Traits of the domestication syndrome such as loss of seed dispersal,
loss of seed dormancy or loss of protection against herbivores
are considered disadvantageous in wild plants (e.g. Crawley and Brown, 1995
These are often recessive alleles so their effects would be
masked in the heterozygotes that make up the bulk of many wild
populations. However, these recessive alleles become exposed
through the inbreeding associated with domestication in many
In maize, pleiotropic effects associated with zfl2 are suchthat selection for increased yield via increases in row numbercontrolled by zfl2 would probably select also for earlier floweringand fewer ears placed lower on the plant (Bomblies and Doebley, 2006).This led Bomblies and Doebley (2006) to suggest that, in general,undesirable secondary effects associated with pleiotropic genescould limit selection for favourable ‘domestication alleles’during early stages of the differentiation of a crop from itswild progenitor. On the other hand, selection for beneficialtraits controlled by pleiotropic genes could result in associatedneutral or even detrimental traits being concurrently selected.This may explain, at least partially, the presence, in wildpopulations, of alleles for traits of the domestication syndromethat apparently evolved prior to domestication and surviveddespite their possibly deleterious effects in the wild. Examplesof this include alleles of the ‘hidden QTL’ fw2·2for increased fruit size in cultivated tomatoes (Solanum lycopersicum)that are also found in the wild cherry tomato (S. lycopersicumvar. cerasiforme; Nesbitt and Tanksley, 2002; Bai and Lindhout, 2007).Alleles of the regulatory locus CAULIFLOWER (BoCAL) in Brassicaoleracea that contribute to, but are insufficient to cause,development of abnormal inflorescence are present in moderatefrequency in wild populations of B. oleracea subsp. oleracea(Purugganan et al., 2000).
A key gene responsible for some differences between maize andits wild progenitor is the teosinte branched 1 (tb1) mutantthat has pleiotropic effects on apical dominance, length oflateral branches, growth of blades of leaves on lateral branches,and development of the pedicillate spikelet in the female inflorescence(Hubbard et al., 2002). In the progenitor of maize, teosinte(Zea mays var. parviglumis ), a tb1 region haplotype with sequencesidentical to that of the major maize tb1 haplotype was found.This result suggested that haplotypes that confer maize-likephenotypes could predate domestication (Clark et al., 2004).Thus, the high-speed evolution represented by crop domesticationcan be the result of strong selection pressures on pre-existingvariation.
Humans caused a major shift in the morphological traits of wild
plants by selecting genes of both large effect and small effect
to create crops with higher yield of desired product. In azuki
bean (Vigna angularis
) domestication has reduced seed yield
on a per plant basis because farmers have selected determinate
plants with larger pods and fewer large seeds per pod than its
progenitor wild relative (A. Kaga, NIAS, Japan, unpubl. res.).
Mathematical analysis of the functional and structural components
of yield, including harvest index – a systems' biology
approach – have great potential to indicate future directions
for selection (Guo et al., 2006
). The wild relatives of crops
continue to be an important reservoir of genes for potential
use in agriculture. Sometimes, the genes they have furnished
have had a dramatic effect on yield, as shown by Tanksley and McCouch (1997)
and by Cheng et al. (2007)
in this Special Issue. Therefore,
there is a continued urgency to conserve these wild genetic
resources appropriately, both in situ
and ex situ
, and to characterize
them for future crop improvement.