期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：36
页码：E5029-E5037
DOI：10.1073/pnas.1511878112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceSymbionts can provide hosts with many advantages including defensive capabilities and novel nutrients. However, symbionts may begin as pathogens that only subsequently become beneficial. In the Dictyostelium discoideum farming symbiosis some amoebas stably associate with bacterial partners. We demonstrate that amoeba-associated Burkholderia can initiate a farming symbiosis with naive amoeba hosts. Burkholderia decreases amoeba spore productivity in food-rich conditions but, because of the induction of bacterial food carriage, sometimes increases spore productivity in food-scarce conditions. Detrimental effects of Burkholderia colonization differ among Burkholderia genotypes and, in some cases, between new and old amoeba hosts, suggesting some coevolution within the association. These results suggest that Burkholderia exerts both pathogenic and mutualistic effects on its host in conditionally dependent ways. Symbiotic associations can allow an organism to acquire novel traits by accessing the genetic repertoire of its partner. In the Dictyostelium discoideum farming symbiosis, certain amoebas (termed "farmers") stably associate with bacterial partners. Farmers can suffer a reproductive cost but also gain beneficial capabilities, such as carriage of bacterial food (proto-farming) and defense against competitors. Farming status previously has been attributed to amoeba genotype, but the role of bacterial partners in its induction has not been examined. Here, we explore the role of bacterial associates in the initiation, maintenance, and phenotypic effects of the farming symbiosis. We demonstrate that two clades of farmer-associated Burkholderia isolates colonize D. discoideum nonfarmers and infectiously endow them with farmer-like characteristics, indicating that Burkholderia symbionts are a major driver of the farming phenomenon. Under food-rich conditions, Burkholderia-colonized amoebas produce fewer spores than uncolonized counterparts, with the severity of this reduction being dependent on the Burkholderia colonizer. However, the induction of food carriage by Burkholderia colonization may be considered a conditionally adaptive trait because it can confer an advantage to the amoeba host when grown in food-limiting conditions. We observed Burkholderia inside and outside colonized D. discoideum spores after fruiting body formation; this observation, together with the ability of Burkholderia to colonize new amoebas, suggests a mixed mode of symbiont transmission. These results change our understanding of the D. discoideum farming symbiosis by establishing that the bacterial partner, Burkholderia, is an important causative agent of the farming phenomenon.
关键词：Burkholderia ; Dictyostelium ; symbiosis ; mutualism ; social amoeba
