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.