A sex-intergrade strain of Cladocera

The above-mentioned strain appeared about a year ago. For four years the writer had been breeding a number of strains of Simocephalus vetulus. For 130 generations reproduction was entirely parthenogenetic. The young were all females, there being neither males nor sexual eggs. In the 131st generation of one of the eleven strains of the species reared under laboratory conditions for so long there suddenly appeared, in addition to normal females, males and sex intergrades of many sorts. This strain has continued to produce sex intergrades for a year—more than twenty generations—and the character of the intergrades produced does not seem different now from what it was when the sex intergrades first appeared.

In this species, in addition to the character of the gonads, eight morphological secondary sex characters are recognized. In the sex-intergrade strain the sex array may be roughly classified into normal females, female intergrades, hermaphrodites with various combinations of male and female secondary sex characters, male intergrades, and normal males.

The female intergrades range from females with a single, perhaps poorly developed, male secondary character to those with all the secondary sex characters male. The hermaphrodites have various combinations of male and female secondary characters. There are male intergrades with as many as five secondary sex characters, though ordinarily the male intergrades have only one or two female characters.

The male intergrades usually have incompletely developed reproductive organs. Sperm is produced in various amounts. The amount is usually smallest in the male intergrade with the larger number of female characters; and usually males with a single female secondary sex character produce fewer sperm than normal males. The female intergrades with one or two male secondary sex characters usually possess a high fertility, those with as many as four or five male characters are in general much less prolific, while those with six or more male characters are usually sterile or nearly so.