The trimorphic life cycle in foraminifera: Observations from cultures allow new evaluation

Foraminifera are the only Protozoa in which an heterophasic alternation of generations between gamont and agamont (dimorphic cycle) occurs. First desc ribed by Schaudinn in 1895 [37] it is analogous to the gametophyte/sporophy te alternation of algae, pteridophytes and spermatophytes. With the discove ry in some foraminifera of a schizont generation, a trimorphic sequence, ga montagamont-schizont-gamont, was suggested by Rhumbler in 1909 [28] (hypoth esis of trimorphism). However, not until 1938 and 1990 respectively the ste ps schizont-gamont and agamont-schizont were described, each step in a diff erent species [18, 34]. The production of a gamont from a schizont, as well as the cyclic production of schizonts directly from schizonts, is now docu mented here in the larger foraminifer Amphistegina gibbosa cultivated in th e laboratory over four generations, and a reappraisal of the trimorphic lif e cycle is made. We suggest that multiple schizogony within the trimorphic cycle, including as it does the beneficial transfer to the offspring of sym biotic algae from the parent, explains the preponderance of megalospheric p opulations of larger foraminifera, both now and in the fossil record.