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4. GENETIC DIVERSITY...
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agreement between molecular and morphological diversity (Chapter 3) and high inter-fertility
of both species as reported by breeders champion strongly for lowering their taxonomic rank.
It should be noted however, that prior to more detailed mapping studies some mechanisms
of “sudden speciation” can not be absolutely excluded. Especially, that the lower transposonbased
similarity in comparison with the other markers might be the first signs of speciation.
4.4.4. Role of transposons in differentiation of L. multiflorum and L. perenne
Mobile genetic elements also called transposons or transposable elements (TE) are
prevalent in the genomes of all plants contributing up to 90% of the genome (Kazazian 2004).
Most of the moderately repeated sequences are mobile genetic elements. Transposons are
classified into two groups according to their transposition mechanism and mode of propagation.
Class I elements (retrotransposons) move via an RNA intermediate that is reverse
transcribed prior to integration into the genome (“copy-paste”). Once inserted, they can not
excise. This mode of reproduction contributes significantly to the genome expansion. Retroelements
are usually flanked by long terminal repeats (LTRs). In plants, the members of two
groups are predominately observed; the most studied Ty1-copia, to which Lolcopia1 and Lolcopia2
belong, and Ty3-gypsy. Both groups were named after the best studied elements in
Saccharomyces cerevisiae and Drosophila melanogaster. Plant Ty1-copia retrotransposons
show a considerable degree of sequence heterogeneity compared to fungal and animal elements.
Class II elements, often termed DNA transposons, are excised from the genome and
integrated elsewhere (“cut-paste”). They typically possess terminal inverted repeats (TIRs)
on the basis of which they have been subdivided into several super-families. One of them,
called the CACTA family, received its name because it is flanked by inverted repeats that
terminate in a conserved CACTA motif. The Tpo1 transposon analysed in the present studies
belongs to this super-family (Flavell et al. 1992; Turcotte et al. 2001; Vershinin et al.
2003; Wicker et al. 2003). Transposons generate genetic diversity by altering the size and
organization of the host genomes and by inducing insertional mutations. The evidence from
H. spontaneum has indicated that both active amplification and losses of transposons are
likely to generate genome diversity (Kumar and Hirochika 2001). A multiplex PCR approach,
SSAP has been developed to detect insertional polymorphism of transposons in plants. In
SSAP the products are derived from a DNA fragment between retrotransposon terminal sequences
and a restriction site in the sequence flanking the transposable element thus, the
method reveals the sites of transposon insertion.
Retrotransposons make up a large proportion of the genome of higher plants and they
contribute to more than 50% of the nuclear DNA being present even in millions copies. They
are commonly found in intergenic regions what reduces their detrimental effect (Bennetzen
2005). This is not surprising therefore, that Lolcopia1 and Lolcopia2 retrotransposons are
highly abundant in L. multiflorum and L. perenne. One of the most convincing cases of Lolcopia
abundance in ryegrasses involves the comparison with the other species. The only
three primer combinations have revealed from 160 to 251 insertion sites and it is four times
more than it is observed in tomato (46) and pepper (40) with the same number of primer
combinations (Tam et al. 2005). The present data indicate as well that primers designed on
the basis of 3’region of LTR sequences derived from other Poaceae species can be used in