Abstract
In predatory ladybirds male embryos in clutches of eggs infected with male-killing bacterial endosymbionts do not hatch and are consumed by female hatchlings. Moreover, it is predicted that infection with male-killing bacteria should be prevalent in a ladybird population if the fitness advantage to female ladybirds due to the resulting reallocation of resources is high. We compared male-killer bacterial prevalence in two sibling species of the ladybird genus Harmonia that use different host ranges. Harmonia yedoensis feeds mostly on the highly elusive pine aphid and its hatchlings can greatly enhance their ability to capture prey and thus their survival by consuming un-hatched eggs in the clutch. In contrast, Harmonia axyridis feeds on a wide range of prey and consumption of un-hatched eggs by the hatchlings does not necessarily increase their foraging success. In the study area where these two species of ladybird occur sympatrically 14 of the 22 females of H. yedoensis (64%) laid clutches of eggs of which approximately only a half hatched and did not produce male offspring, whereas only one of 12 of the females of H. axyridis (8%) exhibited symptoms indicating infection with male-killing bacteria. In the H. yedoensis that exhibited symptoms of being infected with male-killing bacteria, the total allocation of resources to each hatchling, which was provided by laying larger eggs and eating the un-hatched eggs in the clutch, was considerable. These results suggest that the difference in the prevalence of infection with male-killing bacteria in these two species of ladybird may reflect differences in the quality of their prey and their foraging behaviour.
Highlights
Bacterial endosymbionts can enhance their own transmission efficiency by manipulating the reproductive mode of their arthropod hosts (O’Neill et al, 1997)
With one exception (53.91%) the percentage of eggs laid by the MK lines of H. yedoensis that hatched was less than 50%, and significantly lower than that of the NSR lines
Antibiotic treatment and diagnostic polymerase chain reaction (PCR) results indicate that bacterial endosymbionts belonging to Spiroplasma spp. were responsible for killing the male embryos
Summary
Bacterial endosymbionts can enhance their own transmission efficiency by manipulating the reproductive mode of their arthropod hosts (O’Neill et al, 1997). An early male-killing strategy, in which endosymbiont bacteria kill their male hosts during embryogenesis, is reported in a range of insect hosts, including Coleoptera, Lepidoptera and Diptera (Hurst & Jiggins, 2000). This strategy is beneficial to maternally inherited bacteria because the death of the male hosts confers a fitness advantage on the remaining female hosts due to the reallocation of resources from male embryos to female hatchlings (Hurst, 1991; Hurst et al, 1997). Even though there is considerable variation in the prevalence of infection of different host species and populations (Hurst & Jiggins, 2000; Jaenike, 2009), the relationship between male-killer prevalence and ecological factors affecting the fitness of infected hosts remains unclear
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