These are novel structures that have some aspects of spikelet identity and some of branch identity (see below). It includes all panicoid species in which some inflorescence branch meristems are converted to setae or bristles. The bristle clade has been found to be monophyletic in morphological ( Zuloaga, Morrone, and Giussani, 2000) and molecular studies ( Gómez-Martinez and Culham, 2000 Giussani et al., 2001 A. We have chosen to concentrate on the so-called “bristle clade” (Paniceae), which has 25 genera and approximately 310 species, 110 of which are in the genus Setaria.
Therefore, it may be preferable to look for homologies at the level of changes in development rather than in morphology of mature inflorescences.Ī study of inflorescence development across the entire grass family would be impracticable, but analysis of a smaller group of taxa may lead to results that are applicable to the rest of the family. Named inflorescence architectures, such as panicle or spike, have no particular meaning in and of themselves, but are merely convenient descriptors for the outcomes of these developmental processes the mature inflorescence is an historical summary of development, rather than a fixed or ideal type. The inflorescence is the outcome of a temporal and spatial pattern of gene expression, which drives the process of morphological development. Homoplasy in the distribution of inflorescence forms across the grass phylogeny obscures the mechanisms by which inflorescence diversity has arisen. These differences can be characteristic of certain tribes ( Clayton and Renvoize, 1986), but analysis of the phylogenetic distribution of inflorescence variation in the grasses suggests that different forms have arisen independently many times ( Kellogg, 2000a). Of these, the most obvious morphological distinctions among many genera and species are differences in inflorescence architecture, ranging from spikes to many-branched panicles ( Clayton and Renvoize, 1986). There are approximately 10 000 species and 600–700 genera ( Clayton and Renvoize, 1986 Watson and Dallwitz, 1992), and taxa have been morphologically distinguished one from another primarily by differences in inflorescence and spikelet morphology, leaf and culm anatomy, and photosynthetic pathway ( Clayton and Renvoize, 1986). They are the dominant component of the savanna, veldt, or prairie lands that cover about 20% of the globe in dry and semidry habitats ( Clayton and Renvoize, 1986).
The grasses (Poaceae) feed the world, either directly via crops such as wheat, rice, maize, and other grains, or indirectly as the primary fodder of most livestock. Changes in just a handful of developmental events account for inflorescence evolution in the bristle clade, and similar changes may account for inflorescence diversity in the grasses as a whole. Most developmental characters were congruent with the molecular phylogeny except for three reversals in the subclade containing S. Characters derived from comparisons of developmental sequences were optimized onto one of the most parsimonious trees. Changes in axis ramification, primordial differentiation, and axis elongation account for most variation in mature inflorescence morphology. Developmental morphology of these groups is very similar at early stages.
In this tree Cenchrus is monophyletic, but both Setaria and Pennisetum are paraphyletic. We have constructed a chloroplast DNA phylogeny of the three main genera, which finds three well-supported clades, two comprising species placed in Setaria and one of Pennisetum + Cenchrus. The clade is morphologically defined by inflorescences bearing both spikelets and sterile bristles and is monophyletic in both morphological and molecular phylogenetic analyses. We have reduced the complexity of the problem by examining one group of grasses, the panicoid “bristle clade,” which exhibits a less complex pattern of variation. The overall pattern is sufficiently complex that it is difficult to analyze inflorescence evolution. Grasses exhibit a great variety of inflorescence forms and these appear homoplasious when mapped onto cladograms.
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