Krieg Christopher, Watkins James E, Chambers Sally, Husby Chad E
Department of Biology, Colgate University, Hamilton, NY 13346, USA.
Department of Biology, University of Florida, Gainesville, FL 32611, USA.
AoB Plants. 2017 Apr 5;9(2):013. doi: 10.1093/aobpla/plx013. eCollection 2017 Mar.
Selective pressures acting on plant life histories can drive extreme specialization. One example of such specialization is the evolution of dioecious breeding systems. Evolutionary and ecological theory posits that dioecy may subject male and female individuals to different selective pressures and result in unique sex-mediated adaptive traits related to resource allocation and ecophysiology. Cycads are the earliest diverging lineage of seed plants with strict dioecy, yet we know almost nothing about the ecology and physiology of this group. Especially limited is our understanding of potential sex-specific differences and how such differences may influence species ecology. Here we examine the ecophysiology of male and female cycads to understand better, the role that dioecy plays in this group. We evaluated sex-specific differences in ecophysiological traits and resource acquisition in five species. Specifically, we compared photosynthetic physiology, nitrogen and carbon content, isotope discrimination (δN and δC), and stomatal density. In some cycads, (i) males and females have similar investments in leaf nitrogen but females exhibit greater incorporation of nitrogen from nitrogen-fixing soil bacteria, (ii) males display higher photosynthetic capacity but females show decreased [corrected] water-use efficiency, and (iii) males have higher stomatal conductance but similar stomatal density to females. This study is the first to examine the ecophysiological differences that have evolved in the oldest dioecious lineage of seed-bearing plants. Our results show unexpected differences in photosynthetic physiology and highlight the co-evolution with nitrogen fixing soil bacteria as a potential new key player in an old lineage.
作用于植物生活史的选择压力可推动极端特化。这种特化的一个例子是雌雄异株繁殖系统的进化。进化和生态理论认为，雌雄异株可能使雄性和雌性个体面临不同的选择压力，并导致与资源分配和生态生理学相关的独特性别介导的适应性状。苏铁是种子植物中最早分化出的具有严格雌雄异株的谱系，但我们对该类群的生态学和生理学几乎一无所知。我们对潜在的性别特异性差异以及这些差异如何影响物种生态的了解尤其有限。在这里，我们研究了雌雄苏铁的生态生理学，以更好地理解雌雄异株在该类群中所起的作用。我们评估了五个物种在生态生理特征和资源获取方面的性别特异性差异。具体而言，我们比较了光合生理学、氮和碳含量、同位素分馏（δN和δC）以及气孔密度。在某些苏铁中，（i）雄性和雌性在叶片氮素上的投入相似，但雌性表现出从固氮土壤细菌中吸收更多的氮，（ii）雄性具有更高的光合能力，但雌性的水分利用效率降低，（iii）雄性的气孔导度更高，但气孔密度与雌性相似。这项研究首次考察了在最古老的种子植物雌雄异株谱系中进化出的生态生理差异。我们的结果显示了光合生理学方面出人意料的差异，并突出了与固氮土壤细菌的共同进化，这可能是一个古老谱系中的潜在新关键因素。
