One hundred parthenogenetic generations of Daphnia without sexual forms

November 17, 1911, the writer began rearing a number of pure lines of Daphnia pulex from large females taken in out-door ponds. The females were reproducing parthenogenetically and no males or “winter” eggs were found in the pond.

Each line is propagated by selecting from the first brood of a young female on the day this first brood is released from the brood pouch. The selected young are placed each in an individual bottle with standard food and though examined daily are otherwise undisturbed until the first brood of the next generation appears, when the selections are made as before. Several of these lines have passed the 95th generation and one has just reached the one hundredth generation without the appearance of sexual forms in any generation. All the individuals of the first broods of each generation have been under more or less close scrutiny until they themselves reproduced. If any, or at any rate, many males had occurred they must certainly have been noticed. The method of rearing the daphnids (in individual bottles) has precluded the possibility of sexual reproduction even had males been abundant in the cultures.

There is no evidence of decreased vigor or loss of vitality in the lines. Hence it appears that there is not a necessary sexual cycle in the reproduction of this daphnid. Male daphnids, apparently of this species, have been collected at Cold Spring Harbor since this work was begun. These facts would lend evidence (if additional evidence were necessary) that the sexual cycle in Daphnia pulex is not an inherent necessary thing but that it is determined by environment.

Simocephalus, presumably Simocephalus vetulus, has been reared for 76 generations in one line, likewise without the appearance of sexual forms.