Symbiosis and the Environment Final

Aphid Lifecycle

• Females hatch from eggs in spring

• Females go through parthenogenetic cycles (eggs develop within female asexually while female is growing), results in explosive population growth

• If stressed or toward end of season, produce winged offspring and males

• pairs mate and females lay eggs that overwinter

Aphids and Buchnera life history

• association is obligate in both directions, buchnera not found free-living, vertical transmission

• buchnera found intracellularly in bacteriocytes arranged in bacteriome structure toward posterior

• aphid and symbiont growth is tightly regulated

  • during growth, host weight and symbiont # increase together

  • Host stops growing before reproduction, symbiont # remains at euqilibirum

  • at end of reproduction, symbiont # starts decreasing

  • symbionts are exocytosed from maternal bacteriocytes and endocytosed by embryo

Buchnera genome reduction

• very small and stable genome

• loss of key genes: anaerobic respiration, synthesis of amino sugars/fatty acids/phospholipids/complex carbs

• no evidence of gene transfer to host

factors leading to Buchnera genome reduction

• loss of free-living state

• reduction of population size (from vertical transmission)

• loss of machinery for DNA repair and recombination (little/no ability to acquire new DNA in host environment)

• bias toward deletions vs insertions

• relaxed selection pressure (genome streamlining)

Aphid/Buchnera metabolic complementarity

plant sap is poor in N, host cant produce several amino acids which symbionts provide.

Over time, symbiont lost overlapping metabolic pathways bc it could rely on the host to provide it.

Tryptophan

Amino acid that host cant produce.

Tryptophan operon in Buchnera modified so it is always produced. Located on a plasmid instead of chromosome, meaning operon can be replicated independently from chromosome.

Regiella

Aphid secondary symbiont

• provides resistance to fungal pathogen found on clover, allowing host access to new food source/habitat

Hamiltonella

Aphid secondary symbiont

• protects host from lethal parasitoid wasp egg deposition by attacking developing wasp larvae with toxins

Serratia

Aphid secondary symbiont

• protects Buchnera from heat damage, improving host fitness

Cost/benefit of aphid ant farming for ants

Honeydew from aphid is 90-95% sugar with some amino acids and vitamins, help ants meet carbohydrate demands

potential cost is energy caring for aphids and reliance on aphids to meet nutritional needs

cost/benefit of ant farming for aphids

Reduced predation and parasitism, reduced risk of fungal infection

Ants decrease dispersal by reducing number of winged aphids on plants (produce a hormone to prevent wing development or bite them off). Ants also eat aphids if they’re not producing honeydew.

Aphid/ant/butterfly symbiosis

• butterflies lay eggs on plants w/ aphids and ants

• caterpillars hatch and eat aphids

• caterpillars produce pheromone to make them seem like ants, ants don’t attach and instead bring to their nest and feed them

• caterpillars produce honeydew for ants

• when butterfly emerges, ants attack but they have a substance on wings to disable ant jaws

Leafcutter ants

• ants domesticated fungus and maintain a culture within colony, larval ants exclusively eat fungus

• ants cut leaves and feed to fungi, keep fungi safe from mold and pests

• fungi can’t survive w/o cut leaves and produce extra-nutritious hyphae for ants to feed on

• ants have bacterial symbiont which produce antimicrobial pesticides that kill fungal pathogen

Importance of N

• essential for amino acids

• limiting nutrient in many ecosystems

• N2 is mostly useless biologically and must be converted to more easily assimilated forms (fixation)

N cycle inputs and outputs

inputs from atmosphere: electrical storms, microbial fixation, fertilizer and farming practices

outputs to atmosphere: microbial denitrification

Nitrogen cycle

Fixation - microbes convert N2 to ammonia/ammonium

Nitrification - nitrifying microbes convert ammonium into nitrates, which plants can absorb

Denitrification - nitrates in soil converted by bacteria back into N2

Nitrogenase

complex of two enzymes which facilitates N2 fixation. Conformation only allows for N-sized molecules, but O2 is a similar size and can poison the reaction, so reaction must occur in anaerobic environment (provided by plant host)

Rhizobium

• can live heterotrophically in soil but most species don’t fix N2 in this state

• free living (rod-shaped and motile)

• in symbiosis (become elongate, lose flagella, and some form Y shape)

Root nodule

Swellings along tap roots, bacteria housed in high density within cortical cells inside vacuoles. Nodules maintain low O2 concentrations to facilitate N2 fixation by nitrogenase

Leghemoglobin

• special hemoglobin that both partners produce a component of

• buffers O2 so there is just enough for bacterial respiration without poisoning nitrogenase

• production induced by symbiosis

Cost/benefit of nodulation for plant host

• plant gives large amount of photosynthate to nodules/Rhizobium to provide energy for N2 fixation

