Verdú Miguel, Pausas Juli G, Segarra-Moragues José Gabriel, Ojeda Fernando
Centro de Investigaciones sobre Desertificación (CSIC-UV-GV), Apdo Oficial, 46470 Albal (Valencia), Spain.
Evolution. 2007 Sep;61(9):2195-204. doi: 10.1111/j.1558-5646.2007.00187.x.
Mediterranean-type ecosystems are among the most remarkable plant biodiversity "hot spots" on the earth, and fire has traditionally been invoked as one of the evolutionary forces explaining this exceptional diversity. In these ecosystems, adult plants of some species are able to survive after fire (resprouters), whereas in other species fire kills the adults and populations are only maintained by an effective post-fire recruitment (seeders). Seeders tend to have shorter generation times than resprouters, particularly under short fire return intervals, thus potentially increasing their molecular evolutionary rates and, ultimately, their diversification. We explored whether seeder lineages actually have higher rates of molecular evolution and diversification than resprouters. Molecular evolutionary rates in different DNA regions were compared in 45 phylogenetically paired congeneric taxa from fire-prone Mediterranean-type ecosystems with contrasting seeder and resprouter life histories. Differential diversification was analyzed with both topological and chronological approaches in five genera (Banksia, Daviesia, Lachnaea, Leucadendron, and Thamnochortus) from two fire-prone regions (Australia and South Africa). We found that seeders had neither higher molecular rates nor higher diversification than resprouters. Such lack of differences in molecular rates between seeders and resprouters-which did not agree with theoretical predictions-may occur if (1) the timing of the switch from seeding to resprouting (or vice versa) occurs near the branch tip, so that most of the branch length evolves under the opposite life-history form; (2) resprouters suffer more somatic mutations and therefore counterbalancing the replication-induced mutations of seeders; and (3) the rate of mutations is not related to shorter generation times because plants do not undergo determinate germ-line replication. The absence of differential diversification is to be expected if seeders and resprouters do not differ from each other in their molecular evolutionary rate, which is the fuel for speciation. Although other factors such as the formation of isolated populations may trigger diversification, we can conclude that fire acting as a throttle for diversification is by no means the rule in fire-prone ecosystems.
地中海型生态系统是地球上最显著的植物生物多样性“热点”之一，传统上火灾被认为是解释这种特殊多样性的进化力量之一。在这些生态系统中，一些物种的成年植株能够在火灾后存活（萌蘖植物），而在其他物种中，火灾会杀死成年植株，种群仅通过有效的火灾后更新（种子植物）得以维持。种子植物的世代时间往往比萌蘖植物短，尤其是在火灾返回间隔较短的情况下，因此可能会提高它们的分子进化速率，并最终增加其物种分化。我们探究了种子植物谱系的分子进化速率和物种分化是否实际上高于萌蘖植物。在45个系统发育配对的同属分类群中，比较了来自火灾频发的地中海型生态系统、具有不同种子植物和萌蘖植物生活史的不同DNA区域的分子进化速率。在来自两个火灾频发地区（澳大利亚和南非）的五个属（山龙眼属、蝶豆属、拉氏草属、银桦属和灯心草属）中，采用拓扑学和年代学方法分析了差异物种分化。我们发现，种子植物的分子速率和物种分化均不高于萌蘖植物。种子植物和萌蘖植物在分子速率上缺乏差异——这与理论预测不符——可能是由于以下原因：（1）从种子繁殖转变为萌蘖繁殖（或反之亦然）的时间点发生在分支末端附近，因此大部分分支长度是在相反的生活史形式下进化的；（2）萌蘖植物遭受更多的体细胞突变，因此抵消了种子植物复制诱导的突变；（3）突变率与较短的世代时间无关，因为植物不经历确定性的种系复制。如果种子植物和萌蘖植物在分子进化速率上没有差异，而分子进化速率是物种形成的驱动力，那么预期不会出现差异物种分化。尽管其他因素（如隔离种群的形成）可能会引发物种分化，但我们可以得出结论，在火灾频发的生态系统中，火灾作为物种分化的抑制因素绝非普遍规律。
