Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 31.3 The Ancestor of Fungi Was an Aquatic, Single-Celled, Flagellated Protist
Molecular and fossil evidence reveals that fungi evolved from an aquatic, unicellular ancestor with a flagellum. Though most modern fungi lack flagella, their evolutionary roots connect them closely to animals and certain protists within the opisthokont clade.
Origin of Fungi
- Fungi and animals share a common ancestor, making them more closely related to each other than to plants.
- The ancestor of fungi was likely:
- Unicellular
- Aquatic
- Flagellated
- Closest protistan relatives:
- Nucleariids: Amoeboid protists that feed on algae and bacteria.
- DNA evidence shows fungi are more closely related to nucleariids than to animals.
- Animals are more closely related to choanoflagellates—this shows that multicellularity evolved independently in fungi and animals.
Opisthokonts: The Shared Clade
- Fungi, animals, and their protistan relatives form a monophyletic group called opisthokonts.
- Name comes from the posterior location of the flagellum in these organisms.
- Two basal fungal groups (chytrids and cryptomycetes) retain flagellated stages, supporting their aquatic origins.
Fossil Record and Colonization of Land
- The oldest widely accepted fungal fossils date to 440 million years ago.
- Some controversial marine eukaryote fossils may be as old as 1.5 billion years.
- Fossils of Tortotubus, a fungus with filamentous hyphae, show early adaptation to land.
- Chemical signatures in ancient soils suggest fungi may have colonized land before plants (~505 mya).
Symbiosis with Early Plants
- Fossils of early plants like Aglaophyton (405 mya) show mycorrhizal associations:
- Fungal hyphae penetrated root cells, forming arbuscules like modern mycorrhizae.
- Helped early plants absorb nutrients before roots evolved.
- Molecular studies show early plants carried genes for mycorrhizal symbiosis:
- Sym genes found across major plant lineages (even liverworts).
- Transferring a liverwort sym gene to a mutant flowering plant restored its ability to form mycorrhizae.
- Suggests this symbiotic relationship dates back hundreds of millions of years.
Fungal Radiation and Diversity
- Molecular analyses have redefined fungal phylogeny:
- The former phylum Zygomycota was paraphyletic and is now split into multiple groups.
- Microsporidians, once enigmatic parasites, are now classified as fungi.
- Recent metagenomic studies have discovered entirely new fungal lineages.
- Although ~145,000 species are described, estimated diversity ranges from 2.2 to 3.8 million species—more than all currently described eukaryotic organisms combined.
In a Nutshell
Fungi evolved from an aquatic, flagellated, unicellular ancestor and are closely related to animals and certain protists in the opisthokont group. Fossil and molecular data show they colonized land early and played a vital role in helping plants adapt through mycorrhizal symbiosis. With a massive, largely undiscovered diversity, fungi continue to reshape our understanding of life’s evolution.