Transposable elements (TEs) are small DNA sequences that increase their number in host genomes by replicative transposition (moving between different parts of the genome). Because of their negative fitness impacts on hosts, TEs are considered as genomic parasites and their interaction with host genomes is a fundamental and ubiquitous class of host-parasite coevolution. Even though happening infrequently, invasion of TEs into new hosts is expected to reveal a large evolutionary impact on host genomes over a short period because of the high rates of TE movements following the invasion. This study proposes to investigate host-TE coevolutionary dynamics from the host?s perspective by focusing on the evolutionary effects of P elements, which invaded Drosophila melanogaster less than a century ago. The research will identify candidate genes that might have an important role in the coevolutionary dynamics followed by an exploration of the functional consequences of those genes.
Understanding the coevolution of hosts and TEs is crucial to human health because human genomes are also infested with TEs and they may be the cause of several human diseases. Also, TEs have been proposed as plausible biological control agents because of their replicative and invasive nature. This proposed study will further the understanding of the evolutionary impacts of introduced TEs on the targeted organisms, which is crucial for assessing the feasibility of such proposals.
Unlike most genes that have fix positions on an organism’s chromosomes, transposable elements (TEs) have the ability to move between different locations on the chromosomes. They are generally considered genomic parasites because they can selfishly increase their number and, when moving around, can disrupt functions of essential genes. TEs are found in almost all organisms surveyed and their interaction with their "host" is a fundamental example of host-parasites coevolution. TEs of an organism can be further divided into different "families" according to variation in their interaction with the host. Most TE families are inherited from parents to offspring. In rare cases, TE families can move to a new host species and spread epidemically. During the spread of a newly invaded TE family, the hosts were observed suffering from reduced fertility, which is the result of increased activities of the newly invaded TE family. This is expected to impose strong selective pressure on host. A host gene that can ameliorate such impact would be favored and become prevalent in the population. However, there have been no studies formally testing such hypothesis. When studying the evolutionary impact of a specific factor, the most ideal approach would be to directly observe the progress of evolution, which is generally not feasible. P element is a TE family that invaded fruit fly (Drosophila melanogaster) early last century. Fruit flies collected before the invasion of P elements and maintained since then in the laboratories are still free of P elements while all the current wild fruit flies have P elements. We utilized this rare opportunity and compared the fruit fly populations collected before and after the P element invasion to investigate the evolutionary impacts of newly invaded TE families on host gene. We focused on genes that are known to have functions related to interaction with/suppression of P elements. However, none of the surveyed genes showed convincing evidence supporting that the invasion of P elements did lead to differences in gene sequences between fruit flies collected before and after the P element invasion. We also developed a model to further corroborate and to understand the potential reason for our observations. Our modeling result is consistent with our empirical observations: we found that the spread of the newly invaded P element is fast in the fruit fly population, leading to a short time period during which a host gene can enjoy its selective benefits. This leads to no detectable evolutionary influence of the P element invasion on host genes. Accordingly, we concluded that, even though invasion of a new TE family can have strong selective impact on the host, its influence is ephemeral and unlikely to be long enough to have an observable impact on the host. Our study not only formally addressed the long-standing question but also found an unexpected result, laying an important step towards further investigation of host-TE coevolution. There were two undergraduate researchers involved in the project. Both of them gained skills in molecular experiments and experimental design. By involving in the project, one of the students developed further interests in becoming a scientist and is now preparing for graduate school application.
