The Relation between Chromosomes and Sex-determination in “Abraxas grossulariata.”

Abstract

IN a paper on this subject in the Journal of Genetics (vol. iv., June, 1914, p. 1) I gave evidence that in a strain of A. grossulariata which I have bred for several years two kinds of eggs are produced, having respectively twenty-eight and twenty-seven chromosomes. Since the somatic chromosome-number is fifty-six in the male and fifty-five in the female, it seemed evident that the eggs with twenty-eight were male-determining, those with twenty-seven female-determining. In this strain some families in each generation consist entirely of females, so it was hoped to prove the correctness of the conclusion with regard to sex-determination by finding that in families consisting entirely of females all the eggs contain twenty-seven chromosomes. I have now examined the eggs of several such families, and find, contrary to expectation, that the equatorial plate of the inner polar spindle contains twenty-eight chromosomes about as frequently as twenty-seven. The new material confirms the observation that twenty-seven occur in one spindle and twenty-eight in the other, but it seems to make it certain that the presence of twenty-eight chromosomes in the inner spindle does not necessarily cause the production of a male—at least, in the strain which produces all-female families. A possible explanation of the anomaly is that in all-female families a chromosome is eliminated at a later stage, but at present I have no direct evidence for this. I have material preserved in the hope of testing this suggestion, but the investigation is likely in any case to be a lengthy one, and circumstances may prevent my continuing it for some months. I therefore make this short statement of the facts as at present known, in order that it may not be assumed that the existence of male and female-determining chromosomes has been finally demonstrated in Abraxas.