Histocompatibility as adaptive response to discriminatory within-organism conflict: a historical model

Micro-to multi-chimerism: the multiple facets of a singular phenomenon

Natural chimeras are prevalent in nature (> 10 phyla of protists, plants, invertebrates, and vertebrates), disrupting the conventional believe that genetically homogeneous entities are selected to prevent conflicts within an organism. Chimerism emerges as a significant ecological/evolutionary mechanism, shaping the life history characteristics of metazoans, and it develops in various forms, one of which is called 'microchimerism'. Furthermore, chimerism is a pivotal phenomenon, presenting complex biological and ecological expressions akin to a "double-edged sword", bypassing both innate and adaptive immune responses. Considering the proportionate contribution of chimeric partners and their spatial arrangements within chimeras, unveils six somatic states of chimerism (purged-chimerism, sectorial-chimerism, mosaic-chimerism, mixed-chimerism, microchimerism and multi-chimerism) and three states of germline chimerism (mixed-chimerism, male/female chimerism and parasitic germline chimerism). These diverse chimeric states are categorized into two distinct series of continua, namely 'somatic cell chimerism' and 'germline chimerism' scenarios where dynamic chimeric states transit into other states, and vice versa, within a specific continuum that relies on the concept of an endless 'Escherian stairwell' of chimerism states. Also, the same chimera may portray simultaneously, different chimeric states in various parts/organs. We further reviewed the evolutionary perspectives for chimerism, raising five commonly shared features of chimerism (multichimerism, ontogenic windows, reproductive chimerism, transmissible chimerism, germline hitchhiking) and 'costs' and 'benefits' accrued to chimerism, shared between invertebrates and vertebrates, including humans. We contest that 'microchimerism' lacks any quantitative definition, represents just a single facet in the multi-facet panorama of chimeric phenomena that demonstrate transitions over time into other states. All of the above carry evolutionary and clinical implications.

Crozier's paradox and kin recognition: Insights from simplified models

Crozier's paradox suggests that genetic kin recognition will not be evolutionarily stable. The problem is that more common tags (markers) are more likely to be recognised and helped. This causes common tags to increase in frequency, eliminating the genetic variability that is required for genetic kin recognition. In recent years, theoretical models have resolved Crozier's paradox in different ways, but they are based on very complicated multi-locus population genetics. Consequently, it is hard to see exactly what is going on, and whether different theoretical resolutions of Crozier's paradox lead to different types of kin discrimination. I address this by making unrealistic simplifying assumptions to produce a more tractable and understandable model of Crozier's paradox. I use this to interpret a more complex multi-locus population genetic model where I have not made the same simplifying assumptions. I explain how Crozier's paradox can be resolved, and show that only one known theoretical resolution of Crozier's paradox - multiple social encounters - leads without restrictive assumptions to the type of highly cooperative and reliable form of kin discrimination that we observe in nature. More generally, I show how adopting a methodological approach where complex models are compared with simplified ones can lead to greater understanding and accessibility.

Host-parasite coevolution and the stability of genetic kin recognition

Crozier's paradox suggests that genetic kin recognition will not be evolutionarily stable. The problem is that more common tags (markers) are more likely to be recognized and helped. This causes common tags to increase in frequency, eliminating the genetic variability that is required for genetic kin recognition. Two potential solutions to this problem have been suggested: host-parasite coevolution and multiple social encounters. We show that the host-parasite coevolution hypothesis does not work as commonly assumed. Host-parasite coevolution only stabilizes kin recognition at a parasite resistance locus if parasites adapt rapidly to hosts and cause intermediate or high levels of damage (virulence). Additionally, when kin recognition is stabilized at a parasite resistance locus, this can have an additional cost of making hosts more susceptible to parasites. However, we show that if the genetic architecture is allowed to evolve, meaning natural selection can choose the recognition locus, genetic kin recognition is more likely to be stable. The reason for this is that host-parasite coevolution can maintain tag diversity at another (neutral) locus by genetic hitchhiking, allowing that other locus to be used for genetic kin recognition. These results suggest a way that host-parasite coevolution can resolve Crozier's paradox, without making hosts more susceptible to parasites. However, the opportunity for multiple social encounters may provide a more robust resolution of Crozier's paradox.

