Fig. 2.

Selection at the host level can maintain helper cells. (A) Within each host, helper cells (blue) decrease in frequency, because microbe-level selection favors the faster-growing neutral cells (red). However, helper cells can increase in frequency due to host-level selection: hosts with more helper cells have more offspring. These offspring in turn also have high helper frequencies because of vertical transmission. (B) Helper cells are only maintained at high frequencies when host birth rates depend on microbiome composition ($s_b=1$); the average frequency of helper cells over the total host population (mean helper frequency $⟨f⟩$) is shown. (C) Frequency of helper cells $f_i$ varies widely between hosts; the distribution is shown for t = 5,000. Parameters as shown in SI Appendix, Table S1, except for $G_H=10$ and $K_H=\mathrm{5,000}$.