endosymbiosis (lecture 3)

Question 1

what is an endosymbiont?

Answer 1

any organism living within the body or cells of another organism

usually nutritional in basis

e.g. human gut flora, rhizobia, aphid symbiosis

Question 2

how do arbuscular mycorrhizal symbioses form?

Answer 2

• inducting a stable symbiosis requires signalling between organisms
• arbuscular mycorrhizal spores produce myc factors (maillet et al, 2011) which diffuse across plant membranes
• initiate root branching to enhance mycorrhizal colonisation (greater s.a. means greater chance of interacting with fungal partner)
• plant roots produce strigolactones (cameron et al, 2013) which induce hyphal branching for increase fungal s.a

Question 3

how do legumes form associations with n2 fixing bacteria?

Answer 3

• legumes (and some other plants) associate with - rhizobia, a catch all paraphyletic group of n2 fixing bacteria
• fix n2 in return for c and other nutrients
• legumes signal to rhizobia by releasing alkaloids which induce positive chemotaxis in the bacteria and causes them to produce nod-factors
• root hairs detect nod factors, curl up, end falls off to form a tube that the infection thread of rhizobia enters
• rhizobia-infected root nodules form
• myc + nod factors share a similar pathway

Question 4

why must root nodules be gas tight?

Answer 4

• enzyme that needs n2 needs low oxygen levels to operate
• but rhizobia are aerobic bacteria and need o2 to grow
• leghaemoglobin buffers o2 in root nodules to keep levels at optimum

Question 5

what is the tragedy of the commons in terms of mutualism regulation?

Answer 5

• once an endosymbiotic genotype cheats, immense selective pressure on others to cheat also
• creates evolutionary arms race
• red queen hypothesis - faster treadmill goes, faster red queen must run
• endosymbionts evolving to try and cheat, plants evolving to try and stop this

Question 6

what did kiers et al (2003) find about mutualism regulation between legumes and n2 fixing bacteria?

Answer 6

• forced a group of symbionts to cheat by supplying Ar instead of n2
• in cheating group, massive reduction in root nodules and free living bacteria
• plants ‘punish’ cheating bacteria by lowering o2 levels in nodules to below optimal via leghaemoglobin
• less dramatic, but also less carbohydrates sent
• easier to quickly lower o2 levels than control carbs due to source-sink dynamics
• plants avoid tragedy of the commmons through ability to discriminate between contributing and non contributing nodules

Question 7

what is the endosymbiosis in pea aphids?

Answer 7

(Douglas, 1998)

• pea aphids feed on phloem sap, mostly sugar, v few nutrients, lack essential amino acids
• endosymbiotic bacteria synthesises amino acids, especially tryptophan, in return for carbohydrates
• reproductive strategy encompasses symbionts
• host Buchnera symbionts in bacteriocytes
• born with fully formed embryos, growing own embryos
• endow embryos with infection on birth
• Buchnera lost free-living genes, obligate endosymbionts
• reduced genome, amino acid encoding genes became overrepresented
• pea aphids and Buchnera have congruent evolutionary trees, are co-evolving