Sniff and run—the chemistry of attraction

Abstract

Fitness represents the gold standard of evolution. This scintillating term generally describes the efficiency by which organisms can multiply their genetic constitution. The first step in the life cycle of any sexual organism is simultaneously the one most crucial for this task: Since the formation of an egg cell with its rich resources represents a much larger investment than the production of the smaller sperm cells, the number of male cells usually exceeds those of their female targets. This leads to harsh competition between male germ cells providing strict selection for efficient targeting and efficient translocation. In animals, it is the efficiency of chemotaxis that will decide over the fate of a given spermatozoid. In higher plants that have replaced mobile sperm cells by a mobile gametophyte, selectivity is provided by the efficiency of chemotropism. Two contributions in the current issue address the molecular and cellular mechanisms establishing the targeted transport of the male nucleus to the egg cell in animals and plants. Since the mechanism for sperm mobility is so fundamentally different (free swimming versus polar growth), it is interesting to have a look on the commonalities and differences between the two types of organisms.In their review on sperm guidance in animals, Sugiyama and Chandler (2014) focus on the role of calcium signalling for sperm guidance and integrate most recent results on the calcium signature in the flagellum into the context of classical work reaching back for more than a century. They dissect the molecular events that are responsible for the asymmetric flagellar bend that is responsible for directional movement. Although the model for the targeting mechanism dates back several decades, it has been difficult to test the proposed mechanisms experimentally, because the events are extremely fast. In their historical survey of the field, the authors work out very clearly how conceptual advances have been enabled by methodological progress. The fact that calcium influx and cGMP signalling are relevant for the correct processing of the chemoattractive signal had been elucidated some time ago, but this did not really help to understand what is happening because it is not so much the molecular nature of these signals that matters. It is rather the issue, where and when they are generated, that defines the efficiency of chemotaxis. To elucidate the spatiotemporal signature of these signals required sophisticated methodology, such as specific mixing chambers that allowed to obtain extremely precise application of chemical cues or ultrashort excitation devices that allowed to obtain extremely precise readouts from calcium fluorophores. A further methodological aspect is worked out in this reviewthe need to quantify the cellular response (chemotaxis), in order to be able to correlate signalling events and biological effect. The combination of these techniques allowed to dissect signalling and also to uncover that the effect of chemoattractors is multilayered. For instance, proton-de...