Data from: Medial pith cells per meter in twigs as a proxy for mitotic growth rate (Phi/m) in the apical meristem

The Φ model for plant mating system evolution proposes a causal link between Φ, the number of mitoses that occur within a plant’s lifetime from zygote to gamete production, and constraints on the evolution of inbreeding depression and thereby on the evolution of plant mating systems. Through its use of plant stature, the Φ model emphasizes the important role of morphology in creating developmental and genetic constraints on plant evolution. However, the estimation of Φ itself is likely to be extraordinarily complex. Here I describe a protocol for estimating Φ per meter linear growth by an apical meristem (Φ/m) using medial pith cells from mature internodes of twigs. While such cells are produced by the apical meristem, during internode elongation these pith cells also undergo further mitoses and thus their measurement can only approximate a ‘true’ Φ/m via the application of a multiplier (the adjustment ratio) that partially corrects for the occurrence of cell divisions and cell growth beyond the apical meristem. I apply this method to Delonix regia (Caesalpiniaceae) and derive several adjustment ratios from the literature. As variation in Φ/m can have profound evolutionary implications, I also examine interspecific and intraspecific variation as well as within-individual variation in Φ/m. Conifers apparently have lower Φ/m than angiosperms, while 20% of the total variance in Φ/m for D. regia was found among individual trees, with the remainder found within trees. Given the large differences in stature between ‘high-Φ’ plants such as trees and ‘low-Φ’ plants such as herbs, these results support the idea that the total per-generation mutation rate for high-Φ plants is likely to be many times higher than that for low-Φ plants.

用于植物交配系统演化的Φ模型提出，Φ（即植物一生中从合子到配子发生所经历的有丝分裂次数）与近交衰退演化的约束存在因果关联，并进而作用于植物交配系统的演化。该模型通过引入植物株高这一参数，强调了形态学在塑造植物演化的发育与遗传约束中的关键作用。然而，Φ本身的估算过程往往极为复杂。本文描述了一种基于细枝成熟节间的髓中部细胞，估算顶端分生组织单位线性生长量对应的Φ（记为Φ/m）的实验方案。尽管这类细胞由顶端分生组织产生，但在节间伸长过程中，这些髓细胞还会发生额外的有丝分裂。因此，需通过应用校正比例（即乘数）来部分弥补顶端分生组织以外的细胞分裂与细胞生长带来的偏差，才能使测得的结果近似于“真实”的Φ/m。本文将该方法应用于凤凰木（Delonix regia，云实科 Caesalpiniaceae），并从已发表文献中推导得到若干校正比例。鉴于Φ/m的变异可能对演化产生深远影响，本文同时考察了Φ/m的种间、种内以及个体内变异。研究结果显示，裸子植物的Φ/m显著低于被子植物；而在凤凰木中，Φ/m总变异的20%来自个体间差异，剩余部分则来自个体内部。考虑到乔木等“高Φ”植物与草本等“低Φ”植物之间存在显著的株高差异，上述结果支持“高Φ植物的每世代总突变率大概率远高于低Φ植物”这一假说。