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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Ref ID : 7715

Stephen F. NG and Ann Newman; The Role of the Micronucleus in Stomatogenesis in Sexual Reproduction of Paramecium tetraurelia: Micronuclear and Stomatogenic Events. Protistologica XX(1):43-64, 1984

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A detailed, parallel study of stomatogenic and micronuclear activities in sexual reproduction of Paramecium tetraurelia was undertaken with the aim to define the time when micronuclear activities might influence stomatogenesis. Eight stomatogenic stages were discerned: (1) Formation of an oral anlage field with disorganized basal bodies. (2) Alignment of anlage basal bodies into parallel rows in the process of formation of oral membranelles. (3) Formation of an oral primordium of 6 rows of basal bodies in the shape of a hook. (4) Addition of basal bodies to form a 12 row primordium comprising 3 membranellar bands, each of 4 packed rows of basal bodies; relaxation of the posterior end of the hook to form a C-shape structure following the separation of the conjugants. (5) Further stretching out of the primordium into a banana-shape followed by elongation; quadrular rows beginning to spread out in the anterior end; the new buccal cavity becoming prominent. (6) Expansion of the buccal cavity; maturation of quadrular pattern (1-1-2 spacing of the rows and posterior S-spiral); differentiation of the ribbed wall; the appearance of new endoral kinety and the new oral anlage field. (7) Appearance of the postoral fibres. (8) Food vacuole formation. The micronucleus was dispensable in the establishment of the oral anlage field (stage 1), since the latter was also formed in amicronucleates in sexual reproduction. Stage (2) occurred during the exchange of gametic nuclei between the conjugants and formation of the synkaryon following gametic fusion. Since stomatogenesis in the sexual reproduction of amicronucleates was arrested at stage (2) it was concluded that micronuclear activity in the early part of the sexual cycle, i.e., during the 3 prezygotic divisions and synkaryon formation, was essential in the alignment of oral membranelles. In particular, two stages in this period were accorded some importance in this regard: the meiotic prophase I crescent stage, when micronuclear transcription might take place, and, after the second prezygotic division when the pregametic nucleus, or its gametic products of the third prezygotic division, came to reside in the vicinity of the oral anlage field. In addition, the macronuclear anlagen started to differentiate in stage (6), when the oral assemblage assumed their final pattern. At least one structure, the postoral fibres, appeared shortly afterwards. This raised the interesting possibility that the anlagen play a role in the final steps of stomatogenesis.