Sexual evolution in plants: genetic network co-option and reproductive innovation

Sexual reproduction in plants encompasses a remarkable suite of developmental, genetic, and molecular processes that originated early in eukaryotic evolution and diverged extensively throughout the colonization of terrestrial environments. Fossil and molecular clock data suggest that the earliest components of sexual reproduction emerged in ancestral green algae over 1 billion years ago, well before the evolution of land plants. Key innovations such as alternation of generations, gametophytic development, and specialized reproductive organs evolved progressively across lineages, driven by both gene duplication and neofunctionalization, the progressive assembly and rewiring of regulatory gene families such as MADS -box transcription factors, epigenetic, and hormonal systems. In flowering plants (angiosperms), complex genetic networks governing floral organ identity, including the ABCDE model genes, were assembled via ancient duplication events approximately 300–200 million years ago during the diversification of seed plants. Understanding the molecular evolution of these pathways provides insight into the origins of plant reproductive diversity and the mechanisms that underlie speciation and adaptation in extant taxa. The review analyses major evolutionary transitions from algae to bryophytes, vascular plants, seed plants, and angiosperms in the light of the origin and diversification of plant reproductive systems, emphasizing molecular innovation, gene family evolution, lineage-specific adaptations and key elements including the origin of sexual reproduction in Archaeplastida. This review proposes an integrative evolutionary framework in which plant reproductive systems arose through iterative co-option, duplication, and functional divergence of conserved genetic modules across green lineages.