Table 1.

Parameters.

Parameter	Description
a	Investment in cooperation by the host.
b	Investment in cooperation by symbionts.
B	Expression of trait associated with cooperation by symbionts.
c	Investment in symbiont control by the host.
f	Cost of host control on symbionts: captures how much symbiont population size is reduced by control, which will indirectly affect the host by reducing its benefits.
g	Cost of control to host e.g. physiological cost of an immune system.
H(a, c)	Probability density of host individuals with cooperation level a and control level c.
S(b)	Probability density of symbiont genotypes with cooperation level b.
M	Rate of microbial immigration each host generation from a fixed environmental source population (proportion of resident population).
m	Rate of environmental microbe immigration each symbiont generation within the host.
pa	Effect of partner fidelity feedback on host individuals.
pb	Effect of partner fidelity feedback on symbionts.
q	Effect of host control on symbionts.
W	Fitness of focal individual.
x	Benefit to hosts of receiving symbiont cooperation.
y	Benefit to symbionts of receiving hosts cooperation.
R	Relatedness among symbionts (genetic similarity in the host within a species relative to the population mean).