All land plants undergo an alternation of generations between a haploid gametophyte phase that produces the gametes (sperm and eggs) and a diploid sporophyte phase that reproduces by the production of spores. In the early diverging land plant lineages--liverworts, hornworts, and mosses--the gametophyte is the dominant photosynthetic form and the sporophyte is transient and nutritionally dependent on the gametophyte. In all other land plants, such as ferns and seed plants (including gynmosperms and angiosperms), the sporophyte is the dominant form and the gametophyte is transient. In angiosperms, there has been an extreme reduction of the gametophyte; the mature male microgametophyte and the mature female megagametophyte consist of only three and seven cells, respectively, and develop fully contained within the tissues of the sporophyte. We are using both forward and reverse genetics in Physcomitrella patens, along with comparative transcriptomics among embryophytes, to understand the genetic basis of alternation of generations and of gametophyte/sporophyte dominance.
Stephen is a Berkeley Fellow and received a 2012 UC MEXUS grant.