Examining Drivers Of Phenotypic Variation In The Perennial Herb Showy Milkweed (Asclepias Speciosa).

Sussman, Kira, B.S., December 2017 Ecology and Organismal Biology Faculty Mentor: Dr. Lila Fishman Many plant species show variation in phenotypic traits, such as traits related to growth and defense against herbivores, across environmental gradients. Differences in phenotypic traits can be due to selection, leading to local adaptation, or due to the random process of genetic drift. To examine the driver of phenotypic variation, I conducted a Qst (a measure of phenotypic variation among populations) vs. Fst (a measure of genetic variation among populations) analysis for 13 populations of A. speciosa grown in a common garden, using five growth and defense traits and seven microsatellite markers. I found relatively low differentiation at the neutral markers (mean Fst = 0.005), and population differentiation of plant height, leaf shape, and latex production traits (but not trichome density or specific leaf area). These results suggest that the three highly differentiated growth and defense traits are responding to population-specific selection pressures, indicating local adaptation of A. speciosa distributed across an environmental gradient. Examining Drivers Of Phenotypic Variation In The Perennial Herb Showy Milkweed (Asclepias Speciosa). Introduction Plant species are often distributed as separate populations that occur in a variety of different environments, with each population experiencing different abiotic and biotic conditions (Gould et al., 2014; Kooyers et al., 2015). One common feature of spatially separated populations is that they often evolve and exhibit different growth and defense traits, called phenotypic traits (Anderson et al., 2015). If selection by the biotic or abiotic environment is strong, then the population should reflect traits that match the environmental conditions. For instance, if selection by a biotic factor, such as high herbivore pressure, is strong in a certain population then the population should evolve strong defense traits (Abdala-Roberts et al., 2016). If selection by an abiotic factor is strong, such as rainfall, then the population should evolve strong growth or stress tolerance traits. Such selection for traits that match the environmental conditions leads to locally adapted populations (Kawecki and Ebert, 2004; Blanquart et al., 2013; Richardson et al., 2014). ). Local adaptation is the process through which one population develops higher fitness in its environment compared to another population of the same species introduced to that environment (Kawecki and Ebert, 2004; Blanquart et al., 2013; Richardson et al., 2014). Differences in phenotypic traits between populations can also be due to other evolutionary forces, but this has received considerably less attention as a mechanism for describing differentiation of traits among populations than local adaptation. Genetic drift, which occurs due to random genetic changes by sampling error during reproduction, can also create differences in traits among spatially separated populated (Hendry et al., 2001; McKay and Latta, 2002; Kawecki and Ebert, 2004). Some separated populations may show little or no trait variation between populations, which can occur due to the stabilizing effects of gene flow or similar environmental conditions (Kawecki and Ebert, 2004). In order for local adaptation to occur, traits must be affected to a greater degree by selection than by drift or gene flow (Hendry et al., 2001; McKay and Latta, 2002; Kawecki and Ebert, 2004). Both biotic and abiotic factors can act as selective forces; biotic factors tend to act on defense traits, while abiotic factors tend to act of growth traits (Abdala-Roberts et al., 2016). While studies have addressed the individual effects of either biotic or abiotic factors as selective forces, there has been little work has compared how these factors might select for different traits. Showy Milkweed (Asclepias speciosa) is a self-incompatible perennial plant that occurs across much of the western part of North America, and can reproduce through underground clonal roots as well as seed production (Wyatt and Broyles, 1994). Pollination occurs mainly by bees and flies which visit the flower for its abundant nectar (Wyatt and Broyles, 1994). Pollen is dispersed in pollinia, units of hundreds of pollen grains, and dispersal distance largely depends on pollinator behavior, but can be upwards of several kilometers (Wyatt and Broyles, 1994). Fruits typically contain 50-100 plumed seeds (Bookman, 1983), all of which are full siblings (Morse and Schmitt, 1985). Seeds are wind dispersed, and the dispersal distance depends on seed mass, wind speed, and height of release, and can result in long-distance dispersal events (Morse and Schmitt, 1985). Asclepias speciosa is attacked by a suite of specialist herbivores, including the Monarch Butterfly, which uses A. speciosa for ovipositation, cardenolide sequestration, and food (Ackery and Vane-Wright, 1984; Malcolm, 1994). A. speciosa is an ideal species to study local adaptation, as it occurs across a variety of environmental conditions and exhibits diverse growth and defense traits between populations. Previous work in other milkweed species has shown a genetic component to defense trait variation, but the mechanism behind the variation has not been determined. This study builds on previous work that address the mechanisms of phenotypic variation between populations of Asclepias found on different continents (Agrawal et al., 2016). In this study, I analyzed variation phenotypic traits between 13 different populations across the Northern United States grown in a common garden. I tested whether growth and physiological traits (specific leaf area, leaf length:width ratio, and height) and defense traits (latex production and trichomes) were differentiated between the populations, and calculated Qst. In order to test for the effects of selection on differentiation of these traits, relative to other evolutionary processes, I analyzed the population genetic structure using microsatellite DNA markers. Microsatellites are 1-6 nucleotide repeats that are found in nuclear DNA (Selkoe and Toonen, 2006). Because they are generally found in noncoding regions and have high mutation rates (Selkoe and Toonen, 2006), microsatellites are putatively neutral and often highly variable, which makes them suitable for population genetic analysis. As putatively neutral markers, microsatellites reflect demographic processes (drift and migration or gene flow, mating system) that affect all parts of the genome equally (Selkoe and Toonen, 2006). I then conducted a Qst vs. Fst analysis to determine which evolutionary mechanism is more important in the resultant trait differentiation. If Qst (measure of phenotypic trait differentiation) is greater than Fst (measure of neutral genetic variation), populations are more distinct that expected from drift alone, indicating local adaptation as the cause of the trait differentiation (Hendry et al., 2001; Whitlock and Guillaume, 2009). If Qst is equal to Fst, the result indicates that the amount of phenotypic variation between populations is equal to the amount of neutral genetic variation, so drift cannot be ruled out as the cause of differentiation. If Qst is less than Fst, the result indicates that there is less phenotypic variation than neutral genetic variation, which could indicate stabilizing or balancing selection. Overall, this work integrates selection by biotic and abiotic factors with population genetic structure to analyze the mechanisms and forces behind the phenotypic differentiation between different populations of A. speciosa.