Heterogony

HETEROGONY (from Greek heteros—other and gonos—offspring), the alternation of bisexual generations with unisexual generations, or of generations reproducing through fertilization with generations reproducing parthenogenetically (see Parthenogenesis), i.e., from unfertilized eggs. In some animals, between normal females and parthenogenetic ones there are no differences whatsoever in structure or organization (some rotifers), while in others, especially insects, significant changes are observed in the structure and mode of life of these parthenogenetic females, and sometimes of the sexual individuals as well. Thus, in the grapevine phylloxera, a pest of viticulture, wingless females emerge in spring, laying parthenogenetic eggs from which wingless parthenogenetic females arise once more, living above the surface of the ground and feeding on the sap of the grapevine. From such females there originate likewise parthenogenetic, wingless females, differing somewhat in their structure, which live underground on the roots, where they cause pathological swellings that are the cause of disease and death of grapevines. At the end of summer, from these parthenogenetic "root" females there originate winged ones, which also parthenogenetically lay two sorts of eggs. From some (large) females emerge, while the smaller ones give rise to males. Both males and females are also devoid of wings; furthermore, their structure is simplified: the gut is rudimentary, and there is no proboscis for piercing the plant. After fertilization they quickly perish, the and the female lays a single overwintering egg, from which the following year that parthenogenetic female will emerge which will start the cycle anew. Such cycles of heterogony have been well studied in rotifers and cladocerans (Cladocera). In the latter, so-called monocyclic species are encountered, living usually in lakes and generally in larger bodies of water. In spring, from winter eggs formed through fertilization, parthenogenetic females emerge, yielding a series of generations of females reproducing parthenogenetically all the time (amictic females). By autumn, eggs yielding males are formed, as well as larger eggs capable of fertilization and becoming winter eggs. Females producing eggs from which males will emerge, or eggs capable of fertilization, are called mictic females. Polycyclic species are those in which the alternation of generations occurs several times a year (species found in small bodies of water that dry up in summer). Weismann was the first to draw attention to heterogony in Cladocera, considering such alternation to be hereditarily fixed by natural selection. Lauterborn first established the same for rotifers. Since then, the cycles of these animals have been the subject of many carefully designed experiments aimed at resolving the question to what extent these cycles can change under the influence of changing conditions (Woltereck, Scharfenberg, Whitney, Scholl, et al.). Most zoologists previously believed that this change (for example, the appearance at will of males and females instead of parthenogenetic reproduction, or vice versa) can be induced only during a certain period of time, whereas at other times the cycle is hereditarily fixed, and changes are impossible. However, recently M. Hartmann, based on the finer experimental methodology of Luntz and the studies of Tauson on rotifer cycles, came to the conviction that, at least in these animals and Cladocera, the alternation of generations depends entirely on external conditions, especially on the nutrition of the animals and on the hydrogen ion concentration in the culture fluid. Heterogony is also frequently observed in parasitic worms.