Developmental analysis of a female-specific 16S rRNA gene from mycetome-associated endosymbionts of a mealybug, Planococcus lilacinus.

A clone showing female-specific expression was identified from an embryonic cDNA library of a mealybug, Planococcus lilacinus. In Southern blots this clone (P7) showed hybridization to genomic DNA of females, but not to that of males. However, P7 showed no hybridization to nuclei of either sex, raising the possibility that it was extrachromosomal in origin. In sectioned adult females P7 hybridized to an abdominal organ called the mycetome. The mycetome is formed by mycetocytes, which are polyploid cells originating from the polar bodies and cleavage nuclei that harbour maternally transmitted, intracellular symbionts. Electron microscopy confirmed the presence of symbionts within the mycetocytes. Sequence analysis showed that P7 is a 16S rRNA gene, confirming its prokaryotic origin. P7 transcripts are localized to one pole in young embryos but are found in the pole as well as in the germ band during later stages of development. P7 expression is detectable in young embryos of both sexes but the absence of P7 in third instar and adult males suggests that this gene, and hence the endosymbionts, are subject to sex-specific elimination.