Date of Award
Master of Science
Jeff Karron, Michael Graham
Giant Kelp, Isolation By Adaptation, Mating Systems, Plasticity
Organisms living along environmental gradients often utilize phenotypic plasticity to maximize their survival across a range of conditions. Wherever gradients occur, there is potential for divergence through isolation-by-adaptation (IBA) to build-up between genotypes experiencing different selective pressures. Plasticity in traits pertaining to mating systems in particular are likely to constitute an interesting and revealing model for the study of the underlying mechanisms behind parapatric speciation. Giant kelp, Macrocystis spp., shows striking plasticity in holdfast morphology and reproductive strategy when colonizing intertidal (M. integrifolia morph) versus subtidal (M. pyrifera morph) areas along temperate rocky coastlines of the eastern Pacific Ocean. In the intertidal, high photosynthetically-active radiation (PAR) and UV radiation limit development of spores, recruitment of microscopic gametophytes, and growth and survival of embryonic sporophytes of M. pyrifera. Although depth of parent sporophytes influences spore survival in irradiance-stressed environments, few studies have examined the effects of irradiance stress on M. integrifolia’s developmental stages. This study focuses on understanding the roles of IBA and plasticity in maintaining Macrocystis morphs along the California coastline. To test for genetic isolation caused by ecological divergence in the intertidal, we performed fine scale spatial sampling and molecular analysis of parapatric intertidal and subtidal populations off of the Central Californian coast. Using seven microsatellite markers, we compared genetic differentiation between morphs within sites and among morphs across different sites. Furthermore, we identified the presence of clonal replicates in intertidal populations. Results show higher differentiation between adjacent subtidal and intertidal morphs than between the same morph at larger spatial scales, suggesting isolation-by-adaptation. Several potential mechanisms could explain this result: assortative or other non-random mating, longer generation times promoted by asexual growth (intertidal morph), and differential mortality due to early adaptive divergence. Spatial analyses of clonal structure do not indicate asexual reproduction as the dominant strategy in the intertidal. To explore the hypothesis of differential mortality due to adaptive divergence, we will experimentally test assortative mating at different early development stages using controlled crosses of the two morphs under different treatments of irradiance (PAR and UV) stress. Surviving embryonic sporophytes will be genotyped and a paternity analysis will be conducted. Specifically, we hypothesize offspring from M. pyrifera parents will experience higher than expected mortality under irradiance stress, such that paternity analyses will reveal lower than expected numbers of M. pyrifera offspring among surviving embryonic sporophytes.
The overarching goal of this research program is to determine if phenotypic plasticity in mating system traits observed in giant kelp may be facilitating incipient parapatric speciation in the intertidal zone. This thesis will consist of three chapters. The first will concentrate on understanding the impact of alternative methods M. pyrifera utilizes for dispersal at both ecological and evolutionary scales. The second will focus on characterizing genetic differentiation and structure patterns between adjacent populations of each morph. The third will develop additional hypotheses focused on understanding differential mortality between morphs under stress.
Hargarten, Heidi, "A Tale of Two Morphs: Genetic and Genotypic Structure Between Macrocystis Pyrifera and Macrocystis Integrifolia" (2015). Theses and Dissertations. 953.