Department of Ecology and Evolutionary Biology, Brown University, Providence, RI, USA.
Ann Bot. 2019 Jan 1;123(1):145-152. doi: 10.1093/aob/mcy151.
Structures that simultaneously perform many functional roles are likely to show a variety of morphological solutions to these demands, and thus probably exhibit high morphological disparity. In contrast, specialization for a few simple functions should result in a more limited suite of morphologies. We explore this idea using lycopsid reproductive structures, which, throughout their history, have performed a limited set of functional roles compared with the reproductive structures of other plant groups such as seed plants.
We scored living and fossil lycopsid taxa for 18 discrete character measurements and several continuous traits, including sporangium size, supporting axis diameter, and strobilus length and width. We used the discrete characters to construct a multivariate morphospace for lycopsid reproductive morphology through time, and the continuous characters to test whether fossil and extant lycopsids show similar patterns of tissue allocation within reproductive structures.
Lycopsids occupy similar areas of reproductive morphospace and show similar patterns of tissue allocation over most of their history, alternating between diffuse fertile zones with leaf-like sporophylls and compact strobili with specialized sporophylls that allow sporangia to be closely packed while also protected during their development. Growth habit also plays an important role in lycopsid reproductive evolution, broadly influencing the size and shape of reproductive structures.
Lycopsid reproductive structures are primarily specialized for densely packaging sporangia, and are consistent with the idea that performing limited functional roles is associated with reduced morphological disparity. Morphologies similar to lycopsid strobili are also found in other groups with simple, wind-dispersed propagules, suggesting that the same processes occur across plant lineages.
同时执行多种功能的结构可能会针对这些需求展现出多种形态学解决方案，因此可能表现出高度的形态差异。相比之下，少数简单功能的专业化应该会导致形态学的套件更为有限。我们通过石松类生殖结构来探索这个想法，与种子植物等其他植物群的生殖结构相比，石松类生殖结构在其历史上一直执行着有限的功能角色。
我们对活体和化石石松类分类群的 18 个离散特征测量值和几个连续特征进行了评分，包括孢子囊大小、支撑轴直径以及孢子体的长度和宽度。我们使用离散特征通过时间构建了石松类生殖形态的多元形态空间，使用连续特征来测试化石和现存石松类是否在生殖结构内表现出相似的组织分配模式。
石松类在生殖形态空间中占据相似的区域，并在其历史的大部分时间内表现出相似的组织分配模式，在具有叶状孢子叶的弥散可育区和具有专门孢子叶的紧凑孢子体之间交替，从而可以在发育过程中紧密地包装孢子囊并同时保护它们。生长习性也在石松类生殖进化中起着重要作用，广泛影响生殖结构的大小和形状。
石松类生殖结构主要是为了密集包装孢子囊而专门化的，这与执行有限功能角色与减少形态差异相关的观点一致。与石松类孢子体相似的形态也存在于其他具有简单风传播繁殖体的类群中，这表明相同的过程发生在植物谱系中。
