Drastic times call for drastic measures: how timing affects host-parasite interactions
Westwood, Mary Lynn
Earth’s daily rotation causes predictable cycles of day and night, which all life has evolved to cope with. Circadian clocks (i.e. daily, biological timekeepers) are ubiquitous and allow organisms to schedule activities, from gene expression to physiologies to behaviours, according to the time-of-day they are best undertaken. Most research on circadian rhythms has focussed on uncovering the genes and molecular pathways involved in the workings of circadian clocks. However, there is increasing interest in the evolution and ecology of circadian rhythms – particularly, in how rhythms affect interactions between organisms. One of the most fundamental ecological interactions is that between parasites and hosts. In this thesis, I explore how circadian rhythms mediate infection through the lens of evolutionary ecology. My chapters consider how the rhythms of both hosts and parasites evolve in response to each other, with a focus on how rhythms mediate activities underpinning sexual reproduction. Specifically, I have outlined how to examine evolutionary ecology from a chronobiological framework and why it matters to do so, asked questions about the role of rhythms in mating behaviours using the pacific field cricket Teleogryllus oceanicus, and about the role of rhythms in reproductive effort using the rodent malaria parasite Plasmodium chabaudi. First, I wrote a perspective paper (Chapter 2) demonstrating the value of integrating evolutionary ecology and chronobiology. This is the first paper detailing the role of rhythms as mediator to natural and sexual selection, including the development of hypotheses examined in Chapters 4 and 5. Further, I challenge conventional wisdom emerging in chronobiology that immune rhythms mediate susceptibility, and propose how parasite manipulation of host rhythms may explain unusual host behaviours that have so far defied explanation. Moreover, this paper is the first to consider a periodic environment from the parasites “point of view”, because most work to-date has focussed on host rhythms in immune defence. In chapters 3 and 4 focussed on a cricket-parasitoid fly system (Teleogryllus oceanicus – Ormia ochracea) to examine whether hosts can evolve altered rhythms in mate-seeking behaviours as a parasite avoidance strategy (“temporal escape”). I expected this as the parasitic fly locates its cricket host by following the sound of male crickets when they sing to attract female mates, and then homing in using visual cues. Thus, singing in male crickets is a sexually selected trait that individuals must balance with natural selective pressure from the fly. To begin to ask whether temporal escape could have evolved, I had to first make T. oceanicus into a tractable system for chronobiology and characterise its singing rhythm. Thus, in Chapter 3, I performed experiments to uncover to what extent singing in T. oceanicus is clock-controlled. To derive data to analyse in a robust circadian context, I developed a pipeline which combines machine learning and high performance computing. The circadian phase markers I extracted showed conclusively that singing is circadian in T. oceanicus and variation amongst individuals suggests natural and sexual selective pressures may shape singing rhythms. Next, in chapter 4 I performed an experiment to compare the circadian singing rhythms of an ancestral, unparasitised population of T. oceanicus (from the Cook Islands) with a population from the Hawaiian island of Oahu that has experienced sufficiently high parasitism by O. ochracea to evolve several forms of morphological defence. Specifically, I tested whether the timing of singing by males from Oahu differs from the singing rhythm of males from the Cook Islands, hypothesising that Oahu males should be less likely to sing at dusk because that is when the fly is thought to be most likely to host-seek. I found that while both populations have similar entrained and free-running periods, circadian phase markers vary between the populations. Males from Oahu sing nearly twice as much as Cook Island males, but Oahu males are much less likely to sing during the light phase and around dusk. While many other selection pressures will differ between the Oahu and the Cook islands and the population introduced to the Hawaiian islands has experienced a strong bottleneck, which may influence singing rhythms, the timing differences I observe are consistent with temporal escape as a parasite avoidance strategy. In Chapter 5, I switched to malaria parasites to test whether host rhythms influence parasite investment into sexual reproduction. When out-of-synch with host rhythms, P. chabaudi parasites suffer a 50% reduction in the density of both asexual and sexual stages (termed “gametocytes”) in the host’s blood. I focused on asking whether reduced investment in gametocytes and/or increased mortality of gametocytes might explain their lower density in out-of-synch infections. I first analysed data from a previous experiment on reproductive effort (called the “conversion rate”), which is known to be plastically down-regulated when parasites experience stressful situations. Second, I carried out experiments to test whether a key aspect of the innate immune response (the inflammatory cytokine tumor necrosis factor, TNF) varies in its gametocytocidal efficacy according to host time-of-day and gametocyte age. I found that neither plasticity in conversion rate or rhythms in TNF-caused gametocyte mortality explain the reduction in gametocytes observed in out-of-synch infections and suggest alternative explanations. Gametocytes are required for between-host transmission of malaria parasites so understanding why it matters for gametocytes to be synchronized to host circadian rhythms might suggest novel approaches to blocking parasite transmission. Decades of research into the molecular underpinnings of circadian clocks has highlighted the disconnect between progress in understanding the mechanisms driving rhythms and their evolutionary and ecological significance. Infections are ubiquitous in nature, so understanding how rhythms in parasite offense interact with rhythms in host defences are an excellent arena for integrating circadian biology with evolutionary ecology and may uncover novel strategies for controlling infections.