Insect/Bacterial Symbioses Aphid/Buchnera association

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Ambrose Bishop
6 years ago
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1 Insect/Bacterial Symbioses Aphid/Buchnera association I. Introduction A. Intracellular symbioses are common in the order Homoptera, which includes aphids, mealy bugs, whiteflies, and cicadas, Blattaria, cockroaches and Coleoptera, beetles B. Common thread is that many of these animals have diets deficient in one or several key classes of nutrients 1. Endosymbionts make up for these deficiencies with their biosynthetic pathways that provide these nutrients II. Aphids A. In an obligate symbiosis with Buchnera spp. - can t survive or reproduce without symbiont and visa versa B. Life history 1. Feed on plant sap by tapping into phloem tissue 2. Secrete enzymes that dissolve part of plant material to penetrate 3. Diet extremely high in sugars and nitrogen-poor and don t fix nitrogen - so they eat a lot of food and excrete excess as honeydew a) fits in with our discussion of how don t need gut symbionts to help with cellulose breakdown if nitrogen limited - just eat a lot of food b) honeydew is farmed by many ants 4. Populations can reach huge sizes - are real pests - in part because they transmit plant viruses Aphid lifecycle Explosive growth zygote End of season or stress Telescoped development meiosis 5. Life cycle complex show slide a) Females hatch in spring from eggs Parthenogenesis 3-15 cycles 1 female bears 60 live young 1

2 b) go through up to 15 parthenogenetic cycles - usually wingless c) females have telescoped generations (1) parthenogenetic eggs develop in female (2) embryos are undergoing development while female is growing (3) results in explosive population growth d) if stressed and/or towards end of season - produced winged offspring and males e) Mate and females lay eggs that overwinter III. Buchnera A. Found only in aphids, have never been found free living B. Can t be cultured in the lab C. gram negative 2-5µm oval cells D. Phylogenetics: 1. Many originally thought that symbionts were more like organelles a) maybe with some genes controlled in host b) but: symbionts have large genome size - comparable to free-living c) no apparent gene transfer 2. molecular phylogenies group Buchnera with the gamma proteobacteria a) same branch as the chemoautotrophs b) close to E. coli c) another close relative is symbiont from the tsetse fly d) symbionts from other Homoptera, such as mealy bug IV. Description of the symbiosis A. Bacteria are intracellular in vesicles (symbiosomes) show slide B. Housed in mycetocytes - these cells are polyploid and huge C. Cells arranged in a bilobed structure towards the posterior - called a mycetome 1. Mycetome breaks up as animal ages - and mycetocytes form groups throughout abdomen D. As an aside - mycetome surrounded by another bacteria-containing structurewith a different largely undescribed bacteria 2

3 Regulation of biomass ratios between host and symbiont Relative number of symbionts Relative weight of host time Cessation of reproduction Host death E. Aphid and symbiont growth is exquisitely regulated show slide 1. During parthenogenetic growth - weight of animal and number of bacteria mirror each other very closely 2. As aphid reaches maximal reproductive rates symbiont numbers remain high 3. At end of host reproduction - symbiont numbers decrease 4. By end of life - degrading symbionts can be seen in mycetocytes V. Transmission A. Symboints present in both developing embryos and adults B. Buchnera is vertically transmitted in both parthenogenetic and meiotic eggs C. Some mycetocytes become associated with eggs D. Buchnera leave symbiosomes and move into eggs where new mycetocytes are started VI. Coevolution of host and symbiont A. Closed, vertical transmission has likely led to coevolution of the partners B. Good evidence for this from recent molecular phylogenetic studies 1. Branching patterns and times of branching are similar for host and symbiont Show slide C. Supports theory that an ancient aphid ancestor - around 250 mya - acquired bacteria, developed vertical transmission and as aphids have diverged, so have their symbionts D. Very similar to mirror-image phylogenetic patterns between host and symbiont seen in some chemoautuotrophs and in squid-bacterial symbioses VII. Host symbiont energetics A. Hypothesis is that symbiont supplying the host with one or more essential nutrients. 1. Sap is poor in nitrogen 3

4 2. animals cannot synthesize 10 amino acids - these are essential B. Lots of studies looking at survival of host, in presence of antibiotics and on enriched media. 1. Emerging evidence that symbiont does supply aphid with the essential aas and more recently leucine. Evidence Aphid Nutrition Experiments Controls w/ mycetome Add antibiotics to remove symbionts Feed regular food Food + add essential amino acids Food + all but 1 aa dead a) If added to media of symbiont-free aphid - it can survive b) Tryptophan synthase activity present in intact association and isolated symbionts but not in aphid, free of their symbionts grown on antibiotics c) 4

5 in E. coli vs Buchnera E. coli negative feedback loop No negative feedback Always on Buchnera (1) Control show slide (a) In E. coli this is controlled by a negative feedback loop (i) As increased tryp. is synthesized - pathway is shut down (b) This loop absent in Buchnera (2) Location of operon (a) In E. coli on chromosome (b) In Buchnera on a plasmid (i) Number of plasmids can increase greatly independent of chromosome replication (c) Introduction of an important concept in molecular biology of symbiosis - many times symbiosis genes in the symbiont occur on plasmids that are separate from the chromosome (i) It s thought that this allows for greater control of genes on plasmid (a) true in luminescent bacteria and in N-fixing Rhizobium too (3) Both of these mechanisms allow for overexpression of 5

6 Location of operon in E. vs Buchnera Bacterial chromosome E. coli on plasmid Buchnera Independent replication of operon Results in increased production of 6

6 2 Insects and plants

6 2 Insects and plants Insect DIY 1. Find plant habitat 2. Find plant 3. Accept plant 4. Eat survive, reproduce Plant characteristics Shape structure Mechanical defenses trichomes Chemical defenses sap,