Spermine and thermospermine synthases emerged multiple times during eukaryote evolution.

Area of Science:

• Evolutionary Biology
• Biochemistry
• Genomics

Background:

• Polyamines spermine and thermospermine exhibit differential distribution across eukaryotic phyla.
• Their biosynthetic enzymes, spermine synthase (SpmSyn) and thermospermine synthase (TspmSyn), likely emerged during eukaryotic evolution rather than being present in the Last Eukaryotic Common Ancestor.

Purpose of the Study:

• To elucidate the distinct evolutionary trajectories and phylogenetic distribution of functionally validated SpmSyns and TspmSyns in eukaryotes.
• To investigate the origins and evolutionary mechanisms of these key polyamine biosynthetic enzymes.

Main Methods:

• Phylogenetic analysis of SpmSyn and TspmSyn homologs across diverse eukaryotic lineages.
• Comparative genomics to trace the evolutionary history and gene duplication events.
• Functional validation of enzyme activities where applicable.

Main Results:

• Animal SpmSyn was acquired via horizontal gene transfer from bacteria as a fusion protein.
• Fungal and plant SpmSyn evolved through gene duplication of spermidine synthase followed by neofunctionalization.
• Plant TspmSyn likely originated from endosymbiotic gene transfer from a cyanobacterial ancestor, with homologs found in various protists and other algal lineages.

Conclusions:

• SpmSyn and TspmSyn evolved through a combination of horizontal gene transfer, gene duplication, and neofunctionalization, leading to their varied distribution in eukaryotes.
• The evolutionary pathways of these enzymes are linked to major events such as the emergence of animals, fungi, plants, and plastids.