Interestingly, they also found that both bi- and tricellular lineages gave rise to each other. Thus, their analyses debunked the long-standing assumption that pollen states could only evolve in one direction, namely from bi- to tricellular, and that tricellularity was a "dead end."
"Furthermore, our study showed that despite the recurrent evolution of tricellular pollen, those lineages with tricellular pollen actually had slower evolutionary rates," adds Williams. "Tricellular lineages had both reduced net speciation rates (speciation minus extinction) and reduced rates of reverting to the bicellular state."
In other words, even though tricellular species are formed often, suggesting an advantage to this dispersal state, tricellular lineages evolve slowly. And the net effect is that bicellular species are more common than tricellular species.
The authors speculate further that ecology plays an important role in these findings.
"Tricellular pollen develops rapidly after pollination, and so it would be favored in many of the unique lifestyles of angiosperms that demand rapid reproduction, such as herbs, annuals, and herbaceous aquatics," Williams notes.
"But acquiring those kinds of habits has consequences. The pattern of tricellular lineages rarely re-evolving the bicellular state suggests a reduced ability to respond to changing pollen dispersal conditions over evolutionary time, which in turn has slowed their rate of diversification."
One of the ideas that Williams is interested in continuing
|Contact: Richard Hund|
American Journal of Botany