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6. DO ANY SPECIES BOUNDARIES....
on AFLP or SSR maps. Furthermore, several gaps are usually found. For example, the AFLP
map derived from p150/112 contains five gaps (Bert et al. 1999) while as many as five chromosomes
are deficient of SSR markers on the map based on the F 2
population derived from
contrasting cultivars of L. perenne (Gill et al. 2006).
Certainly the uniform distribution of markers on the present map resulted from the application
of various techniques. These findings raise the more fundamental point that a well
saturated genetic map can only be constructed based on several marker types. A vast of different
markers ensures that various types of genomic sequences are matched. Undoubtedly,
the application of transposon-based markers is one of the major improvements preventing
from marker clustering. Successful transposable elements insert in both plant and animal
species in the centric heterochromatin as well as into the others transposons. Although the
former are tagged to some extent by AFLP markers, the latter are rarely recognised due to
high accumulation of LTR sequences. However, both regions can be successfully filled by
transposon based markers. The low efficiency of AFLP markers to discover clusters of transposons
is indirectly confirmed by their more frequent clustering with RAPD markers than with
transposons.
Difficulties in alignment of all maps based on a single p150/112 population but employing
a single marker category in a given experiment illustrate well that high quality genetic
map can only be produced if various markers are used at once. The most dramatic example
includes the SSR and AFLP maps, both constructed in p150/112 by the same authors (Jones
et al. 2002a; b) but both with different linkage group assigning and moreover any attempts to
align both maps have not been undertaken. Similarly, at least two maps are based on the F 2
population derived from a cross between two cultivars of L. perenne, Perma and Aurora. The
first employs RFLP (Armstead et al. 2002) while the second SSR (Gill et al. 2006). Unfortunately,
no consensus map is created. Each map with different marker categories is surely
advantageous over those mono-markers and it has greater utility both in QTL mapping and
evolutionary studies. Even fewer markers ensure quite uniform genome coverage (Hayward
et al. 1998; Warnke et al. 2004). Employment of different marker categories is of the highest
importance if a map has to be used for evolutionary considerations. For example, among
all markers employed, those based on transposons (SSAP) proved to be the most powerful
tool for mapping of species boundaries. Strong directional distortions demonstrate the extent
and nature of evolutionary processes that would be difficult to deduce from the other marker
categories also because all others segregate according to the expected Mendelian ratios
typical of intraspecific crosses. Furthermore, mapping of insertion sites demonstrates clearly
that distorted regions are predominantly associated with transposons. Transposon markers
are among the most unique features of the present maps.
6.4.3. Location of selected traits on the map of L. multiflorum and L. perenne
One of the most obvious outcomes of genetic mapping is the location of genes of interests.
In comparison with major crops including rice, maize or barley, the knowledge about genetics
of morphological characters in Lolium is scarce. Only recently some traits have been
mapped including flowering time, self-incompatibility locus and few enzymatic loci (Yamada
et al. 2005). However, existing information is relatively underdeveloped. The current genetic