Schriften zu Genetischen Ressourcen - Genres

Diversity of African vegetable Solanum Species
disintegrates into a juice, and an abscission layer develops between the fruit and the
stalk. It is well adapted to consumption by birds and is effectively dispersed by them
(LESTER et al. 1986). In contrast, the highly domesticated derivative, Solanum
aethiopicum Kumba Group, has an ovary with many outer locules and also several
inner locules. After pollination it grows irregularly to about 5-10 cm diameter, the colour
turns slowly to scarlet, but the fruit stays strongly attached to the stalk and the
flesh stays firm even after many months of storage. Some of these and several other
changes during domestication of S. aethiopicum, notably loss of prickles and loss of
stellate hairs, have been proved more or less conclusively to be due to single or multiple
recessive genes, the F1 hybrids resembling much more closely the wild parent
than the domesticated one. The F2 progeny shows Mendelian segregation for the
dominant wild type genes for simply inherited characters, or a skew distribution towards
the wild type for some polygenic characters (LESTER and THITAI 1989).” It is
particularly interesting that in this case the traits of domestication are mostly due to
genes that are recessive to the dominant genes of the wild species. This is also true
for most other cases that have been studied, such as pearl millet, barley, musk
melon and many others (SCHWANITZ 1967, LESTER 1989, PONCET et al. 2000).
But what is a recessive gene? The classic definition of a recessive gene is that it is
one that expresses itself only in the absence of the dominant gene, and a classic example
is that of Mendel’s wrinkled peas, where this character is only expressed in the
homozygous recessive (rr), but not if the dominant round (R) gene is present. The
difference between the wrinkled and round morphological characters is very clear to
see. However, it has been found more recently that the basis of the wrinkled phenotype
is the lack of a starch-branching enzyme, due to the non-function of the appropriate
gene, due to the insertion of some extra DNA that appears like a transposable
element (LESTER 1989). Thus in this case, and in many others where investigated,
the so-called ‘recessive’ gene is in fact an incapacitated normal gene, incapable of
expressing itself at all, and therefore is a null allele in terms of biochemical genetics.
This loss of gene function is easily appreciated as the basis of many traits of domesticated
plants, such as loss of articulation in the infructescence of cereals and other
plants and hence the loss of the natural dispersal mechanisms, which thus enables
humans to harvest the crop well. Another example is the loss of seed dormancy
mechanisms, which result in all the seeds germinating together after they have been
sown. These and many other traits of domestication syndromes (HAMMER 1984, in
KOINANGE et al. 1996) can easily be explained in the same way, for example the loss
of protective mechanisms such as prickles, or the loss of secondary metabolites that
inhibit or disable herbivores, or even the loss of pigmentation and patterns in flowers
and seeds. The gain of pigmentation in domesticates, such as purple potato tubers or
eggplant fruits may also be due to altered or lost gene regulation, so that the synthesis
of anthocyanins, which in the wild ancestors was restricted to the corolla, now
occurs also in the tubers or fruits. Apparently new colours such as a change from red
to yellow, are often due to lack of production of a dark pigment thus revealing a paler
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