Role of Seasonal and Geographic Temperature Variation in the Life Cycle of the Clonal Sea Anemone Diadumene Lineata (Verrill)

Clonality is the general term that encompasses all manner of pinching, splitting, budding, and fragmenting behaviors by which organisms divide their somatic body tissues into more or less independent units. It can be as straight forward as fragments of a sponge surviving after being rent apart by a hurricane or as convoluted as the telescoping generations of parthenogenic aphids. Clonality is a widespread feature of animal life cycles and the degree of clonal investment is expected to affect everything from spatial genotypic and genetic structure to evolutionary dynamics and ecology interactions. Yet, the shear diversity and complexity of clonal behavior has hampered efforts to develop a general understanding of how and why clonality evolves as the adaptive benefits of these behaviors may be as idiosyncratic as the mechanisms by which cloning occurs. Contrary to some past formulations of the problem, the production of clonal progeny is not typically an alternative to sexual reproduction, as most clonal organisms also reproduce sexually. While there is often an immediate tradeoff where a unit of energy can either be invested in gametes or clonal progeny at any given time, there is not inherently a tradeoff between asexual and sexual reproduction over the span of a lifetime. Dividing somatic tissue in to separate units can be a way of increasing total lifetime fecundity by increasing total biomass, more efficiently colonizing open space or promoting longevity by spreading the risk of mortality over spatially-separated somatic units. With this perspective, understanding the adaptive value of clonality becomes a matter of analyzing the holistic suite of fitness effects that arise from variation in allocating energy among unitary growth, clonal propagation and gametogenesis. The amount of energy available and the fitness value of a particular investment strategy are governed in large part by the environment and so understanding the environmental context is key to understanding the forces shaping life cycle evolution. Temperature, in particular, affects the metabolic cost of maintaining body tissues and is key in determining the energetically optimal body size for a unitary animal. Where temperatures fluctuate seasonally or where clonal replicates may spread across a heterogeneous landscape, the reaction norm of fission rate, body size or traits associated with gamete production may be an important target of selection, influencing which life cycle patterns can evolve. In this dissertation I examine the influence of seasonal and geographic temperature variation on fission rate, body size and gamete production of the clonal sea anemone, Diadumene lineata (Verrill 1869), to better understand the constraints and tradeoffs that govern the evolution of resource allocation strategy; and ultimately, the factors that drive the evolution of clonality in this species. Through a combination of laboratory experiments, field observations, optimality modeling and genetic tools I demonstrate that (1) fission rates are strongly temperature dependent, resulting in seasonal and geographic variation in clonal behavior, (2) the production of gametes is closely tied to body size and shows an inverse latitudinal pattern with fission rate, (3) the observed reaction norm of fission rate with temperature is consistent with selection to maximize gamete production across the locally experienced range of temperatures, as opposed to selection for maximum clonal proliferation, per se, and (4) there is a latitudinal decrease in genotypic richness and diversity that corresponds with changes in fission rate, suggesting that variation in fission rate leads to changes in the spatial structure of genetic variation among sites. Together, these results are consistent with the hypothesis that clonality is adaptive under conditions where individual body size is constrained by the environment. Under these conditions more gametes may be produced over a lifetime by genets dividing somatic tissue into multiple small units rather than remaining a single large unit. In this species, there is an immediate cost to dividing a large body into two pieces as the number of gametes produced by two small individuals sums to less than those produced by a large individual, yet, the lost reproductive potential may be able to be compensated for over time by an increased growth rate at a smaller body size. Additional costs and benefits imposed by changes in mortality rate, competitive ability or mate choice as fission rate changes remain to be investigated and may be equally important in understanding and predicting the evolution of clonal behavior in this and other species.