The impact of male-killing bacteria on the evolution of aphidophagous coccinellids

Michael E.N Majerus

doi:10.14411/eje.2006.001

Abstract

Many aphidophagous coccinellids harbour male-killing bacteria. These maternally inherited bacteria kill males early in embryogenesis, female offspring of infected mothers gaining a large resource advantage from the consumption of their dead brothers. In this paper, the diversity of male-killing bacteria and their coccinellid hosts will be briefly reviewed. Thereafter, the impact that invasion by male-killers has on coccinellid hosts will be addressed in two ways. First, the selective effects due to the loss of male progeny, and the intra-genomic conflict resulting from the cytoplasmic inheritance of the bacteria will be considered. Reduc- tions in mitochondrial diversity, effects on clutch sizes and the evolution of male-killer suppresser systems will be discussed. Second, the impact of female biased population sex ratios on the evolution of reproductive strategies and the primary sex ratio will be investigated. Preliminary results suggesting changes in male investment per copulation, reductions in female testing behaviour of males, and male biases in the progenic sex ratios of some uninfected male lines will be presented. Avenues for future work will be outlined.

Highlights

Many aphidophagous coccinellids harbour male-killing bacteria (Majerus & Hurst, 1997)
The bacteria that kill males are taxonomically diverse, having been found within the genus Spiroplasma (Mollicutes) (Hackett et al, 1986, Hurst et al, 1999a, Majerus et al, 1999), the Flavobacteria-Bacteroides group (Hurst et al, 1997a, 1999b) and the gamma and alpha subdivisions of the Proteobacteria (Werren et al, 1986, 1994, Hurst et al, 1999c, Majerus, 2003a). Examples from all these groups have been recorded in aphidophagous coccinellids (Table 1)
The basic parameters that determine whether a male-killer will invade a host population are a, the vertical transmission efficiency of the bacterium; s, the direct fitness effect of the bacterium on female hosts; and b, the fitness compensation that accrues to daughters of infected females as a result of the death of their brothers (Hurst et al, 1997b)

Summary

Introduction

Many aphidophagous coccinellids harbour male-killing bacteria (Majerus & Hurst, 1997). *Evidence given as: f-bsr = female biased sex ratio; lhr = low egg hatch rate in infected lines; mi = maternal inheritence; as = antibiotic sensitive; hs = heat sensitive; m = microscopy; sDNA = DNA sequencing; PCR = detection of symbiont using symbiontspecific PCR reaction. Such effects may be a direct consequence of male-killer infection of individual coccinellids, or a response to changes in coccinellid population sex ratios and demography as a result of male-killer presence.

Results

Conclusion