#1 Symbiosis

Costa Rica Science Research Program

Module #1

Hopefully, this document illustrates the utility of using the leaf-cutter ant system as a model for addressing various biological principles. This first module has been created to give some general background and core concepts of evolution, to be used as a teaching resource.  Learning From Nature: From Genes to Ecosystems is being developed a six module teaching resource to help give students insight into the world around them, understand basic biological process, and prepare them for research based inquiry in natural product and novel biofuel enzyme discovery. In bold are terms that have been defined in this module or will be defined in future modules. At the end of Module 1 text is a list of the other modules and key concepts that will be included. In addition, there are model systems that will be used to aid in learning about these key concepts and potential incorporation of field biologists. 

Learning From Nature: From Genes to Ecosystems

Module 1: Leaf Cutter Ants, a model for studying evolution and symbiosis

Core Concept-Species have changed and continue to change on Earth (Evolution). No one organism lives in isolation and relies on forming relationships with other organisms for survival (Symbiosis). Understanding the relationships that have formed between organisms and how they have changed (Co-evolution) can provide insight processes that can help us, as humans, understand our own natural history.

Key Terms

Evolution: descent with modification. Two major forces are considered when looking at the evolution of life on Earth; continual change in traits and the processes that either favors these changed traits or not. A trait is defined as a particular characteristic, such as eye color, hair color, or size that is expressed when an organism's genes interact with its environment. Genes vary within and between populations, so organisms show heritable differences (variation) in their traits. Genes, or alleles, are then inherited by offspring from their parents. Natural selection works on the traits that are inherited. If the variation that is inherited lends itself to a trait that is favorable (i.e. having more muscle mass to run faster, having a longer neck to reach leaves on high branches) then the trait will be passed on to the next generation of offspring and will become more common in a population. Traits that hinder survival and/or reproduction will become more rare. Symbiosis: obligate interaction between two or more different organisms living in close physical association. Symbiosis encompasses a spectrum of relationships.

Parasitism at one extreme is a situation where one organism benefits to the detriment of another.

Mutualism at the other end is a type of symbiotic association where all parties benefit from the association.

Co-evolution: the influence of closely associated species (symbionts) on each other’s evolutionary trajectory

Useful resources:

http://www.youtube.com/watch?v=wKbj3ZDmvdU&feature=related this a great intro for evolution!!!

http://www.youtube.com/watch?v=RH3KYBMpxOU  this a fanstatic Leaf Cutter Ant movie

http://www.youtube.com/watch?v=emSHL03NkF8&feature=related so is this one

http://evolution.berkeley.edu/evolibrary/search/topicbrowse2.php?topic_id=41 this has a more in depth lessons about evolution

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The Leaf Cutter Ant System-An Overview

The Players

The ant is covered with a bacterium that produces anti-fungals and help protect the fungal garden against parasitic fungi

Agriculture is the practice of cultivating an organism for the reliable production of food. Humans cultivate lots of different plants and animals (i.e. corn, rice, cattle, mushrooms, etc). Agriculture is only known to have evolved four times in nature. Human agriculture is the most obvious example, but agriculture has also evolved in termites, beetles, and ants. In all four of these agricultural systems, the animals must find a way to grow their fungal crop, keep it safe from infection, and provide it nutrients. Here we will focus on the agricultural practices of leaf-cutter ants, to date this is the most well studied system outside of human agriculture.

As a system, leaf-cutter ants provide a model for exploring many aspects of ecological and evolutionary principles. Leaf-cutter ants use fresh plant material to grow a mutualistic fungal cultivar, like mushrooms that humans eat, (all parties benefit from the association) that serves as the ants’ primary food source (mutualism). Within fungus gardens, plant compounds (i.e. cellulose) are metabolized and transformed into nutrients that the ant can eat. In turn, the ants protect their fungal cultivar from pathogens and parasites (parasitism), provide the fungus with a constant source of nutrients (mutualism), and aid in its growth and dispersal (mutualism). Fungal-agriculture in ants, or fungiculture, originated once approximately 60 million years ago (co-evolution).

The leaf-cutters are now one of the most dominant herbivores in their native habitats (ecosystem engineering, ecology). A mature colony contains millions of workers living in an elaborate subterranean system of hundreds of interconnected fungal chambers. A single nest can harvest up to ~1100 pounds of plant material (including leaves, fruit, nuts, and flowers) per year (ecosystem engineering, ecology). This association produces a large amount of refuse, consisting of spent fungal biomass and partly degraded plant material. This effectively divides a leaf-cutter ant colony into two spatially and chemically distinct environments that represents a plant biomass degradation gradient (natural composting, animal behavior) analogous to compositing.

Leaf-cutter ants are not without their problems. As in human agricultural systems, the ant’s crop (the fungal cultivar) is susceptible to many pathogens, including infection by a specialized fungal parasite called Escovopsis (parasitism). Escovopsis has co-evolved with the leaf-cutter ant system over millions of years, meaning that it is specialized to infecting the fungus garden. In fact, the only place in nature that scientists have found Escovopsis is in association with these ants. Over time, the ants have developed many mechanisms to control the growth of Ecovopsis. Perhaps the most interesting is an association the ants have forged with a bacterium called Pseudonocardia. Scientists at the UW – Madison discovered that the ants grow this bacterium on their exoskeleton (outside skeleton). The ants provide the bacteria with nutrient necessary for growth. In turn, the bacteria produces potent anti-fungal compounds that suppress the growth of Escovopsis and thus provide protection to the fungal garden (mutualism).

Chemists working at Harvard University in conjunction with UW-Madison scientists discovered that these anti-fungal compounds have many implications for human medicine (natural product discovery). These findings have led to a renewed interest in exploration of nature for useful and novel chemical compounds, particularly in symbiotic systems. By focusing on symbiotic associations, scientists have a powerful search strategy for the discovery of compounds that may aid in human protection against pathogens.

The Squid-Vibrio Symbiosis-An overview

Bacteria have formed symbiosis with every organism on Earth. We do not fully understand the extent to the function of many of these associations, but we do know that bacteria aid and protect many animals. Bacteria help protect squid in ocean from predators.

The Players

Euprymna scolopes is a small marine squid that hunts at night. And so do bigger predators. This little squid has many methods to escape predation. This squid, like other Cephalopods, releases ink blobs to distract predators, giving themselves enough time to escape while the predator can not see through the ink cloud. Or the little squid can hide in the sand on the ocean floor, and change color by expanding or contracting light-reflecting cells in its skin. However, the most complex means of escaping predation is through a symbiosis with the bacterium Vibrio fischeri.

Vibrio fischeri is a marine bacteria species that can be mutualistic or parasitic with host animals. This bacterium can be found throughout the world existing as a member of the microbial gut community in many marine mammals, or a light organ symbiont in several species of squid and fish (Nealson and Hastings, 1991). As is the case with the little squid and Euprymna scolopes. The light produced by the bacterial symbiont is emitted downward, and the squid can manipulate the intensity of the light to match the intensity of down-welling moon and starlight, thus masking its silhouette to evade bottom-dwelling predators (Jones and Nishiguchi, 2004). As in the case with the Leaf Cutter Ants, the squid maintains it’s symbionat in a specialized crypt where it supplies the bacteria nutrients.