All-Female Broods in the Butterfly Hypolimnas bolina (L.)

It has been known since 1924 that a proportion of females in the butterfly H. bolina produce only daughters, whereas others produce a 1:1 sex ratio. The present results confirm this, and show also that occasionally a few males are produced. The production of broods with a disturbed sex ratio is inherited entirely through the female line. By sexing the embryos and larvae cytologically and observing mortality in the embryos and late pupal stage we have obtained evidence that the deficiency of males is due to their very high mortality in the pre-adult stage. It is suggested that the abnormal sex ratio is due to an infective cytoplasmic factor (the presence of spirochaetes, as in Drosophila, having been ruled out), in contrast to Acraea encedon where meiotic drive of the Y chromosome has been postulated. The view is put forward that the polymorphism in H. bolina is maintained by the ‘infected’ females being at a slight disadvantage and that their numbers are maintained by contagion from an unidentified reservoir species.