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Textbook examples of multicellular organisms vary in their scale and complexity but are typically composed of a single species. The prevalence of entities such as lichens, however, suggest that two different species may be capable of forming a type of multi-species multicellularity-though it may not resemble its clonal counterparts. In this chapter, we consider the possibility of multi-species multicellularity and in particular its origins. Drawing upon previous studies of the evolutionary origins of clonal multicellularity, we focus on the emergence of simple reproducing groups that have the capacity to gain adaptations. We present a framework for organizing these initial multi-species group life cycles based on whether the constituent species are unicellular or multicellular and whether the groups reproduce via fragmentation or cycles of dissociation and re-association. We discuss characteristics of each type of multi-species multicellularity and representative examples to assess their likely evolutionary trajectories. Ultimately, we conclude that the multi-species groups that most resemble textbook multicellular organisms are composed of unicellular and multicellular species and reproduce via cycles of dissociation and re-association.

Multicellular organisms, such as plants and animals, have independently evolved several times over the last hundreds of millions of years. The evolution of multicellularity has significantly shaped modern ecosystems, yet its origins remain largely unknown. Due to the ancient history and the small size scale of early multicellular organisms, few intact fossils have been preserved. To uncover the origins of large and complex life, researchers have turned to alternative methods such as phylogenetic modeling, experimental evolution, and theoretical frameworks. While these approaches have provided novel insights in the early steps of multicellular evolution, few studies have considered the role of adaptation in these novel life cycles. This thesis addresses the gap in our knowledge by employing mathematical modeling and computer simulations to study adaptation in novel multicellular life cycles.

The first paper investigates the effects of unicellular reproduction modes, such as budding or binary fission, on the spread of growth rate mutations. It demonstrates that unicellular history significantly influences the adaptation rate, with budding cells exhibiting greater sensitivity to the spatial distribution of mutations.

In Paper II, the role of multicellular reproduction mode for the adaptation of altruistic and selfish mutations is explored. Specifically, the study examines how adaptation is affected when the filaments are exposed to a size-based selective pressure. It reveals that while the adaptation of altruistic mutations is favored by large offspring, the spread of selfish mutations depends on both offspring size and selection strength.

While Papers I and II assume deterministic life cycle structures at the multicellular level, paper III investigates the evolution of life cycle regulation when cells use internal information. The model demonstrates that when cells only have access to a limited amount of information, there is significant variation in the types of life cycles that emerge. This suggests that to evolve regulated life cycles, additional mechanisms beyond internal information may be necessary, such as cell communication.

Papers I-III explore multicellular life cycles where all cells are of the same type, yet most multicellular organisms have evolved cell differentiation, with specialized cells performing various tasks. In Paper IV, the evolutionary paths leading to differentiated multicellularity are investigated when a unicellular population is exposed to an abiotic (non-evolving) selective pressure. The model reveals that while a wide range of phenotypic backgrounds and environmental conditions may induce differentiation and multicellularity, continued adaptation to the stress eventually leads to reversion to unicellularity. This reversion occurs because as cells adapt to the stress, the costs associated with differentiation and group formation may no longer be justified. One potential strategy to prevent reversion could involve considering biotic selective pressures that can co-evolve with the population.

Lastly, paper V delves into organisms composed by combinations of uni- and multicellular species. Utilizing this framework to examine present multi-species multicellularity reveals that the species composition influences both the ease of partnership establishment and its stability. Additionally, these chimeric groups can reproduce through various strategies, including fragmentation and complete dissociation. Leaving the constellation endows organisms with a memory of prior partnerships, enhancing their adaptability in forming new ones. This extension opens up novel evolutionary pathways for further exploration.

In summary, this thesis offers new insights into how the life cycle structures of simple multicellular organisms impact mutation accumulation and the acquisition of new traits. The adaptability of organisms plays a pivotal role in fostering higher complexity and paving the way for further evolution. Enhancing our understanding in this domain will continue to illuminate the origins of complex life and elucidate the evolutionary factors underlying the rich diversity of multicellular organisms we encounter today.

Multicellularity has evolved several independent times over the past hundreds of millions of years and given rise to a wide diversity of complex life. Recent studies have found that large differences in the fundamental structure of early multicellular life cycles can affect fitness and influence multicellular adaptation. Yet, there is an underlying assumption that at some scale or categorization multicellular life cycles are similar in terms of their adaptive potential. Here, we consider this possibility by exploring adaptation in a class of simple multicellular life cycles of filamentous organisms that only differ in one respect, how many daughter filaments are produced. We use mathematical models and evolutionary simulations to show that despite the similarities, qualitatively different mutations fix. In particular, we find that mutations with a tradeoff between cell growth and group survival, i.e. "selfish" or "altruistic" traits, spread differently. Specifically, altruistic mutations more readily spread in life cycles that produce few daughters while in life cycles producing many daughters either type of mutation can spread depending on the environment. Our results show that subtle changes in multicellular life cycles can fundamentally alter adaptation.

Early multicellular organisms must gain adaptations to outcompete their unicellular ancestors, as well as other multicellular lineages. The tempo and mode of multicellular adaptation is influenced by many factors including the traits of individual cells. We consider how a fundamental aspect of cells, whether they reproduce via binary fission or budding, can affect the rate of adaptation in primitive multicellularity. We use mathematical models to study the spread of beneficial, growth rate mutations in unicellular populations and populations of multicellular filaments reproducing via binary fission or budding. Comparing populations once they reach carrying capacity, we find that the spread of mutations in multicellular budding populations is qualitatively distinct from the other populations and in general slower. Since budding and binary fission distribute age-accumulated damage differently, we consider the effects of cellular senescence. When growth rate decreases with cell age, we find that beneficial mutations can spread significantly faster in a multicellular budding population than its corresponding unicellular population or a population reproducing via binary fission. Our results demonstrate that basic aspects of the cell cycle can give rise to different rates of adaptation in multicellular organisms.

With climate change, droughts are expected to be more frequent and severe, severely impacting plant biomass and quality. Here, we show that overexpressing the Arabidopsis gene AtFtsHi3 (FtsHi3OE) enhances drought-tolerant phenotypes without compromising plant growth. AtFtsHi3 encodes a chloroplast envelope pseudo-protease; knock-down mutants (ftshi3-1) are found to be drought tolerant but exhibit stunted growth. Altered AtFtsHi3 expression therefore leads to drought tolerance, while only diminished expression of this gene leads to growth retardation. To understand the underlying mechanisms of the enhanced drought tolerance, we compared the proteomes of ftshi3-1 and pFtsHi3-FtsHi3OE (pFtsHi3-OE) to wild-type plants under well-watered and drought conditions. Drought-related processes like osmotic stress, water transport, and abscisic acid response were enriched in pFtsHi3-OE and ftshi3-1 mutants following their enhanced drought response compared to wild-type. The knock-down mutant ftshi3-1 showed an increased abundance of HSP90, HSP93, and TIC110 proteins, hinting at a potential downstream role of AtFtsHi3 in chloroplast pre-protein import. Mathematical modeling was performed to understand how variation in the transcript abundance of AtFtsHi3 can, on the one hand, lead to drought tolerance in both overexpression and knock-down lines, yet, on the other hand, affect plant growth so differently. The results led us to hypothesize that AtFtsHi3 may form complexes with at least two other protease subunits, either as homo- or heteromeric structures. Enriched amounts of AtFtsH7/9, AtFtsH11, AtFtsH12, and AtFtsHi4 in ftshi3-1 suggest a possible compensation mechanism for these proteases in the hexamer.