parasitic-mutualistic-commensal similarities

Parasitic-mutualistic similarities

• Different outcome of co-evolution, but many similarities. Like pathogens, mutualists must:

  • Locate partner

  • Establish associations

  • Disperse afterwards

    • Horizontal transmission

      • Ex. Free-living ants find free-living aphid mutualists

    • Vertical transmission

      • Ex. Mother injects mutualistic bacteria into aphid offspring at birth

antagonism → mutualism

There are symbionts that once were fatal to the hosts but evolved to mutualists

• ex. Kwan Jeon ~ Lab colony of amoeba became infected with pathogenic bacteria

  • Almost all died

  • A few survived

    • These amoeba now CANNOT survive without the bacteria!

      • The bacteria makes an essential protein)

Endosymbiotic theory: origin of mitochondria and chloroplasts

• Endosymbiont (or endobiont)

  • Any organism that lives within the body or cells of another organism often (but not always) to mutually benefit

Mutualism → Antagonism

• Given that mutualists try to do as little for their partners they can get away with, mutualisms seem fragile to breaking down

• Evidence for mutualisms

  • Mutualisms are quite stable over evolutionary time

  • Many mutualisms have clear origins as antagonisms

  • Ex. Plant traits evolved that turned a pollen consumer into a pollen consumer + pollinator

  • Ex. Convolution has shifted Rhizobium bacteria from plant pathogens to plant nutritional mutualists

Mutualism-parasitism-commensalism

• every organism carries massive and diverse communities of symbionts → microbiome

• Millions of bacteria and viruses live on/inside ever body (mostly non-pathogenic as commensalism or mutualists) representing complicated ecosystems

• Microbes receive nutrients from hosts and provide nutrients back to the host

• Impact on host is diverse in function

  • Nutrition

  • Immune development

  • Pathogen resistance

  • Possibly behavior

Host specificity

• mutualistic and pathogenic symbionts may evolve to be very closely adapted to a particular host

• After a long (co-)evolutionary path, become highly specialized to a particular host environment

• Difficult to infect and be sustainably transmitted among other host species representing completely different environments

• Natural host: the host species an infectious organism “normally” infects

• Host switch/shift: terms usually reserved for establishing sustainable transmission in a new host species, usually because of evolutionary adaptation to that new host species

• Spillover: a parasite winds up in a new host to which it is not adapted to be transmitted among

• Cross-species transmission: any transmission event from the natural host species to a novel host species

Case Study: Heliobacter pylori

• Bacterium in human stomach

• 50% of people carry it (mostly asymptomatic, but sometimes associated with stomach inflammation/ulcers)

• Co-evolved with humans since origin of modern humans: tracked humans in their migration around the world in the past +/-60,000 years

brief history of human evolution

• Contemporary human clades

  • Diverged early in Africa ~150,000 years ago

• Out of Africa migration

  • Establishment of Eurasian clade (some subclasses within Eurasian clade)