Investigating the effects of a ubiquitous endosymbiotic bacteria and its relationship with a threatened butterfly species

Background/Question/Methods

Populations of Oregon silverspot butterfly, (Speyeria zerene Hippolyta), a federally listed threatened species historically inhabited 17 sites spanning from Longview, WA to Del Norte, CA. Extirpation has occurred at 12 of the sites and was likely due to multiple stressors including habitat fragmentation and reduction, invasive species out competing the butterfly’s obligate oviposting plants, and pollution. An additional, yet understudied, stress to the species is a ubiquitous endosymbiotic bacteria, Wolbachia, thought to infect 65% of arthropods. Wolbachia induces fascinating reproductive manipulations to its host with the most common being cytomplasmic incompatability. The goals of this study were to determine if Oregon silverspot butterfly is infected with Wolbachia, if infected to determine the evidence of effects of Wolbachia on the butterfly, and to determine if the infection rate changes over time. We conducted a screen for Wolbachia infection by collecting samples (removing one leg from each female butterfly used in the captive rearing program from the years 1999 and 2001-2011, n=234), extracting DNA from the samples, and employing polymerase chain reaction protocols using Wolbachia specific primers. Reproduction data, eggs laid and eggs hatched, for infected and uninfected individuals were analyzed. Proportion of infected individuals per year was compared to population indices.

Results/Conclusions

Oregon silverspot butterfly is infected with the endosymbiotic bacteria Wolbachia. Infected individuals, on average, laid more eggs than uninfected individuals however, the difference was not statistically significant (p=0.0968). Proportion of successful eggs hatched of infected and uninfected individuals were compared and it was determined that infected individual’s eggs successfully hatch significantly more often than uninfected individuals (p=0.0472). We also found that more uninfected individuals had occurrences of laying zero eggs compared to infected individuals. In addition, infection rates of butterflies have changed over time. Further analysis of population changes, including abiotic and other biotic variables such as temperature, need to be conducted before correlation between infection rates and population changes can be determined although, our data shows evidence of Wolbachia being a potential strong contributor to population declines. While preliminary, these results elucidate some of the effects Wolbachia has on Oregon silverspot butterfly reproduction. Further investigations into the potential of multiple infections and of rapid speciation of the Oregon silverspot butterfly are underway.