Are males and females produced in equal proportions in the sexually cryptic coco de mer?

Around 6 % of all flowering plants are dioecious, meaning that each individual is functionally either male or female. Dioecy occurs in many families of flowering plants, suggesting that it likely evolved independently many times. Genetic mechanisms for dioecy vary widely, with the most common being the XY system as found in mammals — in which an X chromosome, conferring recessive femaleness, pairs with a Y chromosome bearing the dominant genes associated with maleness. Other genetic mechanisms associated with dioecy are the ZW system (also found in birds) and multiple sex chromosome systems (like that found in Drosophila). Many long-lived dioecious plants are economically or ecologically important, including the endangered coco de mer palm (Lodoicea maldivica), the largest seeded plant in the world (seeds can often weigh in excess of 8.5 kg). Previous studies have shown that adult populations of Lodoicea often exhibit biased sex ratios, but it is unclear whether this bias is due to unequal numbers of males and females being ‘born’ or to differential rates of subsequent mortality.

A recent Editor’s Choice study in AoBP by Morgan et al. explores sex ratios in populations of L. maldivica in the Seychelles. Males and females of this species are visually indistinguishable when immature, only becoming sexually dimorphic as adults, so the authors developed male-specific sex-linked markers using ddRAD sequencing. This enabled them to determine the gender of immature individuals and explore how sex ratios change with both environmental conditions and age. They concluded that dioecy in Lodoicea is genetically determined, and probably involves X/Y sex chromosomes. The two sexes are produced in equal numbers and biased adult sex ratios are the result of differential mortality of mature plants, though neither male or female dominance was found consistently across the populations. The reason for the localised sex discrepancies found in adult populations is almost certainly human interference, with female plants either felled for their reputedly superior wood or preserved for the valuable nuts they produce. The authors state that this result bodes well for the future of these populations and for maintaining genetic diversity in the species, because future generations of the species will have a balanced sex ratio. They also suggest that Lodoicea could be grown commercially, which as well as increasing revenue might reduce poaching of nuts from protected areas, further conserving natural populations.

Researcher highlight

Emma Morgan grew up in Wales and in 2013 moved to Switzerland to conduct a PhD at ETH Zürich, focusing on the conservation genetics and reproductive biology of the coco de mer palm in the Seychelles. Emma currently holds a postdoctoral research position with Dr Filip Kolář at the Department of Biology at Charles University in Prague.

Emma is interested in reproductive ecology, and in using population genetics to guide and inform the management of endangered plant species in the face of human disturbance and habitat fragmentation. Emma’s new research focusses on the ecological and evolutionary consequences of polyploidisation in the plant species Arabidopsis arenosa. She combines genome resequencing, ecological field surveys and crossing experiments to identify forces promoting or inhibiting gene flow between diploids and tetraploids at ploidy contact zones.