Evolution: Welcome to Symbiont Prison

Symbiont-mediated immune priming in animals through an evolutionary lens

Protective symbionts can defend hosts from parasites through several mechanisms, from direct interference to modulating host immunity, with subsequent effects on host and parasite fitness. While research on symbiont-mediated immune priming (SMIP) has focused on ecological impacts and agriculturally important organisms, the evolutionary implications of SMIP are less clear. Here, we review recent advances made in elucidating the ecological and molecular mechanisms by which SMIP occurs. We draw on current works to discuss the potential for this phenomenon to drive host, parasite, and symbiont evolution. We also suggest approaches that can be used to address questions regarding the impact of immune priming on host-microbe dynamics and population structures. Finally, due to the transient nature of some symbionts involved in SMIP, we discuss what it means to be a protective symbiont from ecological and evolutionary perspectives and how such interactions can affect long-term persistence of the symbiosis.

The Evolution of Mutualistic Dependence

While the importance of mutualisms across the tree of life is recognized, it is not understood why some organisms evolve high levels of dependence on mutualistic partnerships, while other species remain autonomous or retain or regain minimal dependence on partners. We identify four main pathways leading to the evolution of mutualistic dependence. Then, we evaluate current evidence for three predictions: ( a) Mutualisms with different levels of dependence have distinct stabilizing mechanisms against exploitation and cheating, ( b) less dependent mutualists will return to autonomy more often than those that are highly dependent, and ( c) obligate mutualisms should be less context dependent than facultative ones. Although we find evidence supporting all three predictions, we stress that mutualistic partners follow diverse paths toward—and away from—dependence. We also highlight the need to better examine asymmetry in partner dependence. Recognizing how variation in dependence influences the stability, breakdown, and context dependence of mutualisms generates new hypotheses regarding how and why the benefits of mutualistic partnerships differ over time and space.

Compartmentalization drives the evolution of symbiotic cooperation

Across the tree of life, hosts have evolved mechanisms to control and mediate interactions with symbiotic partners. We suggest that the evolution of physical structures that allow hosts to spatially separate symbionts, termed compartmentalization, is a common mechanism used by hosts. Such compartmentalization allows hosts to: (i) isolate symbionts and control their reproduction; (ii) reward cooperative symbionts and punish or stop interactions with non-cooperative symbionts; and (iii) reduce direct conflict among different symbionts strains in a single host. Compartmentalization has allowed hosts to increase the benefits that they obtain from symbiotic partners across a diversity of interactions, including legumes and rhizobia, plants and fungi, squid and Vibrio, insects and nutrient provisioning bacteria, plants and insects, and the human microbiome. In cases where compartmentalization has not evolved, we ask why not. We argue that when partners interact in a competitive hierarchy, or when hosts engage in partnerships which are less costly, compartmentalization is less likely to evolve. We conclude that compartmentalization is key to understanding the evolution of symbiotic cooperation. This article is part of the theme issue 'The role of the microbiome in host evolution'.

The Life of an Insect Endosymbiont from the Cradle to the Grave

Host-beneficial endosymbioses, which are formed when a microorganism takes up residence inside another cell and provides a fitness advantage to the host, have had a dramatic influence on the evolution of life. These intimate relationships have yielded the mitochondrion and the plastid (chloroplast) - the ancient organelles that in part define eukaryotic life - along with many more recent associations involving a wide variety of hosts and microbial partners. These relationships are often envisioned as stable associations that appear cooperative and persist for extremely long periods of time. But recent evidence suggests that this stable state is often born from turbulent and conflicting origins, and that the apparent stability of many beneficial endosymbiotic relationships - although certainly real in many cases - is not an inevitable outcome of these associations. Here we review how stable endosymbioses form, how they are maintained, and how they sometimes break down and are reborn. We focus on relationships formed by insects and their resident microorganisms because these symbioses have been the focus of significant empirical work over the last two decades. We review these relationships over five life stages: origin, birth, middle age, old age, and death.

