Vertical transmission dynamics of pea aphid symbionts in natural settings

Danielle Irene Rock

doi:10.17918/etd-7288

Several bacterial lineages contain members that are known only as symbionts of insects. The pea aphid (Acyrthosiphon pisum) harbors eight such bacterial symbionts, some of which have been shown to mediate interactions between aphids and natural enemies like parasitoids, predators and pathogens. These symbiotic relationships persist primarily through maternal transmission, with prior lab-based estimates suggesting near perfect passage of symbionts from mothers to offspring. Yet studies in other systems have noted imperfect maternal transfer of bacterial symbionts, with factors such as temperature and the presence of co-infecting microbes playing a role in this fidelity. Since the prevalence of symbionts in natural host populations will depend on their transmission efficiencies and effects on the hosts' fitness, it is important to understand transmission in a more natural context. In the pea aphid system, transmission rates could conceivably vary between symbiont species, across seasons, or based on the presence of co-infecting symbiont species. This would have implications for the known seasonal dynamics of these symbionts and their overall prevalence in aphid populations. In this study, we performed field sampling, lab rearing and extensive PCR screening to help identify transmission efficiency of pea aphid symbionts. Observations indicate imperfect maternal transfer in the field. While we find no strong evidence for an impact of temperature on transmission rates, the identities of co-infecting symbionts have a large impact on the efficiency of maternal transfer. Interestingly, symbionts living together often in the field appeared to improve each others' transmission upon coinfection. In contrast, pairings of rare symbiont partners were associated with reduced rates of transfer. In particular, results show statistically significant differences in transmission rates for Rickettsiella and Serratia inhabiting a pea aphid with and without the presence of Serratia and Rickettsiella, respectively. Evidence also suggests the presence of Spiroplasma in pea aphid microbiomes decreases transmission efficiency of other pea aphid facultative endosymbionts. Given the roles of these symbionts in aphid defense, and the frequent occurrence of inherited, defensive symbionts across plants and invertebrates, our findings on natural symbiont transmission dynamics in pea aphids could have broader implications of importance for many economically, medically, and agriculturally important organisms.

Vertical transmission dynamics of pea aphid symbionts in natural settings

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Vertical transmission dynamics of pea aphid symbionts in natural settings

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