Mycelium as Steady Eddie and the Hopeful Monster
Phenotypic Shape and ‘Instability’
Engineering with mycelium often begins in a place of confidence. A strain colonizes cleanly and responds predictably to known parameters. Texture, morphology, mechanics, and bioefficiency align with expectation. The system starts to feel stable.
And then something changes. A dense pigmented crust forms where it shouldn’t. What was once diffuse is now rhizomorphic. Fruiting initiates under noninductive conditions. The phenotype changed, not subtly, but radically and with operational and economic consequences.
These events aren’t necessarily failures. More often than not, they are the fungus responding rationally to the instructions it has been given. A shift in humidity, a gradient in gas exchange, a slight nutrient imbalance; what appears to us as instability is often the mycelium interpreting those signals and adjusting its development accordingly (it might, in fact, reflect stability). The phenotype hasn’t gone wrong; it has simply gone somewhere we didn’t intend, revealing that what we observe in day-to-day cultivation is only a narrow slice of what the organism is capable of expressing.
This bears the concept of the global phenotype: the full range of morphological and functional outcomes available to a fungal genotype across the practical vocabulary of environment, nutrition, space, and time. Within this range we can imagine:
Steady Eddie; the stabilizing force that preserves nuclear fidelity, maintains consistent morphology, and suppresses drift.
The Hopeful Monster; the rare but instructive emergence of extreme phenotypes that lie just outside the bounds of what’s expected; the ridges, points, and edge-cases of the topology of the global phenotype.
These are coexisting features of a system that can be both highly stable and highly plastic.
The Global Phenotype
Phenotype is not a fixed object. It is the product of interaction between genotype, environment, and unfolding conditions through space and time. In fungi, this interaction is particularly expansive. Even within a single strain, mycelial form can shift dramatically in response to substrate density, humidity, nutrient availability, gas composition, and more. Morphology is not merely influenced by these parameters; it is composed of them.
The global phenotype refers to the full range of morphological, physiological, and functional expressions that a fungal genotype is capable of producing across its viable environmental range. In this framing, phenotype is not a single trait, but a topology. A landscape of possibility shaped by genetic potential and environmental vocabulary. Trait-based ecologists have long used this kind of language to describe functional diversity in plants and soil fungi (Krause et al., 2014; Aguilar-Trigueros et al., 2015). But in mycelium engineering, this topology is not just ecological, it is operational. It defines the boundary of what is achievable, and the range of drift possible with uncontrolled processes.
Fungal systems are notoriously plastic, with some strains able to shift between linear and diffuse growth modes, aerial or substrate-dominant morphologies, or high- versus low-density tissues in response to minor changes in context. In Neurospora crassa, for example, environmental triggers can induce dramatic changes in colony architecture and branching frequency without any underlying genetic change (Watters et al., 2000). In basidiomycetes, traits like hyphal aggregation, density, pigment expression, or secreted matrix formation are often highly inducible, even within clonal populations.
The challenge is that this plasticity is not always symmetrical. For some strains, a given phenotype may be highly robust, with a dampened ‘shape’ of response across a wide range of conditions. For others, useful phenotypes may live only in narrow folds of the global surface, requiring precise alignment of inputs to access and easily lost through drift or minor misalignment.
The global phenotype is not simply a catalog of what a fungus can do, it is a terrain of accessibility, sensitivity, and stability. Some areas are smooth and wide. Others are steep, high, and hard to find again. This poses an essential question: not just what is the phenotype, but where is it?
Steady Eddie
In common usage, “Steady Eddie” refers to someone who can be counted on: dependable and even-tempered. Someone with a comforting and functional predictability. There appears to be something of a Steady Eddie principle to the biology of the basidiomycota.
A striking example is Armillaria gallica, a forest-dwelling basidiomycete with at least one clonal individual in Michigan that spans over 75 hectares and is thought to be at least 2,500 years old. Genomic sequencing of samples taken across this enormous body revealed an astonishingly low number of somatic mutations, only 163 SNPs across the genome along with a handful of small loss-of-heterozygosity events (Anderson et al., 2018). Most of these were singletons that had not spread, indicating that mutations are not only rare but also rarely fixed within the expanding network. The data suggest that Armillaria hyphae may undergo only one cell division for every meter of rhizomorph growth, a rate far below what would be expected in a typical filamentous fungus. Over millennia, this organism has expanded without compromising nuclear fidelity, maintaining continuity across vast spatial and temporal scales. The authors propose that this extraordinary stability may be supported by mechanisms such as quiescent apical cells, high-fidelity DNA repair, or asymmetric strand retention, though these remain hypotheses rather than demonstrated features. What is clear is that stability itself functions as an adaptive strategy.
