Transposable elements are DNA sequences capable of mobilizing or proliferating in the genomes of their host. The natural conditions under which transposable elements become active and proliferate, and the consequences of proliferation events on host species evolution, are poorly understood. This research will investigate the genomic and environmental conditions that likely facilitated massive and independent proliferation events of gypsy-like long terminal repeat (LTR) retrotransposons in three diploid hybrid sunflower species adapted to extreme environments. The evolutionary history of the three hybrid sunflower species is especially relevant because both hybridization and environmental stress have been implicated as drivers of retrotransposon activation. Mobilization and proliferation activity of retrotransposons (as well as genomic modifications that can facilitate such events) will be investigated and characterized both in naturally occurring sunflower hybrids as well as in hybrid individuals synthesized and subjected to stress in the greenhouse. This project will greatly increase our understanding of the role of transposable elements in organismic evolution.

Students involved in this project will be recruited through the KSU Developing Scholars Program. This program pairs KSU undergraduates from under-represented groups with faculty mentors to provide students with early opportunities for research endeavors. Other students will come through the KSU Summer Research Opportunities Program (SUROP) which also targets students from under-represented groups. The PI will participate in outreach through the NSF-funded GROW (Girls Researching Our World) Summer Workshop. This is a three-day summer science workshop for middle school age girls held on the KSU campus and at the nearby Konza Prairie Biological Station. Sunflowers play a prominent role in the natural history of the Flint Hills tall grass prairie in eastern Kansas. Outreach efforts will be directed towards the design and deliver a short workshop explaining basic aspects of sunflower natural history,domestication, and the use of these plants by Native American Peoples. This workshop will be delivered each summer for the duration of the proposed research.

Project Report

Native protein-coding genes make up an astonishingly small fraction of the nuclear DNA of most plant and animal species. In plants such as corn, barely, sunflower, and many others, the majority (80+%) of nuclear DNA consists of repeating DNA sequences known as long terminal repeat (LTR) retrotransposons. These super-abundant sequences are related evolutionarily to infectious retroviruses such as HIV and represent a general class of DNA known as transposable elements or TEs. Long terminal repeat retrotransposons have the capacity to proliferate in copy number within a genome because a single sequence element can replicate and give rise to numerous daughter copies each of which can insert at a new location in the genome (and subsequently replicate again). Unlike infectious retroviruses, however, long terminal repeat retrotransposons cannot be transmitted horizontally from plant to plant (or from cell to cell) and thus are limited to vertical transmission from parent plant to offspring. Understanding the natural phenomena that trigger activation and proliferation of these super-abundant sequences is important for understanding the structure and evolution of plant and animal genomes. Wild species in the sunflower genus Helianthus provide an excellent group in which to investigate how and why these sequences proliferate and come to represent such a large fraction of the genome. Three sunflower species within this group arose, independently, via ancient hybridization events between the same two parental sunflower species. The three hybrid species possess genomes that are 50-75% larger in DNA content than the parental species despite the fact that all species have the same number of chromosomes. These large scale changes in genome size are attributable to proliferation events of specific variants of LTR retrotransposons. In addition to having arose via hybridization, the three hybrid sunflower species also inhabit environmentally harsh conditions: two of these species grow in desert-like conditions and the third grows in salt marshes. Both hybridization and stress have been hypothesized as 'triggers' of LTR retrotransposon activation and proliferation. Utilizing both natural populations and controlled greenhouse experiments, we have examined the potential roles of hybridization and abiotic stress as natural triggers of LTR retrotransposon activation and proliferation. Despite the large scale proliferation events that occurred historically in the sunflower hybrid species, neither hybridization nor stress were shown to activate LTR retrotransposons and lead to their proliferation. Interestingly, however, we have identified the specific sublineages of LTR retrotransposons that proliferatied in the sunflower hybrid species and have described some of their properties. Unlike most other LTR retrotransposons in these sunflower species, these specific proliferative sublineages remain transcriptionally active, meaning they continue to possess the capacity to proliferate. This capacity appears to be higher in the sunflower hybrid species (i.e., transcription rates are higher) than in the parental species from which they're derived, indicating additional forces either promoting their proliferation (or hindering their repression). Work describing these and other results have been published in five different peer-reviewed publications with a sixth paper currently in preparation. Regarding scientific outreach, I participated in the Kansas State University GROW (Girls Researching Our World) summer workshop in 2008, 2009, and 2010. This is a three day summer science workshop for middle school age girls held on the KSU campus. I designed and delivered workshops explaining basic aspects of sunflower biology, natural history, domestication, and the use of these plants by Native American Peoples. Sunflowers play a prominant role in the natural history of Flint Hills tall grass prairie here in eastern Kansas. During the period of this NSF grant, training was provided to two postdocs, a PhD graduate student, a technician, and several undergraduate researchers.

National Science Foundation (NSF)
Division of Environmental Biology (DEB)
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Samuel M. Scheiner
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Kansas State University
United States
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