Abstract

Summary The grass family (Poaceae) has been the subject of intense research over the past decade. Although other angiosperm families contain more species and more genera, the Poaceae exceed all other families in ecological dominance and economic importance. Research has focused on the evolutionary relationships among grasses as well as the structure of grass genomes. Here I examine the evolutionary dynamics of grass genomes in a phylogenetic framework. It is clear that grass genomes are evolutionarily labile for many characteristics, including genome size and chromosome number. Variation in genome size among grasses probably reflects fluctuations in the amount of repetitive DNA per genome, but the history and causes of chromosome number changes remain unclear. Despite substantial variation among genomes, comparative maps suggest that grass genomes retain extensive regions of colinearity. By reanalyzing some comparative map data and also by reviewing comparative sequence data, I argue that the current colinearity paradigm requires reassessment. Contents Summary 15 I. Introduction 16 II. Grass relationships, chromosome numbers and genome size 17 1. Grass relationships 17 2. The timescale of grass evolution 17 3. The evolution of chromosome number 18 4. The evolution of genome size 19 III. Comparative maps and sequencing 21 1. Comparative maps of the grasses 21 2. Limitations of map data for evolutionary analyses 22 3. Synteny among grass genomes: a reanalysis 23 4. Microsynteny: comparative grass sequences 24 IV. Conclusions 25 Acknowledgements 26 References 26