Embryogenic mitoses, mitoses in females and spermatogenesis are described in the predatory mite Metaseiulus occidentalis (Nesbitt). At 22° C, egg development lasts approximately 4 days. Six chromosomes are seen in mitotic metaphases and anaphases of 0-24 h eggs. Toward the end of this period some embryo squashes have patches of cells containing nuclei which are partially heteropycnotic. These patches of cells apparently increase in size with the age of the embryo. In approximately 1/2 of all 24-48 h-old eggs they encompass all or most cells of the embryo. In these embryos metaphases involved 6 chromosomes, anaphases 3. Either prior to, or following metaphase, a pairing of chromosomes appeared to take place to form 3 units which resembled meiotic diplotene chromosomes where there is opening out between homologues. At metaphase, two sets of 3 chromosomes were slightly differentially stained. One, designated the H set, was darker and slightly more contracted than the other, the E set. At anaphase, 3H and 3E chromosomes segregated in a reductional division retaining the differential contraction until telophase. No cytokinesis appeared. The H set appeared to remain contracted while the E set decontracted to assume the appearance of an interphase nucleus. Both of these entities, side-by-side, created the partially heteropycnotic nucleus mentioned above. The H set then appeared to be excluded from the cell. Mitotic meta and anaphases involving 6 chromosomes were noted in female deutonymphs. Spermatogenesis appeared to encompass an equational division of 3 chromosomes, with the formation of a binucleate spermatid. Two tail structures appeared juxtaposed at the edge of each spermatid and thereafter a separation into two individual sperms occurred. -While mitosis was not studied in known males, we believe that the embryos exhibiting heterochromatinization and elimination of chromosomes in most or all cells were in fact demonstrating parahaploidization.
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