The Origin and Behaviour of Chiasmata XIII

(1). The chromosome number in Culex pipiens is 2n=6. Tetraploid cells have been observed in both mitotic and meiotic divisions.(2). In somatic divisions the homologous chromosomes show strong association throughout all stages of division, being in pairs in the diploid and in fours in the tetraploid. Somatic pairing in relation to the anomalous type of meiosis in the male Drosophila is discussed.(3). At meiosis in tetraploid cells, quadrivalents, bivalents and univalents occur, the number of multivalents being low. The failure of complete pairing may be due to three causes:(a) Low number of pairing blocks per chromosome.(b) Failure of chiasma formation in parts of chromosomes which have paired over a short distance.(c) Mechanical interference preventing chromosomes pairing at zygotene.(4). The chiasma frequency at Metaphase I differs in different individuals from 1.06 to 1.64 per bivalent. These differences are considered to be due to the action of different genotypes an chromosome pairing at meiosis.