The inherited bacterial symbiont influences the sex ratio of an insect host.

In many intracellular symbioses, the microbial symbionts provide nutrients advantageous to the host. However, the function of , a symbiotic bacterium localized in specialized host cells (bacteriocytes) of a whitefly , is uncertain. We eliminate this bacterium from its whitefly host by two alternative methods: heat treatment and antibiotics. The sex ratio of the host progeny and subsequent generations of -free females was skewed from 1 : 1 (male : female) to an excess of males, often exceeding a ratio of 20 : 1. is haplodiploid, with diploid females derived from fertilized eggs and haploid males from unfertilized eggs. The status of the insect did not affect copulation frequency or sperm reserve in the spermathecae, indicating that the male-biased sex ratio is unlikely due to the limitation of sperm but likely to be associated with events subsequent to sperm transfer to the female insects, such as failure in fertilization. The host reproductive response to elimination is consistent with two alternative processes: adaptive shift in sex allocation by females and a constitutive compensatory response of the insect to -mediated manipulation. Our findings suggest that a bacteriocyte symbiont influences the reproductive output of female progeny in a haplodiploid insect.