The rhythm of the mold
Long before scientists understood our own internal clocks, they observed a curious pattern in a common orange bread mold, Neurospora crassa. When grown in a long glass tube, this fungus produces rings of bright orange spores—called conidia—in a predictable daily rhythm. This visible, 22-hour cycle of asexual development, first confirmed to be a true circadian rhythm in 1959, provided a simple way to study the genetics of timekeeping. Researchers, including Jerry Feldman at the California Institute of Technology in the late 1960s, began searching for the genes that controlled this rhythm. They exposed the mold to mutagens and looked for individuals with clocks that ran too fast, too slow, or not at all. This work led to the discovery of a single gene they named frequency (frq).
Neurospora proved to be an ideal model organism. Its simple, haploid life cycle means that even recessive genetic traits are immediately visible in its offspring, making genetic analysis straightforward. This fungus, which first gained scientific attention by infesting French bakeries in 1843, had already helped George Beadle and Edward Tatum establish their "one gene-one enzyme" hypothesis, earning them a Nobel Prize in 1958. Its well-understood genetics provided the perfect foundation for dissecting the molecular components of a biological clock.