Symbiosis Definitions and Diversity PDF

29 – Symbiosis I: Definitions and Diversity BioMi 2900 Yeti Crab => Found associated with deep-sea hydrothermal vents Kiwa hirsuta http://www.plosone.org/article/info:doi%2F10.1...

29 – Symbiosis I: Definitions and Diversity BioMi 2900 Yeti Crab => Found associated with deep-sea hydrothermal vents Kiwa hirsuta http://www.plosone.org/article/info:doi%2F10.1371%2Fjournal.pone.0026243#s4 Video available at the website listed on slide 3 Giant, gutless worms (1 - 2 m long) – Riftia pachyptila Symbiosis I – Definitions and Diversity Yeti crab and Riftia 1. Defining an association 2. Lichens 3. Aphids & Buchnera 4. Rhizobium & Agrobacterium 5. Euprymna scolopes and Aliivibrio fischeri By the end of the symbiosis series you should be able to… Define three major types of symbiotic associations Define terms used to describe symbiotic associations Identify the type of association between two or more different organisms based on cost/benefit analysis Give examples of symbioses where symbionts and their hosts have evolved specialized structures or functions to enhance the establishment or maintenance of a mutualism Using Buchnera and aphids as an example, describe the benefits and drawbacks of a vertically inherited intracellular endosymbiont lifestyle  Contrast different associations with plants that evolved in closely related bacterial symbionts (Rhizobia & Agrobacteria) Compare the nutritional strategies of different herbivores and how mutualisms with bacterial symbionts supplement host nutritional needs Define the kinds of bacterial metabolic products that can benefit an animal host, and identify the benefits a bacterium may receive when associated with an animal List the phyla of bacteria normally associated with the human body  Symbiosis Microorganisms (particularly bacteria) are metabolically diverse. Most multicellular life forms are more limited. By developing a symbiotic relationship with a microorganism, an insect or animal or plant or microbe can gain functions that they otherwise could not obtain by mutation or recombination (horizontal gene transfer). (Co-)EVOLUTION Benefits of a symbiotic association can include: – Defense, protection – comes in many forms including enzymes for detoxification, enzymes to make toxins… (Lichens; Euprymna scolopes and Aliivibrio fischeri; Yeti crab and its symbionts?) – Nutritional benefits – can provide vitamins, essential amino acids, enzymes to access nutrients from difficult Host is the larger member of the association (not necessarily the provider), the smaller is the Symbiont Ectosymbiosis - symbiont external to (or on) the host body Endosymbiosis - internal symbiont usually intracellular (inside a cell)  1. Symbiosis “the living together of two lichens differently named organisms” -- de Bary 1879 I like to use this broad definition because… – For some associations, costs & benefits may be difficult to Laboratory grown lichen observe Cladonia cristatella – Relationships may change over “British Soldiers” a lifetime or with changes in  2. Lichens The partnership allows these microbes to live in more inhospitable places (the lichen is tolerant of extremes in temp. and can survive drying) An extreme example, endolithic lichens of the Antarctic  Cross section through a lichen Fungi (one or more species) and algae (one or two -- green alga and/or cyanobacteria), form unique structures Fungal mycobiont(s) provides home/protection; Algal photobiont(s) provides sugars (may secrete as much as 90% of its photosynthate), some lichenized Lichens reproduce by fragmentation cyanobacteria even fix nitrogen and or release of soreidia may release 95% of the fixed N  Types of Associations – defined by cost/benefit analysis Mutualism (a.k.a. beneficial symbiosis) All members benefit from the association Commensalism One member benefits, the other is unharmed Parasitism One member benefits to the detriment of the other – Often considered separately from symbiosis  More Terminology & Considerations: Obligate An organism cannot survive without its partner Facultative Survival is not dependent on the symbiosis Direct transmission of symbionts from one host to another, from parent to offspring (vertical) Acquire symbiont from the  What type of symbiotic relationship is this? A. Obligate mutualism B. Facultative mutualism C. Obligate commensalism D. Facultative commensalism E. Parasitism Syntrophy one member provides nutrition to the other/ one lives off of the metabolic products of the other  3. Buchnera and the Aphid Many insects harbor bacterial symbionts within their body inside specialized cells called bacteriocytes Buchnera aphidicola (proteobacterium) is an obligate, intracellular endosymbiont of the pea aphid >150 Million years old Aphids die when treated with antibiotics that kill Buchnera  Direct transmission of Buchnera Aphid embryos are inoculated with Buchnera inside their mother Buchnera are stained black in these photomicrographs, note stream of Buchnera entering the developing aphid embryo Wilkinson et al. Arthropod Structure & Development (2003)  Buchnera and the Aphid Glucose Nutritional mutualism Non-Essential Amino Acids Aphids feed on the plant phloem contents (sap) is rich in