Definition: reciprocally beneficial relationship between two organisms (+/+ relationships) .
Mutulaistic interactions can be seen in every ecosystem.
How can mutualism evolve?
Definition: reciprocally beneficial relationship between two organisms (+/+ relationships) .
Mutulaistic interactions can be seen in every ecosystem.
How can mutualism evolve?
The organism or species that is negatively affected (host) is basically faced with an ultimatum: either run from the situation or adopt the attitude "if you can't beat them join them!" And if the latter occurs then both organisms evolve into more reciprocally beneficial interactions that try to minimize the damage to the host.
Early evidence of this evolutionary transition was documented by K. W. Jeon in 1966. Development of Mutualism between Bacteria and Amoeba proteus (Laboratory Setting)
Jeon was a professor that spent decades working with strains of Amoeba proteus In 1987, he observed black dots that began to develop with in his collection of Amoebae cultures.
The dots turned out to be bacteria The bacteria quickly began to kill off most of his collection, but many of the amoebas did survive.
The populations of infected amoebas that had survived were the "least sick" ones that happened to be more resistant to the bacteria. These affected populations had slower growth rates, become more fragile than before the infection, small size, high mortality, slow clone formation, and were very sensitive to adverse conditions.
Jeon began to keep a record of these heavily infected amoeba populations and his results were surprising…..
Parasitism -> Mutualism 5 years later… More experiments on the infected Amoebae population concluded that the Amoebae were no longer capable of producing a specific enzyme vital for survival and normal functioning Joen and others showed that the intra-cellular bacteria were now the source of the enzyme that the Amoebae no longer produced Thus this accidental discovery proves that Amoebas became dependent on the parasites, the bacteria, that had infected them in the first place. The exact mechanism for the transition in this experiment remains unknown.
Hypotheses
1) The parasitic infection is of low virulence and leaks out nutrients that benefits the Amoebas survival and in turn increases the parasites' fitness.
2) There could have been environmental constraints where the parasite helped the host overcome them and then selection favored close dependence.
This is a laboratory example of a parasitism that evolved into a mutualistic relationship.
A more recent parasite to mutual transition has been seen between Wolbachia (endosymbiotic parasite) and Asobara tabida (parasitic wasp).
Wolbachia are transmitted maternally, thus their reproductive success lies in the females they inhabit, and males are an evolutionary dead end. They specifically sabotage host reproduction for their own selfish advantage through male killing or by high jacking the sperm of the host and genetically tricking them into having females. Also involved in cytoplasm incompatibility and can cause distortion of sex ratio's in other host.
Although the bacteria are usually harmful, many cases are arising in natural populations where the bacteria has become beneficial and evolved into an obligate mutualism.
Compensatory Evolution: parasitism -> obligate mutual dependence -> mutualism.
Definition of Compensatory Evolution: Evolution in which a second substitution (or mutation) compensates for the deleterious effects of an earlier substitution.
Basically…. First the Wolbachia bacteria infected the host.
The immune system of the host responded against the bacteria by causing apoptosis (programmed cell death) in the infected cells, trying to eliminate further spread of these bad genes.
The Wolbachia bacteria then tried to inhibit or suppress the apoptosis. Such suppression by the bacteria can lead to decreased functions in the infected tissues of the host (in this case the ovaries). The host therefore must compensate for this suppression of apoptosis in order to recover some ovarian functionallity .
How does the host compensate for suppression of apoptotic pathways and try to gain gain back ovarian functionallity?
Two scenarios that will lead to complete host dependence on the bacteria, and will restore function of the ovaries are as follows:
Accumulation of mutations in genes controlling apoptosis are no longer selected for, and Wolbachia is then able to permanently control these apoptotic pathways.
The wasps gene expression are modified to account for the manipulation of the bacteria. These mutants can become fixed in the host population if the pressure created by the parasite is high. - If the parasite is absent, then these new mutant types will be harmful to the host. Thus through time, these compensatory mutations can become fixed, and without the presence of the parasite the host will be negatively impacted as it was in the first place - "reciprocal addiction" phase
Thus the evolution of mutualism between the two species seems to originate from the manipulation of the apoptotic pathways of the wasp
Works Cited
Aanen D.K., Hoekstra R (2007) The evolution of obligate mutualism:if you can't beat 'em, join 'em. Trends in Ecology and Evolution 15: 844-847
Herre EA, Knowlton N, Mueller UG, Rehner SA (1999) The evolution of mutualisms: Exploring the paths between conflict and cooperation. Trends in Ecology and Evolution 14:49-53
Jeon, K.W. (1972) Development of cellular dependence on infective organisms: micrurgical studies in amoebas. Science, 176, 1122–1123.
Pannebaker, B.A. (2007) Parasitic inhibition of cell death facilitates symbiosis. Proceedings of the National Academy of Sciences. 213-215 Weeks, A.R. (2007) From parasite to mutualism: rapid evolution of Wolbachia in natural populations of Drosophila. PLoS Biol. 5,997-1005
