Symposium IV: Evolution of Plant Development
Abs #
40004: Grass spikelets and gene expression – what do we know
The grass spikelet is an evolutionary novelty – no other plant bears its flowers in precisely comparable structures. Efforts to understand how the spikelet originated have led to pages of discussion on exactly which parts are comparable to more conventional flowers and inflorescences. These efforts have intensified as the genes underlying floral development have been identified and their functions worked out, mostly in Arabidopsis. Many labs have now shown that the same genes used to pattern Arabidopsis flowers are also used in the grasses. For example, B-class genes are expressed in stamens and the next outer whorl of floral parts, as expected. AGAMOUS (AG) orthologues are expressed in stamens and carpels, the standard C-class pattern. SEPALLATA-like (SEP) genes are expressed in various parts of the spikelet, more or less as expected. These similarities, however, may lead to oversimplified homology statements – grass spikelets are not the same as Arabidopsis flowers. Some of this may be related to gene duplication. We find that AP2-like, AG-like, and SEP-like genes duplicated early in the evolution of the family. Reports in the literature show that the AG genes have specialized for carpel and stamen function respectively. We find that AP2-like genes have specialized for distinct roles in leaves and spikelets. Grasses have gene homologous to SEP3, plus three other genes (known in rice as OsMADS1, 5, and 34), which may not be orthologous to SEP1/2. The expression patterns of the SEP-like genes vary from gene to gene, and for at least OsMADS1, vary among the grasses.
These data taken together lead to several conclusions: 1. The grass spikelet is an evolutionary novelty, and the genetic interactions controlling its development are likely to be novel as well. 2. The parts of the grass spikelet do not have one-to-one correspondence with the parts of the Arabidopsis flower, but share some similarities of organization and gene expression. 3. Availability of extra gene copies correlates with diversification in function, as predicted by theory. 4. Diversification of gene function appears to correlate with diversification of morphology.
