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4. GENETIC DIVERSITY...
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of organisms. Many authors have concluded that L. multiflorum and L. perenne are indeed
distinct in spite of high allozyme similarity because there are few statistically significant differences
in frequency of enzymatic alleles. They are also distinguished by taxonomists based
on morphology (Loos 1993a; b; Bennett et al. 2002). One intriguing possibility is that insufficient
time has elapsed for accumulation of greater de novo mutational differences. Indeed,
the time-depended aspect of allozyme divergence permitted reassessments of speciation
dates. For example, L. multiflorum and L. perenne prove to exhibit an allozyme similarity of
0.98 that is equal to genetic distance of 0.02. If we assume that the average codon substitutions
per a locus per a year that is detectable by enzyme electrophoresis is 10 -5 , the probable
time of their divergence is only 3000 years. If this value is correct, then it fits historical
processes such as the emergence of primitive agriculture in the Middle East and its expansion
towards Europe. The record for magnitude of genetic similarity among taxonomic species
that has been considered conspecific involves the Agave deserti complex, representing
a group of species and subspecies with near allopatric distribution and clear differences in
morphology. The average Nei’s genetic similarity based on RAPDs between species is 0.96
and no correlation with taxonomic division is observed (Navarro-Quezada et al. 2003). Other
noteworthy studies using allozymes have demonstrated little genetic differentiation between
two subspecies of herbaceous plant Delphinium variegatum (Dodd and Helenurm 2002).
However, the taxonomic revision has been proposed in most such evidences. Furthermore, it
is important to bear in mind that some taxonomists do not use the biological species concept
to define their taxonomic species, but rely on morphological differences, without explicitly
considering whether or not morphological differences provide evidence for reproductive isolation.
From the data presented here, it can be the case of L. multiflorum and L. perenne as
well. Despite a huge amount of data demonstrating that they posses the same gene pool,
that there is no a reproductive barrier, for years taxonomists have been used to classify them
as distinct species.
Among other explanations, the so called “sudden speciation” that entails little or no
change in genetic composition at the allelic level should be taken into account. Is such a scenario
plausible for L. multiflorum and L. perenne Recent theories have stressed several
possibilities by which species can arise rapidly with minimal molecular genetic divergence
overall. Several known pathways include polyploidization, chromosomal rearrangements,
and changes in mating system.
First, polyploidization is the best known mechanism of sudden speciation and usually is
associated with hybridization between species that differ in chromosomal constitution. The
hybrid sterility is removed then by the doubling of chromosomes and a new polyploid hybrid
is produced. Although it is a very frequent mechanism in plants (Zielinski 1987; Ramana and
Jacobsen 2003), it is not a case of L. multiflorum and L. perenne because they both normally
are diploids with the same chromosome number.
Second, closely related taxa may differ in a variety of structural chromosomal features
including micro-deletions, translocations, inversions that may cause improper
chromosome pairing, disjunctions during meiosis resulting in slightly lower hybrid fertility.
Although it has previously been shown that hybridization between L. multiflorum and
L. perenne results in some loss of fitness (Naylor 1960), it has not been confirmed and a lot
of hybrid cultivars are characterized by high vigour and fertility. Structural rearrangements