The Openness–Constraint Spectrum: Module Integrity and the Evolutionary Reversibility of Fungal Traits

Fungal traits differ sharply in whether evolution can retrace its steps. Hymenophore morphologies transition bidirectionally across phylogenies, whereas gasteroid fruiting bodies, arbuscular mycorrhizal symbioses, and obligate parasitism show little or no macroevolutionary return. We propose module integrity — the preservation state of a gene regulatory module relative to its ancestral baseline — as a unified variable, measurable in principle, that predicts where a trait falls on this reversibility spectrum. Integrity erodes when a non-compensable bottleneck core, a top-level regulatory node with no substitute, is lost: downstream coordination collapses, and once degradation passes a critical zone, reversal becomes statistically inaccessible. The resulting constraint is trait-specific; a lineage locked in one trait continues to explore in others. A parallel diversity-generating pathway — variation supply, quasi-neutral retention, somatic expansion, selective recycling — sets how widely a lineage explores, and module integrity sets what it can return to. The two meet at the quasi-neutral plateau, which serves both as the medium of exploration and as the entry channel of erosion. Module integrity has not yet been measured. We specify how to estimate it from gene content, regulatory topology, and expression response; the association test that would support or refute the core claim; and the decay-sequence prediction that separates this account from gene-count alternatives. Drift, weak selection, adaptation, and horizontal gene transfer each enter the causal narrative at a specified point.