This study demonstrates that fungal zygospores of Mucor mucedo contain sporopollenin, a structural biopolymer previously known only from plant pollen. Through exhaustive chemical extraction and analysis, the researchers identified a chemically similar residue in the outer warty layer of the zygospore wall. Radioactive precursor experiments incorporating acetate, mevalonate, and β-carotene revealed that sporopollenin is biosynthesized through oxidative polymerization of β-carotene. This finding provides the first identified function for the well-documented increase in carotenoid accumulation observed during sexual reproduction in Mucorales fungi. The sporopollenin comprises approximately 1-3% of zygospore dry weight and likely serves a protective role. Notably, Rhizopus sexualis, a related fungus that does not accumulate carotenoids, produces zygospores lacking detectable sporopollenin, suggesting the compound is not structurally essential but provides additional protection.

Key findings

• Mucor mucedo zygospores contain sporopollenin chemically identical to plant pollen sporopollenin and synthetic β-carotene polymers
• Radioactive tracer studies show sporopollenin is synthesized from β-carotene through oxidative polymerization in developing zygospores
• Sporopollenin deposition represents a major function for the increased carotenogenesis observed during sexual reproduction in Mucorales
• Sporopollenin is localized to the outer warty layer of the zygospore wall and comprises 1-3% of dry weight
• Absence of sporopollenin in carotenoid-deficient Rhizopus sexualis suggests the polymer provides protection rather than essential structural support