Constant Light and Frequent Schedule Changes Do Not Impact Resistance to Parasites in Monarch Butterflies.

Organisms have evolved internal biological clocks to regulate their activities based on external environmental cues, such as light, temperature, and food. Environmental disruption of these rhythms, such as caused by constant light or frequent light schedule changes, has been shown to impair development, reduce survival, and increase infection susceptibility and disease progression in numerous organisms. However, the precise role of the biological clock in host-parasite interactions is understudied and has focused on unnatural host-parasite combinations in lab-adapted inbred models. Here, we use the natural interaction between monarch butterflies (Danaus plexippus) and their virulent protozoan parasite, Ophryocystis elektroscirrha, to investigate the effects of constant light and frequent light schedule changes on development, survival, and parasite susceptibility. We show that constant light exposure slows the monarchs' rate of development but does not increase susceptibility to parasitic infection. Furthermore, frequent schedule changes decrease parasite growth, but have no effect on egg-to-adult survival of infected monarchs. Interestingly, these conditions are usually disruptive to the biological clock, but do not significantly impact the clock of monarch larvae. These unexpected findings show that constant light and frequent schedule changes can uncouple host and parasite performance and highlight how natural relationships are needed to expand our understanding of clocks in host-parasite interactions.

Genomic evidence for gene flow between monarchs with divergent migratory phenotypes and flight performance.

Monarch butterflies are known for their spectacular annual migration in eastern North America, with millions of monarchs flying up to 4,500 km to overwintering sites in central Mexico. Monarchs also live west of the Rocky Mountains, where they travel shorter distances to overwinter along the Pacific Coast. It is often assumed that eastern and western monarchs form distinct evolutionary units, but genomic studies to support this notion are lacking. We used a tethered flight mill to show that migratory eastern monarchs have greater flight performance than western monarchs, consistent with their greater migratory distances. However, analysing more than 20 million SNPs in 43 monarch genomes, we found no evidence for genomic differentiation between eastern and western monarchs. Genomic analysis also showed identical and low levels of genetic diversity, and demographic analyses indicated similar effective population sizes and ongoing gene flow between eastern and western monarchs. Gene expression analysis of a subset of candidate genes during active flight revealed differential gene expression related to nonmuscular motor activity. Our results demonstrate that eastern and western monarchs maintain migratory differences despite ongoing gene flow, and suggest that migratory differences between eastern and western monarchs are not driven by select major-effects alleles. Instead, variation in migratory distance and destination may be driven by environmentally induced differential gene expression or by many alleles of small effect.

Environmental circadian disruption elevates the IL-6 response to lipopolysaccharide in blood.

The immune system is regulated by circadian clocks within the brain and immune cells. Environmental circadian disruption (ECD), consisting of a 6-h phase advance of the light:dark cycle once a week for 4 weeks, elevates the inflammatory response to lipopolysaccharide (LPS) both in vivo and in vitro. This indicates that circadian disruption adversely affects immune function; however, it remains unclear how the circadian system regulates this response under ECD conditions. Here, we develop an assay using ex vivo whole-blood LPS challenge to investigate the circadian regulation of immune responses in mice and to determine the effects of ECD on these rhythms. LPS-induced IL-6 release in whole blood was regulated in a circadian manner, peaking during subjective day under both entrained and free-running conditions. This LPS-induced IL-6 release rhythm was associated with daily variation in both white blood cell counts and immune cell responsiveness. ECD increased the overall level of LPS-induced IL-6 release by increasing immune cell responsiveness and not by affecting immune cell number or the circadian regulation of this rhythm. This indicates that ECD produces pathological immune responses by increasing the proinflammatory responses of immune cells. Also, this newly developed whole blood assay can provide a noninvasive longitudinal method to quantify potential health consequences of circadian disruption in humans.