Where did the seed evolve from? An investigation of SPOROCYTLESS homologs in the fern Ceratopteris richardii
K MCCREADY, DR J.C. COATES, DR A.R.G PLACKETT,
School of Biosciences, University of Birmingham, Edgbaston B15 2TT

Seeds are a recent evolutionary innovation that arose once from an ancestral spore-based reproductive mechanism that is still present in living seedless plant groups (e.g. mosses, liverworts and ferns). How this dramatic transition to seed-based reproduction occurred is currently unknown. SPOROCYTELESS/NOZZLE (SPL/NZZ) is a crucial regulator of seed development in Arabidopsis, but related genes (homologs) also exist in plants that do not make seeds. Identifying the functions of SPL/NZZ genes in seedless plants may therefore help to explain how seeds first evolved. Here we use a model seedless plant, the fern Ceratopteris richardii, to investigate the function of such SPL homologs. We have identified three SPL-like genes in Ceratopteris richardii. Phylogenetic analysis places two of these genes within the Arabidopsis SPL/NZZ clade, providing evidence that SPL function may predate the evolutionary origin of the seed. We further compare the expression of these three homologs in different reproductive and non-reproductive tissue stages. We demonstrate that these homologs have different patterns and levels of expression across tissue types, suggesting that there are functional differences between them. This work provides the foundation upon which to begin further investigation into a potential role for SPL-like genes during the evolution of the seed.