Are Offspring-Heteromorphic Species Hedging Their Bets?

Abstract

Most organisms have canalized the production of their offspring to one form or shape which is presumably best adapted for the environment they inhabit. Nevertheless, at least 500 species of plants, among other organisms, defy this general pattern and instead produce multiple, morphologically different seeds simultaneously. Researchers have proposed numerous drivers of this phenomenon. Theoretical work strongly suggests bet hedging as the major underlying mechanism and this has become a widely accepted but empirically untested assumption. At its core, seed heteromorphism can be attributed to only three underlying mechanisms or some integrated strategy among them: bet hedging, plasticity, and adaptive tracking. In this thesis I focus on empirically testing the long-standing assumption that seed heteromorphism serves as a bet hedging strategy. First, I take a broad perspective in summarizing the occurrence of seed heteromorphism in southwestern North America. Then I use this dataset to phylogenetically evaluate seed heteromorphism as a bet-hedging strategy across an unprecedented 96 seed-heteromorphic species from 51 genera and 9 angiosperm families. In so doing, I evaluate the pervasive, and to date empirically untested, assumption that the occurrence of seed heteromorphism is spatially associated with environmental unpredictability as measured, quantitatively, by aridity. This is the first study to statistically evaluate large-scale evidence for bet hedging among seed-heteromorphic species. Second, I narrow my focus and use field and experimental work to evaluate seed heteromorphism as a bet hedging strategy across multiple populations in a particularly tractable winter, desert annual species, Pectocarya heterocarpa (I.M. Johnston) I.M. Johnston (Boraginaceae). This species produces visually distinct and spatially separated seed morphs which allow for the assessment of phenotypic expression in reproductive investment and facilitate experimental manipulation. I couple my field collections with greenhouse studies to evaluate the role of plasticity in the expression of seed heteromorphism. I demonstrate that P. heterocarpa hedges its bets by plastically adjusting its ratio of low risk to high risk seed morphs across the aridity gradient in the direction predicted by bet hedging theory. I also show that seed germination fraction across the aridity gradient follows patterns expected from bet hedging with seeds from more variable sites displaying increased dormancy. Third, I focus even more locally on just one population of P. heterocarpa to quantify the fitness consequences of its different seed morphs across years, environmental conditions, and germination cohorts. Using a field and greenhouse experiment I evaluate how the natural germination timing of P. heterocarpa seed morphs translate to fitness differences across different rainfall regimes and years. Overall, I find evidence for bet hedging in that seed types never had significant average fitness differences across treatments, years, or germination cohorts, but instead exhibited complex interactions with these factors such that each seed type had highest fitness under different combinations of conditions. My results demonstrate that the different seed morphs of P. heterocarpa, by virtue of these fitness differences, act to reduce variance in fitness both within and across years as predicted by bet hedging theory. The assumption that offspring heteromorphism serves as a bet hedging strategy is pervasive in the literature despite the lack of rigorous empirical tests. In this dissertation I combine large- and narrow-scale analyses to present a rigorous analysis of bet hedging among offspring-polymorphic species. Overall, this work helps us gain a better understanding of how organisms cope with change, which is crucial for gaining any predictive power for population dynamics amidst global climate change.