Elizabeth
A. Kellogg, Paulo E. A. S. Camara, Paula J. Rudall, Philip Ladd, Simon
T. Malcomber, Clinton J. Whipple and Andrew N. Doust
Abstract
The shoot apical meristem of grasses produces the primary branches of
the inflorescence, controlling inflorescence architecture and hence seed
production. Whereas leaves are produced in a distichous pattern, with
the primordia separated from each other by an angle of 180°,
inflorescence branches are produced in a spiral in most species. The
morphology and developmental genetics of the shift in phyllotaxis have
been studied extensively in maize and rice. However, in wheat, Brachypodium,
and oats, all in the grass subfamily Pooideae, the change in
phyllotaxis does not occur; primary inflorescence branches are produced
distichously. It is unknown whether the distichous inflorescence
originated at the base of Pooideae, or whether it appeared several times
independently. In this study, we show that Brachyelytrum, the
genus sister to all other Pooideae has spiral phyllotaxis in the
inflorescence, but that in the remaining 3000+ species of Pooideae, the
phyllotaxis is two-ranked. These two-ranked inflorescences are not
perfectly symmetrical, and have a clear “front” and “back;” this
developmental axis has never been described in the literature and it is
unclear what establishes its polarity. Strictly distichous
inflorescences appear somewhat later in the evolution of the subfamily.
Two-ranked inflorescences also appear in a few grass outgroups and
sporadically elsewhere in the family, but unlike in Pooideae do not
generally correlate with a major radiation of species. After production
of branches, the inflorescence meristem may be converted to a spikelet
meristem or may simply abort; this developmental decision appears to be
independent of the branching pattern.
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