Genes involved in asexual sporophyte development in Ceratopteris richardii and Arabidopsis thaliana

The life cycle of land plants alternates between the haploid gametophyte and the diploid sporophyte generations. Asexual reproductive strategies, that bypass meiosis and fertilization, have evolved in diverse land plant taxa. Apogamy is one such strategy that produces a sporophyte directly from a gametophyte cell. The genes that drive the process of apogamy are unknown. Knowledge of these genes and their functions will provide insight into the evolution of asexual reproduction, the sporophyte body plan and the alternation of generations in land plants. My Ph. D. research has focused on identifying the genes function in apogamy commitment, and understanding the functions of their counterparts in angiosperms.

First, I successfully induced apogamy from the fern Ceratopteris richardii and discovered that the gametophytes begin to become committed to apogamy after 10 days of culture on inductive medium. I then created a cDNA library that represents genes with enhanced expression during commitment. Comparison of the Gene Ontology terms mapped to this cDNA library with that of the gametophyte transcriptome of the fern Pteridium aquilinum showed that this library is enriched in genes that function in stress response and metabolism. This library contains many sequences whose homologues in Arabidopsis are specifically expressed or upregulated in flower organs or seed structures, both of which are absent in ferns. One of these genes, UNC93-like, is expressed in the eggs of C. richardii gametophytes, as evidenced by in situ hybridization. Functional egg cells are implicated as necessary for C. richardii gametophytes to undergo apogamy.

In Arabidopsis, an AtUNC93-like mutant, which produces a partial AtUNC93-like transcript, has vegetative and reproductive defects. Embryo abortion and gametophyte lethality contribute to a small seed set in these plants. Reciprocal crosses indicate that the mutant allele does not affect gametophyte function but cause a maternal failure to maintain viability of all gametophytes. Thus, the AtUNC93-like gene is necessary for normal sporophyte vegetative growth and reproductive success, but is dispensable for the direct function of gametophytes.

My research has provided insight into the mechanisms that induce apogamy in a fern. The apogamy library is a valuable resource for future investigations into apogamy.