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4. GENETIC DIVERSITY...
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well documented for Trifolium (Bulińska-Radomska 2005), longevity has no significant effect
on distribution of variation and a wide range of variation was observed in perennial species.
Moreover, analyses of extensive data compilations have demonstrated that long-lived perennial
taxa have rather higher diversity than annual taxa (Nybom 2004).
The similar level of genetic variation is somewhat unexpected for species that are
thought to be adapted for diverse ecological conditions i.e., L. multiflorum for mild, warm
climate and L. perenne for colder, north climate. Very often even closely situated conspecific
populations differ in allele frequencies as a result of adaptation to slightly different conditions
(soil, shadow etc). The variation that arises within population becomes transformed into
variation among populations and species. In wild emmer wheat (Triticum dicoccoides), for
example, enzymatic and ribosomal DNA polymorphism displays considerable regional and
local differences (Nevo 1983; Flavell et al. 1986). The same is true for enzymes and hordein
polymorphism in wild barley (Hordeum spontaneum), which is adaptive and selected by soil
and topographic differences over very short distances. About 56% of all variant enzymatic
alleles are not widespread but reveal localized and sporadic distribution (Nevo et al. 1983).
Commonly, the farther apart populations are, the more different they are in genetic structure.
However, the results from highly variable, out-crossing species can greatly differ. Populations
of distant geographic origin can show a very little divergence if any. This is known for Pinus
sylvestris, whose populations are very similar on enzymatic level (Goncharenko et al. 1994)
as well as on DNA level (Polok et al. 2005a). Based on the present results, two out-crossing
Lolium species, L. multiflorum and L. perenne clearly fall within this group of species.
Southern Italian ryegrass ecotypes are grouped together with Polish ecotypes of perennial
ryegrass based on enzymatic allele frequencies. On the other hand such results understandably
attribute to high gene flow resulting from rather sympatric distribution of ryegrasses over
the huge temperate areas of Europe, Asia, America and Australia.
While the majority of marker systems used display the same level of polymorphism in
L. multiflorum and L. perenne, there are some exceptions. Unexpectedly low level of cpDNA
polymorphism in L. multiflorum (40%) has strong roots in extremely low polymorphism of cultivars
of this species. Another important differences between Italian and perennial ryegrass
i.e., higher transposon diversity in the former may be attributed to the role of transposons
in species differentiation from one side and, in the case of Lolcopia1 to its origin from L.
multiflorum. Quite different can be the basis of a higher number of ISJ alleles in L. perenne.
One possible explanation is that it may be a function of the adaptation to more northern
environments where the ability to grow during dark periods can decide about reproductive
success. North ecotypes of perennial ryegrass are able to produce normal short stems (not
etiolated) and opened expanded shoots long in the autumn and even during the whole winter
if it is relatively mild. Several evidences from fine-scale mutagenic studies have proved
that these features are very often caused by point mutations at intron splice junctions as for
example in the light-independent photomorphogenesis mutant lip1 in pea (Sullivan and Gray
2000). Light-independent forms occur neither in more southern L. perenne populations nor
L. multiflorum. In other words, the presence of light-dependent and light-independent forms in
L. perenne correlates with mutations in intron splice junctions and this transforms into higher
number of ISJ alleles in L. perenne because this type of marker reveals polymorphism at
intron splice junctions.