Few studies quantify evolutionary processes in populations of domesticated plants in traditional farming systems. In February's Ecology Letters, Pujol, David and McKey show that these systems offer unusual opportunities for studying microevolution. Cassava (Manihot esculenta) is clonally propagated, but Amerindian cassava farmers also regularly incorporate volunteer plants from sexually produced seeds into their clonal stocks (cuttings) at harvest time.
These new genotypes renew diversity lost under clonal propagation. However, whereas multiplied clones are highly heterozygous, many of the volunteer plants are inbred. How does high heterozygosity persist despite their incorporation? The authors demonstrate a novel case of selection for heterozygosity that explains this paradox, showing that humans inadvertently favour heterozygous volunteers.
When farmers weeded fields, they killed small volunteers, but retained large ones, which were also the most heterozygous. Demonstrating heterosis in nature usually requires large samples, but novel features of this system allowed escape of this constraint.
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