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6. DO ANY SPECIES BOUNDARIES...
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a sign of diversification. Taken together, the strong selection against insertions in F 2
population
derived from a cross between L. multiflorum and L. perenne resulting from the presence
of repression genes in both parents contributes to the lack of functional incompatibilities that
would underlie speciation and birth of reproductive isolation. However, it can not be excluded
that some of observed transposon distortions play an important role in genetic divergence
and the route for speciation is still opened. The data indicate that the molecular clock for
transposable elements is at least twofold more rapid than synonymous base substitutions
within the genes (Ma and Bennetzen 2004).
6.4.2. Genetic map of L. multiflorum and L. perenne
A major question is to what extent the current set of markers covers the whole Lolium
genome. The theoretical genome length is estimated to be 1 190 cM based on the mean
chiasma frequency per chromosome of 1.7 observed in the interspecific cross of L. perenne x
L. multiflorum (Naylor 1960). The map obtained here spans 1 014 cM and covers 85% of the
whole genome. Thus, it is the most complete map among all published until recently. Their
genome coverage ranges from 47% to 78% (Table 6.12). The average distance between
markers of 2 cM also positively distinguishes the present map from all others Lolium maps
with the most common distance about 3 cM (Jones et al. 2002a; Hirata et al. 2006).
The first glimpse at the present map demonstrates that the most striking feature is
the uniform distribution of markers between all linkage groups. Although some small AFLP
or SSAP clusters can be observed, the gaps between them are filled by the other marker
types. This is a considerable progress with all other Lolium maps, on which strong marker
clustering is observed. For example SSR loci tend to group around the putative centromeric
region on LG1 of the p150/112 map resulting in sub-optimal genome coverage (Jones et
al. 2002a). Several SSR clusters are also reported on the L. multiflorum map, however it
is not clear if they are related with centromers (Hirata et al. 2006). Similarly, large clusters
are observed on AFLP maps (Bert et al. 1999; Jones at al. 2002b). The obvious outcome
of marker clustering is the presence of gaps i.e., long parts of linkage groups without
any marker. The maximum distance of 10.4 cM found on LG3 between ISJ12-1 marker
and Lolcopia2 specific marker, Lc2Pat18 is the only gap on the present L. multiflorum and
L. perenne map. Indeed, it is twofold smaller in comparison with about 20 cM gaps observed