Institut für Biologie-Botanik, Freie Universität Berlin, Altensteinstrasse, Berlin, Germany.
Botanischer Garten und Botanisches Museum Berlin, Freie Universität Berlin, Berlin, Germany.
Ann Bot. 2018 Jun 28;122(1):69-85. doi: 10.1093/aob/mcy055.
Many African genera of the Amaranthaceae exhibit unique inflorescences that include sterile flowers modified to hooks or spines. Considering that the abundance of large terrestrial herbivores increased on the African continent with the expansion of grassland and savannah ecosystems, modified sterile flowers could have been an innovation that boosted the diversification of an African achyranthoid clade of Amaranthaceae, with large animals serving dispersal.
We generated an extensively sampled phylogeny comprising 26 of the 31 achyranthoid genera as well as representatives of all other lineages of Amaranthaceae. Phylogenetic tree inference employed four genomic regions, using parsimony, likelihood and Bayesian inference methods. We estimated divergence times, evaluated trait-dependant changes and species diversification rates using state-dependent speciation and extinction models, and reconstructed ancestral character states for modified sterile flowers.
The achyranthoids were found to be a major clade of the Amaranthaceae, comprising mostly African members. Phylogenetic relationships within this clade were well resolved and supported two main subclades. Several genera were found to be polyphyletic. Our results indicate that the achyranthoids started to diversify ~28 million years ago, and that modified sterile flowers evolved multiple times. An asymmetry in transition rates towards the gain of sterile flowers was observed, whereas no trait-dependent increase in species diversification rates was detected. Bayesian rate heterogeneity analyses indicated that the achyranthoids diversified without significant rate shifts.
The accumulation of modified sterile flowers within achyranthoids appears to result from the higher transition rates in favour of modified sterile flowers. Multiple gains suggest an adaptive value for this trait. However, epizoochory does not appear to fuel species diversification, possibly due to extensive gene flow through regularly migrating mammals, which limits the possibility of speciation by isolation.
苋科的许多非洲属具有独特的花序，其中包括被改造为钩状或刺状的不育花。考虑到随着草原和稀树草原生态系统的扩张，大型陆生草食动物在非洲大陆上的数量增加，改造后的不育花可能是促进苋科 Achyranthoid 分支多样化的创新，大型动物充当了传播者。
我们生成了一个广泛采样的系统发育树，其中包括 31 个 Achyranthoid 属中的 26 个以及苋科所有其他谱系的代表。使用简约法、似然法和贝叶斯推断方法，对四个基因组区域进行了系统发育树推断。我们使用状态相关的物种形成和灭绝模型来估计分歧时间、评估与性状相关的变化以及物种多样化率，并重建了改造后的不育花的祖先性状。
Achyranthoids 被发现是苋科的一个主要分支，主要由非洲成员组成。这个分支内的系统发育关系得到了很好的解决，并支持两个主要的亚分支。一些属被发现是多系的。我们的结果表明，Achyranthoids 大约在 2800 万年前开始多样化，并且改造后的不育花多次进化。观察到向获得不育花的转变率的不对称性，而没有检测到与性状相关的物种多样化率的增加。贝叶斯率异质性分析表明，Achyranthoids 没有明显的率变化而多样化。
Achyranthoids 中改造后的不育花的积累似乎是由于向改造后的不育花的更高过渡率所致。多次获得表明该性状具有适应性价值。然而，动物传粉似乎并没有促进物种多样化，可能是由于经常迁徙的哺乳动物广泛的基因流，限制了隔离导致物种形成的可能性。