Another example lies in the clamp connections of dikaryotic hyphae. These structures act as a physical quality-control system during cell division. By ensuring that each daughter cell inherits one nucleus from each parent lineage, the clamp preserves the dikaryotic state and filters out nuclear imbalances that might otherwise destabilize growth. They operate as a spatially encoded filter, where the geometry of the clamp and the choreography of the cytoskeleton together enforce fidelity. Microtubules guide nuclei into position, actin dynamics shape the clamp branch, and septa close in a tightly regulated sequence. If nuclei don’t migrate properly, or if division is mistimed, the clamp doesn’t fuse and the error is effectively quarantined. In this way, the physical layout of the clamp functions as a checkpoint that only permits forward growth when the nuclear arrangement is correct (Aanen et al., 2023).
Taken together, these examples show that higher fungi possess effective mechanisms for preserving continuity across time and space. Which is why it is worth asking whether instability is always what it appears to be. A change in morphology or bioefficiency can quickly be labeled as drift, when in many cases it may actually be a stable organism responding rationally to inconsistent handling. Inoculum quality, passage control, or environmental fluctuation may just as likely be the source. The strain itself may be steady while the processes around it are not.
The Hopeful Monster
In evolutionary biology, the idea of the “hopeful monster” comes from Richard Goldschmidt, who argued in the 1940s that major evolutionary leaps could arise from large, abrupt mutations (Dietrich, 2003). Most of these so-called monsters would be maladaptive, but a rare few might prove advantageous and give rise to new forms. The idea was long dismissed, but later work in evolutionary developmental biology reframed it with more nuance, showing that major changes in body plan can sometimes be traced to regulatory shifts rather than slow accumulation of small differences.
In the context of mycelium engineering, the hopeful monster principle can be recast in a way that is less about mutation and more about expression. The fungus does not need a new genotype to produce a radical new form if it already contains latent extremes within the terrain of its global phenotype. These are triggered not by genetic but by contextual novelty; by the precise combination of perhaps atypical environmental, nutritional, or spatial inputs. In this sense, unique environmental extremes may then produce opportunities for unique and acute phenotypic response that may be functionally rare in normal ecological terms, but provide a momentary Hopeful Monster that provides a competitive advantage (Virágh et al., 2021).
In Agaricus bisporus, the formation of ‘stroma’ (dense, thickened mats of mycelium that resist fruiting) represents a major diversion from the expected crop morphology. Fruiting bodies can also emerge in highly etiolated forms, with elongated, blanched stems, distorted caps, or uniquely formed fertile surfaces when grown under light or gas conditions that deviate from inductive norms (Sakamoto, 2018). In some polypores, shelf fungi that usually form robust three-dimensional structures with well-organized pore surfaces, instead flatten into resupinate forms that still generate pores but lack the architecture of the full sporocarp. In other cases, strains that typically grow with a diffuse colonial morphology can switch under stress or gas limitation to produce organized rhizomorphic cords, fundamentally altering how the colony distributes and transports resources (Mihail, 2002).
These excursions are often narrow in their environmental triggers, and not easily reproduced. They are not failures, but are the expression of edge conditions that live on ridgelines rather than valleys. Most of the time, these expressions are unhelpful or costly. Occasionally, they may prove useful. They may embody a trait (structural toughness, porosity, transport efficiency) that is valuable to us from a materials development or process innovation standpoint, or provide a competitive advantage for the fungus. In both senses this aligns with the spirit of the concept of the hopeful monster.
The Steady and the Strange
The Steady Eddie and the Hopeful Monster are not in conflict or opposite ends of a spectrum. They are coexisting properties of fungal systems that reveal themselves in different contexts, with different implications for how we build, and together serve a cohesive competitive role. To engineer with mycelium is to hold both principles in view.
Steady Eddie is what allows a strain to perform reliably across passages and batches, to carry its morphology from plate to bag to bioreactor. It is the engine of reproducibility, the reason inoculum protocols can be standardized, and the foundation for any architecture that aims to scale. If inoculation is inconsistent, if strain passage is poorly tracked, or if culture age or inoculum density drifts, then so will the outcomes, not necessarily because the strain is unstable, but because a potentially stable strain is rationally tracking the instability surrounding it.
The Hopeful Monster is what makes discovery possible. Outlying and unusual condition excursions can access physical responses and properties that live at the edges of the global phenotype. They may be rarely repeatable on the first try, but they are not random. They are the result of input conditions that happen to align in a way the strain knows how to respond to, even if we did not intend it, and can surface meaningfully useful latent traits.
Imagine the global phenotype as a topographic landscape. The valleys are stable and wide phenotypes that appear often and predictably under broad ranges of conditions. The peaks and ridges are extreme expressions, less accessible and narrow in trigger, but sometimes uniquely valuable. Understanding that topography is what allows the engineer to make use of both the stability and the surprises. The steady regions of the phenotype enable reproducibility, scale, and confidence. The rare excursions, the hopeful monsters, are where novel traits can surface and new materials emerge. Together, they define the full working space of mycelium as a material system.
References
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