Repeated evolution of tricellular (and bicellular) pollen

Area of Science:

• Evolutionary biology
• Plant reproductive biology
• Phylogenetics

Background:

• Male gametophytes (pollen) are dispersed before sexual maturity.
• Approximately 30% of flowering plants exhibit tricellular pollen with mature sperm at anthesis.
• Previous research suggested irreversible, parallel evolution of tricellular pollen in angiosperms.

Purpose of the Study:

• Re-evaluate the irreversibility of tricellular pollen evolution using modern phylogenetic methods.
• Investigate the evolutionary dynamics of pollen cell number in angiosperms.
• Test hypotheses on the origins and transitions of bicellular and tricellular pollen.

Main Methods:

• Expanded a dataset of pollen traits for 2511 angiosperm species.
• Utilized comparative phylogenetic methods, including Binary State Speciation and Extinction (BiSSE).
• Employed the hidden rates model (HRM) to analyze trait evolution across a time-calibrated phylogeny.

Main Results:

• Seventy percent of species studied possessed bicellular pollen.
• The transition rate from bicellular to tricellular pollen was 1.9 times higher than the reverse.
• Bicellular lineages exhibited an 1.8-fold higher diversification rate compared to tricellular lineages.

Conclusions:

• The tricellular pollen condition is evolutionarily labile, not irreversible.
• Pollen cell number frequencies are maintained by a balance between diversification rates and transition rates.
• Slower diversification and state shifts in tricellular lineages suggest a link between sporophyte lifestyle evolution and male gametophyte developmental plasticity.