TJ Bliss and Dr. Byron J. Adams, Microbiology and Molecular Biology
Molecular evolution is defined as any change in the genetic makeup of an organism. Since organisms of the same species and community tend to undergo similar molecular changes, rates of molecular evolution are often measured at the species level. It is well established that different species appear to evolve at different rates across evolutionary time. Comparing the rates of evolution among different species determines the relative rate of molecular evolution in those species. Many things are known to affect the rate of molecular evolution of a species. These influences include, but are not limited to: body size, generation time, and latitude.
Specifically, larger individuals tend to evolve more slowly than smaller ones; organisms that take longer to pass on their genetic information (have longer generation times) evolve more slowly than those with shorter generation times; and species at higher latitudes tend to evolve more slowly than those at lower latitudes. A phenomenon known as the Latitudinal Diversity Gradient hypothesis (LDG) describes an increase in species richness from higher to lower latitudes. This pattern is one of the most common in nature and is thought to have major affects on rates of molecular evolution in a number of ways. As you move away from the equator, for example, body size and generation time increase generally, both negatively affecting the overall rate of molecular evolution of a species. In addition, a molecular biologist named Rohde predicted that lower temperatures and decreased solar radiation at higher latitudes may also lead to slowed molecular rates (Rohde, K. 1978 Latitudinal gradients in species diversity and their causes. Biologisches Zentralblatt 97:393-403). Therefore, determining and understanding relative rates of molecular evolution is critical to our understanding of the LDG and a correct understanding of the LDG is important in almost every field of science.
Nematodes are microscopic round worms. They are predicted to make up 4/5 of all animal life on the planet yet are commonly overlooked because relatively few people study them. Most nematodes live in the soil and feed on bacteria, fungus or plants. Nematodes are proving to be exceptional model organisms for understanding many of the main scientific processes currently under investigation, and our research is a novel approach to studying the latitudinal Diversity Gradient hypothesis.
In our study, we examined and compared rates of molecular evolution in nematodes from the McMurdo Dry Valleys of Antarctica to rates of evolution in closely related nematodes from areas closer to the equator. We predicted that the Antarctic nematodes living at higher latitudes should evolve more slowly than their closely related sister taxa living at lower latitudes. This prediction was based on generation time, a key component of the Latitudinal Diversity Gradient. Antarctic nematodes generally complete fewer generations than do nematodes at lower latitudes over the same period of time. Inverting that concept, we predict that nematodes that are able to complete more generations over the same period of time should undergo a relatively higher rate of molecular evolution.
Only three nematode species live in the McMurdo Dry Valleys: Eudorylaimus antarcticus, Plectus antarcticus and Scottnema lindsayae. Their rates of molecular evolution were compared to representatives from the entire phylum and to representatives of genus and family only.
For Scottnema lindsayae we found that when compared to all nematodes, it has slower rates of molecular evolution. Moreover, we found that when compared to just closely related nematodes it continues to exhibit relatively slower rates of molecular evolution, as predicted. We can conclude that S. lindsayae has rates of molecular evolution that support the Latitudinal Diversity Gradient hypothesis. For Eudorylaimus antarcticus we found that when compared to all nematodes it has slower rates of molecular evolution but that it doesn’t evolve any more quickly or slowly than closely related nematodes. This result does not support the LDG so we conclude that latitude is not the factor affecting the evolution of E. antarcticus. We suggest that it evolves more slowly because of some genetic trait(s) it inherited from an ancestor common to the nematodes in the constrained test, and that they all evolve more slowly than nematodes in general. For Plectus antarcticus we found that it is not evolving significantly faster or slower than other nematodes in general but that it is evolving more rapidly than closely related nematodes of lower latitudes. This finding directly opposes the LDG. We predict that these interesting differences in evolutionary rate may be due to the mode of reproduction in P. antarcticus. Other species in the genus are amphimictic whereas P. antarcticus is unisexual and does not have the evolutionary advantages of conventional reproduction. We suggest that it utilizes increased rates of molecular evolution to compensate for the evolutionary constraints of unisexual reproduction.
Overall, our findings show that although latitude can be a factor in determining rates of molecular evolution, many other factors can play major roles as well. The most important thing that must be considered, however, is the usefulness of nematodes in understanding the Latitudinal Diversity Gradient. Nematodes can be found at more latitudes than any other metazoans on the planet. Scottnema lindsayae, for example, is the most southern metazoan known. Yet, it has close relatives near the equator that can be studied and compared in our quest for understanding the underlying principles of the LDG.
This project has taught me a lot about research and learning in the field of microbiology. I’ve stretched my mind and expanded my talents. My mentor has been a great “guide on the side” rather than a “sage on the stage”. He let me work things out on my own and then gave proper correction and helps as I needed them. I plan on continuing my research in this area as I have developed a great passion for the work I am doing.