Data from: A continuous morphological approach to study the evolution of pollen in a phylogenetic context: an example with the order Myrtales

The study of pollen morphology has historically allowed evolutionary biologists to assess phylogenetic relationships among Angiosperms, as well as to better understand the fossil record. During this process, pollen has mainly been studied by discretizing some of its main characteristics such as size, shape, and exine ornamentation. One large plant clade in which pollen has been used this way for phylogenetic inference and character mapping is the order Myrtales, composed by the small families Alzateaceae, Crypteroniaceae, and Penaeaceae (collectively the “CAP clade”), as well as the large families Combretaceae, Lythraceae, Melastomataceae, Myrtaceae, Onagraceae and Vochysiaceae. In this study, we present a novel way to study pollen evolution by using quantitative size and shape variables. We use morphometric and morphospace methods to evaluate pollen change in the order Myrtales using a time-calibrated, supermatrix phylogeny. We then test for conservatism, divergence, and morphological convergence of pollen and for correlation between the latitudinal gradient and pollen size and shape. To obtain an estimate of shape, Myrtales pollen images were extracted from the literature, and their outlines analyzed using elliptic Fourier methods. Shape and size variables were then analyzed in a phylogenetic framework under an Ornstein-Uhlenbeck process to test for shifts in size and shape during the evolutionary history of Myrtales. Few shifts in Myrtales pollen morphology were found which indicates morphological conservatism. Heterocolpate, small pollen is ancestral with largest pollen in Onagraceae. Convergent shifts in shape but not size occurred in Myrtaceae and Onagraceae and are correlated to shifts in latitude and biogeography. A quantitative approach was applied for the first time to examine pollen evolution across a large time scale. Using phylogenetic based morphometrics and an OU process, hypotheses of pollen size and shape were tested across Myrtales. Convergent pollen shifts and position in the latitudinal gradient support the selective role of harmomegathy, the mechanism by which pollen grains accommodate their volume in response to water loss.长期以来，花粉形态学研究一直助力进化生物学家解析被子植物（Angiosperms）的系统发育关系，同时深化学界对化石记录的认知。此类研究中，学界主要通过离散化花粉的核心特征（如大小、形状及外壁纹饰）开展相关分析。桃金娘目（Myrtales）是采用该方法开展系统发育推断与性状映射的大型植物支系之一，该类群涵盖苞芽树科（Alzateaceae）、隐翼木科（Crypteroniaceae）与管萼木科（Penaeaceae）（三者合称“CAP支系”），同时包含使君子科（Combretaceae）、千屈菜科（Lythraceae）、野牡丹科（Melastomataceae）、桃金娘科（Myrtaceae）、柳叶菜科（Onagraceae）及沃希西亚科（Vochysiaceae）等多个大科。本研究提出了一种全新的花粉演化研究范式，即采用定量的大小与形状变量展开分析。我们借助形态计量学与形态空间分析方法，结合时间校准的超级矩阵系统发育树，对桃金娘目的花粉演化变化进行系统评估。随后，我们检验了花粉的形态保守性、性状分化与形态趋同假说，并探讨了纬度梯度与花粉大小、形状之间的相关性。为获取花粉形状的量化数据，研究团队从已发表文献中提取了桃金娘目的花粉图像，并采用椭圆傅里叶方法（elliptic Fourier methods）对其轮廓进行分析。在此基础上，我们基于奥恩斯坦-乌伦贝克过程（Ornstein-Uhlenbeck process）的系统发育框架，对桃金娘目演化历程中花粉大小与形状的演化偏移情况进行检验。研究仅发现桃金娘目花粉形态存在少量演化偏移，这表明该类群花粉形态具有较强的保守性。具异沟孔的小型花粉为桃金娘目的祖先性状，而柳叶菜科（Onagraceae）类群的花粉尺寸最大。桃金娘科（Myrtaceae）与柳叶菜科（Onagraceae）出现了花粉形状的趋同演化偏移，但大小未发生显著变化，且该偏移与纬度及生物地理区系的转变存在显著相关性。本研究首次将定量分析方法应用于大时间尺度下的花粉演化研究。我们借助基于系统发育的形态计量学与奥恩斯坦-乌伦贝克过程，对桃金娘目内的花粉大小与形状相关假说进行了全面检验。花粉的趋同演化偏移及其与纬度梯度的关联，证实了花粉体积调节机制（harmomegathy）的选择作用——该机制指花粉粒应对水分流失时调节自身体积的生理过程。