Shannon Hedtke
Research Interests

This page is still under construction.

Life is amazing. There are millions of species, all with different characteristics and morphologies. And all these species arose from a single, bacteria-like common ancestor! Evolutionary biologists have identified several forces which can cause changes between one population of organisms and another, but the details are still being hammered out. My research uses experimental techniques to look at what causes a random mutation in the genome to spread throughout a population, causing diversification between that population and other populations. This diversification is what ultimately leads to the formation of new species.

• Androgenesis in the Asian Clam Corbicula
• Mating experiments
• Phylogenetic analyses
• Taxon sampling in large data sets
• Population sampling in phylogeographic studies
• Conservation genetics of the Coachella Valley Fringe-Toed Lizard

Androgenesis in the Asian Clam genus Corbicula collaborators: Dr. David Hillis, University of Texas, Austin Dr. Kathrin Stanger-Hall, University of Texas, Austin Dr. Matthias Glaubrecht, Humbolt University Dr. Robert Baker, Texas Tech University Introduction Conflict is pervasive across life. Conflict occurs between species, such as when a wolf wants to eat a rabbit, which in turn would prefer not to be eaten. Conflict occurs within a population, when individuals compete for the same resources such as food or mates. Conflict can even occur between the sexes ­ a male, for example, might prefer that the female put all her energy into bringing up his offspring, while the female would prefer to have some energy left over for survival and future reproduction.

I've used the word "prefer" here ­ but in actuality, organisms are not necessarily making conscious choices. Rather, those organisms with the genes to exploit a resource better are going to be more likely to survive and pass on those same genes to offspring. A very fast rabbit isn't choosing to be faster than other rabbits; rather, it has genes which contribute to it being fast. Since it's fast, it escapes predators, and passes some of those fast rabbit genes to its children.

The focus of my PhD thesis is a system with very interesting sources of genetic conflict. In the clam genus Corbicula, there are some species which mate via androgenesis. This is a fascinating mating system in which the sperm causes the egg to eject the entire maternal genome. All the offspring are then clones of the father!

The first level of conflict, then, is between males and females ­ or, since androgenetic Corbicula are hermaphrodites (have both male and female reproductive organs), between sperm and eggs. The genes within the sperm "want" to be spread at the expense of the genes in the egg.

This produces conflict between individuals. One individual would rather make sperm and take over the eggs of another individual. This is because Corbicula are internal brooders ­ the eggs are larger and more expensive to make, and grow up using the resources of the mother, even when the mother doesn't share any genes of the offspring! You could think of sperm from one individual parasitizing or stealing the egg of another individual.

What is interesting is that in Corbicula we have preliminary evidence that egg parasitization can occur between species. The first part of my thesis explores this possibility. The second part tests the idea that egg parasitization led to the infectious spread of androgenesis throughout the genus.

There are two species of Asian Corbicula which have invaded the U.S. freshwater river systems. Although my preliminary phylogenetic work indicates that these species are probably C. leana and C. fluminea, for simplicity I will refer to them as "purple" and "white", which refer to the interior color of their shells.

Corbicula is a nuisance. They foul industrial intake pipes. They also probably outcompete native American bivalves, at the least in areas which have been disturbed by human activity, and possibly elsewhere. They are practically impossible to get rid of, because since they are hermaphrodites only one can produce offspring by itself, and because one clam can have about 68,000 offspring in one year !! (McMahon 1999)

We have preliminary evidence that one species can parasitize the eggs of another species. This has interesting ecological and genetic implications. From an ecological standpoint, eggs become another resource two species compete for. If one species is better at exploiting a resource than another, then it will spread relative to the other. The reason this is interesting is that originally only the whites were found in Texas. But now, the purples have been spreading. Is this just stochastic, or are the purples better at exploiting some resource? What if that resource were eggs?

I am running mating experiments at Brackenridge Field Station in Austin, Texas, to determine the extent to which purples and whites are able to parasitize one another. Through genotyping, I will be able to tell whether one species has used the egg of another to make a clone of itself. These mating experiments will also serve as a pilot study to see whether there are interesting ecological questions to be further examined.

Phylogenetics is the study of the relationships between species. My preliminary phylogeny using only one mitochondrial locus indicates that androgenesis is not monophyletic within Corbicula. What that means is that either: 1) androgenesis arose more than once, 2) that androgenesis arose once and the species reverted back to normal sexual reproduction, or 3) that androgenesis is infectious and can spread between species through polyploidy: the sperm would penetrate the egg but be unsuccessful in ejecting the maternal genome, making offspring with more DNA than either of its parents. This has been observed in the lab (Komaru et al. 2001). We have preliminary evidence that the purple is a polyploidy hybrid between the white and some other species.

To test this hypothesis, I will be performing phylogenetic analyses using genes from both the mitochondrial and the nuclear DNA. Mitochondria are inherited only from the mother. Nuclear DNA is inherited from only the father, or from both parents. Thus, I have different expectations about the shape of the gene trees depending on which hypothesis is correct.

collaborator: Dr. Ted Townsend, University of California, San Diego

Given finite resources, are you going to get a more accurate picture of the relationships between species if you invest your time and money sequencing lots of genes, or sequencing the same few genes in lots of taxa? Rokas et al. (2003, 2005) argue using a data set with a huge number of genes and very few taxa that the answer is: use lots of genes and few taxa. We use simulations to demonstrate that one of the reasons they reach this conclusion is that any analysis will converge on an answer as you add more sequence data. But, using real data, you can't be sure that that answer is the correct answer! We argue that you can never be sure that you have the correct phylogeny using real data. However, you can solve phylogenetic problems such as long branch attraction by increasing the number of taxa. You still need a good number of genes too ­ the more data, the better ­ but you can get a good phylogeny using fewer genes than the twenty recommended by Rokas et al. (2003).

This project is still early in the development stage. The field of phylogeography examines the diversification of populations on a geographic landscape. Many techniques commonly used have underlying assumptions about completeness of sampling and diversification between populations. But how much sampling is good sampling? How much diversification is too much diversification? We don't have a good understanding of when these methods can be applied without violating the assumptions of the models. I will be developing or adapting simulations of populations on a geographic landscape so that we can test the performance of various techniques using different sampling schemes. Ideally, I would like to move towards developing techniques to statistically test alternative phylogeographic hypotheses.

The Coachella Valley Fringe-Toed Lizard has astonishing adaptions for sand dune living, including including toe scales for locomotion on sand, ear scales to block blowing sand, a dorso-ventrally flattened body and wedge-shaped head for burrowing, and specialized nostrils for breathing below sand.

This lizard historically occupied approximately 250 square kilometers of dune habitat in the Coachella Valley, Riverside County, California. Human development in the Coachella Valley has fragmented this once continuous habitat, and has altered the input and size of wind-born sand particles required for the persistence of the dunes (Barrows 2000, Turner et al. 1984). Unfortunately, their specialized morphological features make them extremely vulnerable to changes in dune composition and structure (Turner et al. 1984, Barrows 1997). As a result of the impacts of human-mediated habitat destruction, U. inornata is currently classified as threatened by the U.S. government (USFWS 1980b) and endangered by the State of California. I examined the effect this reduction and fragmentation of habitat has had on the genetic variability of the species using four microsatellite loci. Overall genetic variability is low, and populations are not strongly differentiated. This means that either the lizard had pervasive gene flow before the fragmentation occurred, or (evolutionarily) frequent local extinctions followed by recolonizations. Conservation plans, therefore, need to take into consideration whether these lizards will use habitat corridors, or whether translocation of animals will be required to maintain population genetic processes.