Schriften zu Genetischen Ressourcen - Genres

R.N. LESTER and M.-C. DAUNAY
pigment which previously was not obvious although it was already there, as can be
proved by chromatography. Truly novel pigments may result from the interactions of
biochemical pathways previously restricted to different tissues or organs but now reacting
together in the same tissue. Loss of genetic control may also be significant in
the pre-adaptation of ecological weeds. Many wild species have rigid genetic control
appropriate to plants in strict natural habitats: they show narrow plasticity and cannot
respond to favourable conditions by greater growth. However, many weedy species
seem to have less rigid genetic control, allowing them to be very plastic and adaptable
so they can take advantage of favourable conditions in disturbed and unpredictable
habitats, especially cultivated fields. This pre-adaptation has allowed some
weeds to become the progenitors of all our annual crop plants: tomato is a prime example
(HAWKES 1983).
However, a major phenomenon of domestication remains to be explained and that is
‘gigantism’ or ‘Gigaswuchs’ (SCHWANITZ 1967), where particular parts of the plant,
which we use for food or ornament or other purposes, grow much bigger than in the
wild species, and often in an irregular way. This is obvious in the change from the
small spherical berry of S. anguivi to the monstrous fruits of S. aethiopicum Kumba
Group. Although of course plant breeders may think that they are breeding for bigger
and better crops (which they are), we can realise that these changes can also be due
to loss of gene function, particularly in regulator genes. Thus in wild S. anguivi, after
pollination and fertilisation, plant hormones are produced and the ovary grows. However,
at the appropriate time hormone production is turned off and this growth is
stopped, resulting in rapid production of a neat, small, red, juicy, easily detached
berry exactly 1 cm in diameter, precisely the right size, colour and composition for the
birds that eat these fruits and disperse their seeds in the natural environment. On the
other hand it seems that in S. aethiopicum Kumba Group, hormone production is not
turned off, and growth is not stopped but continues and thus produces the monstrous
fruits. Similar explanations, involving loss of gene function and/or regulation probably
apply to other cases of gigantism in domesticates (LESTER 1989). This also has implications
for our understanding of QTLs (quantitative trait loci): if increased size results
from loss of gene function or regulation, then each QTL we seek is the absence
rather than the presence of a factor. Thus gigantism and various other characteristics
of domestication might be accounted for by loss of genetic control mechanisms,
which could explain in part the phenomenally rapid evolution of crop plants over a
mere few thousand years (LESTER and THITAI 1989). (The genetics of disease resistance
is much more complicated and we do not attempt to explain that here.)
The discussion above, perhaps a new paradigm (i.e., a different conceptual framework),
can help us further to explain the spectacularly rapid domestication of crop
plants, and to consider whether they should be classified in the same way as wild
species, and even whether molecular markers are unlikely to help with such classification.
It is generally appreciated that the evolution of wild species is a very slow
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