Evolution of Phase-Biased Gene Expression in Land Plants

The most intriguing aspect of the land plant life cycle is the alternation of haploid and diploid phases. The relative size, morphology and interdependency of the haploid and diploid phases differs greatly among major groups of plants. In the land plants alone, some groups have haploid-dominant biphasic life cycles with an ephemeral diploid phase (e.g. bryophtes), whereas others have diploid-dominant life cycles in which the haploid phase is reduced to just a few cells (e.g. pollen in seed plants). It is widely agreed that land plants evolved from a haplontic green algal ancestor, subsequently elaborating of the diploid phase while reducing the haploid phase. However, very little is known about the large-scale genomic changes that have accompanied evolution of the land plant sporophyte and the corresponding shift in life history. In particular, specialization of expression patterns to the sporophyte and gametophyte phases, the relationship between gene family expansion/contraction and phase dominance and the appearance and evolution of the embryophyte sporophyte is fairly hypothetical. Toward this end we investigate the evolution of generation-specific gene expression evolution in major groups of land plants using high-throughput next-generation expression profiling. We report our preliminary results obtained using bryophytes, extant representatives of early diverging land plants, ferns and angiosperms. By combining next-generation sequencing data with the powerful toolbox of phylogenomics, we provide a quantitative assessment of the evolutionary processes that led to the elaboration of the sporophyte and reduction of the gametophyte in land plants.