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In a Goldilocks mushroom, the gills are not too close, not too far apart, but just the right distance for the spores to be shot to the open midplane. The following figure from a forthcoming review article shows the reciprocal changes in gill separation and discharge distance that might have played out during mushroom evolution. (Too esoteric for a blog post? Yes.)
Caption: Hypothetical evolutionary seesaw that matches gill separation to the distance of ballistospore discharge. In this diagram, a modification to the range of the spore discharge mechanism precedes an increase in gill spacing which is followed by a second adjustment to gill spacing that boosts spore output or escape from the fruit body. Goldilocks mushrooms in which the spore output is optimized to gill spacing are indicated by the level seesaws diagrammed in (A) and (D). (A) Goldilocks I: In the starting condition, optimal spore output is supported from fruit body whose spores are shot to the midplane between opposing gills. (B) Spore size begins to increase within a fungal population in response to an ecological opportunity for larger spores. The larger spores produce larger Buller’s drops in this model and are shot further than the smaller spores. These developmental adjustments happen incrementally and reduce spore output as more and more spores hit the opposing gill. This outcome is a potential feeder toward the loss of ballistospory in favor of dispersal by animals in gasteroid descendants from the population in (B). (C) Ballistospory is maintained in other populations by an increase in gill spacing that limits the wastage of spores. In this case, spores are launched short of the midplane because the distance between gills is unnecessarily large, and the efficiency of the fruit body decreases. (D) Goldilocks II: Optimal spore output is restored by a slight reduction in gill spacing so that the larger spores are shot to the midplane between gills. The same model applies to poroid mushrooms if we replace changes in gill spacing with modifications in tube size.