Balanophora is a plant that no longer photosynthesizes. It abandoned sunlight roughly 100 million years ago, becoming an obligate parasite — tapping into host roots for all its carbon. It looks more like a fungus than a flower. Most botanists who encounter it in tropical forests don't immediately recognize it as a plant.
Its plastids should be gone. The chloroplast's reason for existing is photosynthesis. Remove photosynthesis, remove the organ. That's the logic. And Balanophora followed it — partially. Its plastid genome has collapsed from roughly 200 genes in a typical plant to about 20. The photosynthesis machinery is entirely deleted. The genome is among the smallest and most AT-biased known in any organism. By every genomic measure, the plastid is vestigial.
But it persists. Recent phylogenomic work (Lim et al., New Phytologist, 2025) found that over 700 nuclear-encoded proteins still carry chloroplast transit peptides — molecular addresses that route them into the plastid. The organelle is metabolically active. It still produces essential fatty acids, amino acid precursors, and isoprenoids. Seven hundred proteins are targeted to a room whose original purpose was demolished.
This is not vestigiality. It is functional reassignment at the compartmental level. The plastid was built for photosynthesis. It was kept for metabolism. Its genome was gutted but its membrane was preserved because downstream biosynthetic pathways had been constructed around its physical existence. Deleting the organelle would require rebuilding those pathways elsewhere — a reorganization more costly than maintaining the emptied room.
The structure outlived its function because the structure had become load-bearing for functions it was never designed to serve.