Polyploidy and Types of Development in the TricladDugesia LugubrisS.L.: Interaction Between the Ploidy Level and the Origin of Amphimixis or Pseudogamy

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Abstract
The planarian Dugesia lugubris s. 1. has various karyological races with peculiar chromosome cycles; one of these is characterized by a triploid somatic set, while the female line is hexaploid and the male line is diploid. The development is pseudogamic. A few specimens of this race after repeated regenerative processes have undergone some variations in this cycle, showing also diploid somatic and tetraploid female cells. The diploid and tetraplo'd oocytes are all amphimictic, while the hexaploid oocytes are pseudogamic, as in the normal specimens. A few of the descendents of these regenerates, born from diploid oocytes, are exclusively diploid with amphimictic development; the majority of them may be diploid or polyploid (triploid or tetraploid) in the somatic line and tetraploid or hexaploid in the female line. The tetraploid oocytes seem all amphimictic; a low percentage of the hexaploid oocytes develop amphimictically, while the majority of them develop pseudogamically. The modality of this development frequently differs from that characterizing the hexaploid oocytes of the natural biotype. The following topics are discussed: 1) origin of the individuals with various ploidy levels and with different modes of development (amphimixis or pseudogamy); 2) heterozygous conditions of the factors that determine the ploidy level and the modality of development; 3) relationship between ploidy level and modality of development; 4) appearence of a new mechanism of pseudogamy, due to a different action of the egg upon the sperm, or a new egg-sperm interaction.