The Determination of Sex in Habrobracon.

In short, testis size in F1 hybrids is determined by an interaction between the chromosome complement and the cytoplasm. We are dealing either with a real cytoplasmic effect (determination by the intrinsic properties of the cytoplasm, independent of the chromosomes it carries or carried), or with a maternal effect (determination of the properties of the cytoplasm by those of the chromosomes it carried before fertilization). The data available permit a discrimination between these two possibilities. A backcross male having one half of its autosomes from race A and the other half from race B has small testes irrespective of the source of its cytoplasm. Furthermore, back-cross males having either kind of X-chromosomes and either kind of cytoplasm may have large or small testes depending upon the combination of the autosomes they carry. Hence, in Drosophila pseudoobscura, the difference between the F1 generations of the reciprocal crosses is due to a maternal effect and not to a plasmatic inheritance. I Lancefield, D. E., "A Genetic Study of Crosses of Two Races or Physiological Species of Drosophila Obscura," Zeits. ind. Abst. Vererbungsl., 52 (1929). 2 Unpublished data of the writer. 3 Morgan, T. H., Bridges, C. B., and S^hultz, J., "The Constitution of the Germinal Material in Relation to Heredity," Carnegie Inst. Year Book, 29 (1930). 4 Dobzhansky, Th., "Role of the Autosomes in the Drosophila pseudoobscura Hybrids," Proc. Nat. Acad. Sci., 19, 950-953 (1933).