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6. DO ANY SPECIES BOUNDARIES....
throughout the genome of L. multiflorum and L. perenne all indicate that distortions are connected
with particular markers, hence confirming the role of genic interactions. Changes in
gene regulations or negative epistatic interactions between divergent alleles at different loci
may reduce fitness and embryos carrying them are more likely to be aborted (Harushima et
al. 2001). Conversely, positive interactions are responsible for increased fitness and greater
proportion of embryos carrying them. Distorted RAPD markers indicate both types of interactions
owing to equal representation of markers selected toward the presence of a band as
well as the lack of it. Another noteworthy example includes positive selection towards the
presence of a band at three distorted katG loci. Because katG markers are predominantly
linked to enzymatic loci it is plausible that the selection towards a band means the selection
towards a specific enzyme. For instance katG5-9 is tightly linked to Mdh2 encoding malate
dehydrogenase. Concepts that protein variation is largely adaptive and selected by different
environmental conditions are well documented in plants (Nevo et al. 1983). The same sort of
interactions can be proposed for ISJ markers. Because they are specific to junctions between
exons and introns they likely mark genes, among which some increase fitness. That is why
they are predominantly selected towards the presence of a band. The examples of katG and
ISJ markers illustrate the kinds of positive genic interactions that can act at the species level.
They are not so severe in a sense that not so many markers are distorted.
At the opposite extreme, more than half of transposon markers are distorted. Indeed,
transposons are prevalent in all distorted regions on L. multiflorum x L. perenne genetic map.
Although, having in mind the results from the genetic diversity studies, it could be expected,
the magnitude of distortion inevitable demonstrates the unprecedented power of mobile elements
in driving the evolutionary processes. Importantly, nearly all transposon markers were
selected against the presence of a band. In other words, individuals without insertions were
favoured in the interspecific F 2
population. Each genome represents a compromise between
transposon movement and the mechanisms of their immobilisation. Transposon activity may
differ between closely related lineages leading to differences in accumulation of retrotransposons
blocks between genes. Within these blocks retrotransposons are commonly nested by
insertion into each other (Fedoroff 2002). Presumably, a higher number of insertion sites in
L. multiflorum (Chapter 4) result from such differences in transposon activities. Transposons
are known to be both segregation and post-segregation distorters. Many of them reduce
the frequency of non-carrier individuals after fertilisation, the phenomenon opposite to that
observed in the present studies. On the other hand, host genomes develop mechanisms that
prevent the action of distortion driving elements. One possibility is the spread of unlinked
genes that inhibit the selfish phenotype. Unlinked mutations that prevent the action of the
driving element may spread in natural populations of a species and increase the number of
non-carrier individuals. However, the distortion driving phenotypes will re-emerge in hybrids
between species or even distant populations if one of the parents lacks repressor genes
(Hurst and Werren 2001). If we turn again to L. multiflorum and L. perenne it is plausible that
repressor genes present in both parents contribute to strong selection against insertion. Under
this hypothesis the potential lack of the repressor genes in one of the parents would result
in the rapid spread of transposons in the F 2
population and the individuals with transposons
would be favoured. Because repressor genes evolve as an answer for the action of selfish
elements specific to a species, the lack of such genes in a given population is regarded as