Variable Incidence of Spiroplasma Infections in Natural Populations of Drosophila Species

Thomas Watts,Nancy A Moran,Therese A Markow,Tamara S Haselkorn,Trine Bilde

doi:10.1371/journal.pone.0005703

Abstract

Spiroplasma is widespread as a heritable bacterial symbiont in insects and some other invertebrates, in which it sometimes acts as a male-killer and causes female-biased sex ratios in hosts. Besides Wolbachia, it is the only heritable bacterium known from Drosophila, having been found in 16 of over 200 Drosophila species screened, based on samples of one or few individuals per species. To assess the extent to which Spiroplasma infection varies within and among species of Drosophila, intensive sampling consisting of 50–281 individuals per species was conducted for natural populations of 19 Drosophila species. Infection rates varied among species and among populations of the same species, and 12 of 19 species tested negative for all individuals. Spiroplasma infection never was fixed, and the highest infection rates were 60% in certain populations of D. hydei and 85% in certain populations of D. mojavensis. In infected species, infection rates were similar for males and females, indicating that these Spiroplasma infections do not confer a strong male-killing effect. These findings suggest that Spiroplasma has other effects on hosts that allow it to persist, and that environmental or host variation affects transmission or persistence leading to differences among populations in infection frequencies.

Highlights

Based on recent molecular surveys, heritable bacterial symbionts are widespread in arthropods, but, in most cases, their effects on hosts are unknown
We examined infection status in wild-caught females and males of 19 Drosophila species from localities (Figure 1) in the southwestern United States and northwestern Mexico in order to (1) ask how the incidence of infected flies varies in nature and (2) assess the sex ratio of infected flies in order to detect evidence of male killing infections
Infection incidence ranged from under 1% in D. simulans and D. melanogaster to an average of 37% in D. mojavensis

Summary

Introduction

Based on recent molecular surveys, heritable bacterial symbionts are widespread in arthropods, but, in most cases, their effects on hosts are unknown Drosophila species harbor only two types of heritable bacterial endosymbionts [3,4]. The most widely studied, and the most common, is Wolbachia [5,3]. The other heritable bacterial endosymbiont in Drosophila is Spiroplasma, reported in a total of 16 species [6,7,8,9,3,10] and, curiously, rarely found to coinfect with Wolbachia. In some Drosophila species, Spiroplasma causes male-killing [11,12,13,8], while in others it does not [14,3,11]. Spiroplasma has been studied far less than Wolbachia, and factors underlying its distribution among and within Drosophila species are unknown

Methods

Results

Conclusion