Final Activity Report Summary - ANKWOL (Ankyrin-encoding genes in Wolbachia)
Wolbachia are maternally inherited symbiotic bacteria that reside in a large number of invertebrate species. They manipulate the reproduction of their insect hosts in a variety of ways in order to spread themselves through populations. The best studied of these is cytoplasmic incompatibility (CI), patterns of sterility seen when different populations interbreed. Wolbachia strain wPip is found in mosquitoes from the Culex pipiens complex where it induces CI, in one or both directions of the cross and involving either partial or complete sterility, causing a multitude of different crossing types. At the publication of the first Wolbachia genome, strain wMel from Drosophila melanogaster, a large number of genes encoding ankyrin (ANK) repeat domains were found. Because of the relative rareness of this motif in bacterial sequences and previously described functions, these genes became top candidates for involvement in CI. Additionally, these ANK genes were seen to be variable in sequence in different variants of the strain wPip. The aim was hence to investigate the variation both in sequence and in expression of these genes in Wolbachia strain wPip.
The genome of wPip was sequenced in collaboration with the Sanger Institute. We have identified as many as 60 genes in the genome of wPip that encode ANK motifs, as compared to 23 ANK genes found in wMel, the largest number of ANK genes found in any prokaryotic genome to date. The expression of all 60 ANK genes has been investigated in host male and female mosquitoes. Several ANK genes were found to be expressed specifically in female mosquitoes and one was found to be part of a sex-specifically expressed operon of co-regulated genes.
A thorough comparison between the ANK genes in wPip and in two previously sequenced genomes of Wolbachia were performed. It seems clear that this group of genes evolve rapidly and with diverse mechanisms including deletions, insertions and duplications. Several of the genes in the wPip strain appear to have arisen by strain specific expansion, but remnants of other ANK genes could be found in one or both of the other strains and are therefore more ancient. Additionally, five copies of a prophage (bacteriophage or bacterial virus that is integrated into the genome) previously named WO, was found in the genome of wPip, a considerably larger portion of the genome compared to other sequenced Wolbachia genomes. All were in general most similar to at least one of the other prophage copies in wPip, which suggests that they are actively recombining.
Two adjacent genes in wPip and a Wolbachia strain from Drosophila flies were seen to be similar to two adjacent genes in the mosquito Aedes aegypti, a species that is not infected by Wolbachia, but in no other sequenced insect genome. The level of similarity and the fact that the two genes are next to each other on the chromosome indicate that a horizontal gene transfer event has occurred between Wolbachia and the mosquito, or vice versa. The two genes were also found to be present in a related species, Aedes mascarensis, indicating that the horizontal transfer predates the split of these species. These findings show for the first time that genes transferred from bacteria to higher animals can persist over extended periods and apparently become incorporated into the biology of the recipient (the genes are clearly expressed), producing evolutionary innovation.
The genome of wPip was sequenced in collaboration with the Sanger Institute. We have identified as many as 60 genes in the genome of wPip that encode ANK motifs, as compared to 23 ANK genes found in wMel, the largest number of ANK genes found in any prokaryotic genome to date. The expression of all 60 ANK genes has been investigated in host male and female mosquitoes. Several ANK genes were found to be expressed specifically in female mosquitoes and one was found to be part of a sex-specifically expressed operon of co-regulated genes.
A thorough comparison between the ANK genes in wPip and in two previously sequenced genomes of Wolbachia were performed. It seems clear that this group of genes evolve rapidly and with diverse mechanisms including deletions, insertions and duplications. Several of the genes in the wPip strain appear to have arisen by strain specific expansion, but remnants of other ANK genes could be found in one or both of the other strains and are therefore more ancient. Additionally, five copies of a prophage (bacteriophage or bacterial virus that is integrated into the genome) previously named WO, was found in the genome of wPip, a considerably larger portion of the genome compared to other sequenced Wolbachia genomes. All were in general most similar to at least one of the other prophage copies in wPip, which suggests that they are actively recombining.
Two adjacent genes in wPip and a Wolbachia strain from Drosophila flies were seen to be similar to two adjacent genes in the mosquito Aedes aegypti, a species that is not infected by Wolbachia, but in no other sequenced insect genome. The level of similarity and the fact that the two genes are next to each other on the chromosome indicate that a horizontal gene transfer event has occurred between Wolbachia and the mosquito, or vice versa. The two genes were also found to be present in a related species, Aedes mascarensis, indicating that the horizontal transfer predates the split of these species. These findings show for the first time that genes transferred from bacteria to higher animals can persist over extended periods and apparently become incorporated into the biology of the recipient (the genes are clearly expressed), producing evolutionary innovation.