NIELD, BECKY A.* AND JEFFREY P. HILL. Department of Biological Sciences, Idaho State University, Pocatello, ID 83209-8007. - Molecular genetic studies of gender expression in angiosperms: early ovule development in Nicotiana tabacum.
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