title: Differentiation of a free-living alga into forms with ecto- and endosymbiotic associations with heterotrophic organisms in a 5-year microcosm culture.
abstract: The ecological mechanisms underlying the diversification of autotrophic species into endosymbiotic lifestyles and the ways in which the evolution of endosymbiotic species is ecologically and evolutionarily affected by sister lineages/lines that are adapted to extra-host environments remain unclear. In this paper, we investigated a differentiation process of algal species in which an endosymbiotic type was differentiated phenotypically from a free-living ancestral clone, by using an experimental model called the CET microcosm, which contains a green alga (Micractinium sp.), a bacterium (Escherichia coli), and a ciliate (Tetrahymena thermophila) cultured together without an external resource supply for over 5 years. We then analyzed the algal diversification process by comparing algal phenotypic properties regarding cell-aggregate formation and their effects on the survival of Tetrahymena (using a clone isolated on day 2668) in the absence of bacteria. We examined 13 Micractinium clones, including both ancestral and derived clones isolated from long-term (day 1819-1847) CET microcosm cultures. The results revealed that the free-living ancestral algal strain diversified in sympatry into an aggregate-forming type that associates with E. coli, and a non-aggregate-forming type that associates with Tetrahymena. Furthermore, a competition experiment revealed that the endosymbiotic (non-aggregate-forming) type was less adapted to the extracellular environment than the aggregate-forming type. This result suggests that severe competition with a nonendosymbiotic sister line in the extra-host environment favors the host-benefiting phenotype in an endosymbiotic line, because such symbionts can enhance the longevity of the host, thereby enabling the survival and reproduction of the symbiont within the host.
authors: Department of Biology, Ehime University, Bunkyo-cho 2-5, Matsuyama, Ehime 790-8577, Japan. Electronic address: nakajima@sci.ehime-u.ac.
authors: Toshiyuki
authors: T
authors: Nakajima
authors: Department of Biology, Ehime University, Bunkyo-cho 2-5, Matsuyama, Ehime 790-8577, Japan.
authors: Yoshiyuki
authors: Y
authors: Fujikawa
authors: Department of Biology, Ehime University, Bunkyo-cho 2-5, Matsuyama, Ehime 790-8577, Japan.
authors: Toshiyuki
authors: T
authors: Matsubara
authors: Department of Biology, Ehime University, Bunkyo-cho 2-5, Matsuyama, Ehime 790-8577, Japan.
authors: Mimi
authors: M
authors: Karita
authors: Department of Biology, Ehime University, Bunkyo-cho 2-5, Matsuyama, Ehime 790-8577, Japan.
authors: Akiko
authors: A
authors: Sano
mesh_terms: D002449:Cell Aggregation; D000460:Chlorophyta / Q000235:genetics* / Q000502:physiology; D002999:Clone Cells; D046508:Culture Techniques; D004926:Escherichia coli / Q000235:genetics*; D049810:Genetic Speciation; D010641:Phenotype; D012641:Selection, Genetic; D013559:Symbiosis; D016808:Tetrahymena thermophila / Q000235:genetics*
keywords: Algae; Bacteria; Ciliates; Evolution; Selection; Speciation; Symbiosis