Understanding spikelet orientation in Paniceae (Poaceae)

Area of Science:

• Botany
• Evolutionary Biology
• Plant Morphology

Background:

• Spikelet structure and arrangement are crucial for grass identification.
• The Andropogoneae and Paniceae tribes exhibit distinctive spikelet pairs.
• Understanding spikelet evolution is key to grass phylogeny.

Purpose of the Study:

• To investigate if spikelet pairs are a precursor to secondary single spikelet conditions.
• To explain the variation in spikelet orientation within Paniceae genera.
• To clarify the homologies and developmental origins of spikelet structures in selected grass species.

Main Methods:

• Studied inflorescence development in *Paspalum simplex*, *P. stellatum*, and *Axonopus suffultus*.
• Analyzed spikelet morphology and orientation.
• Discussed potential genetic pathways (RAMOSA, polar auxin) and apical meristem inhibition.

Main Results:

• Solitary spikelets in *A. suffultus* are homologous to subsessile spikelets of *P. simplex*.
• Solitary spikelets in *P. stellatum* are homologous to pedicellate spikelets of *P. simplex*.
• Spikelet orientation changes result from differential reduction/abortion of spikelet primordia.

Conclusions:

• The single spikelet condition in Paniceae is likely homoplasic, evolving independently.
• At least two distinct genetic mechanisms underlie the evolution of single spikelets.
• Apical meristem inhibition appears to be controlled by different genetic factors in *A. suffultus* and *P. stellatum*.