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Genome Reduction and Relaxed Selection is Associated with the Transition to Symbiosis in the Basidiomycete Genus Podaxis

Summary

This research explores how fungi adapt when they transition from living freely in the environment to living in partnership with termites. By studying fungal specimens collected over 240 years ago alongside modern samples, researchers discovered that fungi living with termites undergo significant genetic streamlining – losing genes they no longer need in their protected termite home. This is similar to how bacteria evolve when living inside insects. Impacts on everyday life: • Helps us understand how organisms adapt when forming partnerships in nature • Demonstrates the value of preserving historical specimens for modern research • Provides insights into evolution that could inform agricultural and medical applications • Shows how organisms can become dependent on each other over time • Reveals how protected environments can lead to loss of survival abilities

Background

The genomic consequences of symbiosis for basidiomycete fungi associated with social insects remain poorly understood. The genus Podaxis includes free-living desert species as well as facultative and specialist termite-associated fungi, making it an ideal system to study genomic changes during the evolution of symbiosis.

Objective

To investigate genomic changes associated with the transition from a free-living lifestyle to becoming a specialist termite symbiont by analyzing genomes from historical herbarium specimens spanning different lifestyles within the Podaxis genus.

Results

The transition to termite association was accompanied by significant reductions in genome size and gene content, accelerated evolution in protein-coding genes, and reduced functional capacities for oxidative stress responses and lignin degradation. Termite specialists performed worse under oxidative stress while retaining some lignin-cleaving ability. Their mitochondrial genomes were significantly larger, possibly due to smaller population sizes or reduced competition.

Conclusion

The findings indicate relaxed selection in termite-associated Podaxis that mirrors genome traits observed in obligate endosymbiotic bacteria of insects. The genome reduction and loss of certain functions appears linked to the stable, protected environment within termite mounds compared to free-living conditions.
  • Published in:iScience,
  • Study Type:Comparative Genomics Study,
  • Source: 10.1016/j.isci.2021.102680
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