Morfología Vegetal, Facultad de Ciencias Agracias, Instituto de Agrobiotecnología del Litoral (UNL-CONICET), Kreder 2805, S3080HOF Esperanza, Santa Fe, Argentina.
Ann Bot. 2013 Nov;112(8):1613-28. doi: 10.1093/aob/mct027. Epub 2013 Mar 10.
Inflorescence forms of panicoid grasses (Panicoideae s.s.) are remarkably diverse and they look very labile to human eyes; however, when performing a close inspection one can identify just a small subset of inflorescence types among a huge morphospace of possibilities. Consequently, some evolutionary constraints have restricted, to some extent, the diversification of their inflorescence. Developmental and genetic mechanisms, the photosynthetic type and plant longevity have been postulated as candidate constraints for angiosperms and panicoids in particular; however, it is not clear how these factors operate and which of these have played a key role during the grass inflorescence evolution. To gain insight into this matter the macroevolutionary aspects of panicoid inflorescences are investigated.
The inflorescence aspect (lax versus condensed), homogenization, truncation of the terminal spikelet, plant longevity and photosynthetic type were the traits selected for this study. Maximum likelihood and Bayesian Markov chain Monte Carlo methods were used to test different models of evolution and to evaluate the existence of evolutionary correlation among the traits. Both, models and evolutionary correlation were tested and analysed in a phylogenetic context by plotting the characters on a series of trees. For those cases in which the correlation was confirmed, test of contingency and order of trait acquisition were preformed to explore further the patterns of such co-evolution.
The data reject the independent model of inflorescence trait evolution and confirmed the existence of evolutionary contingency. The results support the general trend of homogenization being a prerequisite for the loss of the terminal spikelet of the main axis. There was no evidence for temporal order in the gain of homogenization and condensation; consequently, the homogenization and condensation could occur simultaneously. The correlation between inflorescence traits with plant longevity and photosynthetic type is not confirmed.
The findings indicate that the lability of the panicoid inflorescence is apparent, not real. The results indicate that the history of the panicoids inflorescence is a combination of inflorescence trait contingency and order of character acquisition. These indicate that developmental and genetic mechanisms may be important constraints that have limited the diversification of the inflorescence form in panicoid grasses.
panicoid 草类（Panicoideae s.s.）的花序形式非常多样，在人眼看来非常不稳定；然而，当进行仔细观察时，人们可以在巨大的形态空间中仅识别出一小部分花序类型。因此，一些进化约束在某种程度上限制了它们花序的多样化。发育和遗传机制、光合作用类型和植物寿命被认为是被子植物和 panicoid 草类的候选约束条件；然而，这些因素如何运作以及哪些因素在草类花序进化过程中发挥了关键作用尚不清楚。为了深入了解这一问题，研究了 panicoid 花序的宏观进化方面。
选择花序方面（松弛与密集）、均匀化、终端小穗的截断、植物寿命和光合作用类型作为本研究的特征。最大似然法和贝叶斯马尔可夫链蒙特卡罗方法用于测试不同的进化模型，并评估特征之间是否存在进化相关性。在进化背景下，通过在一系列树上绘制特征来测试和分析模型和进化相关性。对于那些被证实存在相关性的情况，进行了拟合优度检验和特征获取顺序检验，以进一步探索这种共同进化的模式。
数据拒绝了花序特征进化的独立模型，并证实了进化相关性的存在。结果支持了均匀化作为主轴终端小穗丢失的先决条件的总体趋势。在均匀化和密集化的获得中没有时间顺序的证据；因此，均匀化和密集化可以同时发生。花序特征与植物寿命和光合作用类型之间的相关性没有得到证实。
研究结果表明，panicoid 花序的不稳定性是表面的，而不是真实的。结果表明，panicoid 草类花序的历史是花序特征偶然性和特征获取顺序的结合。这表明发育和遗传机制可能是限制 panicoid 草类花序形式多样化的重要约束条件。
