Parthenogenesis in Marbled Crayfish
Another cambarid freshwater crayfish, called marbled crayfish is reported to reproduce parthenogenetically (Martin et al., 2007). The conclusion that the marbled crayfish propagates apomictically is based on the finding that 19 selected individuals of various generations of this crayfish showed identical allelic composition. In addition, histological studies of the ovaries also indicated the absence of meiosis. The absence of any genetic differences over a number of generations suggests that no recombination had occurred during oogenesis.
In summarizing the foregoing discussion on sexual systems, it becomes evident that crustaceans provide a good model system to study the relationship between sexual patterns, reproductive strategy, and the environmental adaptation. Although crustaceans are generally gonochoristic, several alternative sexual systems are found among different crustacean taxa. Assumption of different sexual systems reflects on the control mechanisms of sex differentiation. Unlike insects, the genetic factors involved in sex determination are greatly influenced by epigenetic factors such as the androgenic gland hormone. This hormone is responsible for the differentiation and maintenance of male sex characters. Any developmental disturbance in the activity of this sex hormone could bring about an array of sexual patterns. As a consequence, a variety of hermaphroditic conditions, ranging from sequential to simultaneous hermaphroditism has been realized among decapods. The increased frequency of hermaphroditism among certain groups such as the caridean shrimps point to their adaptive ability to the precarious environmental conditions to achieve greater success in their reproduction. Even more reproductive patterns and strategies are found among lower crustacean orders such as the branchiopods. ESD and the absence of androgenic gland to control sex differentiation in the nonmalacostracan crustaceans have resulted in peculiar sexual patterns such as androdioecy and parthenogenesis, in addition to the production of sexual anomalies like intersexuals and gynandromorphs. In the absence of androgenic gland, other hormonal factors such as methyl farnesoate play a role in the control of sex differentiation in forms such as the water flea, Daphnia. In Crustacea, there exists a strong interlink between sexual system and the formation of mating systems, leading to the evolution of several social systems in these aquatic arthropods.
