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7. ROLE OF QTLs IN THE EARLY EVOLUTION...
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make plausible that heterosis in L. multiflorum and L. perenne resulted rather from interactions
between specific genes than from the overall heterozygosity.
The view emerging from the analysis of QTLs in the L. multiflorum x L. perenne population,
BR3 x NZ15 is that QTLs associated with heterotic traits and with positive effects
on phenotypes are dispersed between parents. Similar conclusions can be drawn by inspecting
parental phenotypes in the other populations. Floret number per a spikelet is
just an example of a trait for which the hybrid exceeds the better parent of 35%, but the
parental values are very alike and rather low in comparison with the other crosses. This
character is an important separator of L. multiflorum and L. perenne in many dichotomous
keys. The observed heterosis resulting in values similar to those in L. multiflorum seriously
questions the utility of the number of florets for taxonomic purposes. Notably, six major
QTLs are responsible for this trait in the L. multiflorum x L. perenne F 2
population. The
L. perenne hybrid performance makes plausible that all of them also segregate in this species
and presumably, QTLs increasing the number of florets are dispersed between parents.
According to the dominance theory the dispersion of alleles with positive influence on a phenotype
is sine qua non of heterosis (Polok 1996).
Referring to the possible genetic basis of heterosis it is likely that dominance with additive
and non-additive interactions between loci are the most important in L. multiflorum and
L. perenne hybrids. The dominance effects for the majority of QTLs associated with heterotic
traits in the L. multiflorum x L. perenne population (BR3 x NZ15) in addition to asymmetric
distribution of F 2
individuals support the above opinion. This hypothesis has strong roots
in earlier wide explorations of mechanisms responsible for heterosis in crosses between
barley mutants. The homozygous lines derived from heterotic crosses, performing as good
as heterotic hybrids and significant estimations of parameters informing about additive gene
actions and interactions between homo- and heterozygous loci prove that the interaction
between dominant loci is the most important mechanism underlying heterosis. In the most
striking examples it is possible to connect heterosis with recessive epistasis. Another important
outcome from barley studies is that even a few genes differentiating mutants from
their parental cultivars are suitable to express heterosis in hybrids provided the effects of all
dominant genes and interactions have positive influence on phenotypes. To exemplify, most
heterotic traits involve from two to six genes or groups of linked genes, however for some
traits ten genes can be found (Polok 1996, Polok et. al. 1997). The present data for L. multiflorum
and L. perenne are in good agreement with those cited above. Heterotic traits can
be controlled by either a few QTLs or quite a lot. For instance, spike length, flag leaf length
in the L. multiflorum x L. perenne hybrid, BR3 x NZ15 are associated with only three QTLs,
the number of QTLs responsible for green and dry weight of tillers is twofold bigger (six),
whereas spikelet number and flag leaf width are controlled by as many as 14-16 QTLs both
with major and minor effects.
In summary, there are no differences with respect to the occurrence, magnitude and
mechanisms underlying heterosis in inter- and intraspecific Lolium crosses. It demonstrates
clearly that L. multiflorum and L. perenne perhaps represent two extreme phenotypes but still
constituting a single biological species. On the other hand, the heterotic hybrids can have
higher adaptive values that under certain circumstances might lead to rapid adaptation to
new environments and result in future diversification.