Genetic and cytogenetic bases of Cardinium-caused cytoplasmic incompatibility

Cytoplasmic incompatibility (CI) is a reproductive failure of terrestrial arthropods that occurs when maternally- inherited endosymbiotic bacteria in males affect sperm such that they fail to fertilize eggs of uninfected females. CI causes the spread of the symbiont in the host population because uninfected female fitness is depressed relative to infected females. Requiring both "modification" of the sperm of infected males, and "rescue" of the CI-affected paternal chromosomes in the egg cytoplasm of symbiont-infected females, CI symbionts make subtle changes in early embryogenesis that determine life or death for zygotes. CI has been studied extensively in the Wolbachia bacteria, yet the underlying molecular mechanism remains elusive. We propose to conduct the first study of the genetic and cytogenetic bases of CI caused by Cardinium, a bacterium evolutionarily distant to the better-known Wolbachia. This proposal builds on our team`s analysis of the first Cardinium genome, isolated from a parasitic wasp of whiteflies, Encarsia pergandiella. First we will produce a Cardinium transcriptome in male and female Cardinium-infected wasps, testing the hypothesis that symbiont genes involved in manipulation show sex-specific patterns of expression. Second, with quantitative RT-PCR, we will examine expression of candidate Cardinium genes and their orthologs in a panel of five Cardinium strains housed in different Encarsia parasitic wasp populations. These Cardinium strains cause different host phenotypes (two cause CI, two cause parthenogenesis, and one is asymptomatic). Genes of particular interest will be those that genome and/or transcriptome analyses suggest may influence CI, for example genes with differential expression in males and females, are upregulated in CI strains, have eukaryotic domains, and/or show similarity to CI Wolbachia genes. Third, we will conduct the first microscopic analysis of embryonic mortality in a CI Cardinium cross. Finding similarity to embryonic mortality caused by CI Wolbachia will support the hypothesis of a common mechanistic basis to CI in the two lineages, and lead to a greater focus on genes with common functions in our genetic analyses. CI Wolbachia is arguably the most common animal symbiont in nature, and is likely to have powerful ecological and evolutionary effects; for example CI symbionts may cause population collapse or reinforce reproductive isolation and eventual speciation of two host populations. Yet, in spite of considerable interest in CI caused by the dominant bacterial lineage, Wolbachia, and three completed CI Wolbachia genomes, the basis of host manipulation is not well understood. A study of CI in Cardinium offers a comparative approach likely to reveal fresh insights into the mechanism of CI.

Cytoplasmic incompatibility (CI) is a reproductive failure of terrestrial arthropods that occurs when maternally-inherited endosymbiotic bacteria in males affect sperm such that they fail to fertilize eggs of uninfected females. CI causes the spread of the symbiont in the host population because uninfected female fitness is depressed relative to infected females. Requiring both modification of the sperm of infected males, and rescue of the CI-affected paternal chromosomes in the egg cytoplasm of symbiont-infected females, CI symbionts make subtle changes in early embryogenesis that determine life or death for zygotes. CI has been studied extensively in Wolbachia bacteria, yet the underlying molecular mechanism remains elusive. In this project, we conducted the first study of the genetic and cytogenetic bases of CI caused by Cardinium, a bacterium evolutionarily distant to the better-known Wolbachia. Using transcriptome (the entire RNA of a given sample) sequencing of Cardinium in male and female Cardinium-infected wasps, revealed sex-specific patterns of expression of candidate symbiont genes possibly involved in manipulation. Here we present not only the first global gene expression data of Cardinium, but also identified many CI candidate genes. This project is thus the basis for detailed in-depth analyses of CI candidate genes identified here in future studies.