Geographic variation in reaction norms of phenological traits in the greater duckweed, Spirodela polyrhiza

Abstract

Variability is a ubiquitous feature of natural environments. Organisms can adapt to this through several methods such as adaptive phenotypic plasticity, changes in phenotype in response to reliable cues predicting fitness outcomes across environments, and bet hedging, the maximization of geometric mean fitness. The greater duckweed Spirodela polyrhiza is an ideal system to study the evolution of these strategies. Phenology of production of overwintering structures called turions, a phenotypically plastic trait that can prevent reproductive failure, is thus vital to fitness. Despite clonal reproduction, offspring show phenotypic variability in both turion phenology and size, mediated through the order in which they are produced, suggesting the expression of diversification bet hedging. Here I use S. polyrhiza populations collected from a latitudinal gradient to study how life-history traits evolve in response to variable environments. I make two hypotheses; first, I hypothesized that reaction norms in turion formation differ across latitudes due to differences in season length and environmental predictability. Second, I hypothesised that populations from northern latitudes would trade off offspring size for number to allow for greater diversification potential at northern latitudes where environments are expected to be more variable. I found support for the first hypothesis, showing that reaction norms in turion phenology do differ such that turions are produced earlier at higher latitudes under warmer, but not colder experimental treatments. I was unable to evaluate my second hypothesis due to premature frond mortality but did show that, while size was only weakly correlated with latitude, significant differences between some populations were present, suggesting offspring size is affected more by local environmental conditions that those that are correlated with latitude. There are a number of people I would like to thank for their support and assistance. First and foremost, I would like to thank my supervisor Dr. Andrew Simons for all his support throughout my master's degree and for his expert advice that was crucial to my research. Under his supervision, I have acquired many invaluable skills and I am grateful for everything he has done for me in my time at Carleton. I would also like to thank my advisory committee, Dr. Tom Sherratt and Dr. Risa Sargent for their guidance and helpful comments. I would like to thank Dr. Catherine Cullingham and Dr. Naomi Cappuccino for serving as external examiner and chair for my thesis defence. I acknowledge the financial support I have received from Andrew's NSERC Discovery grant as well as financial support I received from the Faculty of Graduate Studies.