title: Flow cytometry as a strategy to study the endosymbiosis of algae in Paramecium bursaria.
abstract: BACKGROUND
  The stable symbiotic association between Paramecium bursaria and algae is of interest to study such mechanisms in biology as recognition, specificity, infection, and regulation. The combination of algae-free strains of P. bursaria, which have been recently established by treating their stocks of green paramecia with herbicide paraquat (Hosoya et al.: Zool Sci 12: 807-810, 1995), with the cloned symbiotic algae isolated from P. bursaria (Nishihara et al.: Protoplasma 203: 91-99, 1998), provides an excellent clue to gain fundamental understanding of these phenomena.
  
  
  METHODS
  Flow cytometry and light microscopy have been employed to characterize the algal cells after they have been released from the paramecia by ultrasonic treatment. Algal optical properties such as light scattering and endogenous chlorophyll fluorescence intensity have been monitored for symbiotic and free-living strains, and strains at stages of interaction with a host.
  
  
  RESULTS
  Neither algal morphology nor chlorophyll content has been found to be altered by sonication of green paramecia. This fact allows to interpret in adequate degree changes in the optical properties of symbiont that just has been released from the association with a host (decreased forward light scatter and chlorophyll fluorescence signals). Optical characterization of both symbiotic and free-living algal strains with respect to their ability to establish symbioses with P. bursaria showed that chlorophyll content per cell volume seems to be a valuable factor for predicting a favorable symbiotic relationship between P. bursaria and algae.
  
  
  CONCLUSIONS
  Flow cytometry combined with algae-free paramecia and cloned symbiotic algae identifies algal populations that may be recognized by host cells for the establishment of symbioses.
authors.affiliation: Department of Biological Science, Faculty of Science, Hiroshima University, Higashi-Hiroshima, Japan.
authors.forename: B I
authors.initials: BI
authors.lastname: Gerashchenko
authors.forename: N
authors.initials: N
authors.lastname: Nishihara
authors.forename: T
authors.initials: T
authors.lastname: Ohara
authors.forename: H
authors.initials: H
authors.lastname: Tosuji
authors.forename: T
authors.initials: T
authors.lastname: Kosaka
authors.forename: H
authors.initials: H
authors.lastname: Hosoya
mesh_terms: D000818:Animals; D002708:Chlorella / Q000737:chemistry / Q000166:cytology / Q000502:physiology*; D002734:Chlorophyll / Q000032:analysis; D005434:Flow Cytometry / Q000379:methods*; D008954:Models, Biological; D010247:Paramecium / Q000382:microbiology*; D013010:Sonication; D013559:Symbiosis
chemical_list: D002734:Chlorophyll