## 5 Description of the Brown Trout _Salmo trutta_ Complex in Italy  
In its wide distribution range, the _S. trutta_ complex shows extreme phenotypic diversity and considerable variation in biological characteristics. Italy, due to its latitude, central position in the Mediterranean area and abundant mountains, is a very important hotspot of the _S. trutta_ complex. In Italy, the _S. trutta_ complex is widely distributed in all suitable waters. From north to south, country ranges from the rivers, streams and lakes of the Alps, which are characterized by seasonality, to the streams of the southern Apennines, which are characterized by summer heat waves at the survival limits of salmonids. In Italy, available habitats range from the cold springs of the great rivers of the central Apennines to the streams that flow through the middle of the Mediterranean scrub on the main Italian islands. In recent decades, knowledge of the state and distribution of native populations of the _S. trutta_ complex in Italy has received a considerable boost, especially for conservation purposes. This attention has led to the publication of numerous scientific papers that have revealed a certain degree of introgression of domestic populations of northern European origin (AT lineage) into native strains [80; 81]. Clearly, this characteristic makes the systematic classification of the _S. trutta_ complex even more complicated. In recent decades, several taxonomic classification methods have been proposed. [PERSON] in 1995 [82] identified three species, _S. trutta_, _S. marmoratus_ and _S. carpio_, considering _S. murmortatus_ and _S. carpio_ to be the only Italian endemic species. More recently, [PERSON] [83] identified six species, _S. fibreni_, _S. carpio_, _S. cttii_, _S. marmoratus_, _S. ceneritus_ and _S. rhodanus_, the latter of which is endemic to the Roja River on the Italian-French border. In 2004, [PERSON] [84] identified two species (_S. fibreni_ and _S. carpio_) and a superspecies consisting of three semispecies: _S. trutta_ semi species, _S. trutta macrostigma_, and _S. trutta marmoratus_. In this case, the endemic species of the Garda and Fibreno lakes were identified as species, and the other forms present in Italy were combined into a single superspecies (_S. trutta_). It is worth noting that this approach was based on the hypothesis existing at the time, which was that the native trout of central-northern Italy were similar to the trout in northern Europe and that in the Tyrrhenian area, some populations similar to _S. macrostigma_ occurred, as suggested by [PERSON] in the early 1900s [84]. Later, according to the first studies based on molecular markers, it became clear that the brown trout of Italy were phylogenetically separated from northern European trout [14]. During this period, Mediterranean Alpine and northern-central Apennine trout were referred to as \"Mediterranean brown trout\", distinct from _S. macrostigma_[42]. The taxonomic classification of the _S. trutta_ complex in Italy was revised by [PERSON] and [PERSON] in 2007 [85]. According to their study, the term _S. macrostigma_ should beused only for the populations of Morocco, and the correct name for the \"Tyrrhenian and Apennine trout\" was _S. cettii_ [PERSON], [PERSON] 1810, which was originally used by the Franco-German zoologist to describe trout living in the rivers of eastern Sicily. Additionally, these authors proposed the name _S. centerinus_ [PERSON], 1847 for the populations of the _S. trutta_ complex living along the Adriatic side of the Apennines (up to the Vomano River in Abruzzo) and in the high portions of Alpine streams in the Po basin. Using several mitochondrial markers (the mtDNA control region) and nuclear markers (11 microsatellites and eight intronic genes), [PERSON] et al. [86] identified two main native evolutionary lineages: a \"pensular lineage\", comprising samples of _S. centerinus_, _S. cettii_ and _S. fibreni_, and the lineage of _S. carpio_ and _S. marmoratus_. _S.