Abstract
Numerous living systems are hierarchically organized, whereby replicating components are grouped into reproducing collectives—e.g., organelles are grouped into cells, and cells are grouped ...into multicellular organisms. In such systems, evolution can operate at two levels: evolution among collectives, which tends to promote selfless cooperation among components within collectives (called altruism), and evolution within collectives, which tends to promote cheating among components within collectives. The balance between within- and among-collective evolution thus exerts profound impacts on the fitness of these systems. Here, we investigate how this balance depends on the size of a collective (denoted by N) and the mutation rate of components (m) through mathematical analyses and computer simulations of multiple population genetics models. We first confirm a previous result that increasing N or m accelerates within-collective evolution relative to among-collective evolution, thus promoting the evolution of cheating. Moreover, we show that when within- and among-collective evolution exactly balance each other out, the following scaling relation generally holds: Nmα is a constant, where scaling exponent α depends on multiple parameters, such as the strength of selection and whether altruism is a binary or quantitative trait. This relation indicates that although N and m have quantitatively distinct impacts on the balance between within- and among-collective evolution, their impacts become identical if m is scaled with a proper exponent. Our results thus provide a novel insight into conditions under which cheating or altruism evolves in hierarchically organized replicating systems.
The first step in the evolution of complex multicellular organisms involves single cells forming a cooperative group. Consequently, to understand multicellularity, we need to understand the costs and ...benefits associated with multicellular group formation. We found that in the facultatively multicellular algae Chlorella sorokiniana: (1) the presence of the flagellate Ochromonas danica or the crustacean Daphnia magna leads to the formation of multicellular groups; (2) the formation of multicellular groups reduces predation by O. danica, but not by the larger predator D. magna; (3) under conditions of relatively low light intensity, where competition for light is greater, multicellular groups grow slower than single cells; (4) in the absence of live predators, the proportion of cells in multicellular groups decreases at a rate that does not vary with light intensity. These results can explain why, in cases such as this algae species, multicellular group formation is facultative, in response to the presence of predators.
Understanding the evolutionary transition to multicellularity is a key problem in biology.1,2,3,4 Nevertheless, the ecological conditions driving such transitions are not well understood. The first ...known transition to multicellularity occurred 2.5 billion years ago in cyanobacteria,5,6,7 and today’s cyanobacteria are characterized by enormous morphological diversity. They range from unicellular species; unicellular cyanobacteria with packet-like phenotypes, e.g., tetrads; and simple filamentous species to highly differentiated filamentous species.8,9,10 The cyanobacterium Cyanothece sp. ATCC 51142, an isolate from the intertidal zone of the U.S. Gulf Coast,11 was classified as a unicellular species.12 We report a facultative life cycle of Cyanothece sp. in which multicellular filaments alternate with unicellular stages. In a series of experiments, we identified salinity and population density as environmental factors triggering the phenotypic switch between the two morphologies. Then, we used numerical models to test hypotheses regarding the nature of the environmental cues and the mechanisms underlying filament dissolution. While the results predict that the observed response is likely caused by an excreted compound in the medium, we cannot fully exclude changes in nutrient availability (as in Tuomi et al.13 and Matz and Jürgens14). The best-fit modeling results show a nonlinear effect of the compound, which is characteristic of density-dependent sensing systems.15,16 Furthermore, filament fragmentation is predicted to occur by connection cleavage rather than cell death of each alternating cell, which is supported by fluorescent and scanning electron microscopy results. The switch between unicellular and multicellular morphology constitutes an environmentally dependent life cycle that is likely an important step en route to permanent multicellularity.
•A unicellular cyanobacterium with a facultative multicellular life cycle•The phenotypic changes in morphology depend on salinity and population density•Phenotypic switch is likely mediated by compounds in the medium in a nonlinear way•Filament fragmentation happens via connection cleavage and not via cell death
Tang et al. report a facultative life cycle of a unicellular cyanobacterium in which multicellular filaments alternate with unicellular stages depending on salinity and population density. Filament fragmentation is likely mediated in a nonlinear way by compounds in the medium and occurs through connection cleavage rather than cell death.
The function of the glutaminyl-tRNA synthetase and Glu-tRNAGln amidotransferase might be related to the origin of the genetic code because, for example, glutaminyl-tRNA synthetase catalyses the ...fundamental reaction that makes the genetic code. If the evolutionary stage of the origin of these two enzymes could be unambiguously identified, then the genetic code should still have been originating at that particular evolutionary stage because the fundamental reaction that makes the code itself was still evidently evolving. This would result in that particular evolutionary moment being attributed to the evolutionary stage of the progenote because it would have a relationship between the genotype and the phenotype not yet fully realized because the genetic code was precisely still originating. I then analyzed the distribution of the glutaminyl-tRNA synthetase and Glu-tRNAGln aminodotrasferase in the main phyletic lineages. Since in some cases the origin of these two enzymes can be related to the evolutionary stages of ancestors of archaea and eukaryotes, this would indicate these ancestors as progenotes because at that evolutionary moment the genetic code was evidently still evolving, thus realizing the definition of progenote. The conclusion that the ancestor of archaea and that of eukaryotes were progenotes would imply that even the last universal common ancestor (LUCA) was a progenote because it appeared, on the tree of life, temporally before these ancestors.
•The function of the glutaminyl-tRNA synthetase and Glu-tRNAGln amidotransferase might be linked to the origin of the genetic code.•If the evolutionary stage of the origin of these two enzymes could be unambiguously identified, then at that particular evolutionary stage the genetic code should still have been originating.•That particular evolutionary moment would be attributed to the evolutionary stage of the progenote.•I have analyzed the distribution of the glutaminyl-tRNA synthetase and Glu-tRNAGln aminodotrasferase.•Since the origin of these two enzymes can be related to the evolutionary stages of ancestors of archaea and eukaryotes, this would indicate these ancestors as progenotes.
