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6. DO ANY SPECIES BOUNDARIES...
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6.4. DISCUSSION
6.4.1. At what stage of speciation are L. multiflorum and L. perenne
Taxonomists very often classify species based on one or two visible morphological differences.
The idea behind this is quite simple. Species that are separated by reproductive
isolation also differ by more “ordinary” morphological characters that play no necessary role
in blocking gene flow (Orr 2001). Despite the fact that reproductive barriers and morphological
differences are often connected, there is a good reason to think that this association is
partly casual. Cryptic species that are morphologically very alike, yet reproductively isolated
are good examples that evolution is not necessarily correlated with phenotypic diversification.
By contrary, phenotypic diversification does not always go hand-in-hand with a birth
of reproductive barriers. Huge mutant collections of crop species are the best examples of
“ordinary” morphological differences that are not related with speciation. On the other hand,
speciation is gradual and it is somehow uncoupled from processes of intraspecific population
differentiation. It is a challenging task to find out whether observed differentiation is coupled
or not with speciation especially at the very early stages at which no reproductive barriers
exist or eventually their birth can be hardly observed. L. multiflorum and L. perenne belong
to these “troublesome” species. One, the most traditional approach to study the reproductive
barrier formation often involves the analysis of intra- and interspecific hybrids with respect
to fertility. The easy of hybridisation and the lack of any signs of hybrid sterility coupled with
possibility of backcrossing observed in all analysed Lolium crosses states for the lack of
reproductive barriers between incipient species. Obviously, species that are fully interfertile
can not be classified as biological species, however crossability does not exclude that they
are diversifying and that the birth of a reproductive barrier can be observed only in certain
loci. The difficulties are that black and white situations are rare and very often, even hybrids
between cultivars exhibit some level of sterility due to different mechanical damages during
artificial crossing. Furthermore, a lesson learnt from maize has shown that fully interfertile
species in crossing experiments, may be reproductively isolated in nature through the action
of a single allele, Tcb1-s differing in frequency in species under question (Kermicle 2006). In
L. multiflorum and L. perenne at least two self-incompatibility genes were identified and they
may play a role in limiting the recombination frequency at certain loci and distorting segregation
of adjacent markers (Thorogood et al. 2005). Thus, the crucial distinction in a case of
L. multiflorum and L. perenne is not whether a reproductive barrier exists (obviously it does
not), but whether or not they are undergoing speciation, and more exactly if any signs of
a species boundary can be found. Another traditional method to study it involves the comparison
of the level of polymorphism in the crosses between related species that could be
hybridized and distribution of polymorphic markers between parents. It is believed that parents
belonging to different species have fixed different alleles in many loci thus they differ in
so called multilocus genotypes. Such differences have been observed in L. multiflorum and
L. perenne at several enzymatic loci, but - surprisingly in intraspecific crosses. Conversely,
enzymatic and DNA alleles in interspecific crosses were equally split between parents. Thus,
there is another reason to throw away the thesis about species differences between L. multiflorum
and L. perenne.