# Self-Incompatibility in Devil’s Potato (Echites umbellatus Jacq., Apocynaceae) May Explain Why Few Flowers Set Fruit

## TL;DR

This study finds that Devil’s Potato plants mostly need pollen from other plants to produce fruit, which explains their low fruit production in nature.

## Contribution

The study identifies self-incompatibility as a key factor in low fruit set and shows variation in self-compatibility across habitat types.

## Key findings

- Cross-pollinations between unrelated plants resulted in the highest fruit set (59%).
- Self-pollinated flowers had the lowest fruit set (9%) and fewer viable seeds.
- Self-compatible individuals were more common in smaller habitat fragments.

## Abstract

Echites umbellatus is a plant with tubular white flowers that are pollinated by hawkmoths. Few fruits are produced in nature, so we investigated the breeding system of E. umbellatus by growing plants from different populations in a greenhouse and hand-pollinating flowers with self-pollen, pollen from siblings (plants grown from seeds in the same fruit), and pollen from plants in other populations. Pollinations between unrelated plants were most successful, and we conclude that this species is mostly self-incompatible, though most populations had a few individuals that were somewhat self-compatible. There were more self-compatible individuals in smaller habitat fragments, perhaps a function of limited mating opportunities. We conclude that self-incompatibility in this species contributes to its limited fruit set, though other factors such as low pollinator activity may also be important.

Pollinators are needed for the reproduction of Echites umbellatus, and only sphingid moths have mouthparts long enough to reach the nectar at the bottom of the species’ long, twisted floral tube. Though plants produce many flowers over a period of several months, one observes very few fruits in nature. We asked: (1) Are plants self-compatible, or do they need pollen from another individual to set fruit and seed? (2) Are cross-pollinations between unrelated individuals more successful than crosses with relatives? (3) How does the relatedness of pollen and ovule parent plants affect fruit set, seed number, and seed quality? We investigated the breeding system of E. umbellatus by collecting fruits from seven sites, growing plants and performing hand pollinations over a period of several years, collecting and measuring fruits and counting seeds. Echites umbellatus is self-incompatible, though some individuals produce fruit by self-pollination. Cross-pollinations between unrelated individuals set the most fruit (59%), and those that were self-pollinated set the least (9%). Fruit set from cross-pollinations between related individuals was intermediate (32%). Although the number of seeds per fruit did not differ significantly among pollination treatments, fruits from self-pollinations had substantially fewer viable seeds than outcrossed fruits, with fruits from sibling crosses being intermediate. There were higher levels of self-compatibility in the fragment populations compared with plants from intact habitats. Self-incompatibility may explain why fruit set is low in this plant species; future investigation into the breakdown of self-incompatibility in smaller populations is warranted.