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7. ROLE OF QTLs IN THE EARLY EVOLUTION...
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A huge number of 145 QTLs are responsible for differences in 21 quantitative traits
between L. multiflorum and L. perenne. A comparable huge number of QTLs are related with
the domestication of T. dicoccoides (Peng et al. 2003). More QTLs are predominantly associated
with traits that likely have not been under strong selection pressure generated by human
activities. The examples involve basal leaf length (11 QTLs), flag leaf width and area (16
and 14, respectively) and spikelet number (14). In contrast, the characters that might have
been selected during the domestication of L. multiflorum are governed by just a few QTLs
(3-6). These QTLs are both minor and major in respect to explained phenotypic variances.
Similarly, in Q. robur and Q. petraea more QTLs are related with traits of low economic value
(Saintagne et al. 2004).
Most traits appear to be controlled in part by at least one major QTL explaining more
than 25% of the total phenotypic variance. Some of them explain even more than half of the
phenotypic variance. Green and dry weight of vegetative parts, floret number, and winter
survival are just a few examples. This implies that either single genes of individually large
effects or clusters of linked genes with a large summary effect can play a role in diversification
of L. multiflorum and L. perenne. In addition only a modest number of mutations in QTLs
may be required for their diversification. The pattern demonstrating that many morphological
characters in plants are controlled by genes of large effects is common both in domesticated
crops and wild species. The most famous major QTL is teosinte branched1 (tb1) separating
teosinte from maize (Hubbard et al. 2002). However, many other examples are known such
as a cluster of eight major QTLs associated with the domestication syndrome factor in T. dicoccoides
(Peng et al. 2003) or several major QTLs controlled panicle and spikelet structure
in rice (Paterson 2002). Among wild plants, each trait responsible for differences in flower
morphology between M. lewissii and M. cardinalis is associated with at least one major QTL
(Bradshaw et al. 1998). The most plausible hypothesis explaining the prevalence of major
QTLs in domesticated plants is strong artificial selection followed by fixation of QTLs with
large effects.
All QTLs involved in L. multiflorum and L. perenne morphological evolution are scattered
throughout the genome and they have no strong tendency to clustering in comparison
with such crops as maize, rice and wild emmer wheat. The initial events of Lolium domestication
were quite recently and there has not been enough time to fix all QTLs. Moreover,
the selection pressure has surely not been so strong like in cereals. However, some regions
most strongly associated with differences between L. multiflorum and L. perenne can be observed.
In total nine regions predominantly related with domestication syndromes can be distinguished
on six linkage groups. Two groups are identified on LG1, LG4 and LG6, and one
group on LG2, LG3, and LG5. None domestication syndrome region is present on LG7. Each
of two regions on LG4 is connected with different characters; the first lying on the more distal
part is responsible for differences in vegetative traits, whereas the second located in the
proximal part controls generative characters including spike morphology and weight of tillers.
It is likely, that each group is connected with different arms of a chromosome. The other linkage
groups carrying two domestication syndrome regions, LG1 and LG6 are associated with
generative traits and winter survival. The regions controlling mostly generative characters are
also located on LG2 and LG5. By contrary, the LG3 region governs the diversification of both
morphological and vegetative traits. It can be assumed that these syndromes are related with