Research

Host-parasite interactions in monarch butterflies

Throughout their breeding range, monarch butterflies (Danaus plexippus) are infected with a protozoan parasite, Ophryocystis elektroscirrha (OE; Figure 1). Monarchs are best known for a spectacular long-distance migration that they undertake yearly in both eastern and western North America . Non-migratory populations also occur in Florida , Hawaii , and other tropical locations. Parasite prevalence differs dramatically among populations, with fewer than 8% of monarchs infected in the eastern US, and over 80% infected in S. Florida (Figure 2). Monarch resistance to infection also differs between populations, with monarchs from the eastern US being more resistant than those in Florida . Besides studying the genetic basis of host resistance to infection, we are investigating the effects of parasitism on monarch fitness, and the effect of monarch migration on the evolution of virulence of the parasite.

Figure 1. Dormant spores (smaller objects) of the protozoan parasite O. elektroscirrha form around the developing scales of monarch butterflies. Adults emerge covered with parasites on the outside of their bodies, particularly on the abdomen. Parasites are transmitted when adult butterflies scatter dormant spores on milkweed leaves and eggs. Spores must be ingested by caterpillars to cause a new infection.

Figure 2. Population variation in the prevalence of heavily infected butterflies. From left to right, bars show Eastern North America; Rockhampton, Australia; Northern S. America; Western North America; Sydney, Australia; and S. Florida. This gradient represents longest distance migrants (on L) to resident non-migratory monarchs (on R). From Altizer, Oberhauser and Brower (2000) Ecol. Ent.

Recent and ongoing projects include:

Genetic and environmental determinants of host resistance

PhD student Elizabeth Lindsey has used a series of controlled experiments to examine the genetic and environmental determinants of monarch butterfly resistance, defined to include measures of quantitative parasite load, immune parameters, and lethal and sub-lethal effects of infection. Results have demonstrated significant variation in resistance to infection, and an effect of infection status on wing darkness (melanization). Interestingly, males showed greater parasite loads than females and also had lighter wings. This result and the effect on wing pigmentation are interesting because of the potential relationship between wing melanism (controlled by the ability of an individual to produce the black pigment) and the innate immune parameter of phenoloxidase activity involved in the encapsulation response (melanism of an invasive organism or object). In another experiment comparing immune parameters following controlled infections, females showed higher baseline levels of hemocytes, whereas males showed lower baseline levels but a sharp rise in hemocytes when parasitized (Figure 3). Together with student researchers in the lab, Elizabeth also examined the effects of rearing temperature and larval host density on monarch resistance. In one study, inoculated larvae were reared at cold (21° C), moderate (26° C), and hot (31° C) temperatures. Monarchs reared under cold temperatures had greater survival and fewer deformities, but also had higher mean parasite loads, pointing to an interaction between temperature and host response to infection.

 

Figure 3. Change in hemocyte concentrations in male (blue) and female (green) monarchs in relation to experimental infection with O. elektroscirrha.

 

Virulence evolution of monarch parasites

Postdoctoral researcher Jaap de Roode is investigating whether different migration strategies affect parasite virulence evolution, and more generally is testing several key evolutionary theories for the maintenance of parasite virulence. One study focuses on whether virulence is an adaptive trait of O. elektroscirrha by examining the relationships between transmission rate and virulence of genetically distinct parasite strains. In theory, more virulent parasites should have a higher transmission rate (in part due to greater within-host replication), so that the evolution of virulence could be favored. We also plan to study competition between virulent and avirulent parasite strains within hosts, to test whether virulent parasites have a competitive advantage in genetically mixed infections. In theory, such competition could lead to selection for higher virulence in populations where mixed infections are common. We expect to find a higher incidence of mixed infections in non-migrating monarch populations that inhabit tropical regions, because the milkweed host plants live year-round with a potential build-up of parasites in the environment. Based on theory, virulence levels should therefore be higher in non-migratory than migratory populations.

Ongoing projects include: (1) Documenting population variation in virulence among parasite from the eastern US, western US, South Florida, Hawaii and Australia. (2) Assessing the relationships between parasite virulence and transmission rate across parasite clones from different populations(Figure 4). (3) Studying within-host competition of parasite strains within mixed infections, and assessing the incidence of mixed infections in the wild. (4) Developing genetic markers to distinguish parasite genotypes, and to create a molecular phylogeny of parasites in different populations. (Figure 5)

Figure 4. Variation in the virulence of different clonal strains of OE derived from four different butterfly populations. Virulence here is shown as host longevity (in days), where more virulent strains are associated with reduced host longevity. Strains are ordered from most (L) to least (R) virulent. Each bar shows mean host longevity for each parasite clone.

 

 

Parasites and monarch flight performance

One explanation for reduced parasite prevalence in migratory monarch populations is that long distance migration weeds out infected animals, thus reducing parasite transmission between generations. Graduate student Catherine Bradley spearheaded a study to examine effects on parasitism on monarch flight performance. We experimentally infected monarchs from a migratory population and recorded their long-distance flight performance using a tethered flight mill (Figure ).6 Results showed that parasitized butterflies exhibited shorter flight distances, slower flight speeds, and lost proportionately more body mass per km flown. Differences between parasitized and unparasitized monarchs were generally not explained by individual variation in wing size, shape, or wing loading, suggesting that poorer flight performance among parasitized hosts was not directly caused by morphological constraints. Effects of parasite infection on powered flight support a role for long-distance migration in dramatically reducing parasite prevalence in this and other host-pathogen systems. Read more about this project on www.monarchparasites.org.

Figure 5. Diagram of flight mill apparataus, showing monarch attached to a horizontal arm rotating on a near frictionless pivot.

Visit the Ophryocystis elektroscirrha webpage and MonarchHealth citizen science project - www.monarchparasites.org