Mark Ungerer
Our research is broadly focused on understanding the underlying genetic basis of adaptive natural variation and evolutionary change in plants. Most of our work is conducted on species for which there are considerable and/or growing genetic and genomic resources available. Two current projects are described below:
Clinal variation in freezing tolerance in Arabidopsis thaliana.
Populations of the model plant species Arabidopsis thaliana are found over a broad geographic and latitudinal range. We have documented considerable variation in freezing tolerance among natural populations that follows a latitudinal gradient, with populations from northern latitudes expectedly more tolerant of low temperature than population from southern latitudes. This finding suggests a major role for natural selection in shaping this variation. We are currently investigating the underlying genetic and physiological basis of these natural differences by examining functional variation in important freezing tolerance candidate genes. We are additionally examining variation in global metabolite and lipid profiles, as previous work has indicated an important role of various metabolites/lipids in conveying increased freezing tolerance. These latter experiments are utilizing the Kansas Lipidomics Research Center at KSU.
Hybrid speciation, transposable elements and genome evolution in wild sunflowers.
The class I transposable elements known as long terminal repeat (LTR) retrotransposons are ubiquitous in plants and represent a significant genomic fraction in plant species with large genomes. The vast majority of LTR retrotransposons remain transcriptionally and transpositionally quiescent during normal growth and development. These periods of inactivity, however, can be interrupted by episodic bursts of transpositional activation during which massive genomic expansion and restructuring can occur. The conditions under which such events take place naturally and the consequences of such events on host species evolution are poorly understood.
We are investigating the probable causes and evolutionary consequences of massive LTR retrotransposon proliferations that have taken place independently in three diploid hybrid sunflower species, Helianthus anomalus, H. deserticola, and H. paradoxus. The evolutionary history of these hybrid taxa has been well characterized: each of the hybrid species is independently derived via hybridization between the same two parental taxa (H. annuus andH. petiolaris), and each hybrid species is adapted to an abiotically extreme environment. The evolutionary history of the hybrid sunflower species is extremely relevant because both hybridization and environmental stress have been implicated as natural agents influencing retrotransposon activation. The origin of these taxa via hybrid speciation and the ecological transitions of these hybrid species to abiotically extreme environments raise a very intriguing question: what role has LTR retrotransposon proliferation played in these evolutionary events? Have these proliferations in some way facilitated the evolutionary events that have taken place in this group? Or have these evolutionary events occurred independently of, and in fact notwithstanding the massive transposable element bombardments that have taken place in each hybrid species' genome? Support for either alternative would deeply impact our understanding of the role of transposable elements in organismic evolution.
