Schriften zu Genetischen Ressourcen - Genres

Unconscious selection in plants under domestication
vegetatively propagated counterparts. Sexually reproducing plants are automatically
selected, each generation, for their fertility. Under such a reproductive system, mutations
and/or chromosomal changes affecting fertility will be promptly weeded out. In
other words, in seed planted crops, stabilising selection maintains fertility. As a rule,
cultivars will be fully pollen fertile and seed fertile. In addition, their chromosomes will
pair normally in meiosis. In contrast, shifting to vegetative propagation brings about
drastic relaxation of the stabilising selection that under sexual reproduction keeps
fertility rigorously intact. Under vegetative propagation, sterile genotypes will be able
to maintain themselves as clones. Under such conditions, one would expect to find
numerous cases of semi-sterile and sterile cultivars, as well as intra-crop chromosomal
polymorphism such as several ploidy levels, including triploids, pentaploids,
aneuploidy and other meiotically unbalanced chromosomal situations.
The choice of the plant's part
Different crops are grown for different parts of the plant's body. Some cultivated
plants are raised for their vegetative parts (roots, corms, leaves, stems, etc.). In others,
the reproductive parts (flowers, fruits, seeds) constitute the agricultural products.
In addition, the choice of the part (or parts) used, leads automatically to the operation
of contrasting selection pressures, particularly in traits concerned with the reproductive
biology of the crops.
As already noted, when crops are grown for their seed (or at least when crops are
seed planted) they stay under constant stabilising selection that safeguards their
seed fertility. Grain crops have the most rigid protection of this kind. The evolutionary
fitness in these plants depends decisively on normal development of their flowers
and fruits, on full fertility, and on normal chromosome pairing in meiosis. Deviants are
promptly weeded out, and the reproductive system is kept in balance. It is no wonder
that among cultivated plants, grain crops are the most conservative in this regard.
They are, as a rule, fully fertile. They are also characterised by balanced chromosome
systems, and they show very little chromosome divergence under domestication.
With few exceptions (such as addition hexaploidy in bread wheat), the chromosome
sets in grain crops are identical to those found in their wild progenitors. In seed
planted vegetables grown for their vegetative parts, flowers or fruits, stabilising selection
to maintain pollen and seed fertility might be somewhat laxer; yet the very
maintenance by planting of seeds keeps fertility in these crops intact.
Considerable reduction in pollen and seed fertility (as well as in chromosome stability)
is tolerated in vegetatively propagated crops grown for their fruits. In this group
(the bulk of the fruit crops) the production of fleshy, tasty fruits is at premium; but not
the production of seeds. In fact, growers frequently prefer (and consciously select)
clones with seed-less fruits, or with reduced number of seeds in the fruits. Several
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