Changes in Endosymbiont Complexity Drive Host-Level Compensatory Adaptations in Cicadas

For insects that depend on one or more bacterial endosymbionts for survival, it is critical that these bacteria are faithfully transmitted between insect generations. Cicadas harbor two essential bacterial endosymbionts, "Candidatus Sulcia muelleri" and "Candidatus Hodgkinia cicadicola." In some cicada species, Hodgkinia has fragmented into multiple distinct but interdependent cellular and genomic lineages that can differ in abundance by more than two orders of magnitude. This complexity presents a potential problem for the host cicada, because low-abundance but essential Hodgkinia lineages risk being lost during the symbiont transmission bottleneck from mother to egg. Here we show that all cicada eggs seem to receive the full complement of Hodgkinia lineages, and that in cicadas with more complex Hodgkinia this outcome is achieved by increasing the number of Hodgkinia cells transmitted by up to 6-fold. We further show that cicada species with varying Hodgkinia complexity do not visibly alter their transmission mechanism at the resolution of cell biological structures. Together these data suggest that a major cicada adaptation to changes in endosymbiont complexity is an increase in the number of Hodgkinia cells transmitted to each egg. We hypothesize that the requirement to increase the symbiont titer is one of the costs associated with Hodgkinia fragmentation.IMPORTANCE Sap-feeding insects critically rely on one or more bacteria or fungi to provide essential nutrients that are not available at sufficient levels in their diets. These microbes are passed between insect generations when the mother places a small packet of microbes into each of her eggs before it is laid. We have previously described an unusual lineage fragmentation process in a nutritional endosymbiotic bacterium of cicadas called Hodgkinia In some cicadas, a single Hodgkinia lineage has split into numerous related lineages, each performing a subset of original function and therefore each required for normal host function. Here we test how this splitting process affects symbiont transmission to eggs. We find that cicadas dramatically increase the titer of Hodgkinia cells passed to each egg in response to lineage fragmentation, and we hypothesize that this increase in bacterial cell count is one of the major costs associated with endosymbiont fragmentation.

An Emerging System to Study Photosymbiosis, Brain Regeneration, Chronobiology, and Behavior: The Marine Acoel Symsagittifera roscoffensis

The acoel worm Symsagittifera roscoffensis, an early offshoot of the Bilateria and the only well-studied marine acoel that lives in a photosymbiotic relationship, exhibits a centralized nervous system, brain regeneration, and a wide repertoire of complex behaviors such as circatidal rhythmicity, photo/geotaxis, and social interactions. While this animal can be collected by the thousands and is studied historically, significant progress is made over the last decade to develop it as an emerging marine model. The authors here present the feasibility of culturing it in the laboratory and describe the progress made on different areas, including genomic and tissue architectures, highlighting the associated challenges. In light of these developments, and on the ability to access abundant synchronized embryos, the authors put forward S. roscoffensis as a marine system to revisit questions in the areas of photosymbiosis, regeneration, chronobiology, and the study of complex behaviors from a molecular and evolutionary perspective.

Endosymbiosis: The feeling is not mutual

Endosymbiosis is an idea that provided a remarkable amount of explanatory power about the origins of eukaryotic organelles. But it also promoted a number of assumptions that have also been influential, but are less well-examined. Here we look at two of these to see whether or not they fit current evidence. The assumption we first address is that endosymbiotic relationships such as nutritional symbioses and eukaryotic organelles are mutualisms. We argue instead that they are more one-sided associations that can be regarded as context-dependent power struggles like any other ecological interaction. The second assumption is that during endosymbiotic interactions (such as the origin of organelles), the host genomes will acquire a great many genes from endosymbionts that assume functions in host systems (as opposed to the well-documented genes whose products are simply targeted back to the endosymbiont or organelle). The idea that these genes exist in large numbers has been influential in a number of hypotheses about organelle evolution and distribution, but in the most carefully-examined systems no such mass migration of genes is evident. Overall, we argue that both the nature and impact of endosymbiosis need to be constantly re-evaluated to fully understand what roles it really plays in both cell biology and evolution.