Progenies from some wild-caught females of Drosophila willistoni and three other sibling species are entirely female. The proclivity for production of unisexual female progeny by these flies was named the sex ratio (SR) trait and was originally thought to be genetic. However, experiments in the laboratory of Donald F. Poulson in the early 1960s demonstrated that this 'trait' was vertically transmitted and infectious, in that it could be artificially transferred by injection from infected females to non-infected females. Motile, helical micro-organisms were observed in females showing the trait. In 1979, the SR organisms were designated as group II in the informal spiroplasma classification system. The organisms proved to be extremely fastidious, but were eventually cultivated in a very complex cell-free medium (H-2) after initial co-cultivation with insect cells. Cultivation in the H-2 medium and the subsequent availability of a triply cloned strain (DW-19 permitted comparative studies. Cells of strain DW-lT were helical, motile filaments 200-250 nm in diameter and were bound by a single trilaminar membrane. Cells plated on 1.8% Noble agar formed small satellite-free colonies 60-70 pm in diameter with dense centres and uneven edges. The temperature range for growth was 26-30 "C; optimum growth occurred a t 30 "C, with a doubling time in H-2 medium of 15.8 h. The strain passed through filters with 220 nm, but not 100 nm, pores. Reciprocal serological comparisons of strain DW-lT with representatives of other spiroplasma groups showed an extensive pattern of one-way crossing when strain DW-lT was used as antigen. However, variable, usually low-level reciprocal cross-reactions were observed between strain DWlT and representatives of group I sub-groups. The genome size of strain DW-lT was 2040 kbp, as determined by PFGE. The G+C content was 2621 mol%, as determined b y buoyant density and melting point methods. The serological and molecular data indicate that strain DW-lT is separated from group I representative strains sufficiently to justify retention of its group status. Continued group designation is also indicated by the ability of SR spiroplasmas to induce male lethality in Drosophila, their vertical transmissibility and their extremely fastidious growth requirements. Group II spiroplasmas, represented by strain DW-lT (ATCC 431 533, are designated Spiroplasma poulsonii.

Several lines of evidence implicate small spirochetes, presumably treponemata, as etiologic agents in the production of the maternally transmitted "sex ratio" condition (SR) in Drosophila nebulosa, in D. willistoni, and in strains of D. melanogaster into which the SR condition has been artificially transferred. The presence of treponemata in the hemolymph of adult females of these species is completely correlated with the production of unisexual progenies and like this condition is dependent on the genotype of the host and of the infectious agent.

The eggs of Bombyx mori, both in diapause and nondiapause, were subjected to cytological examination of nucleoli and measurement of RNA precursor incorporation (2 hours) into ribosomal RNA. In diapause eggs, the nucleoli were very small and the rate of ribosomal RNA synthesis was the lowest of the samples tested. Most cells in diapause possessed nuclei with one nucleolus. In contrast, the eggs activated from diapause by long chilling attained the largest size of nucleoli and the highest rate of ribosomal RNA synthesis. A significant proportion of the cell nuclei still had only one nucleolus at this stage. Three days after activation, the eggs exhibited intermediate levels in both the size of nucleoli and the rate of ribosomal RNA sythesis. At this stage, about half of the egg cell nuclei had two nucleoli.

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