Inherited bacteria that parasitically distort the pattern of sex allocation of their host, biasing allocation towards female progeny, are found in many arthropods. One such manipulation is male-killing, where male progeny of infected females die during embryogenesis. We here provide evidence for a male-killing bacterium in the coccinellid beetle, Adonia variegata. We then address 3 questions. First, is this male-killing bacterium one that is found in other hosts, or does it represent a new transition to male-killing within the eubacteria? Using the sequence of the 16S rDNA of the bacterium, we found that the male-killing bacterium is a member of the Flavobacteria–Bacteroides group, most closely related to the male-killing bacterium in another ladybird beetle, Coleomegilla maculata. Secondly, is there any evidence that this bacterium affects female host physiology? In a paired test under nutritional stress, we found no evidence for a physiological benefit to infection, and weak evidence of a physiological cost, in terms of reduced fecundity. Thirdly, is there any evidence of host involvement in the transmission of the bacterium to the germ line? We found no evidence of host involvement. Rather, bacteria migrated to the ovariole independently of host cells. We conclude that the bacterium is a parasite, and discuss how 2 different species of ladybird come to be infected with 1 lineage of bacterium, and why case studies of male-killing bacteria have generally found little evidence of any symbiont contribution to host physiological functioning.
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