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8. MOLECULAR PHYLOGENY OF THE GENUS LOLIUM
8.4.3. Phylogeny of two clades within the genus Lolium
When trying to clarify a sequence of events involved in the evolutionary development of
the genus Lolium, like any other taxonomic group, it should be remembered that alleles are
not only transferred vertically from parents to offspring, but hybridisation and introgression
are quite frequent during a history of a species. Introgression is an important evolutionary
process, by which species can acquire new genes enabled them to adapt to new environments
or simple to overcome stresses. The evolutionary significance of introgression is well
known but only recently it has been shown that hybridisation between distant species or even
genera followed by introduction of genetic material is a regular process (Bachmann 2000). In
California, the hybridisation between Raphanus sativus (cultivated radish) and R. raphanistrum
(jointed charlock) is so extensive that both species have completely merged (Hancock
2005). Thus, there is no reason not to expect introgressions between closely related taxa
such these from the genus Lolium and Festuca. The examples are well documented elsewhere
and the introgression of pollen allergen from F. pratensis to L. perenne observed in the
present work is just one of them. Notwithstanding the evolutionary significance of introgression,
from a practical point of view it is very troublesome and undesired to everybody who
would like to reconstruct phylogenesis. Ideally, it would be to have a single sequence applicable
to each taxon at any level. For the sake of sustaining biodiversity it is only a dream.
Each allele in a genome may represent different stories and as it has been documented
here for the gene encoding pollen allergen, a given allele can originate from an introgression
event in one species (L. perenne), whereas it is inherited from a common ancestor in another
(L. remotum and L. temulentum). It is important therefore, to examine multiple sequences per
species in phylogenetic studies. It should also be kept in mind that genomes predominantly
consist of repetitive sequences, and among them retrotransposons are the most abundant.
Hence, low copy genes or even members of multigene families that are liked so much in
phylogenetic studies represent only a small piece of the whole diversity of the genome,
and unfortunately a relatively conservative and unrepresentative piece. Thus, application of
a sequence or a gene can be only a prelude to phylogenetic studies, enabling for preliminary
diagnosis. With nearly 3 000 molecular markers based on different approaches and sampling
various sequences from individual genes, through multigene families to highly repetitive
transposons, this work can not be compared to any other. Such a wide choice of methodolo-