Self-sterility, self-incompatibility and xenia: a review of the mechanisms of cross-pollination benefits in animal-pollinated crops.

BACKGROUND

While there are multiple mechanisms of self-incompatibility (SI), known to promote outbreeding in angiosperms, these are not well synthesized and described across major global crops. This can lead to misinterpretations of biological processes involved in crop pollination, fertilization and fertility, in particular by confusing them with an additional overlooked phenomenon causing self-sterility (SS), early-acting inbreeding depression (EID). Another overlooked mechanism, called xenia, results in increased quality of seeds and fruits through cross-pollination even in self-compatible and self-fertile crops.

SCOPE

The aim of this review was to describe and synthesize all the known mechanisms of SI and SS encountered in animal-pollinated (zoophilous) crops, and additional mechanisms by which cross-pollination can be beneficial for crop production. All the known zoophilous crops presenting SS, SI or xenia were quantified and described.

KEY RESULTS

One hundred and thirty-four zoophilous crops were found to be self-sterile, including 52 displaying complete SS and 82 displaying partial SS. We identified all the known mechanisms of SI and SS in these crops, including gametophytic SI, sporophytic SI, heteromorphic SI, late-acting SI and EID. In addition, 58 zoophilous crops were found to display xenia, including 22 that are self-compatible and completely self-fertile. In total, 156 zoophilous crops were identified as benefitting from cross-pollination for the quantity and quality of seed and fruit production.

CONCLUSIONS

While previous reviews focused on quantifying the benefit of animal pollinators for crop production, they did not synthesize the mechanisms underlying pollinator dependence for such crops. Our review provides valuable knowledge about crop pollination requirements in general and more particularly the benefits of cross-pollination across crops ranging in self-fertility. This information could help growers make suitable management decisions regarding their field and orchard planting designs, specifically by mixing mutually suitable cultivars in crops displaying SS or SI, or benefiting from xenia.