ABSTRACT
Evolutionary transitions from animal to wind pollination have occurred repeatedly during the history of the angiosperms, but the selective mechanisms remain elusive. Here, we propose that ...knowledge of pollen release biomechanics is critical for understanding the ecological and evolutionary processes underpinning this shift in pollination mode. Pollen release is the critical first stage of wind pollination (anemophily) and stamen properties are therefore likely to be under strong selection early in the transition. We describe current understanding of pollen release biomechanics to provide insights on the phenotypic and ecological drivers of wind pollination. Pollen release occurs when detachment forces dominate resistive forces retaining pollen within anthers. Detachment forces can be active or passive depending on whether they require energy input from the environment. Passive release is more widespread in anemophilous species and involves processes driven by steady or unsteady aerodynamic forces or turbulence‐induced vibrations that shake pollen from anthers. We review empirical and theoretical studies suggesting that stamen vibration is likely to be a key mechanism of pollen release. The vibration response is governed by morphological and biomechanical properties of stamens, which may undergo divergent selection in the presence or absence of pollinators. Resistive forces have rarely been investigated for pollen within anthers, but are probably sensitive to environmental conditions and depend on flower age, varying systematically between animal‐ and wind‐pollinated species. Animal and wind pollination are traditionally viewed as dichotomous alternatives because they are usually associated with strikingly different pollination syndromes. But this perspective has diverted attention from subtler, continuously varying traits which mediate the fluid dynamic process of pollen release. Reinterpreting the flower as a biomechanical entity that responds to fluctuating environmental forces may provide a promising way forward. We conclude by identifying several profitable areas for future research to obtain deeper insight into the evolution of wind pollination.
Interactions among microbial cells can generate new chemistries and functions, but exploitation requires establishment of communities that reliably recapitulate community-level phenotypes. Using ...mechanistic mathematical models, we show how simple manipulations to population structure can exogenously impose Darwinian-like properties on communities. Such scaffolding causes communities to participate directly in the process of evolution by natural selection and drives the evolution of cell-level interactions to the point where, despite underlying stochasticity, derived communities give rise to offspring communities that faithfully re-establish parental phenotype. The mechanism is akin to a developmental process (
) that arises from density-dependent interactions among cells. Knowledge of ecological factors affecting evolution of developmental correction has implications for understanding the evolutionary origin of major egalitarian transitions, symbioses, and for top-down engineering of microbial communities.
Early evolution of the Eukaryota Butterfield, Nicholas J.; Smith, Andrew
Palaeontology,
January 2015, 2015-01-00, 20150101, Letnik:
58, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The evolution of eukaryotes represents one of the most fundamental transitions in the history of life on Earth; however, there is little consensus as to when or over what timescale it occurred. ...Review of recent hypotheses and data in a phylogenetic context yields a broadly coherent account. Critical re‐assessment of the palaeontological record provides convincing evidence for the presence of crown‐group eukaryotes in the late Palaeoproterozic, and stem‐group eukaryotes extending back to the early Archaean. Despite their relatively early establishment, crown‐eukaryotes appear not to have become ecologically significant until the middle Neoproterozoic. I argue that this billion‐year delay was due to the singular, contingent evolution of crown‐group animals and their unique capacity to drive co‐evolutionary change.
Communication among cells was paramount to the evolutionary increase in cell type diversity and, ultimately, the origin of large body size. Across the diversity of Metazoa, there are only few ...conserved cell signalling pathways known to orchestrate the complex cell and tissue interactions regulating development; thus, modification to these few pathways has been responsible for generating diversity during the evolution of animals. Here, we summarize evidence for the origin and putative function of the intracellular, membrane-bound and secreted components of seven metazoan cell signalling pathways with a special focus on early branching metazoans (ctenophores, poriferans, placozoans and cnidarians) and basal unikonts (amoebozoans, fungi, filastereans and choanoflagellates). We highlight the modular incorporation of intra- and extracellular components in each signalling pathway and suggest that increases in the complexity of the extracellular matrix may have further promoted the modulation of cell signalling during metazoan evolution. Most importantly, this updated view of metazoan signalling pathways highlights the need for explicit study of canonical signalling pathway components in taxa that do not operate a complete signalling pathway. Studies like these are critical for developing a deeper understanding of the evolution of cell signalling.
This article is part of the themed issue ‘Evo-devo in the genomics era, and the origins of morphological diversity’.
Major Cooperative Evolutionary Transitions occur when smaller-scale entities cooperate together to give rise to larger-scale entities that evolve and adapt as coherent wholes. Key examples of ...cooperative transitions are the emergence of the complex eukaryote cell from communities of simpler cells, the transition from eukaryote cells to multicellular organisms, and the organization of humans into complex, modern societies. A number of attempts have been made to develop a general theory of the major cooperative transitions. This paper begins by critiquing key aspects of these previous attempts. Largely, these attempts comprise poorly-integrated collections of separate models that were each originally developed to explain particular transitions. In contrast, this paper sets out to identify processes that are common to all cooperative transitions. It develops an alternative theoretical framework known as Management Theory. This general framework suggests that all major cooperative transitions are the result of the emergence of powerful, evolvable ‘managers’ that derive benefit from using their power to organize smaller-scale entities into larger-scale cooperatives. Management Theory is a contribution to the development of a general, “all levels” understanding of major cooperative transitions that is capable of identifying those features that are level-specific, those that are common across levels and those that are involved in trends across levels.
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