Fig. 1: Illustration of seed bank manipulation.
In the + seed bank treatment, we used a strain of Bacillus subtilis that was capable of forming endospores after resources were exhausted by growth (black arrows). In addition, we established an external seed bank (shown in blue) to extend endospore residence time. The first step of this process involved purifying endospores through heat treatment (flame = 80 °C, 20 min), which eliminated phages and vegetative cells from a sample taken from the focal population contained in a flask. Then, we mixed these endospores with endospores preserved from previous transfers that were obtained in the same fashion. This spore mixture (i.e., external seed bank) and an untreated sample taken from a focal population were used to inoculate fresh medium and establish the next transfer. In the - seed bank treatment, serial transfers (black arrows) were conducted with a mutant strain of B. subtilis that was not capable of producing endospores in rich medium after resource exhaustion owing to an engineered mutation in a gene that is essential for sporulation (spoIIE). After establishing the seed bank with an initial serial transfer (t-1), we began the experiment at t0 by infecting half of the populations with phage SPO1. For simplicity, non-infected controls are not shown. See methods for further details.
