Allohormones and sensory traps: a fundamental difference between hermaphrodites and gonochorists?

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Abstract
Multiple mating, sperm storage and sperm digestion cause uncertainty about the fate of donated sperm. In invertebrates these are common processes and increase the selective pressure for tactics that enhance fertilization success. Hence, to assure that donated sperm will be used for their rightful purpose, many different strategies can evolve. For example, biochemical substances—such as pheromones and allohormones—can be employed to improve the chances of fertilizing the partner's eggs. Several recent examples of such substances in gonochoric and hermaphroditic invertebrates are reviewed here. Moreover, I argue that a simple but fundamental difference may exist in the evolution of allohormones between the two modes of gender expression, primarily based on the differences in gene expression between species with separate sexes and hermaphrodites. This idea suggests that a biochemical sensory trap could occur more readily in hermaphrodites, and, depending on the mode of transfer of these allohormones, this should be reflected in a different speed of allohormone evolution.