Data from: The adaptive value of heterospory: evidence from Selaginella

Heterospory was a pivotal evolutionary innovation for land plants, but it has never been clear why it evolved. We used the geographic distributions of 114 species of the heterosporous lycophyte Selaginella to explore the functional ecology of microspore and megaspore size, traits that would be correlated with many aspects of a species’ regeneration niche. We characterized habitats at a global scale using leaf area index (LAI), a measure of foliage density and thus shading, and net primary productivity (NPP), a measure of growth potential. Microspore size tends to decrease as habitat LAI and NPP increase, a trend that could be related to desiccation resistance or to filtration of wind-borne particles by leaf surfaces. Megaspore size tends to increase among species that inhabit regions of high LAI, but there is an important interaction with NPP. This geographical pattern suggests that larger megaspores provide an establishment advantage in shaded habitats, although in open habitats, where light is less limiting, higher productivity of the environment seems to give an advantage to species with smaller megaspores. These results support previous theoretical arguments that heterospory was originally an adaptation to the increasing height and density of Devonian vegetative canopies that accompanied the diversification of vascular plants with leaves.

异型孢子（Heterospory）是陆生植物一项至关重要的演化革新，但学界始终未能明确其演化的动因。我们利用114种异型孢子石松类卷柏属（Selaginella）植物的地理分布数据，探究了小孢子与大孢子尺寸的功能生态学特征——这两类性状与物种再生生态位的诸多方面均存在关联。我们在全球尺度上对生境进行表征：采用叶面积指数（leaf area index, LAI，用以衡量植被密度与遮阴程度的指标）以及净初级生产力（net primary productivity, NPP，用以衡量生长潜力的指标）。研究发现，随着生境LAI与NPP的升高，小孢子尺寸呈现减小的趋势，这一规律可能与抗脱水性，或是叶片表面对风传颗粒物的过滤作用有关。栖息于高LAI区域的类群中，大孢子尺寸往往更大，但该规律与NPP存在重要的交互效应。这一地理分布格局表明：在遮阴生境中，更大的大孢子可赋予物种定植优势；而在光照限制较弱的开阔生境中，环境更高的生产力似乎更有利于大孢子尺寸更小的物种。本研究结果支持了此前的理论假说：异型孢子最初是对泥盆纪时期伴随带叶维管植物辐射演化而出现的、愈发高大茂密的植被冠层的适应。