Summary: Supporting Information 3. Host-parasite simulations
Deterministic computer simulations were performed to evaluate the effect of maternally-
transmitted parasites on the evolution of sex. Briefly, the simulations work as follows.
Hosts are assumed to be diploid and parasites to be haploid. Both species have non-
overlapping generations. Each generation begins with the birth of host offspring. If they
are produced by infected mothers, offspring are exposed to parasites transmitted by their
mother (maternal infection phase). All offspring, regardless of maternal infection status,
experience a global transmission phase in which they are exposed to parasites transmitted
by any infected host of parental generation. Reproduction follows these transmission
phases. Hosts produce some fraction of their offspring sexually and the remainder
asexually. An individual's investment into sex is determined by its genotype at the M
locus. Infected individuals produce less offspring than uninfected individuals.
Generations are non-overlapping, i.e., mothers die immediately following production of
offspring and transmission of parasites. A more detailed description follows.
Let nH and nP be the number of host and parasite genotypes respectively. Let
nI = nP + 1 be the number of possible infection states for a host; a host can be infected by
one of the nP parasite genotypes or be uninfected. Let FM,g,i be the frequency of offspring
in the population that are of genotype g from mothers in infection state i. For 1 i nP,
infection state i means that the host is infected by parasite genotype i. Infection state i =
nP + 1 corresponds to being uninfected.