  It is challenging to apply the tenets of individuality to filamentous fungi:
a fungal mycelium can contain millions of genetically diverse but totipotent
nuclei, each capable of founding new mycelia. Moreover a single mycelium can
potentially stretch over kilometers and it is unlikely that its distant parts
share resources or have the same fitness. Here we directly measure how a single
mycelium of the model ascomycete {\it Neurospora crassa} is patterned into
Reproductive Units (RUs); subpopulations of nuclei that propagate together as
spores, and function as reproductive individuals. The density of RUs is
sensitive to the geometry of growth; we detect 50-fold smaller RUs when mycelia
had expanding frontiers than when they were constrained to grow in one
direction only. RUs fragmented further when the mycelial network was perturbed.
In mycelia with expanding frontiers RU composition was strongly influenced by
the distribution of genotypes early in development. Our results provide a
concept of fungal individuality that is directly connected to reproductive
potential, and therefore to theories of how fungal individuals adapt and evolve
over time. Our data show that the size of reproductive individuals is a dynamic
and environment-dependent property, even within apparently totally connected
fungal mycelia.
