Reproductive biology

Information on rotifer mating behavior is scarce. It has been shown, however, that females of Brachionusplicatilis carry a glycoprotein on the surfaces of the body that acts as a sex pheromone. This protein can bind to chemoreceptors in the corona of conspecific males. Hence, the pheromone probably serves as a mating recognition signal that helps rotifers avoid mating with nonspecific specimens.

The rotifer reproductive cycle differs between the three classes. Whereas seisonids reproduce sexually and bdelloids are solely asexual, the monogonont rotifers have a complex cycle that includes both a sexual and an asexual phase. In seisonids males and females are the same size, and both probably are diploid (contrary to the haploid monogonont dwarf males, discussed later). The male stores sperm in a sper-matophore that is transferred to the female. Fertilization and initial cell divisions occur inside the female's germarium; later, the female attaches the eggs to the host, the crustacean Nebalia, where they stay until the juveniles hatch. In bdelloids there are only females, and they reproduce exclusively by asexual parthenogenesis. This means that the maternal individual produces diploid eggs via mitosis and that these eggs can de velop into new embryos without initial fertilization. As a consequence, the daughters are cloned individuals that always are genetically identical to the mother.

The monogonont reproductive cycle is divided into an asexual (the amictic) and a sexual (the mictic) phase. The am-ictic phase resembles the bdelloid cycle, with parthenogenet-ically reproducing amictic females and complete absence of males; during this phase the population is capable of growing very quickly. Certain physical stimuli may induce the production of another kind of female, named a mictic female. Mictic females are morphologically similar to amictic females, but they produce eggs by meiotic cell division, which means that the eggs become haploid. The haploid mictic egg either waits to be fertilized by a male or, if it is not fertilized, starts to develop into a haploid male. Males are much smaller than females, and internal organs, such as the alimentary canal, often are reduced. The short-lived males seek mictic females immediately after hatching and fertilize eggs by hypodermic impregnation. Fertilization results in a thick-shelled resting egg that can survive extreme conditions, such as freezing and dehydration; after a period of dormancy an amictic female hatches from the egg and enters the amictic phase again. Most rotifers have direct development, and mitosis never occurs after hatching. Larvalike stages are found among permanently attached rotifers, so that the newly hatched animals can move to a suitable place before they settle permanently.

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