Root nodule morphology

• nodule has it’s own meristem formed during initial colonization, it constantly grows new cells that become colonized

• outer section not colonized yet, middle region is area of active N2 fixation, back senescent region is where cells and bacteria are dying and not fixing N2

• vascular region surrounds symbiotic tissue

Four phases of nodulization

A. Plant Signaling

B. Bacterial signaling

C. infection

D. maturation

Plant Signaling

• plants release chemical attractants (flavonoids) into soil

  • different plants produce different flavonoids to attract particular Rhizobium species and strains

• Bacteria proliferate and migrate toward root

Bacterial nod operon

• bacterial nod operon contains genes which encode for nod factors (sugar signaling molecules)

  • operon is activated by flavonoids entering bacterial cell and binding to nodD

• nodD is species-specific transcription factor

  • host-specific flavonoid bonds to it, and nodD changes shape to active form, binds to nod box and activated downstream nod genes

Bacterial Signaling

• nod factors diffuse out of Rhizobium as they approach the plant root

• nod factors bind to plant lectins (sugar-receptor proteins which vary with plant species). Lock-and-key model

• Nod factor attachment triggers root hair curling

• cortical meristem (ultimate site of nodule) begins to form

Infection

• bacteria captured by curling root

• infection begins by hydrolysis of plant cell wall and invagination

• bacteria move into invagination, forming the infection thread that grows through root

• bacteria move down growing thread to cortical meristem

• infection reaches meristem and branches to individual cells, bacteria move into cortical meristem

• bacteria congregate at tip of thread, tip pinches off

maturation

• bacteria differentiate into bacteroids

  • bacteroids= stop dividing, non-motile, grow, distinct gene expression, glutamine synthesis inhibited, N2 fixation

• nif operon turned on, N2 fixation begins

Actinorhizal symbioses

• most similar to legume/Rhizobium, host are trees and shrubs

• symbionts in soil and symbioses in the root

• both form nodules which contain a form of hemoglobin

Cyanobacterial symbioses

• symbionts differentiate to heterocysts (special N2 fixing cells)

• symbionts have limited division in hospite, and host morphology is altered

lHornworts and liverworts

• primitive non-vascular plants

• Nostoc symbiont found extracellularly in cavities under gametophytes

• both partners alter morphology (plant has new structures, Nostoc has more heterocysts)

Plasmids

• extrachromosomal DNA molecule

• typically circular and double-stranded

• DNA on plasmids is more easily regulated bc independent from chromosome

Nod genes

• establish infection in plant by initiating nodulation response in correct host (host-specific response)

• produce nod factors - signal plants to curl root, create infection thread, and divide cortex cells

exo genes

• maintain infection in plant

• nod factor production reduces after Rhizobia enters root hair cell, then exo genes are expressed

• encode for synthesis of polysaccharides (not host-specific) that are important for continued recognition as “good bacterium", thought to sustain nodulation process

fix and nif genes

• required for N2 fixation

• nif found in free-living bacteria, code for synthesis of nitrogenase complex and regulators

• fix found in symbiotic bacteria, some code for leghemoglobin heme group and its release

• low O2 conditions caused by leghemoglobin induces downstream nif and fix genes which produce additional proteins

mos and moc genes

• provide nutrients for free-living Rhizobia that aren’t yet housed in nodules

• nif gene activated mos genes in symbiotic bacteria within the nodules

  • mos gene products synthesize rhizopine (a C and N rich food then released into rhizosphere to sustain nearby free-living Rhizobia)

• moc genes only expressed by free living Rhizobia and used to catabolize rhizopine produced from Rhizobia in the nodules

Legume-Rhizobium and Arbuscular mycorrhizae

• share plant genes for symbiotic establishment, likely that legume-rhizobium signaling evolved from ancient arbuscular-mycorrhizae signaling pathway

• plants release general array of compounds related to flavonoids which AM cue to and migrate up concentration gradient

• AMs product Myc factors similar to nod factors

• Host plant has carb binding receptors similar to those in legumes

• Downstream signaling involving Ca oscillations is very similar between LRs and AMs

Endosymbiotic Theory

• posits that eukaryotes evolved due to symbiotic acquisition of prokaryotes in deep evolutionary past, which eventually became organelles

Shared features of bacteria and mitochondria/chloroplasts

• plasmids

• genomes and gene structure most similar to bacteria

• similar replication and proteins

Primary endosymbiosis events

1. prokaryote develops a nucleus and engulfs an alpha proteobacteria which becomes a mitochondria, forming a eukaryote

2. eukaryote then engulfs a cyanobacteria which becomes a chloroplast, forms a photosynthetic micro-alga