Within the non-seed plants, sex determination either occurs just before meiosis (e.g., Selaginella) or some time after meiosis in the developing gametophyte (e.g., homosporous ferns). In angiosperms, sex determination is generally considered a feature of the sporophyte's life history, because gender-specific organs (i.e., male stamens and female carpels) are initiated well before meiosis. This study presents a different hypothesis about patterns of sex determination in flowering plants, one that is broadly based on the phylogenetic history of this trait. Due to alternation of generations, flowering plants may have elements of two independent sex determination events per sexual cycle. The first widely recognized event dictates gender expression in the sporophyte. The second event, which is an evolutionary vestige of a sex determination mechanism that pre-dates the evolution of the seed habit, reflects the primitive state for controlling gender expression in the gametophyte phase. This research seeks to identify and characterize genes involved in early gender specialization during this second (i.e., haploid) event in Nicotiana tabacum. Genes expressed in early stages of ovule development, including the nascent placenta and pre-meiotic ovule primordia, are being characterized. These structures are, in a developmental morphological sense, close to the phases that appear to evoke differences in haploid gender expression in more primitive heterosporous plants like Selaginella. Differential Display-Polymerase Chain Reaction (DD-PCR) has been employed to initially characterize gene expression patterns among target tissues compared to leaf and stamen primordia. To date, seven putative placental-specific genes and two putative early ovule-specific genes have been found using DD-PCR, and further characterization is in progress. The results should advance the current understanding of genetic mechanisms regulating gene expression in early ovules of an agriculturally significant plant family (Solanaceae), and provide insights into the evolution of gender expression in plants at the same time.

Key words: gender evolution, gene, Nicotiana, ovule development, sex determination