The post-fertilization archegonium gene network of seedless plants contributed to the origin of seeds.

The evolution of seeds was a critical transition in plant evolution, but how ancestral seedless development was modified to form the first seed is only partly-answered through comparative morphology or the fossil record. We investigate seed origins by quantifying gene network conservation between seeds and seedless plant reproductive organs. Characterizing reproduction in the homosporous fern Ceratopteris richardii as a proxy for ancestral seedless reproduction, we create a gene expression atlas of fern diploid and haploid reproductive organ development and test for enrichment of these genes in Arabidopsis thaliana. Here we show ovule gene networks are enriched in genes regulating post-fertilization development of the fern haploid egg chamber (archegonium), a subset of which also show enrichment in sporophyll primordia, suggesting a mechanistic model to explain the origin of the ovule from a sporangium-bearing axis whereby gene networks in the ancestral post-fertilization archegonium became heterotopically expressed during development of the sporangium-bearing axis.