New research published on the coco de mer reveals equal proportions of females and males in early life stages |07 April 2020
A new research paper published by scientists from ETH Zürich, in partnership with the Seychelles Islands Foundation (SIF), has uncovered some of the mysteries surrounding sex determination in coco de mer trees.
The coco de mer palm, endemic to the Seychelles, is dioecious, meaning each individual plant is either male or female. In most wild populations the adult sex ratio (i.e. the proportion of females to males) is skewed – in palm forested areas of Fond Ferdinand and Praslin National Park there are significantly more females than males, whereas in the Vallée de Mai there are more males. The coco de mer can take up to 50 years to reach sexual maturity and start producing flowers, and until then it is impossible to visually distinguish the sexes.
For this reason, we have not known whether the skewed sex ratios in adults are due to different proportions of males and females germinating or to differing mortality rates in later age.
To investigate this, the researchers developed a new genomic method that allowed them to determine the gender of immature plants – something which had not been possible before. The authors screened almost 600 immature plants on Praslin and Curieuse islands and discovered that the two sexes germinated in roughly equal numbers.
This exciting new research shows that sex determination in coco de mer is genetic, and not environmentally determined. There are several plausible reasons that could explain the biased sex ratios in adult populations – the first being human interference. None of the populations in which coco de mer grows are in an entirely natural condition, and unprotected sites may have been subjected to higher levels of selective felling of one sex over the other: either felling of females for their reputedly superior wood, or preserving females for the valuable nuts they produce.
Another explanation for the earlier death of females could be the natural pattern of regeneration in the forest. Because the huge fruits cannot disperse very far, young plants usually grow in dense clusters around the mother trees, and eventually end up competing with her for light and nutrient resources, putting her at a disadvantage.
These results have important implications for the management of the endangered coco de mer. Because they develop slowly, the majority of plants in most populations are immature individuals whose sex could not be previously determined. These immature cohorts have balanced sex ratios, which bodes well for the future of these populations and for maintaining genetic diversity.
Source: SIF February newsletter