Palms, which comprise the monocot family Arecaceae, are vitally important globally due to their high vegetable oil production, which can occur in both fruits and seeds. The full range of lipid storage and composition, however, isn’t well understood across this large and diverse group, which includes approximately 2600 species in 181 genera and 28 tribes. The top two economically important palms, the oil palm and the coconut palm, both belong to the same tribe, Cocoseae, as do several others used regionally for seed oil production. This raises the question of whether oil-rich seeds occur only in this tribe and what lipid traits are present in fruits and seeds elsewhere in the family. It is also unknown whether unsaturated fatty acids (FAs), which have a lower melting point and are produced in higher proportions in the seeds of some palm species, have an adaptive advantage in aiding germination at lower temperatures.
A recent paper published by Chloé Guerin and colleagues in Annals of Botany seeks to better understand fatty acid accumulation and composition in the fruits and seeds of the Arecaceae. The authors analyzed the lipid composition of the seeds of 174 species (101 genera in 26 tribes) and the fruits of 144 (85 genera in 25 tribes). They examined this data in relation to the systematics of the family, considering how lipids varied within and among tribes. The lipid data was also compared to latitude and altitude data in order to determine whether there is any correlation between unsaturated FAs and species that occur in colder environments.
The authors found significant variation in lipid traits across the palm family. While species with very large amounts of oil accumulation in their seeds did all belong to the Cocoseae, those with oil-rich fruits were found in two-thirds of the tribes studied. “Since oil is the most energy-rich form of seed reserve,” the authors write, “increased oil storage may represent a significant adaptive advantage for Cocoseae seed germination and seedling establishment.” There appeared to be no relationship between oil storage in the fruit and the seed, and the FA composition of fruits was highly variable within tribes, while that of seeds was not. Medium-chain FAs were the predominant type in seeds for most species, but also appeared in some fruits. The analyses found a significant correlation between the proportion of unsaturated FAs in the seed and temperature of the coldest environments in which certain species occurred, suggesting a role in aiding seed germination.
The overall picture of lipid storage in palm seeds is one of genetic drift during speciation, rather than adaptive constraints. This makes lipid traits in palm seeds potentially useful for chemotaxonomic purposes. However, while seed lipid traits appear to have been the result of random evolutionary processes, the unsaturated FAs found to be correlated with the coldest parts of the palms’ range may have resulted in a selective advantage. As the authors write, “the adaptive advantage of accumulating unsaturated FAs in the southernmost and northernmost regions occupied by palms and in tropical montane ecosystems might have been of crucial importance for palm species that colonized these areas.” The study also identified several previously uncharacterized species of palm that may represent novel sources of vegetable oil.