Spore size data for free-sporing extant and fossil plants

The evolution of different spore size classes, or heterospory, is a fundamental reproductive innovation in land plants. The appearance of heterospory is particularly notable during the Devonian, when most known origins of the trait occur. Here we provide a perspective on the evolution of heterospory during this time interval, particularly from the late Early Devonian through the Middle Devonian (Emsian to Givetian stages; 408-383 Ma), which shows an unusually high concentration of heterospory origins. We use theoretical considerations and compilations of fossil and extant spore sizes to suggest that the basic features of most heterosporous lineages, large spores and gametophytes that mature within the spore wall, are difficult to evolve in combination because large spores disperse poorly but small spores cannot support a functional gametophyte developing within their walls; evolving spores between 100-200 microns in diameter appears to represent a particularly important barrier for the ...

不同孢子大小类别的演化，即异孢现象（heterospory），是陆生植物的一项基本繁殖创新。异孢现象的出现于泥盆纪尤为显著，该时期是已知该性状起源最多的阶段。本文针对这一时期异孢现象的演化提供了一个视角，尤其是早泥盆世晚期至中泥盆世（埃姆斯期至吉维特期；408-383 Ma），该阶段异孢起源的集中度异常高。我们结合理论分析及化石与现存孢子大小的汇编数据，指出大多数异孢谱系的基本特征——大孢子及在孢子壁内成熟的配子体——难以协同演化；原因在于大孢子扩散能力较弱，而小孢子无法支撑在其壁内发育的功能性配子体。直径在100-200微米之间的孢子演化似乎构成了一个尤为重要的屏障……