Survey of Genomes - Buchnera sp. APS
How about being extreme in how much of your genome you throw away? Obligate endosymbionts have done just that and in doing so have become trapped in their host organisms. Quite often the interdependence is mutual as the host has come to rely on the endosymbiont for critical functions as well. Hannah Mann guest hosts this episode on the first endosymbiont genome sequenced - that of Buchnera sp. APS.
Welcome to Genomics Revolution. This is Hannah Mann from the 2019 Hiram College Genetics course hosting this episode on Buchnera aphidicola sub species Acyrthosiphon Pisum, or Buchnera sp. APS. Named after Paul Buchner, a pioneer in the field of symbiotic microbiology of sap sucking insects.
Buchnera sp. APS is a type of microbe found in the gut of pea aphids (Acyrthosiphon Pisum). The endosymbiotic relationship between these two organisms goes back over 160 million years and is tied to the creation of 5 essential amino acids that two organisms share. The pea aphids maintain this relationship by passing the bacteria through their eggs using a cell called a bacteriocyte (1). The bacteriocyte is really where the study of Buchnera aphidicola begins. In 1858, Thomas Huxley was describing an organ-like structure called a mycetoma – a collection of bacteriocyte cells in an organ like structure (2). Huxley thought that this mycetoma only contained granules of yolk material used for the aphid eggs, but in 1910 Pierantoni and Sule disproved this idea and showed that the granules were Buchnera microorganisms (2).
Paul Buchner entered the scene in the early 20th century alongside Pierantoni and Sule and furthered the understanding of the aphid/ Buchnera symbiosis. Using electron microscopy, Buchner discovered that the bacteriocytes housed the Buchnera cells in its cytoplasm and that there are 3 membranes that separate the Buchnera from the bacteriocyte – an inner and an outer Gram-Negative membrane and a 3rd that envelopes the Buchnera cell known as the symbiosomal membrane that is created by the aphid (2). In addition to structural elements, Buchner theorized the nutrient relationship of the symbionts – meaning he believed that the aphids and the bacteria supplied each other with amino acids that the other couldn’t make on their own. These theories were later confirmed by a series of diet assays, aposymbiotic aphids - meaning living apart from - and whole genome sequencing from the 1960s to the 1990s (2).
The genome of Buchnera sp. APS was sequenced using whole genome random shotgun sequencing – a method which uses a library of the bacteriocytes containing the Buchnera that have been sheared into random fragments, blunted, and subjected to multiple rounds of sequencing and fragmentation to obtain a reading (1). The result of the sequencing revealed that genome consisted of one circular chromosome of 640,861 bp size, and two circular plasmids – the pLeu plasmid of 7,786 bp size with 7 ORFs containing the leu ABCD operon and the pTrp plasmid that contains at least 2 tandem repeats the trpEG operon that generates 12-16 copies of Tryptophan for the host (1). The Buchnera genome codes for 574 protein genes and 36 RNA genes, of these genes 55 genes are used to code nutrients for the host (1).Interestingly enough, while the Buchnera contain so many nutrient based genes, it lacks many genes needed to synthesize structural lipids for its membranes – these elements are provided to the bacteria by the host and further cement the endosymbiotic relationship.
All this information is well and good, but what exactly are we trying to learn from these gut buddies? Why is this important to us? By diving deeper into the Buchnera and pea aphid relationship, we have the potential to learn how co-adaptive evolution occurs and how to separate endosymbionts in ways that could affect our work in gene transfers and protein synthesis. Recent studies in weevils found that endosymbionts would leave the bacteriocytes when antimicrobial genes responsible for symbiont growth and transmission were silenced (3). Aside from the mechanisms, we can learn a bit more about Eukaryotes by studying these endosymbiotic relationships. Think back to introductory biology and the endosymbiotic theory – it is believed that the organelles of our cells and other eukaryotes evolved from bacterial cells that formed similar endosymbiotic relationships to that of the aphid and the Buchnera (4). By studying the aphid and Buchnera, we can see how two species adapt to each other and supply essential products of life.
Now, these are just my thoughts on why I think these discoveries are important, but think about how much further we could push our knowledge of the human body if we understood our organelles or bacterial relationships better – like in areas of mitochondrial mutations or protein mutations – if we can figure out how other organisms share and separately synthesize essential proteins, perhaps we can apply similar systems in health treatment to compensate for these mutations. If we keep furthering our base knowledge, who knows what we could learn or tools we can develop? After all, CRISPr came from yogurt bacteria and now it is being used to modify the human genome, perhaps diabetes treatments could come from the mechanisms of the endosymbionts that would allow us to synthesize insulin outside of the pancreas? While pea aphids and their gut buddies seem insignificant in the scope of human knowledge, what lies in the stomach could be the key to a world of new possibilities.
Thank you for listening to the Genomics Revolution, this is Hannah Mann, signing off.
References
(1) Shigenobu, S., Watanabe, H., Hattori, M., Sakaki, Y., & Ishikawa, H. (2000). Genome sequence of the endocellular bacterial symbiont of aphids Buchnera sp. APS. Nature, 407(6800), 81-86. doi:10.1038/35024074
(2) Shigenobu, S., & Wilson, A. C. (2011). Genomic revelations of a mutualism: the pea aphid and its obligate bacterial symbiont. Cellular and molecular life sciences : CMLS, 68(8), 1297-309.
