Parthenogenesis

The term parthenogenesis was first used in 1849 by the biologist Richard Owens. Although most animals reproduce sexually, some species of vertebrates and invertebrates reproduce by parthenogenesis. Of these species the most frequently studied are fish, reptiles, and insects. Parthenogenetic animals are classified as either facultative or obligate. Facultative parthenogens (usually invertebrates) can reproduce either parthenogenetically or sexually at all times, whereas obligate parthenogens are animals in which individuals of at least one generation reproduce by parthenogenesis. Obligate parthenogens may be further subdivided into either constant parthenogens or cyclical parthenogens. All generations of species showing constant parthenogenesis reproduce by parthenogenetic methods, and are typically composed of only females. Examples of these organisms include species of lizards, minnows, and brine shrimp. Cyclical parthenogens, such as aphids, alternate parthenogenetic generations with a sexual generation. In the summer months aphids reproduce by parthenogenesis, but the onset of the fall acts as a signal for new offspring to develop into males which then mate with available females (sexual reproduction) producing fertilized eggs that hatch in the spring.