Schriften zu Genetischen Ressourcen - Genres

Diversity of African vegetable Solanum Species
process, involving the production of mutations, resulting in variant phenotypes, followed
by natural selection of the fittest. This process is usually slow, and may take
millions of years for the production of new species. In contrast, all the diversity of our
domesticated crops has been developed within the past 10,000 years or so. Of
course this rapid development has been favoured by the rapid life cycles of annual
crops, by the abundance of individuals in these crops, by the ease of gene exchange
in monoculture crops, by the founder effect and genetic drift in small scale agriculture,
by the reduction of the constraints of natural selection when these crops are
cultivated and thus the survival of relatively unfit novelties, and by the artificial selection
by humans, whether conscious or unconscious (HAWKES 1983). Most of these
points apply also to somatic mutations in vegetatively propagated crops such as potatoes
and yams. However, underlying all of this may also be the ease of production
of novelties by loss of gene function or regulation as emphasised in this paper, as
well as the fact that a few mutations can generate the enormous changes of a domestication
syndrome (KOINANGE et al. 1996). Because the abundant phenetic novelties
in domesticated plants produce different patterns of variation than in wild species,
taxonomists have recently produced the International Code of Nomenclature of
Cultivated Plants in addition to the long established International Code of Botanical
Nomenclature. That domesticated plants display much greater diversity in phenetic
traits than in molecular markers, especially amongst advanced cultivars has been
shown clearly for Capsicum using 41 morphological traits, 544 RAPD markers, and
378 AFLP markers (LEFEBVRE et al. 2001, Fig. 2b). This also means that molecular
markers have less resolving power than do phenetic traits for distinguishing cultivars,
and therefore should only be used with great caution when selecting representative
core collections in genebanks. However, for comparing genome evolution amongst
distantly related taxa (e.g., eggplant, pepper, tomato, potato) these molecular tools
are very powerful (DOGANLAR et al. 2002a, b).
A further twist on our path may bring us briefly back to Mansfeld, and especially to
Vavilov. The Law of Homologous Series of VAVILOV (1951) was based on the observation
that similar morphological traits were developed in unrelated crop species
within particular geographical areas, such as “non-ligulate rye as well as wheat in
Bokhara and naked-grained forms of barley, oats and millet in China” (HAWKES
1983). This suggests that the same new mutations have occurred in these different
species, which would be surprising. If however these new morphological traits were
the results of incapacitation of orthologous genes in the different species this is less
surprising, especially now that we know the high degree of synteny in very different
cereals (PATERSON et al. 1995).
Another observation by VAVILOV (1926, in HAWKES 1983), was that there is a higher
incidence of ‘dominant’ genes at the centre of origin of a crop, and that more genes
for disease resistance may be found there. This now seems obvious, that as a crop is
domesticated, it loses the functions of various genes (including those for disease re-
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