Germline Evolution: Sequestered Cells or Immortal Strands?

Longevity of Fungal Mycelia and Nuclear Quality Checks: a New Hypothesis for the Role of Clamp Connections in Dikaryons

This paper addresses the stability of mycelial growth in fungi and differences between ascomycetes and basidiomycetes. Starting with general evolutionary theories of multicellularity and the role of sex, we then discuss individuality in fungi. Recent research has demonstrated the deleterious consequences of nucleus-level selection in fungal mycelia, favoring cheaters with a nucleus-level benefit during spore formation but a negative effect on mycelium-level fitness. Cheaters appear to generally be loss-of-fusion (LOF) mutants, with a higher propensity to form aerial hyphae developing into asexual spores. Since LOF mutants rely on heterokaryosis with wild-type nuclei, we argue that regular single-spore bottlenecks can efficiently select against such cheater mutants. We then zoom in on ecological differences between ascomycetes being typically fast-growing but short-lived with frequent asexual-spore bottlenecks and basidiomycetes being generally slow-growing but long-lived and usually without asexual-spore bottlenecks. We argue that these life history differences have coevolved with stricter nuclear quality checks in basidiomycetes. Specifically, we propose a new function for clamp connections, structures formed during the sexual stage in ascomycetes and basidiomycetes but during somatic growth only in basidiomycete dikaryons. During dikaryon cell division, the two haploid nuclei temporarily enter a monokaryotic phase, by alternatingly entering a retrograde-growing clamp cell, which subsequently fuses with the subapical cell to recover the dikaryotic cell. We hypothesize that clamp connections act as screening devices for nuclear quality, with both nuclei continuously testing each other for fusion ability, a test that LOF mutants will fail. By linking differences in longevity of the mycelial phase to ecology and stringency of nuclear quality checks, we propose that mycelia have a constant and low lifetime cheating risk, irrespective of their size and longevity.

Evolution via somatic genetic variation in modular species

Somatic genetic variation (SoGV) may play a consequential yet underappreciated role in long-lived, modular species among plants, animals, and fungi. Recent genomic data identified two levels of genetic heterogeneity, between cell lines and between modules, that are subject to multilevel selection. Because SoGV can transfer into gametes when germlines are sequestered late in ontogeny (plants, algae, and fungi and some basal animals), sexual and asexual processes provide interdependent routes of mutational input and impact the accumulation of genetic load and molecular evolution rates of the integrated asexual/sexual life cycle. Avenues for future research include possible fitness effects of SoGV, the identification and implications of multilevel selection, and modeling of asexual selective sweeps using approaches from tumor evolution.

Phenotypic plasticity through disposable genetic adaptation in ciliates

Ciliates have an extraordinary genetic system in which each cell harbors two distinct kinds of nucleus, a transcriptionally active somatic nucleus and a quiescent germline nucleus. The latter undergoes classical, heritable genetic adaptation, while adaptation of the somatic nucleus is only short-term and thus disposable. The ecological and evolutionary relevance of this nuclear dimorphism have never been well formalized, which is surprising given the long history of using ciliates such as Tetrahymena and Paramecium as model organisms. We present a novel, alternative explanation for ciliate nuclear dimorphism which, we argue, should be considered an instrument of phenotypic plasticity by somatic selection on the level of the ciliate clone, as if it were a diffuse multicellular organism. This viewpoint helps to put some enigmatic aspects of ciliate biology into perspective and presents the diversity of ciliates as a large natural experiment that we can exploit to study phenotypic plasticity and organismality.

Asymmetry, division of labour and the evolution of ageing in multicellular organisms

Between the 1930s and 1960s, evolutionary geneticists worked out the basic principles of why organisms age. Despite much progress in the evolutionary biology of ageing since that time, however, many puzzles remain. The perhaps most fundamental of these is the question of which organisms should exhibit senescence and which should not (or which should age rapidly and which should not). The evolutionary origin of ageing from a non-senescent state has been conceptually framed, for example, in terms of the separation between germ-line and soma, the distinction between parents and their offspring, and-in unicellular organisms-the unequal distribution of cellular damage at cell division. These ideas seem to be closely related to the concept of 'division of labour' between reproduction and somatic maintenance. Here, we review these concepts and develop a toy model to explore the importance of such asymmetries for the evolution of senescence. We apply our model to the simplest case of a multicellular system: an organism consisting of two totipotent cells. Notably, we find that in organisms which reproduce symmetrically and partition damage equally, senescence is still able to evolve, contrary to previous claims. Our results might have some bearing on understanding the origin of the germ-line-soma separation and the evolution of senescence in multicellular organisms and in colonial species consisting of multiple types of individuals, such as, for example, eusocial insects with their different castes. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'