Experiments on chromosome elimination in the gall midge, Mayetiola destructor

Cleavage in Cecidomyidae (Diptera) is characterized by the elimination of chromosomes from presumptive somatic nuclei. The full chromosome complement is kept by the germ-line nuclei. The course of cleavage in Mayetiola destructor (Say) is described. After the fourth division two nuclei lie in the posterior polar-plasm and become associated with polar granules, and fourteen nuclei lie in the rest of the cytoplasm. All the nuclei possess about forty chromosomes. During the fifth division the posterior nuclei do not divide and the polar-plasm becomes constricted to form primordial germ cells (pole cells). The remaining fourteen nuclei divide and lose about thirty-two chromosomes so that twenty-eight nuclei are formed containing only eight chromosomes. These are the presumptive somatic nuclei. During subsequent divisions the pole cell nuclei retain the full chromosome number; these divisions occur less frequently than those of the somatic nuclei. Experiments were performed on early embryonic stages to elucidate the properties of the posterior end during the time that chromosome elimination was taking place from the presumptive somatic nuclei. Ultraviolet irradiation, constriction, and centrifugation techniques were used. The polar granules are concerned with the non-division of the germ-cell nuclei during the fifth division, since if the granules are dispersed by centrifugation, or if nuclei are prevented by constriction from coming into contact with them before the fifth division, all the nuclei divide with chromosome elimination at this division. With each technique it is possible to obtain embryos possessing germ cells with only eight chromosomes in their nuclei. Individuals possessing germ-line nuclei with only eight chromosomes were allowed to develop to maturity. Abnormalities were confined to the germ cells only and were the same regardless of which technique had been used to produce the deficient germ line. An ovary containing germ-cell nuclei with only eight chromosomes is unable to form both oocytes and nurse cells. A testis containing germ-cell nuclei with only eight chromosomes is unable to form spermatocytes but cells which come to resemble gametes are formed. Experimental males and females are both sterile. The results are discussed in relation to other experimental work on Cecidomyidae and the following main conclusions are reached: (a) the polar granules are responsible for preventing an irreversible loss of chromosomes from the germ-cell nuclei by preventing the mitosis of these nuclei during the fifth division; (b) the chromosomes normally retained in the germ line are required for gametogenesis, particularly for oogenesis. The significance of chromosome elimination is discussed.