Unconscious selection drove seed enlargement in vegetable crops

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Abstract

Domesticated grain crops evolved from wild plants under human cultivation, losing natural dispersal mechanisms to become dependent upon humans, and showing changes in a suite of other traits, including increasing seed size. There is tendency for seed enlargement during domestication to be viewed as the result of deliberate selection for large seeds by early farmers. However, like some other domestication traits, large seeds may have evolved through natural selection from the activities of people as they gathered plants from the wild, or brought them into cultivation in anthropogenic settings. Alternatively, larger seeds could have arisen via pleiotropic effects or genetic linkage, without foresight from early farmers, and driven by selection that acted on other organs or favored larger plants. We have separated these unconscious selection effects on seed enlargement from those of deliberate selection, by comparing the wild and domesticated forms of vegetable crops. Vegetables are propagated by planting seeds, cuttings, or tubers, but harvested for their edible leaves, stems, or roots, so that seed size is not a direct determinant of yield. We find that landrace varieties of seven vegetable crops have seeds that are 20% to 2.5-times larger than those of their closest wild relatives. These domestication effect sizes fall completely within the equivalent range of 14% to 15.2-times for grain crops, although domestication had a significantly larger overall effect in grain than vegetable crops. Seed enlargement in vegetable crops that are propagated vegetatively must arise from natural selection for larger seeds on the occasions when plants recruit from seed and are integrated into the crop gene pool, or via a genetic link to selection for larger plants or organs. If similar mechanisms operate across all species, then unconscious selection during domestication could have exerted stronger effects on the seed size of our staple crops than previously realized.

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