ABSTRACT

Unlike many animals, plants are sessile and unable to escape from unfavorable environments, so they must rely on developmental flexibility to cope with unpredictable changes in their environment. It is well known that plants accomplish this goal by responding to seasonal cues such as temperature and day length, and use these cues to initiate major life history transitions such as, from vegetative growth to flowering at the appropriate time of year. One of the most striking morphological switches is the shift between different flower forms on the same plant. Such species can shift from a flower that is relatively large and attracts pollinators that promotes mating between individual plants to a flower that is relatively much smaller, in part because it does not need to attract an animal to visit it to reproduce, because it is capable of mating with itself. This developmental shift is thought to be triggered when an environmental cue reaches a threshold. However, reproduction by each flower type comes with different consequences; small self-pollinating flowers produce inbred offspring that often suffer from a reduction in vigor compared to progeny produced from matings between individuals. The researchers will uncover genetic components of the pathways that result in populations exhibiting different thresholds for variable environmental responses using Mimulus douglasii, an emerging model plant that is capable of floral morph switching. They will examine the selective advantage of the loci responsible for floral morph shifts under natural field conditions. One prediction is that populations living in harsh environments with limited water and nutrients will be selected to lower their threshold and produce more self-pollinating flowers to save resources despite the potential reduction in vigor following inbreeding. The results of the proposed study could be used to enhance understanding of how agricultural species sense and respond to the environment, and could be used to modify crops to allow them to grow under novel environments. The ways in which plants can adjust to novel habitats will be shared with elementary, middle school, and high school students in the Durham area. Additionally the researchers will mentor Duke undergraduate students in the lab and the field.

This study will (1) determine how a polyphenic environmental response threshold shift can occur, by examining the molecular genetic basis of the difference in threshold responsiveness and (2) assess the fitness consequences of these genetic loci under natural field conditions to understand why the shift in threshold occurred. To accomplish these goals two populations of M. douglasii that occur at the extremes of environmental responsiveness will be hybridized to form F2 individuals. First, a genetic mapping experiment will be done to find the genomic loci that correlate with a change in threshold for the production of particular floral morphs under long-day photoperiods. Mapped loci from this objective will form the basis for the second line of investigation: F3 hybrid individuals will be grown in two field common gardens. Correlates to fitness that will be measured include germination, flower number, flower type, and total seed production per plant. These fitness measurements will be correlated with the genomic loci mapped for floral morph production thresholds. To gain greater insights the researchers have the whole genome sequence data for M. douglasii, its sister species M. kelloggii, and an assembled and roughly annotated genome of a close relative, M. guttatus, where the latter two species exhibit only one flower morph. Collectively these data will aid in the understanding of the genetic basis of the role of threshold responses influencing the evolution of life history strategies.

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