Adaptations to herbivory in alfalfa (Medicago sativa)

Abstract

Alfalfa (Medicago sativa L.) possesses numerous adaptations to herbivores. As with other wild plants, general chemical defences (notably medicagenic acid in alfalfa) and general anti-insect structures (notably trichomes) provide some protection against a wide spectrum of polyphagous insects. Several morphological adaptations protect alfalfa against specialized insect feeders, especially those that consume the seeds. Alfalfa has been disruptively selected by man to possess morphological adaptations and tolerance for two contrasting classes of herbivory: continuous grazing by livestock and intermittent harvesting for hay. Domestication of alfalfa has lowered the effectiveness of several natural adaptations against insects. Occasionally, however, artificial selection for resistance against certain insect species has, often unintentionally, resulted in selection for structural features discouraging these insects. Preadaptations of wild alfalfa for the horse facilitated this plant's domestication as a specialized horse feed, despite the fact that horses are poorly adapted in the wild to such a nutritious forage. Under conditions of domestication, however, alfalfa appears to offer horses several invaluable dietary advantages, and is much more suited to the horse than to man's other grazing herbivores. Anti-herbivorous hemolytic saponins are present in high concentrations in certain alfalfa lineages and cultivars. Horses in southern Europe and Asia have been fed low-saponin alfalfa for millenia, whereas high-saponin alfalfa has been utilized in northern Europe and much of the New World only since about the 16th century. While these compounds are poisonous for most herbivores, they could be beneficial for the horse. A very high degree of herbivorous coadaptation exists between alfalfa and its specialized pollinator, the alfalfa leafcutter bee (Megachile rotundata), and there is evidence that this relationship has coevolved. As the world's most efficient protein crop, alfalfa is preeminently adapted to mankind's future need for increased direct consumption of plant proteins. Keywords: adaptation, alfalfa, fodder, forage, herbivory, Medicago sativa.