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1. INTRODUCTION
are continuously entering new sites, mutations caused by their insertions are probably much
less frequent than are point mutations in most organisms. Over millions of years of evolution,
transposons have achieved a balance between detrimental effects of mobilization and longterm
beneficial effects on a species through genome modifications. As a result, about 90%
of transposons remain reasonable stable (Kazazian 2004) but they can be reactivated by
stress or other unusual events. This can be the beginning of population differentiation leading
to speciation. Therefore, transposons are especially useful in analysis of recently diverged,
closely related taxa. In recent years transposons have been shown to be present throughout
the plant kingdom. They have played a significant role in the evolution of grasses being
responsible for genome size variation. Despite transposons play important role in speciation
of Poaceae they have never been exploited in resolving phylogenetic relationships within the
genus Lolium.
Closely related taxa often tend to be more similar in phenotype than distant taxa. Whenever
a phylogeny is known with reasonable assurance, morphological, behavioral, or other
organismal features may be alternative states of composite attributes (such as awns, perennation
in Lolium). An implication of this is that evolution of higher taxa might have been driven
by rare mutational changes in single major genes (Avise 2004). On opposite, closely related
taxa usually differ in discrete traits whose underlying model of inheritance is multifactorial and
polygenic. Selection would favor new multigenic combinations that would create a discrete
shift in morphology. This model assumes that evolution of closely related taxa is depended
on multiple genes, each with a small effect on phenotype - i.e., quantitative trait loci (QTL).
The use of DNA makers in conjunction with experimental crosses enables to map genomic
position of these “speciation QTLs”. The approach is virtually the same as QTL mapping for
breeding purposes and involves the crosses between two species in question. An argument
against this methodology is that it can be applied only if both interspecific hybrids and their
offspring can be produced. Second, such approach needs a genetic map of at least modest
resolution. Nevertheless, the fruitful results were obtained in maize, in which 22 QTLs
affecting the seven traits responsible for the evolution of maize from teosinte were identified
(Lauter and Doebley 2002). This strategy also resulted in the location of 33 QTLs responsible
for differences in inflorescence architecture between foxtail and green millet (Doust
et al. 2005) and 56 QTL loci contributing to differences in 15 morphological traits between
closely related species of Helianthus (Avise 2004). Considering several taxa studied to date
by QTL mapping, the composite number of genetic changes between even closely related
species must normally be large. An important task will be to conduct similar kinds of QTL
mapping experiments on closely related taxa, especially if their taxonomic status is controversial.
A QTL-mapping approach is a method that could be used to identify chromosome
regions responsible for taxonomic characters differentiating L. multiflorum and L. perenne.
Even though several genetic maps of ryegrass have been reported, none included “speciation
QTLs”. First, the genetic maps have been developed primarily for breeding purposes
and therefore, nearly all of them have been published for the most important member of
the genus, L. perenne (Bert et al. 1999; Armstead et al. 2002; Jones et al. 2002a; Yamada
et al. 2004). Second, the majority of these maps have been based on the P150/112 reference
mapping population consisted of about 150 individuals and derived by crossing a highly
heterozygous L. perenne parent of complex descent, as a pollinator, with a doubled haploid