Schriften zu Genetischen Ressourcen - Genres

D. ZOHARY
solutions how to curtail or prevent seed set without harming fruit development
evolved in horticulture. Under traditional horticulture, conscious preference for seedless
types leads rather frequently to establishment of mutations conferring drastic
decrease in seed set, or rare seed set due to triploidy (e.g., in some pear cultivars),
as well as the incorporation of mutations inducing parthenocarpy (e.g., in bananas or
non-Smyrna-type cultivars of fig).
Crops maintained by vegetative propagation and grown for their vegetative parts exhibit
the most drastic disruption of their reproductive systems; and the most bizarre
chromosomal situations among cultivated plants. Because of their mode of propagation,
the pressures exerted on such crops to increase vegetative output are rarely
counterbalanced by normalising selection to retain sexual reproductive ability. Tropical
root and tuber crops provide us with good examples for this mode of evolution.
Cultivated clones of cassava, yams, sweet potato and taro often show drastic irregularities
in flowering. In some cultivars flowering ceases altogether, or almost altogether.
When flowers do appear they are frequently semi-sterile or sterile. Also
chromosomally, some of these crops are exceptionally polymorphic, and contain
clones showing different levels of polyploidy; including 3x, 5x or even higher meiotically
unbalanced chromosome complements. Thus in the yams Dioscorea alata is
known to contain 2x, 3x, 4x, 5x, 6x, 7x and 8x clones; and D. esculenta 3x, 4x, 6x, 9x
and 10x clones (HAHN 1995). Sugarcanes confront us with even more complex
chromosome variation. Cultivated clones in this crop are all highly polyploid and frequently
aneuploid. Their chromosome numbers range from 2n = 80 to 2n = 125
(ROACH 1995). Another feature of sugarcanes (as well as of several other vegetatively
propagated crops grown for their vegetative parts) is the rather common origin
of cultivars by distant inter-specific hybridisation. Since they do not have to pass
through the sieve of sexual reproduction, such largely sterile and/or chromosomally
unbalanced hybrids are effectively maintained in cultivation.
The impact of sowing and reaping
Traditional grain agriculture depends on the practice of sowing the crop in the tilled
field, reaping the reproductive parts soon after seed maturation, and threshing out
the grains. Sowing and reaping automatically initiate selection towards the following
changes in plants grown for their grains, setting them apart from their wild progenitors.
Loss of the wild-type seed dispersal devices: This is perhaps the most conspicuous
difference that separates grain crops from their wild relatives. It is also one of the
most intensively studied changes under domestication (DARLINGTON 1963, ZOHARY
1969, HARLAN et al. 1973, HEISER 1988, HILLMAN and DAVIES 1990). Seed dispersal is
a most vital adaptation in plants under wild conditions; and in each wild progenitor,
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