(3) Login, F. H., Balmand, S., Vallier, A., Vincent-Monegat, C., Vigneron, A., Weiss-Gayet, M., . . . Heddi, A. (2011). Antimicrobial Peptides Keep Insect Endosymbionts Under Control. Science, 334(6054), 362-365. doi:10.1126/science.1209728
(4) Tamames, J., Gil, R., Latorre, A., Peretó, J., Silva, F. J., & Moya, A. (2007). The frontier between cell and organelle: Genome analysis of Candidatus Carsonella ruddii. BMC Evolutionary Biology, 7(1), 181. doi:10.1186/1471-2148-7-181
Buchnera sp. APS is a type of microbe found in the gut of pea aphids (Acyrthosiphon Pisum). The endosymbiotic relationship between these two organisms goes back over 160 million years and is tied to the creation of 5 essential amino acids that two organisms share. The pea aphids maintain this relationship by passing the bacteria through their eggs using a cell called a bacteriocyte (1). The bacteriocyte is really where the study of Buchnera aphidicola begins. In 1858, Thomas Huxley was describing an organ-like structure called a mycetoma – a collection of bacteriocyte cells in an organ like structure (2). Huxley thought that this mycetoma only contained granules of yolk material used for the aphid eggs, but in 1910 Pierantoni and Sule disproved this idea and showed that the granules were Buchnera microorganisms (2).
Paul Buchner entered the scene in the early 20th century alongside Pierantoni and Sule and furthered the understanding of the aphid/ Buchnera symbiosis. Using electron microscopy, Buchner discovered that the bacteriocytes housed the Buchnera cells in its cytoplasm and that there are 3 membranes that separate the Buchnera from the bacteriocyte – an inner and an outer Gram-Negative membrane and a 3rd that envelopes the Buchnera cell known as the symbiosomal membrane that is created by the aphid (2). In addition to structural elements, Buchner theorized the nutrient relationship of the symbionts – meaning he believed that the aphids and the bacteria supplied each other with amino acids that the other couldn’t make on their own. These theories were later confirmed by a series of diet assays, aposymbiotic aphids - meaning living apart from - and whole genome sequencing from the 1960s to the 1990s (2).
The genome of Buchnera sp. APS was sequenced using whole genome random shotgun sequencing – a method which uses a library of the bacteriocytes containing the Buchnera that have been sheared into random fragments, blunted, and subjected to multiple rounds of sequencing and fragmentation to obtain a reading (1). The result of the sequencing revealed that genome consisted of one circular chromosome of 640,861 bp size, and two circular plasmids – the pLeu plasmid of 7,786 bp size with 7 ORFs containing the leu ABCD operon and the pTrp plasmid that contains at least 2 tandem repeats the trpEG operon that generates 12-16 copies of Tryptophan for the host (1). The Buchnera genome codes for 574 protein genes and 36 RNA genes, of these genes 55 genes are used to code nutrients for the host (1).Interestingly enough, while the Buchnera contain so many nutrient based genes, it lacks many genes needed to synthesize structural lipids for its membranes – these elements are provided to the bacteria by the host and further cement the endosymbiotic relationship.
All this information is well and good, but what exactly are we trying to learn from these gut buddies? Why is this important to us? By diving deeper into the Buchnera and pea aphid relationship, we have the potential to learn how co-adaptive evolution occurs and how to separate endosymbionts in ways that could affect our work in gene transfers and protein synthesis. Recent studies in weevils found that endosymbionts would leave the bacteriocytes when antimicrobial genes responsible for symbiont growth and transmission were silenced (3). Aside from the mechanisms, we can learn a bit more about Eukaryotes by studying these endosymbiotic relationships. Think back to introductory biology and the endosymbiotic theory – it is believed that the organelles of our cells and other eukaryotes evolved from bacterial cells that formed similar endosymbiotic relationships to that of the aphid and the Buchnera (4). By studying the aphid and Buchnera, we can see how two species adapt to each other and supply essential products of life.
Now, these are just my thoughts on why I think these discoveries are important, but think about how much further we could push our knowledge of the human body if we understood our organelles or bacterial relationships better – like in areas of mitochondrial mutations or protein mutations – if we can figure out how other organisms share and separately synthesize essential proteins, perhaps we can apply similar systems in health treatment to compensate for these mutations. If we keep furthering our base knowledge, who knows what we could learn or tools we can develop? After all, CRISPr came from yogurt bacteria and now it is being used to modify the human genome, perhaps diabetes treatments could come from the mechanisms of the endosymbionts that would allow us to synthesize insulin outside of the pancreas? While pea aphids and their gut buddies seem insignificant in the scope of human knowledge, what lies in the stomach could be the key to a world of new possibilities.
Thank you for listening to the Genomics Revolution, this is Hannah Mann, signing off.
References
(1) Shigenobu, S., Watanabe, H., Hattori, M., Sakaki, Y., & Ishikawa, H. (2000). Genome sequence of the endocellular bacterial symbiont of aphids Buchnera sp. APS. Nature, 407(6800), 81-86. doi:10.1038/35024074
(2) Shigenobu, S., & Wilson, A. C. (2011). Genomic revelations of a mutualism: the pea aphid and its obligate bacterial symbiont. Cellular and molecular life sciences : CMLS, 68(8), 1297-309.
(3) Login, F. H., Balmand, S., Vallier, A., Vincent-Monegat, C., Vigneron, A., Weiss-Gayet, M., . . . Heddi, A. (2011). Antimicrobial Peptides Keep Insect Endosymbionts Under Control. Science, 334(6054), 362-365. doi:10.1126/science.1209728
(4) Tamames, J., Gil, R., Latorre, A., Peretó, J., Silva, F. J., & Moya, A. (2007). The frontier between cell and organelle: Genome analysis of Candidatus Carsonella ruddii. BMC Evolutionary Biology, 7(1), 181. doi:10.1186/1471-2148-7-181