Wolbachia are maternally inherited intracellular bacteria known to infect a
wide range of arthropods. The artificial transfer by microinjection of the
'popcom-effect'-inducing Wolbachia from Drosophila melanogaster larvae
to Stomoxys calcitrans pupae identified the germ-line tissue as the limiting
factor in horizontal transmissions across wider phylogenetic distances.
Electron microscopic analysis of adult somatic tissue of S. calcitrans
documented that this nai've host is fully capable of supporting Wolbachia.
However, Wolbachia does not succeed in achieving sufficient germ-line
titres to maintain transovarial transmission beyond five host generations.
This leads to the hypothesis that host-symbiont specificity in the case of
Wolbachia is primarily determined by specific host transport and localisation
phenomena surrounding egg development and not by mechanisms involving
basic life support by the host cell or by host cell-symbiont communication.
Coevolution of Drosophila and Wolbachia seems to have become speciesspecific for only very few and specific cellular interactions.
A further consequence of the interaction of Drosophila with the popcorn
strain of Wolbachia has been identified by confocal microscopy. While the
ancestral phenotype of the popcorn strain is cytoplasmic incompatibility as
evidenced by its capacity to rescue incompatibility, the exclusion of
Wolbachia from developing sperm cysts explains its failure to induce
incompatibility. Wolbachia 's ability to manipulate reproductive phenotypes
of its host seems to be governed by cellular localisation phenomena as well.
The 'popcorn-effect' itself practically halves the adult life span of its
Drosophila host. This was originally attributed to an unrestricted
proliferation of Wolbachia in the host cells of the imago. The 'popcorneffect' was identified as a temperature sensitive trait. A statistical comparison of the quantitative kinetics of Wolbachia with the help of the
real-time polymerase chain reaction between permissive and non-pennissive
temperatures does not support the original description of the 'popcorneffect'. The data suggest heterogeneity of the trait.