Unisexual flowers have evolved multiple times within Poaceae in such diverse lineages as Bambusoideae, Ehrhartoideae, Pooideae, Danthonieae, Chloridoideae and Panicoideae. The conversion from a bisexual to unisexual floral meristem in Zea mays is correlated with programmed cell death and the expression of tassleseed2 (ts2). Based on developmental studies, ts2 is hypothesized to act in a similar manner in other panicoid grasses. Little is known about the genetic basis of unisexual flowers in other grass lineages, although developmental studies in Zizania aquatica suggest that either ts2 is not involved in the production of male flowers or is regulated differently. We have designed primers to amplify a 550 bp fragment at the 3 end of the ts2 gene for a phylogenetically diverse array of grasses. Preliminary results suggest that ts2 sequence is largely conserved within Panicoideae (nucleotide divergence 3.1-6.1%, amino acid divergence 1.7-6.5%) and Bambusoideae (nucleotide divergence 6.2%, amino acid divergence 4.7%), but less so within Ehrhartoideae (nucleotide divergence 17.1% amino acid divergence 16.5%), largely due to the Zizania sequence. Nucleotide sequence divergence between Oryza and Bambusoideae (7.8-9.5%) and Panicoideae (7.8-9.5%) taxa, is less than the estimated divergence between Oryza and Zizania. A hot spot of sequence evolution in all sampled taxa occurs between amino acids 255 and 270 of the Zea mays ts2 protein. Zizania has a large deletion within this area that maps to a helix in the estimated secondary protein structure. Other aspects of how amino acid sequence evolution affects estimated secondary protein structure will be discussed within the context of the GWPG phylogeny.

Key words: developmental genes, Poaceae