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4. GENETIC DIVERSITY...
related genera of the same plant family. The Lolcopia1 is amplified in both Italian and perennial
ryegrass although the primer was designed on the basis of L. multiflorum and O. sativa
LTRs. Similarly, Lolcopia2 based on S. officinarum and T. aestivum LTRs is also present in
ryegrasses.
In opposite to retrotransposons, DNA transposons predominate in or around genes,
increasing the potential phenotypic impact of individual movements. That is why they are
usually less abundant in the genome with the copy number from ten to fewer to hundreds
(Langdon et al. 2003). Unlike the typical case for DNA transposons, Tpo1 is present at a sufficiently
high density in L. multiflorum and L. perenne genome. The copy number is similar
to retrotransposons. This observation is supported however by congruent results obtained
from L. perenne (Langdon et al. 2003) and Triticeae (Wicker et al. 2003). The Tpo1 family is
a lineage of CACTA superfamily whose members are unusual in consistently having a high
copy number in species with large genomes. The reasons behind a mechanism accounting
for the host’s ability to tolerate large numbers of Tpo1 elements have not been understood
so far. One speculative explanation that can be also adopted for the present results includes
the insertion in an active retrotransposon. In that way the Tpo1 ancestor could have become
dispersed to large numbers of intergenic regions at a rate far higher than could be achieved
by conventional transposition. The presence of an RN-aseH-like motif that is typical of retrotrasposon
accounts for the above hypothesis (Langdon et al. 2003).
Being highly abundant in plant genomes, transposons are one of the most important
factors affecting the population structure of a species. The transposition process produces
hundreds of thousands of new insertions in the genome, comprising the driving force of
polymorphism acquisition. These genetic properties have been exploited to study genetic
diversity in L. multiflorum and L. perenne. Unsurprisingly, the insertion sites of all transposons
analysed are highly polymorphic within each species. This is rather a common feature
of transposon-based population genetic parameters as can be deduced from the noteworthy
studies in pea (Vershinin et al. 2003), soybean (Chesnay et. al. 2007), barley (Leigh et al.
2003) and many others (Gribbon et al. 1999). Thus, the crucial question is not what the level
of insertional polymorphism is but what its magnitude in comparison with the other marker
categories is. Unfortunately, such analyses have been hardly done and the present work
is the one of just a few. One notable study demonstrating much higher transposon-based
polymorphism than AFLP involves tomato and pepper (Tam et al. 2005). In tomato SSAP
has shown threefold more diversity than AFLP (P=57 and 15%, H=0.175 and 0.046, respectively).
Even more dramatic, tenfold difference has been observed in pepper (P=76 and 8%,
H=0.229 and 0.026). Another example has shown that in Pisum the SSAP approach based
on PDR1 retrotransposon is more informative and generates more polymorphism than AFLP
(Kumar and Hirochika 2001). The difference is less pronounced in sweet potato, in which
SSAP reveals 18% more polymorphic bands than AFLP and 10% more than RAPD (Berenyi
et al. 2002). These data are in apparent contradiction to the examples from L. multiflorum
and L. perenne, of which the magnitude of transposon-based genetic variation (P=60-83%,
A=1.6-1.8) and this reveals by the other nuclear DNA markers and isozymes (P=75%-96%,
A=1.7-1.9) are very alike. In a sense, the present studies merely affirm that the level of polymorphism
appears not to be attributable to a marker type. The transposon-based parameters
are not biased and can give a good picture of species diversity. Additional advantage of