Uncovering an ancestral green menage a trois: Contributions of Chlamydomonas to the discovery of a broadly conserved triad of plant fertilization proteins

During sexual reproduction in the unicellular green alga Chlamydomonas, gametes undergo the conserved cellular events that define fertilization across the tree of life. After initial ciliary adhesion, plus and minus gametes attach to each other at plasma membrane sites specialized for fusion, their bilayers merge, and cell coalescence into a quadriciliated cell signals for nuclear fusion. Recent findings show that these conserved cellular events are driven by 3 conserved protein families, FUS1/GEX2, HAP2/GCS1, and KAR5/GEX1. New results also show that species-specific recognition in Chlamydomonas activates the ancestral, viral-like fusogen HAP2 to drive fusion; that the conserved nuclear envelope fusion protein KAR5/ GEX1 is also essential for nuclear fusion in Arabidopsis; and that heterodimerization of BELL-KNOX proteins signals for nuclear fusion in Chlamydomonas through early diverging land plants. This review outlines how Chlamydomonas's Janus-like position in evolution along with the ease of working with its gametes have revealed broadly conserved mechanisms.

Automated summary

In the process of sexual reproduction in Chlamydomonas, a unicellular green alga, conserved cellular events including ciliary adhesion, fusion of gametes, and nuclear fusion take place. These events are driven by three conserved protein families. Species-specific recognition in Chlamydomonas activates viral-like fusogen HAP2, while the conserved nuclear envelope fusion protein KAR5/GEX1 is essential for nuclear fusion in Arabidopsis. Heterodimerization of BELL-KNOX proteins signals for nuclear fusion in Chlamydomonas through early diverging land plants.