Gene networks are conserved across reproductive development between the fern Ceratopteris richardii and the flowering plant Arabidopsis thaliana

Seed-bearing plants arose from within seedless vascular plants, with ferns as their closest relatives, but how seed development originated from ancestral reproductive development has remained unresolved by comparative morphology or the fossil record. To investigate this question at the level of gene network evolution, we identified genes associated with reproduction across sporophyte and gametophyte reproductive development of the fern Ceratopteris richardii and tested their conservation with reproductive gene networks in Arabidopsis thaliana.

The flowering signal gibberellin was conserved with the fern shoot sporing transition and sporophyte sporophyll genes were conserved with floral development but not seeds. Post-fertilization gametophyte archegonium genes demonstrated significant conservation with both pre- and post-fertilization seed gene networks, together with two experimentally validated post-fertilization mechanisms. We conclude that the seed may first have arisen through the mis-expression of archegonium developmental programmes within the developing sporophyte sporangium.