Chromosome Behavior, Inheritance and Sex Determination in Sciara

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Abstract
The paper is a general account and review of conditions in Sciara (lower Diptera). 14 spp. have been studied cytologically, 4 genetically. In all but 2, one type of chromosome is "limited" to the germ-line in both sexes, being eliminated from somatic nuclei during cleavage. Eggs transmit the regular haploid chromosome group, but each sperm transmits 2 sister X chromosomes. In production of a [female] one of the latter is eliminated from all nuclei during early embryonic development. In production of a [male] one is eliminated from the germ-line and both from the soma. All fertilized eggs have essentially the same set of chromosomes; the type of elimination from the soma apparently determines the sex of the individual. In some species half the [female][female] are "female-producers" and half "male-producers," their nature being detd. by the sex chromosomes, XX [male]d-producer, XX[image] [female]-producer (X[image] is a modified X). Type of elimination is here largely predetermined by constitution of mother, but numerous "exceptional" individuals appear in some "unisexual" families. Oogenesis is of standard type. In spermatogenesis there is no synapsis; the 1st spermatocyte division is unipolar; maternal autosomes and X and "limited" chromosomes go to the pole and are transmitted; paternal autosomes and X move backward away from the pole and are eliminated. Thus the [male] transmits only maternal chromosomes and genes[long dash]except perhaps for the "limited" chromosome. The 2d spermatocyte division is also aberrant. Features of special cytological interest are discussed.