Tropical islands lead to more diversity in Palm sexual reproduction

Palm trees on tropical islands are associated with exotic holidays, but some people, cunning botanists, have made a career out of studying these plants. Palms do not only grow in alluring locations, but they also present many exciting questions about plant evolution and adaptation. A team of Brazilian and Swedish scientists, led by Dr Cibele Cássia-Silva at the Federal University of Goiás, has been looking at how reproduction for palm trees impacts their evolution. They closely studied the palm trees’ flowers. 

Flowers might look simple and innocent, but they are more complex than they let on. Almost three-quarters of all flowering plants (angiosperms) have bisexual flowers (hermaphrodite). A classic example is the textbook illustration of a flower with the stigma (female) in the middle, surrounded by anthers (male). Then some plants have unisexual (female or male) flowers on the same plant (monoecy), such as the birch, spruce or corn. The third group is characterised by having female and male flowers completely separated and produced on individual plants (dioecy). These plants make up about 6% of all angiosperms. Each of these sexual reproduction systems has advantages and disadvantages. 

Palms can be diecious, monoecious, hermaphrodite. Some species even have a mixture of bisexual and unisexual flowers on the same plant (polygamy). Next, palms have to overcome the challenge of travelling long distances, mainly with the help of frugivore animals’ help; hence, most palm species have fleshy fruit. 

Cássia-Silva and colleagues decided to explore the connection between fruit size, sexual reproduction system and distribution of over 2,500 palm species worldwide. Whilst fruit size did not vary between the four reproduction systems, there were relatively more dioecious, closely-related palms on islands than on the mainlands. Based on evolutionary models, the scientists suggest that ancient palms were hermaphrodite, transitioned to dioecy, followed by polygamy and monoecy. Surprisingly, none of these sexual systems appears to constrain their evolution, but rather the diversity of their reproduction could have fueled their evolution and species richness. 

In 2019, Cássia-Silva and colleagues compiled all palm distribution based on Kew’s World Checklist of Selected Plant Families (WCSP), fruit size based on Cássia-Silva’s previous research, known sexual system (hermaphrodite, dioecy, monoecy and polygamy), and molecular evolution based on an earlier study. The scientists grouped the palms into “island species”, “mainland species”, and “island and mainland species” based upon their distribution. 

The ancestral sexual system was estimated using evolutionary modelling. The botanists estimated the diversification rates for different traits (distribution, sexual system) to investigate if either combination (e.g. dioecy on islands) restricts the speciation of certain palm species. 

As previously mentioned, dioecious species have total separation between female and male plants. This difference means that only half of a population is seed-bearing. So next, the researchers tested whether dioecious have larger fruits to compensate for lower reproduction rates (e.g., more fleshiness might be more attractive to animals). 

Cássia-Silva and colleagues estimated that hermaphroditism is the most likely ancestral state of palms. Only polygamy (having bisexual and unisexual flowers on an individual plant) had lower speciation and diversification rates than the other three sexual systems. Palm fruit size did not vary between sexual systems nor between mainland and island species. Dioecious palms were relatively more frequent on islands (36%) than on mainlands (27%). 

“[C]ontrary to the classic view of dioecy as an irreversible evolutionary trait (i.e. an evolutionary dead-end), our results support the suggestion that dioecy is a labile trait across angiosperms,” Cássia-Silva and colleagues wrote. 

Some dioecious palms “exhibited unconstrained evolution towards other sexual systems (7.63% towards polygamy; 5.91% hermaphrodite and 0.76% towards monoecy)”, so it was clearly not an evolutionary dead-end. 

Interestingly, dioecious and monoecious species were more frequent on islands compared to hermaphrodites on mainlands. This finding goes against an old hypothesis (Baker’s law) that says that hermaphrodites should be the most successful island coloniser species. 

Whilst the researchers could not identify what traits or factors explain the evolutionary success of highly isolated palm species, they did find an “island effect” (there were more frequent and more closely related) on the evolution of dioecy in palms. 

But overall, this study has disproved two classic evolutionary views and shown that having multiple sexual systems have evolutionary advantages. Next time you dream of chilling on a tropical island surrounded by palms – take a minute to ponder how incredible evolution and plants are.