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7. ROLE OF QTLs IN THE EARLY EVOLUTION...
Explained variation and major QTLs
The magnitudes of the QTLs effect ranged from medium to big relative to the trait differences
between L. multiflorum and L. perenne. The average QTL explained 25% of observed
phenotypic variation however, a great range of QTL magnitudes was observed both for each
character and individual QTLs (Table 7.3). Aside from winter survival, the biggest proportion
of phenotypic variance explained by a single QTL was again found for traits not interesting to
breeders. For example, the average QTL controlling the number of florets or leaf colour explained
around 40% of variation observed in BR3 x NZ15 population whereas this value was
of 10% lower for weight of vegetative parts. Traits for which the lowest proportion of variation
was explained included height at ear emergence and flag leaf length.
The average QTL spanned over 27 cM on the genetic map however, the QTL as small
as 2.4 cM was found for the presence of awns. The biggest QTL (95 cM) controlled crown
rust resistance and it covered nearly half of the first linkage group suggesting a lot of underlying
genes. The maximum LOD values were correlated in a certain degree with a QTL size;
smaller QTLs had bigger effect as indicated by maximum LOD values. Presumably, this was
attributable to QTLs responsible for the presence of awns that were among smallest but with
the maximum LOD values up to tenfold bigger than for the other traits. Indeed, it was not
surprising because this trait is not a typical quantitative character and it is frequently controlled
by one or a few major genes. On the other hand, the presence or absence of awns
was not always so obvious in the BR3 x NZ15 population, and many intermediate types were
observed. This again suggested the more complex inheritance and several QTLs identified
confirmed these predictions.
Expectedly, the number, size and magnitudes of QTLs for some traits were very alike
suggesting a single QTL with pleiotropic effect. This was true for characters estimated by recalculation
from the others, for example basal and flag leaf areas obtained by the multiplication
of the leaf length by the leaf width. Note, that QTLs for areas corresponded to QTLs for
width but not for length. Likewise, QTLs for green and dry weight were similar with respect
to their effects and size.
For nearly all traits there were one or a few QTLs of large effect, so called major QTLs,
and several others with smaller effects (minor QTLs). Major QTLs explained threefold more
phenotypic variance than minor genes (Table 7.4). If a threshold of 25% was employed nearly
all quantitative characters were controlled by at least one major QTL. The exceptions were
height at ear emergence, spikelet length and flag leaf area. By contrary, floret number was
controlled only by major QTLs, each with large effect ranged from 29% to 70% of explained
phenotypic variance. Likewise, the most QTLs responsible for leaf colour and winter survival
were major genes. Note, that the number of major QTLs controlling crown rust resistance
in the L. multiflorum x L. perenne population correlated well with the number of qualitative
classes recognised in the intraspecific populations. The size of major and minor QTLs was
very alike. Similarly, the LOD value generally did not correspond to a QTL effect.