Induction of mictic females in the rotifer Brachionus: oocytes of amictic females respond individually to population-density signal only during oogenesis shortly before oviposition

1. One at a time during the reproductive period of amictic females, oocytes fill with yolk and undergo a mitotic maturation division (oogenesis), are oviposited as single cells, and then develop parthenogenetically into females. Sexual reproduction in Brachionus and several other genera is initiated when amictic females are crowded and oviposit some eggs induced to differentiate into mictic females. Mictic females produce haploid eggs that can develop parthenogentically into males or be fertilised and develop into diapausing embryos called resting eggs. 2. This study examines the time when oocytes in amictic females respond to maternal population density. Is the fate of all oocytes in the germarium irreversibly determined during the early postnatal life of the mother, or is each oocyte labile until just before oviposition? In the former case, the probability of an amictic female producing a mictic daughter at any time throughout her reproductive period would reflect the population density she experienced while young and not that at the time she oviposited an egg. 3. Amictic females of two clones of a Florida strain of B. calyciflorus were cultured singly from birth at a low or high density (in a large or small volume) until about halfway through their reproductive period and then switched (experimental treatment), or not (control treatment), to the other density condition. The results indicate that the female fate of an oocyte is determined by maternal population density during oogenesis. Eggs oviposited soon after transfer from low to high density had the same, or a higher, probability of becoming mictic females compared with those produced by control females kept at the high density; eggs oviposited after transfer from the high to the low density had the same low probability of becoming mictic females as those produced by control females kept at the low density. 4. Control females kept at the high density were less likely to produce mictic daughters as they aged. This decline is not because of a decreased propensity of older females to respond to crowding, as older females responded maximally when transferred from a low to a high population density. 5. As oocytes in amictic females respond to maternal population density only during oogenesis, there is a negligible lag between the population‐density signal in the environment and the commitment to sexual reproduction. This minimises the obligatory two‐generation lag between this signal and production of resting eggs, and thus reduces the possibility that crowding will lead to food limitation before production of these eggs.