Schriften zu Genetischen Ressourcen - Genres

D. ZOHARY
genitors by complex syndromes of both morphological and physiological traits. Numerous
adaptations vital to survival under wild conditions broke down under cultivation;
and many domestic ones have evolved.
In comparison, vegetatively propagated crops have had an entirely different history of
selection. "Cultivars" in these crops are not true races but just clonal replications of
very rare "exceptional individuals" with superior fruit or tuber qualities that are - as a
rule - highly heterozygous. They were originally picked up by the cultivator from variable,
panmictic, wild populations; and later also from among segregating progeny of
spontaneous and/or man-made crosses between cultivated × cultivated clones, or
cultivated × wild individuals. In the hands of the grower, vegetable propagation has
been a powerful device to evade genetic segregation, and to “fix” desired types. By
discarding sexual reproduction and inventing clonal propagation the farmer was able
to (i) select rare individuals with desired traits from among a large numbers of variable,
inferior plants, and (ii) duplicate (clone) such superior types to obtain a steady
supply of genetically identical saplings. In the case of fruit trees and corm and tuber
crops, this is no small achievement. Because these vegetatively propagated crops
are mostly cross-pollinated and widely heterozygous, most individuals obtained from
planting seeds of attractive parents (even progeny derived from commercial cultivars
used today) are economically worthless. Thus, the change from seed planting to
vegetative propagation has been the practical solution to assure a dependable supply
of desired genotypes. As stressed by ZOHARY and SPIEGEL-ROY (1975), in most
fruit trees this invention made cultivation possible. This is apparently true also in numerous
corms and tuber crops.
In terms of selection, domestication of vegetatively propagated crops is largely a single
step operation. With the exception of somatic mutations, selection is completed
the moment the clone is picked up. In traditional agriculture, the turnover of clones
was apparently slow, particularly in the perennial fruit trees. Appreciated genotypes
were maintained for long periods of time. Thus, in sharp contrast to sexually reproducing
cultivated plants, vegetatively propagated crops underwent (under domestication)
only few recombination-and-selection cycles. Consequently, as far as basic adaptations
they remained closer to their wild progenitors. Thus, wheat or pea (which
originally were adapted to Mediterranean climatic conditions) evolved cultivars fitting
a wide range of climatic regimes – from Scandinavia to Ethiopia. In contrast, cultivated
apple still needs winter chilling, the date palm is able to fruit only in hot and
very dry environments, and the cultivated olive remained a characteristic Mediterranean
plant, not very different from its wild ancestor. In fact, so strict are the climatic
requirements of the olive that climatologists use it as a reliable indicator for Mediterranean
conditions.
Pollen and seed fertility (including chromosome stability in meiosis) are additional
traits in which one finds wide differences between seed planted crops and their
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