Seed mass is a main determinant of seedling establishment, particularly under stressful conditions, as bigger seeds offer more resources to the emerging seedling. This has led to the expectation of increased seed mass under harsh conditions. Here, we studied the role of different components of environmental harshness on the evolution of seed size in Pinus. We assessed whether differences in cold and arid conditions between subgenera Strobus and Pinus matched differences in seed mass and tested the existence of positive trends between seed mass and cold and aridity, by controlling for evolutionary patterns that might derive from shared ancestry. Our results corroborate that Strobus pines inhabit drier and/or colder environments and have heavier seeds than subgenus Pinus. Variation in seed mass was largely determined by co-ancestry, and this strong phylogenetic signal cancelled the association between cold and seed size. Conversely, the effect of aridity on the size of pine seeds was unambiguous and independent of ancestry. These results strongly support that aridity exerts directional selection on seed size, which might trigger a selective feedback with vertebrate dispersal. Coupled selection on recruitment and dispersal traits will ultimately determine seed evolution.

中文翻译：

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干旱环境选择松树（Pinus spp.）中较大的种子

种子质量是幼苗建立的主要决定因素，特别是在压力条件下，因为更大的种子为新出现的幼苗提供更多资源。这导致人们期望在恶劣条件下增加种子质量。在这里，我们研究了环境严酷性的不同组成部分对松树种子大小进化的作用。我们评估了亚属 Strobus 和 Pinus 之间寒冷和干旱条件的差异是否与种子质量的差异相匹配，并通过控制可能源自共享祖先的进化模式来测试种子质量与寒冷和干旱之间是否存在正趋势。我们的结果证实 Strobus 松树栖息在干燥和/或寒冷的环境中，并且比亚属松树的种子更重。种子质量的变化很大程度上取决于共同祖先，这种强烈的系统发育信号消除了寒冷和种子大小之间的关联。相反，干旱对松树种子大小的影响是明确的，与祖先无关。这些结果强烈支持干旱对种子大小施加定向选择，这可能触发脊椎动物传播的选择性反馈。对招募和传播性状的耦合选择将最终决定种子进化。