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7. ROLE OF QTLs IN THE EARLY EVOLUTION...
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The floret number per a spikelet is another important trait in the evolution of L. multiflorum
and L. perenne that is controlled by six major QTLs. They are mostly located within
regions associated with the domestication (LG1, LG2, LG3, and LG4) and they are very
often co-located with QTLs associated with the spikelet number. Such location champions
the selection of this trait as the selection toward higher yield. Indeed more florets mean more
seeds. It can be also speculated that additional selection pressure has been related with the
elevation of L. multiflorum to a species rank followed by the separation of L. multiflorum from
L. perenne based on the floret number. Then it would be an example of a trait which selection
has been promoted by taxonomists.
One final point about evolution of L. multiflorum and L. perenne is an intriguing discovery
of obvious association between QTLs and transposon-based markers confirming
the possible role of mobile genetic elements in morphological evolution of Lolium. QTLs
are more frequently flanked or linked both with the DNA transposon, Tpo1 and the retrotransposon,
Lolcopia2. Although this association can be coincidental it is in agreement
with the line of evidences from the other plants. Eight from ten yield enhancing QTLs
from O. rufipogon and O. sativa contain transposons from CACTA family and both classes
of retrotransposons, Ty1-copia and Ty1-gypsy (Reddy et al. 2006). A nonautonomous
Mutator-like transposon is also present in the FLC allele responsible for flowering time in
A. thaliana (Gazzani et al. 2003). The data again demonstrate that transposons make genomes
a dynamic structure responding to various evolutionary forces. Although we are far
to understand the role of transposons, the domestication of L. multiflorum, that is in action,
can be a good experimental field to study it. And in the end, the current studies including all
presented within Chapters 2-7, have provided the strongest support that L. multiflorum is
a domesticated form of L. perenne. Therefore, implementing the same philosophy like for
maize and teosinte, or indica and japonica forms in rice, it can be proposed to classify them,
in agreement with the Integrated Taxonomic System of the USA (2007), as subspecies i.e.,
L. perenne ssp. perenne for a more primitive form and L. perenne ssp. multiflorum for a domesticated
form.
7.5. CONCLUSIONS
1. The lack of differences with respect to the occurrence, magnitude and mechanisms
underlying heterosis in intra- and interspecific Lolium crosses, confirms the status of
L. multiflorum and L. perenne as a single biological species.
2. Heterosis in intra- and interspecific crosses results from dominance and gene interactions
and it is not associated with genetic distance between parents. The number of QTLs
responsible for heterotic traits depends on a trait and ranges between three QTLs up
to 16.
3. L. multiflorum is a domesticated form of L. perenne that was selected in the Middle-Ages,
and after that elevated to species status. The domestication process is controlled by
many major QTLs in addition to several minor QTLs that are located within nine domestication
syndrome regions on six linkage groups.