Untitled

4. GENETIC DIVERSITY...
69
L. multiflorum ecotypes again justifies their recent origin. The major drawback of this explanation
is the same level of gene diversity displayed by cultivars. The third transposon Lolcopia2
seems not to play an important role in ryegrass speciation as can be concluded from similar
values of Nei’s diversity parameters for ecotypes. It is plausible that the lower divergence
of Lolcopia2 in perennial ryegrass cultivars resulted from breeding activities. The cultivars
that have to be uniform are derived from a limited number of individuals. This was confirmed
by extremely low cpDNA diversity in L. multiflorum cultivars for which gene diversity indices
were about threefold lower than in ecotypes and sevenfold lower than in L. perenne.
The marker type had some effect on the values of population genetic parameters in Italian
and perennial ryegrass. The largest discrepancy was observed between H S
and D ST
estimated
from enzymes and DNA markers. Firstly, allozyme revealed the two-threefold higher
the within-population gene diversity then did DNA markers. This difference was rather expected
since the latter type of markers can only produce two alleles in each locus, and therefore,
the maximum gene diversity is 0.5 whereas multiallelic markers such as isoenzymes
can produce values up to one. Nevertheless, the H S
obtained by the analysis of transposon
insertion sites (SSAP) was surprisingly low that can suggest the fixation of specific transposon
pools in each population. As a result most of genetic variation was distributed between
populations. Secondly, in the case of enzymes, the within-population gene diversity (H S
) was
higher than the between-population variation (D ST
), which is common for cross-breeding species.
This relationshipwas inverted for all DNA markers that again support the thesis about
the narrow genetic background of each population.