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Tuesday "Noon" Seminar : Sept. 12Nancy Moran on "Effects of symbiont mutations on the ecological range of hosts"
12:30-1:45 p.m. in Biosciences West (map of building location), Room 208 |
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Talk Abstract Symbiosis is a ubiquitous phenomenon generating biological complexity, affecting adaptation, and expanding ecological capabilities. However, symbionts, which can be subject to genetic limitations such as clonality and genomic degradation, also impose constraints on hosts. Aphids are dependent on an ancient symbiosis with the bacterium Buchnera aphidicola, which supplies essential nutrients. We report a mutation in Buchnera that recurs in laboratory lines and that occurs in field populations. This single nucleotide deletion affects a homopolymeric run within the heat shock transcriptional promoter for ibpA, encoding a heat shock protein. We found that that this Buchnera mutation virtually eliminates the transcriptional response of ibpA to heat stress and dramatically reduces host fitness under hot conditions. Following heat exposure, aphids bearing mutant symbionts contained almost no Buchnera, and most failed to reproduce. Results of laboratory competition experiments, as well as allele frequencies in field populations, suggest that loss of ibpA heat stress response improves host fitness under some conditions. This recurring Buchnera mutation governs thermal tolerance of aphid hosts. Similar cases, in which symbiont microevolution has a major effect on host ecological tolerance, are likely to be widespread because of the high mutation rates of symbiotic bacteria and their crucial roles in host metabolism and development. |
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