THE CONTROLLING ELEMENT IN SEX CHROMOSOME BEHAVIOR IN SCIARA

Abstract

ENETIC and cytological studies have been made on a number of species of Sciara (see METZ 1938 for review). Common to all of these species is a series of unusual and remarkable cytological phenomena which involve the sex chromosome and which are related to the differentiation of sex in this genus. The first of these is encountered at the second meiotic division in the male, when, following the selective elimination of paternal homologues at the first spermatocyte division, the maternally derived X chromosome undergoes equational nondisjunction. Thereby, instead of a haploid set of chromosomes, such as the egg transmits (three autosomes and one X) , the sperm nucleus comes to include two identical X’s and three autosomes. Although the sperm transmits two X chromosomes to the zygote, in no cell of the developing embryo are both of them retained permanently: one is eliminated from the somatic nuclei of the females, both from the somatic nuclei of males, and one from the germ cells of each sex. This pattern of sex chromosome behavior constitutes a unique sex determining mechanism. All zygotes begin development under the influence of the same chromosome complement, three X’s and three pairs of autosomes. A differential between male and female is not established until anaphase of the seventh or eighth cleavageand then only in the somatic nuclei-at which time there is an elimination of one or of both paternal X’s in the production of female and male embryos, respectively (Du BOIS 1933). The germ line retains all three sex chromosomes until the germ cells have migrated to the definitive gonad site; then, in both sexes alike, one paternal X is eliminated. This elimination occurs not at anaphase but in resting cells at a stage when the chromosomes are in the form of fairly compact prochromosomes; one of the paternal X’s appears to migrate directly through the nuclear membrane into the cytoplasm and subsequently degenerates (BERRY 1941 ) . This series of interrelated and unusual phenomena demonstrates that the cells of the developing Sciara embryo can distinguish between sex chromosomes and autosomes, between maternal and paternal homologues. The nature of this “recognition” of chromosomes is not understood. One of the first questions to be answered, however, and one which is experimentally feasible, is whether a local1 The studies reported here were supported by grants G-6176 and G-9682 from the National Science Foundation. The studies on the Oak Ridge translocations were initiated in the Biology Division of the Oak Ridge National Laboratory during the tenure of a research participantship (1957-58) sponsored by the Oak Ridge Institute of Nuclear Studies.