Untitled

192
7. ROLE OF QTLs IN THE EARLY EVOLUTION...
gene-rich regions. In the progenitor of wheat, T. dicoccoides approximately 70 domestication
QTLs are nonrandomly distributed among and along chromosomes. They are clustered
into seven domestication syndrome factors, each affecting 5-11 quantitative characters and
showing remarkable association with gene-rich regions (Peng et al. 2003).
A tremendous number of QTLs responsible for differences between L. multiflorum and
L. perenne show the exceptional possibilities for further their evolution. Surely, not all QTLs
are involved in the process of their domestication. However, the connection of some of them
seems to be apparent. For instance, height at ear emergence, probably selected as the adaptation
to hay harvest, is controlled by three minor QTLs, of which two are located within domestication
syndrome regions together with generative characters (LG3, LG6) and the third
is a solo QTL on LG5. The lack of major QTLs may indicate the narrow genetic background
for this trait in modern cultivars and ecotypes. Similarly, spike and spikelet length that could
be unconsciously selected towards bigger values are under control of three, mostly minor
QTLs located within domestication syndrome regions. Nevertheless, each trait evolves rather
independently for the reason that responsible QTLs are rarely co-located.
Six QTLs are responsible for the green and dry weight of tillers, while only three are
associated with green and dry weight of vegetative parts. The QTLs related with weight
of tillers and vegetative parts are not co-located that means two types of selection should
have taken place during the evolution of L. multiflorum and L. perenne. First, it has been
the selection towards higher yield of hay during the domestication of L. multiflorum and this
has apparently been associated with changes in QTLs controlling weight of tillers. Half of all
QTLs responsible for this trait are major QTLs. They are distributed among six linkage groups
with prevalence on LG1 and LG2 within domestication syndrome regions. Second, the selection
towards the resistance to grazing in L. perenne may have been, in a certain degree,
the selection towards higher green weight. Grasses under continuous grazing should have
the ability to grow rapidly to stay alive and if they are allowed to grow continuously without
grazing for a longer period, like in the experimental conditions, they produce higher weight of
vegetative parts. The major QTLs governing the evolution of green and dry weight of vegetative
characters are mapped within the domestication syndrome regions on L3 and another
two are clustered with the others QTLs responsible for vegetative characters on LG4.
Winter survival is another trait connected with domestication. The shift from perennial to
annual forms is of a great importance for the evolution of many crops. It is usually connected
with the lost of rhizomes that permit to survive in harsh environment. This character in Lolium
is mostly controlled by major QTLs (three major and 1 minor QTL) with strong additive
and dominance effect, all mapped within domesticated syndrome regions (LG1, LG2, LG5,
LG6). At least some of these QTLs may be connected with two key genes responsible for
rhizomatousness that diverged from a common ancestor of Poaceae about 50 MYA. In Oryza
longistaminata two dominant genes Rhz2 and Rhz3 are responsible for many rhizome traits.
They also contribute to regrowth and persistence of perennial grasses. The dominant nature
of both genes causes that a single mutation resulting in loss of function shuts off rhizome
expression. The close correspondence between rhizome genes/QTLs in distantly related
rice and sorghum suggests that the convergent evolution of independent mutations at corresponding
loci may be responsible for the domestication of many grasses (Hu et al. 2003).