Decrease in purifying selection pressures on wheat homoeologous genes: tetraploidization versus hexaploidization.

Area of Science:

• Genomics
• Evolutionary Biology
• Plant Science

Background:

• Polyploidization events are key drivers of genome evolution and gene content alteration.
• Understanding the impact of multiple polyploidization rounds on duplicated gene function is crucial but poorly understood.

Purpose of the Study:

• To investigate the causal relationship between multiple polyploidization events and functional divergence of duplicated genes.
• To compare selection pressure on homoeologous gene pairs before and after tetraploidization and hexaploidization in the Triticum-Aegilops lineage.

Main Methods:

• Comparative analysis of selection pressure on homoeologous gene pairs across different ploidy levels (diploid, tetraploid, hexaploid).
• Examination of selection pressure on gene expression patterns.
• Comparison of evolutionary consequences of polyploidization-derived genes with those from other duplication mechanisms.

Main Results:

• Both tetraploidization and hexaploidization decreased selection pressure on homoeologous gene pairs, with initial tetraploidization having a greater impact.
• Relaxation of selection pressure on expression patterns was more pronounced after the initial tetraploidization event.
• Decreased selection pressure was independent of in-paralog presence, and homoeologous genes showed distinct evolutionary trajectories and retained unique functions (e.g., in reproduction, chromosome segregation) compared to other duplication mechanisms.

Conclusions:

• Multiple allopolyploidization events, especially initial tetraploidization, provide a significant source of functional gene divergence in wheat.
• Homoeologous genes possess unique functions that differ between tetraploidization and hexaploidization, contributing to evolutionary novelty.