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4. GENETIC DIVERSITY...
cies and shared bands occur in T. aestivum, S. cereale and A. sativa (Gribbon et al. 1999).
Common bands are observed not only for Triticeae but also for timothy, oat and cordgrass of
the subfamily Chloridoideae. Presumably, the integration events took place in the last common
ancestor of Poideae and Chloridoideae, long before the Triticeae diverged (Vicient et al.
2001). However, only seven shared bands have been observed in the former example, and
probably the similarly low number has been present in the latter. An important drawback to
both studies is the lack of data about frequency of shared bands and it can be only deduced
from gel images. Assuming that an average bands’ number on the SSAP gel is about 40-50,
then the frequency of common bands in cited examples is about 14-17%. This magnitude
is more or less in agreement with the only published frequency data for the Glycine species
(Chesnay et al. 2007). The SSAP profile of SIRE-1 has produced 21% of shared bands in
annual accessions and 27% in perennial accessions i.e., almost fourfold less than in the
present studies. Considering the above values, it is hardly imaginable that almost all insertions
produced by three different transposons in L. multiflorum and L. perenne are derived
from the common ancestor. A follow-up question is whether the time period of evolutionary
history of ryegrasses roughly estimated between one million and 10 000 years is sufficient for
accumulation of a great number of new insertions. For instance, the transposon population is
completely different in sorghum and maize but they had shared last common ancestors less
than 15 million years ago (Bennetzen 2005). The Pisum retrotransposon PDR1 has been
transposing within roughly the last five MYA, with a peak at 1-2.5 MYA but younger insertions
were also found in small subset of accessions (Jing et al. 2005).
A lesson learnt from the genus Pisum is that extremely high diversity both within a species
and between species have been produced during only “minutes” on the evolutionary
timescale providing the transposons could have been acting for a sufficient period of time
and their activity has been combined with the other genetic processes such as introgression.
Cultivated pea (P. sativum) is an Old World legume whose evolutionary history is
about 10 000 years although the whole genus is much older. Likewise ryegrasses, existing
taxonomic classifications of the genus are not congruent and they distinguish from two
to six species including P. abyssinicum, P. elatius, P. fulvum, P. humile, and P. sativum.
Molecular appraisals based on the two major groups of retrotransposons, PDR1 (Ty1-copia)
and Cyclops (Ty3-gypsy), together with Pis1, a member of the CACTA super-family,
have shown clear separation of the P. fulvum lineage (Vershinin et al. 2003). This species
has 77 insertion sites not found in any other species (i.e., six times more than between
L. multiflorum and L. perenne) and occupies the separate position on the trees. Another
interesting finding is the absence of common markers shared by P. abyssinicum and
P. sativum showing that they were brought into cultivation independently. Thus, transposons
have demonstrated that the widely accepted view of P. abyssinicum as an ecotype of
P. sativum does not mirror the evolutionary history of these otherwise different species. On
the other hand, the shared insertional polymorphism between the other three Pisum species,
P. elatius, P. humile and P. sativum and virtually the lack of species specific markers indicate
extensive and intermixing gene flow among them. This picture seems to be very similar to
that in L. multiflorum and L. perenne. According to the idea expressed by Vershinin et al.
(2003) for pea, it can be assumed that recombination, introgression, and segregation have
been responsible for the extremely high fraction of shared bands in ryegrasses preserving