Why Mutualism Is Good - the Evolutionary Significance of Symbiosis

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Another example of a mutualistic relationship is the one between the Euprymna scolopes and Vibrio fischeri. Euprymna scolopes are a nocturnal Hawaiin squid that spend their day buried in the sand at the bottom of the ocean. Prior to the evolution of its light organ, during the night when the squid would come out to feed it would appear as a dark silhouette in the moonlight and, consequently, attract bottom feeding predators. Previously during embryonic development the E. scolopes had the light organ as well as the genes to code for the receptor signals on the organ but only the form of the organ was developed and the genes weren’t expressed. Within the first few hours after being hatched, large amounts of V. fischeri are flushed in and out of the E. scolope’s body cavity as a result of ventilation (McFall and Ruby, 1991). Since there was no light organ to pass through, V. fischeri would simply enter and exit the E. scolope. Because this feature was undeveloped, it served no purpose for the squid and as a result they faced increased predation. When compared to previous generations, some new generations though exhibited a more developed light organ where the epithelial lining was present and the genes were coded for along the gene sequence. Now during ventilation, as V. fischeri passed through the E. scolope, they began to enter the light organ and feed off of the developing epithelial tissue. Over a span of three- five years the light organ became fully developed with each new generation due to the increased interest the bacteria took in the organ (McFall and Ruby, 1991). Every day the squid expels 95 percent of the bacteria in its light organ (McFall and Ruby, 1991). In order to repopulate, the bacterium release a specific chemical that interacts with their genes which helps sense when the population has reached its full capacity within the organ and tells them to stop reproducing. As the chemical interaction becomes more abundant in the organ it begins to emit a soft blue light. This light is what makes the light organ glow which then helps the squid blend in as part of the moonlight rather than an obvious source of prey. As for the bacteria, their benefit from this mutualistic relationship is a continuous source of food and housing which has helped their population exponentially grow. Although nutrients similar to the ones found in the epithelial tissue can be found within the ocean, they aren’t as abundant and readily available as in the tissue (McFall and Ruby, 1991). Also, since the squid expels most of the bacteria in its organ every day, the bacteria react by rapidly reproducing more. This has led to the rapid expansion of the V. fischeri population. Over a short period of time this readily available animal-bacteria mutualistic relationship has benefited the squid by allowing them to avoid predation and it has also allowed the bacterium population to rapidly grow.

Organisms are most successful within their environment when they possess traits that help improve their natural selection. Sometimes, though, organisms do not possess those specific traits and therefore look to mutualistic relationships to help them. Mutual symbiotic relationships are partnerships among organisms within ecosystems that are responsible for improving each organism’s survival which directly correlates to their increased fitness. Relationships such as the one between the Acacias and P. jerruginea or between the V. fischeri and E. scolope are ones in which each member has positively shaped one another to form new characteristics that have benefitted them. These new characteristics have evolved faster over time than if environmental factors were used to initiate them. This rapid change is what makes readily available mutual symbiotic relationships a stronger selective pressure for evolutionary change.

CITATIONS

Hooper, L.V. 2001. Mutualism in the human sciences. Theory of Social Behavior 292: 1115- 1118.

Janzen, D. H. 1996. Coevolution of mutualism between ants and acacias in central america. Evolution 20: 249-275.

McFall, M. J. and Ruby, E. G. 1991. Symbiont recognition and subsequent morphogenesis as early events in an animal-bacterial mutualism. Science 254: 1491-1494.

Watson, R. A. and Pollack, J.B. 1995. How symbiosis can guide evolution. Dynamical and Evolutionary Machine Organization.

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