(This exercise is based on Weeks, A. R., M. Turelli, W. R. Harcombe, K. T. Reynolds, and A. A. Hoffmann. 2007. From Parasite to Mutualist: Rapid evolution of Wolbachia in Natural Populations of Drosophila. PLoS Biology 5: e114.)
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Wolbachia is a genus of bacteria that infect cells of invertebrates. Surveys have found that up to a fifth of insect species have at least some individuals infected with Wolbachia. This endosymbiont (a symbiont that lives inside its host) bacteria can have many effects on its host, including killing offspring and changing the sex of offspring.
Ary Hoffmann from the University of Melbourne in Australia, Michael Turelli from the University of California at Davis, and their associates have been studying Wolbachia infections in populations of Drosophila simulans from California. From the initial infection around 1980, Wolbachia infection spread northward across the state during the 1980s and 1990s. The most striking manifestation of infection in this fly species is cytoplasmic incompatibility: offspring produced in crosses where males are infected and the females are uninfected tend to die early in development. In crosses between infected males and infected females, the offspring develop normally. Females transmit the Wolbachia to nearly all of their offspring. In the lab, infected lines of flies can be made uninfected by treating them with an antibiotic.
Of interest in the current study is the effect of the Wolbachia infection on female fecundity. Female flies in the 1980s infected with Wolbachia showed a noticeable decrease in fecundity when compared to uninfected females, and in this aspect the bacteria acted as a parasite. In the last 20 years (equivalent to about 200 generations for the flies), has this relationship evolved from host–parasite into a commensalism or even a mutualism?
To address that question, the researchers examined flies that had been caught in the wild during the 1980s and raised in the lab since that time, as well as flies collected more recently. For both of these groups of flies, they measured the fecundity of females, both infected and uninfected, by allowing them to mate with males of the same infection state and lay eggs.
Figure 1 The fecundity (the mean number of offspring produced during 5 days of egg laying) by infected (black) and uninfected (white) lines. Riv1988 line was collected in 1988; the other lines were collected in 2002. Asterisks denote statistically significant differences at p < 0.05.
Question 1
What is the effect of Wolbachia infection on the flies of the Riv1988 line?
Question 2
Based on the female fecundity data alone, is Wolbachia behaving as a mutualist, a commensual, or a parasite in the Riv1988 line?
Question 3
What is the effect of Wolbachia infection on the flies from the lines collected in 2002?
Question 4
Based on the female fecundity data alone, is Wolbachia behaving as a mutualist, a commensual, or a parasite in the R10 line? The R24 line?
Figure 2 The researchers repeated the experiment looking at lines collected in 2004. The procedure was the same as above except the female fecundity was tested over 10 days of egg-laying instead of 5. Black bars again denote fecundity of infected females and white bars denote fecundity of uninfected females.
Question 5
Which lines, if any, show statistically higher fecundity in the infected females?
Question 6
Which lines, if any, show statistically higher fecundity in the uninfected females?
Figure 3 Each of the four strains that resulted from a series of genetic crosses is listed with the fly genotype first and then the strain of the Wolbachia as a superscript. Thus, Riv88wIR2 means that the fly genotype is Riv88 and the Wolbachia strain is IR2. The different letters indicate statistically significantly different means.
Question 7
The researchers were interested in whether the change in the relationship between fly and bacterium were due to evolutionary changes in the fly host or in the bacterium or both. To accomplish this, they used a series of genetic crosses to look at the effects of two different strains of Wolbachia in two different genetic backgrounds of the fly. Such a technique is called introgression. In particular, they looked at lines IR2 (a strain collected in 2004 where infected females had higher fecundity) and Riv88 (the strain collected in 1988 where uninfected females had higher fecundity). Describe the results. Is it the fly or the bacterium that has changed?
