Dimorphism

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Fig. 1. Standard dimorphic or trimorphic reproductive cycle in benthic, medium- to large-sized foraminifera according to Goldstein, 1999. Schizogeny, eventually repeated several times, may be widespread in larger foraminifera from oligotrophic habitats. Planktic foraminifera seem to have no dimorphic life cycle. Life cycles are linked in various ways to seasonal cycles. Example: Heterostegina depressa d'Orbigny, equatorial sections, Gulf of Aqaba, Red Sea (from Hottinger, 1977); (Hottinger, 2006; fig. 5 [1] CC/BY-NC-SA)
Fig. 2. Odd association and polar structures in Praealveolina tenuis Reichel. From Alcantara, Lisbon; Cenomanian. Transmitted light micrographs. A: microspheric specimen, axial section; B: megalospheric specimen, axial section; C-E: odd partners: C: axial section of microspheric Simplalveolina sp.; D: axial sections of megalospheric Simplalveolina sp.; E: Ovalveolina sp, axial section. F: microspheric P. tenuis, axial section, detail showing polar structure with floors and shaftsin the basement. G: model of two subsequent chambers near their polar ends where the first floors in the basement appear. H: model of the protoplasmic body filling the cavities in G. Both models schematic, not to scale, after Reichel, 1933. a: aperture: af: apertural face; bchl: basement chamberlets; bl: basal layer; chl: (main) chamberlets; fl: floor; prp: preseptal passage; rp: (incipient) residual pillar; s: septum; sa: supplementary aperture; sh: shaft; sl: septulum; sut: cameral suture; (Hottinger, 2006; fig. 70 [2] CC/BY-NC-SA)

Definition

  • according to Hottinger (2006):

DIMORPHISM - coexistence of two discrete morphotypes representing different generations in the life cycle of a single species. They are expressed in the adult growth stages and/or in the protoconch and in the following nepionic chambers. The protoconch diameter is large (megalospheric = A - form) when the agamont's protoplasm is distributed (including eventual symbionts) among the cloned offspring. The protoconch diameter is small (microspheric = B - form) when the gamont's gametes fuse pairwise to form a zygote whith no protoplasm or symbionts from the mother. If there is dimorphism in the adult shell, the B - form becomes larger than the A - form. The compartments of the microspheric initial phases are small. It takes many growth steps to reach the initial shell size of the megalospheric generation. Reaching the adult oversize of the microspheric generation demands numerous additional instars. Consequently, the dimorphism of foraminiferal generations reflects different life times and thus different strategies of life within the same species: the microspheric generation is adapted to the permanent basic low-level carrying capacity of an oligotrophic and warm environment, while the megalospheric generation with its short life time adapts to both spring and eventual autumn seasonal peaks of carrying capacity. Nepionts with large megalospheres may have a particular architecture different from that of the adult (see megalospheric apparatus). In complex life cycles, a third (schizontic) generation may produce megalospheric shells with a morphology slightly different from that of the gamonts.

See also





References

Goldstein (1999), Foraminifera, a biological overview. In: SEN GUPTA B.K. (ed.), Modern Foraminifera, Kluwer Academic Publishers, Dortrecht, p. 37-55.

Hottinger (1977), Foraminifères operculiniformes, Mémoires du Muséum National d'Histoire Naturelle, Paris, (Série C, Sciences de la terre), t. XL, 159 p. + 66 pls.

Hottinger (2006), Illustrated glossary of terms used in foraminiferal research. Carnets de Géologie, Memoir 2, ISSN 1634-0744

Reichel (1933), Sur une alvéoline cénomanienne du Bassin du Beausset, Eclogae geologicae Helvetiae, Basel, vol. 26, N° 2, p. 269-280.






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