Symbiont survival and host-symbiont disequilibria under differential vertical transmission

Interspecific genetic interactions in host-symbiont systems raise intriguin g coevolutionary questions and may influence the effectiveness of public he alth and management policies. Here we present an analytical and numerical i nvestigation of the effects of host genetic heterogeneity in the rate of ve rtical transmission of a symbiont. We consider the baseline case with a mon omorphic symbiont and a single diallelic locus in its diploid host, where v ertical transmission is title sole force. Our analysis introduces interspec ific disequilibria to quantify nonrandom associations between host genotype s and alleles and symbiont presence/absence. The transient and equilibrium behavior is examined in simulations with randomly generated initial conditi ons and transmission parameters. Compared to the case where vertical transm ission rates are uniform across host genotypes, differential transmission ( i) increases average symbiont survival from 50% to almost 60%, (ii) dramati cally reduces the minimum average transmission rate for symbiont survival f rom 0.5 to 0.008, and (iii) readily creates permanent host-symbiont disequi libria de novo, whereas uniform transmission can neither create nor maintai n such associations. On average, heterozygotes are slightly more likely to carry and maintain the symbiont in the population and are more randomly ass ociated with the symbiont. Results show that simple evolutionary forces can create substantial nonrandom associations between two species.