carbohydrates, Buchnera poor in nitrogen, sap is a poor source of essential amino acids (cannot be synthesized by aphid) Aphids consume mass quantities of 2 µm sap to recover scarce non-essential a.a., provide nutrients to Buchnera Essential (see right) Amino Buchnera overproduces essential Acids amino acids, some are provided to the aphid host  Impact of the association on Buchnera evolution Buchnera has a tiny genome (~0.5 Mb) Massive gene loss – made the association obligate Being sequestered inside host (no free-living stage) has limited genetic exchange (no transformation or transduction). Gaining new functions or replacing a defective gene is next to impossible leading to the Science (2019)  4. Rhizobia & Agrobacterium Both are Gram-negative, proteobacteria that can be found in the soil (free-living) or closely associated with plants Rhizobium and Agrobacterium are very closely related and differ mostly in the genes contained on giant plasmids: – Rhizobium has a Sym (symbiosis) plasmid. This allows it to invade the plant root, forming a root nodule where it fixes N 2, bacterial symbionts of plants - mutualism – Agrobacterium tumefaciens Ti (tumor inducing) plasmid, infects plant cells - parasitism Facultative relationships but in both cases, bacteria thrive when associated with a plant. Rhizobia + Legumes Host provides: – a regulated environment suitable for N2-fixation (uses leghemoglobin to regulate O2 levels) – organic acids (succinate, malate, fumarate) used as e- donors and C source  Rhizobia + Legumes Bacteria provide: NH4+ which is a great source of N for plant to use for biosynthesis of the amino acids, glutamine and asparagine Rhizobia have specific legume partners. A species of rhizobia will not make nodules on the roots of an incompatible plant.  How do the right partners find each other? Legumes secrete organic compounds from roots to enhance growth of rhizobia Rhizobia use chemotaxis to swim toward and locate plant root source Legume also secretes flavonoids, specific signals that activate gene expression of nodulation genes in the bacterium, including genes used in Nod Factor synthesis Nature Reviews Microbiology 7:312  Nod factors Recognized by the plant cell surface receptors => transcriptional changes Induces root hair curling Rhizobia attached to the curled root hairs to initiate infection and induce root cell proliferation => tumor- like nodule Mutualism or controlled parasitism? Once established, the bacteria fix nitrogen and plant maintains a population of bacteria inside its roots. Rhizobia suppress the natural defenses the plant would normally use to kill off bacteria invading its roots. If plant mechanisms to regulate C-flow to nodule are suppressed, the rhizobia can overtake and kill the plant. Agrobacterium tumefaciens Can cause crown gall disease Wounded plant => phenolics Agrobacterium swims toward plant wound exudates Agro attaches to plant cells and transfers DNA from its Ti plasmid into the plant cell where it is integrated into the nuclear DNA.  Ti Plasmid conjugation “Tumor inducing” Two-component system (VirA/VirG) senses phenolics and responds by activating transcription of gene to transfer T DNA VirD – ssDNA nick, VirE transfers to type IV secretion system (VirB) Once in the plant, T-DNA genes are expressed, lead to uncontrolled plant cell division and production of opine (a great C and N source for Agrobacterium) Used in biotechnology to alter genes of plants! What type of symbiotic relationship is this? A. Obligate mutualism B. Facultative mutualism C. Obligate commensalism D. Facultative commensalism E. Parasitism 5. Euprymna scolopes and Aliivibrio fischeri Light production (predator avoidance by counter illumination) Squid is not born with bacteria in its light organ  Acquisition of Symbiotic Microbes from the Environment “1,2,3” crypts “aa” anterior arm “pa” posterior arm “m” mantle “g” gills “f” funnel To improve its ability to catch symbionts, the squid has evolved special anatomical features In addition, it uses its immune system to recognize A. fischeri It secretes mucous which enhances aggregation of A. fischeri but not other bacteria. The A. fischeri What type of symbiotic relationship is this? A. Obligate mutualism B. Facultative mutualism C. Obligate commensalism D. Facultative commensalism E. Parasitism  Summary Symbiotic associations can be defined by looking at cost/benefit to each partner; sometimes it’s difficult to determine from the outside! Many mutually beneficial associations result in a nutritional benefit Some associations provide protection or allow organisms to live in hostile environments. Evolution and Coevolution: In mutualisms with animals, microorganisms may provide complex metabolic functions that the host does not have to evolve or genetically acquire. Partners have coevolved including suppression of immune responses by the host to allow microbial partners intimate access. Endosymbionts become metabolic specialists and often lose many genes, forcing them to become obligate! Vertically transmitted, obligate endosymbionts may accumulate deleterious mutations. In soil environments, chemical signals help a symbiont find a host in facultative associations, diffusible signals don’t work in aquatic systems.