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Study Finds Polyploidy Relatively Common Plant Speciation Mechanism

NEW YORK (GenomeWeb News) – About four times as many vascular plant speciation events as previously believed involved an increase in polyploidy, according to an early online paper in the Proceedings of the National Academy of Sciences this week.

A group of researchers from Germany, the US, and Canada used a combination of cytogenetic and phylogenetic approaches to examine the frequency of polyploidy — the addition of one or more copies of the genome — in plants, specifically with respect to the appearance of new plant species. Their results indicate that some 15 percent of angiosperm (flowering plant) speciation events occurred in concert with a jump in ploidy. In ferns that number was even higher, at 31 percent.

Even so, the team's work did not find evidence to support the notion that plant genera that have arisen through polyploidy have greater species diversity than other groups.

"Although we detect no diversification advantage associated with genome duplication, the high frequency of polyploidy speciation in angiosperms, coupled with loss of chromosomes in polyploidy lines, helps to explain the initially surprising evidence that even flowering plant species with low chromosome numbers have a polyploidy ancestry," lead author Troy Wood and his colleagues wrote.

Wood, who was involved with the project as a graduate student at Indiana University, is now a research scientist at the University of Muenster in Germany.

Because it can lead to reproductive isolation as well as new morphology and characteristic traits, polyploidy is considered a promising potential mechanism for speciation, the authors explained.

But despite the fact that many flowering plants and ferns are related to ancient polyploid groups, previous studies have led researchers to believe that as few as three or four percent of plant species came about through more recent polyploidy-related speciation events.

"I felt that recent estimates of the polyploid speciation rate were too conservative because they did not take genealogical history into account," senior author Loren Rieseberg, an Indiana University evolutionary biologist, also affiliated with the University of British Columbia, said in a statement.

Based on this hunch, Rieseberg, who is reportedly writing a book about plant speciation, got his graduate student Wood to start pulling together information not only on plant phylogeny but also chromosome number.

After the researchers did chromosome counts using data on seed plants and ferns from the Index to Plant Chromosome Numbers database, they found that nearly 35 percent of all plants tested appeared to be polyploid compared with their species at the base of their genera.

And based on 1,813 angiosperm speciation events in 123 phylogenetic trees, the team estimated that about 15 percent of these events involved polyploidy. When they examined 204 fern speciation events in 20 phylogenetic trees, they determined that more than 31 percent were accompanied by ploidy increases.

Nevertheless, based on their subsequent analyses, the researchers say polyploidy does not seem to offer plants an advantage in terms of the species richness within a given genera. Rather, their results indicate that polyploidy could be an evolutionarily neutral event that happens to occur frequently.

"I really thought we would find evidence that polyploids have an advantage," Wood said in a statement. The idea that the large number of polyploid species and the attending high chromosome numbers might be simply due to a neutral process is intriguing."

The team also acknowledged several potential limitations in their analysis, noting, for instance, that the phylogenetic trees they examined tended to be incomplete. Combined with the possibility that some multiple speciation events could appear as just one event in their analysis, the researchers noted that their estimates for polyploidy in speciation are likely somewhat conservative.

To gain a more complete understanding of the process, Loren noted, it would help to examine a broader range of speciation events, including those that occurred long ago. "[O]ur diversification rate analyses only examined recent polyploids," he said in a statement. "A future area of research should be to ask whether more ancient polyploidy events have increased diversification rates."

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