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Unconscious selection drove seed enlargement in vegetable crops
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.
Comparisons of seed mass between landraces and wild accessions of (A)
cereals (annual grass crops), (B) pulses (grain legumes), and (C)
vegetables. The seed mass in domesticated crop plants is expressed as a
multiple of that in wild plants (i.e., a value of two indicates a
twofold increase in seed mass under domestication). Points represent
mean ± 95% confidence interval and the red line denotes a value of 1.0
(i.e., no effect of domestication).
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