interactions with their hosts that persist over long evolutionary timescales. Parasites that are highly host specific tend to interact with their hosts in ways that facilitate a long-term coevolutionary history (i.e., cospeciation), whereas parasites that are not host specific may not cospeciate with their hosts. Comparisons of host and parasite phylogenies facilitate determination of whether or not cospeciation has occurred and how taxa have been associated through time. Although topological comparisons offer a variety of ways to investigate host-parasite associations, they cannot distinguish between trees that are concordant as a result of cospeciation and trees that are concordant for reasons unrelated to cospeciation (i.e., pseudocospeciation events such as sequential colonization). This level of discrimination requires use of a combination of methods that compare not only topological similarities between host and parasite trees, but also timing of putative codivergence events. I therefore supplement my studies with estimates of divergence times in host and parasite lineages because these analyses provide a way to distinguish between cospeciation and other processes that could result in identical branching patterns in host and parasite trees. Tests of cospeciation also offer the possibility for comparisons of rates of molecular evolution to determine if parasites are evolving faster, slower, or at the same rate as their hosts. Fundamental biological differences, such as differences in generation time, metabolic rate, DNA base composition, mitochondrial gene order, and evolution of parasitic lifestyle, have all been suggested as possible causes of rate differences between organisms. Therefore, studies incorporating multiple methodologies, such as phylogenetic comparisons, estimates of divergence times, and comparisons of molecular rates, have the potential to elucidate broad evolutionary processes operating in distantly related taxa.

Future Plans – Along with PI David Reed and Co-PIs Lance Durden and Vince Smith, I recently have been funded by the National Science Foundation to further examine the symbiotic association among mammals, sucking lice, and the bacterial endosymbionts living within the lice. It is believed that the acquisition of bacterial endosymbionts enabled lice to successfully radiate along with their mammalian hosts over the last 60-80 million years. However, it appears from preliminary data that there may be multiple, distantly related endosymbiotic bacteria sustaining this suborder of lice. Only a comprehensive reconstruction of the louse’s evolutionary history, including that of its endosymbiont, will elucidate whether the lice have enlisted the help of more than one lineage of bacterium during their radiation. Using phylogenetic, cophylogenetic, and divergence dating analyses, we hope to determine how often and how long ago sucking lice have entered into obligate mutualistic relationships with their primary endosymbionts. Since the mutualistic relationship between many invertebrates and their endosymbionts is often very old and stable, this research offers a unique opportunity to assess how the process of endosymbiosis occurs and how many times insects can enter into endosymbiotic relationships over a relatively short period of time.