| Abstract |
The genus Origanum belongs to the family Lamiaceae and includes 45 species (with 6
subspecies) and 19 hybrids. The genus is distributed throughout Europe, North Africa
to temperate Asia (Taiwan), but the majority of taxa have an eastern Mediterranean
distribution, with Turkey being the center of speciation. In Greece, the genus Origanum
is present with 12 taxa (10 species with three subspecies), seven of which are endemic
to parts of the country. Due to the wide use of Origanum species by humans, there are
many studies on the properties and uses of different species (spice, essential oils,
medicinal uses) but also studies on the ecology and distribution of the species. Based
on genetic research, existing studies focus on the genetic diversity of species, on their
karyological structure and differences, while there are also studies on population
genetics. Regarding the evolutionary species relationships in the genus, there is only
one research which studied the species relationships of section Majorana (O. onites, O.
majorana, O. dubium and O. syriacum). The goal of the present thesis was to
investigate species relationships and assess their current taxonomy and re-evaluate their
diagnostic morphological characters. The dataset included the majority of Greek
species (except for O. lirium and O. sipyleum), covering a broad range of their
distribution, along with species occurring outside of Greece. The various analyses were
based on phylogenetic analyses of nuclear and chloroplast genes (sanger method with
Bayesian and ML analyses), on morphological phylogenetic analyses (Maximum
Parsimony – MP) and divergence time estimations (BEAST). According to the obtained
results, phylogenetic relationships are moderately resolved; however, the monophyly
of almost all species is confirmed, apart from that of the Cretan endemic species O.
dictamnus. Nevertheless, the latter does not fall out of the range of the “species
concept”. The 10 morphological sections are neither genetically nor morphologically
confirmed, as they do not reflect the evolutionary relationships of the species and do
not include all the species that correspond to them. Also, the clustering of the 10
sections into three large groups, is not genetically and morphologically supported.
Some species (O. majorana, O. dubium, O. syriacum) appear to have a rather confusing
taxonomic position and thus need further analysis. According to morphological
analysis, all species are confirmed to be monophyletic, while specific morphological
characters were proven to be highly informative for species identification, even when
used alone. In contrast, characters that previously were considered to be useful for species identification, turned out to be uninformative, as several species from different
sections were found sharing the same subcategories. Concerning divergence time
estimations, the majority of Origanum species started their diversification between late
Pliocene to mid Pleistocene (from 2,5 to 1,6 mya), although the start of the genus
divergence can be traced back to mid to late Pliocene (4,5 mya). Given the existing
knowledge of palaeogeography and paleoclimatology of Greece, it appears that the
main reasons for species diversification is probably a combination of the effects of the
Pleistocene glacial and interglacial periods, along with dispertion events and
hybridization; a very important phaenomenon for speciation events in the genus
Origanum.
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