Stem Cells and Innate Immunity in Aquatic Invertebrates: Bridging Two Seemingly Disparate Disciplines for New Discoveries in Biology

The scopes related to the interplay between stem cells and the immune system are broad and range from the basic understanding of organism's physiology and ecology to translational studies, further contributing to (eco)toxicology, biotechnology, and medicine as well as regulatory and ethical aspects. Stem cells originate immune cells through hematopoiesis, and the interplay between the two cell types is required in processes like regeneration. In addition, stem and immune cell anomalies directly affect the organism's functions, its ability to cope with environmental changes and, indirectly, its role in ecosystem services. However, stem cells and immune cells continue to be considered parts of two branches of biological research with few interconnections between them. This review aims to bridge these two seemingly disparate disciplines towards much more integrative and transformative approaches with examples deriving mainly from aquatic invertebrates. We discuss the current understanding of cross-disciplinary collaborative and emerging issues, raising novel hypotheses and comments. We also discuss the problems and perspectives of the two disciplines and how to integrate their conceptual frameworks to address basic equations in biology in a new, innovative way.

Genetic signatures of microbial altruism and cheating in social amoebas in the wild

Many microbes engage in social interactions. Some of these have come to play an important role in the study of cooperation and conflict, largely because, unlike most animals, they can be genetically manipulated and experimentally evolved. However, whereas animal social behavior can be observed and assessed in natural environments, microbes usually cannot, so we know little about microbial social adaptations in nature. This has led to some difficult-to-resolve controversies about social adaptation even for well-studied traits such as bacterial quorum sensing, siderophore production, and biofilms. Here we use molecular signatures of population genetics and molecular evolution to address controversies over the existence of altruism and cheating in social amoebas. First, we find signatures of rapid adaptive molecular evolution that are consistent with social conflict being a significant force in nature. Second, we find population-genetic signatures of purifying selection to support the hypothesis that the cells that form the sterile stalk evolve primarily through altruistic kin selection rather than through selfish direct reproduction. Our results show how molecular signatures can provide insight into social adaptations that cannot be observed in their natural context, and they support the hypotheses that social amoebas in the wild are both altruists and cheaters.

Invasiveness, chimerism and genetic diversity

Adaptation for invasiveness should comprise the capability to exploit and prosper in a wide range of ecological conditions and is therefore expected to be associated with a certain level of genetic diversity. Paradoxically, however, invasive populations are established by only a few founders, resulting in low genetic diversity. As a conceivable way of attaining high genetic diversity and high variance of gene expression even when a small number of founders is involved in invasiveness, I suggest here chimerism, a fusion between different individuals-a common phenomenon found in numerous phyla. The composite entity offers the chimeric organism genetic flexibility and higher inclusive fitness that depends on the joint genomic fitness of the original partners. The ability to form a chimeric entity is also applied to subsequent generations, and consequently, the level of genetic diversity does not decline over generations of population establishment following invasion.

Rapid and widespread de novo evolution of kin discrimination

Diverse forms of kin discrimination, broadly defined as alteration of social behavior as a function of genetic relatedness among interactants, are common among social organisms from microbes to humans. However, the evolutionary origins and causes of kin-discriminatory behavior remain largely obscure. One form of kin discrimination observed in microbes is the failure of genetically distinct colonies to merge freely upon encounter. Here, we first use natural isolates of the highly social bacterium Myxococcus xanthus to show that colony-merger incompatibilities can be strong barriers to social interaction, particularly by reducing chimerism in multicellular fruiting bodies that develop near colony-territory borders. We then use experimental laboratory populations to test hypotheses regarding the evolutionary origins of kin discrimination. We show that the generic process of adaptation, irrespective of selective environment, is sufficient to repeatedly generate kin-discriminatory behaviors between evolved populations and their common ancestor. Further, we find that kin discrimination pervasively evolves indirectly between allopatric replicate populations that adapt to the same ecological habitat and that this occurs generically in many distinct habitats. Patterns of interpopulation discrimination imply that kin discrimination phenotypes evolved via many diverse genetic mechanisms and mutation-accumulation patterns support this inference. Strong incompatibility phenotypes emerged abruptly in some populations but strengthened gradually in others. The indirect evolution of kin discrimination in an asexual microbe is analogous to the indirect evolution of reproductive incompatibility in sexual eukaryotes and linguistic incompatibility among human cultures, the commonality being indirect, noncoordinated divergence of complex systems evolving in isolation.