Dioecy is associated with higher diversification rates in flowering plants

Area of Science:

• Evolutionary Biology
• Plant Systematics
• Phylogenetics

Background:

• Dioecious angiosperm clades often exhibit lower species richness than related nondioecious clades.
• This pattern has led to the hypothesis that dioecy is an evolutionary dead end, limiting diversification.
• Previous null hypothesis assumptions in sister clade tests were not suitable for derived traits like dioecy.

Purpose of the Study:

• To re-evaluate the diversification rates of derived dioecious angiosperm clades using a novel sister clade test.
• To determine if dioecy is associated with reduced or increased species diversification.
• To investigate the influence of associated traits on diversification rates in dioecious clades.

Main Methods:

• Compiled a new dataset of 115 sister clade pairs from over 600 dioecious angiosperm genera where dioecy is likely derived.
• Applied a recently developed sister clade comparison test that accounts for derived character states.
• Analyzed the relationship between dioecy and diversification rates, considering traits like growth form and pollination syndrome.

Main Results:

• Contrary to the 'evolutionary dead end' hypothesis, derived dioecy is associated with significantly increased diversification rates.
• Associated traits such as arborescent growth, abiotic pollination, fleshy fruits, or tropical distribution did not influence diversification rates.
• Observed species richness patterns are better explained by a low transition rate to dioecy and frequent losses, not low diversification.

Conclusions:

• Dioecy is not an evolutionary dead end in angiosperms; it is linked to enhanced diversification.
• The apparent lower species richness in some dioecious clades is likely due to infrequent evolutionary transitions to dioecy and subsequent losses.
• The study provides robust evidence supporting increased diversification in derived dioecious angiosperm lineages.