Adaptation of the Spore Discharge Mechanism in the Basidiomycota

Background
Spore discharge in the majority of the 30,000 described species of Basidiomycota is powered by the rapid motion of a fluid droplet, called Buller's drop, over the spore surface. In basidiomycete yeasts, and phytopathogenic rusts and smuts, spores are discharged directly into the airflow around the fungal colony. Maximum discharge distances of 1–2 mm have been reported for these fungi. In mushroom-forming species, however, spores are propelled over much shorter ranges. In gilled mushrooms, for example, discharge distances of <0.1 mm ensure that spores do not collide with opposing gill surfaces. The way in which the range of the mechanism is controlled has not been studied previously.
Methodology/Principal Findings
In this study, we report high-speed video analysis of spore discharge in selected basidiomycetes ranging from yeasts to wood-decay fungi with poroid fruiting bodies. Analysis of these video data and mathematical modeling show that discharge distance is determined by both spore size and the size of the Buller's drop. Furthermore, because the size of Buller's drop is controlled by spore shape, these experiments suggest that seemingly minor changes in spore morphology exert major effects upon discharge distance.
Conclusions/Significance
This biomechanical analysis of spore discharge mechanisms in mushroom-forming fungi and their relatives is the first of its kind and provides a novel view of the incredible variety of spore morphology that has been catalogued by traditional taxonomists for more than 200 years. Rather than representing non-selected variations in micromorphology, the new experiments show that changes in spore architecture have adaptive significance because they control the distance that the spores are shot through air. For this reason, evolutionary modifications to fruiting body architecture, including changes in gill separation and tube diameter in mushrooms, must be tightly linked to alterations in spore morphology.

### 研究背景
在已描述的30000种担子菌（Basidiomycota）中，绝大多数物种的孢子释放过程，均借助孢子表面名为布勒液滴（Buller's drop）的流体液滴的快速运动提供动力。在担子菌酵母、植物致病性锈菌与黑粉菌中，孢子会直接释放到真菌菌落周围的气流中，此类真菌的孢子最大释放距离可达1~2毫米。但在形成子实体的蘑菇类群中，孢子的弹射距离则短得多。例如在具菌褶的蘑菇中，孢子释放距离小于0.1毫米，可避免孢子与对面的菌褶表面发生碰撞。此前学界尚未对该弹射机制的距离调控方式展开研究。

### 研究方法与主要结果
本研究对选取的担子菌类群开展了高速视频分析，涵盖从酵母到具孔状子实体的木腐真菌等类群。通过对视频数据的分析与数学建模，我们发现孢子释放距离由孢子尺寸与布勒液滴的尺寸共同决定。此外，由于布勒液滴的尺寸受孢子形状调控，本实验表明孢子形态看似微小的变化，会对孢子释放距离产生显著影响。

### 结论与意义
本研究针对蘑菇类群及其近缘类群的孢子释放机制开展生物力学分析，尚属首次，为两百余年来传统分类学家所记录的极其多样的孢子微观形态提供了全新视角。新实验结果表明，孢子结构的变化并非随机的微观形态变异，而是具有适应意义——其通过调控孢子在空气中的弹射距离实现适应性进化。因此，子实体结构的演化修饰（包括蘑菇的菌褶间距、菌管直径变化等），必然与孢子形态的改变紧密相关。