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The Mefite in the Ansanto Valley is one of the largest and oldest cold natural carbon dioxide springs (CNCDS) known in the world. This site is characterized by toxic concentrations of CO2 and other harmful gases, and few plant species can survive in this environment. Among these plants, there is an aberrant Genista tinctoria population previously described as G. anxantica. This study aims to advance understanding of evolution over small geographical scales in an extreme environment using G. tinctoria populations from low and high CO2 environments. To investigate differentiation of the Mefite population, we analysed plastid sequences and microsatellite data for populations in the surrounding area. Considering the genetic results, it is evident that the population of the Mefite has an exclusive genetic pattern from both plastid and nuclear points of view with the presence of significant genetic barriers for both genomes analysed, which would point to the isolation of this population from neighbouring G. tinctoria populations. The population of G. tinctoria from the Mefite shows evidence of genetic differentiation from the nearby populations and the genetic indices show discrete variation. Recruitment was confirmed in the Mefite population, an indication that fitness is maintained despite the extreme local conditions. Our results indicate that the potent selection pressure imposed by the Mefite atmosphere has driven genetic differentiation of the local population from nearby G. tinctoria populations. A correct taxonomic rank evaluation and some conservation remarks are also proposed. Our study demonstrates that CO2 concentrations can influence population differentiation and adaptation in plants in a similar way to atmospheric sulphur, herbicides or edaphic conditions.

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The delimitation of Limonium taxa is highly complicated due to hybridization, polyploidy, and apomixis. Many “microspecies” were described and aggregated into groups, most of which are still poorly known from both molecular and morphological points of view. The aim of this study is to investigate four endemic species from the Tyrrhenian coast of central Italy and the Ponziane Archipelago belonging to the L. multiforme group (L. amynclaeum, L. circaei, L. pandatariae, and L. pontium) by means of molecular and morphometric analyses. Molecular data by sequencing ITS and three plastid markers and morphometric data highlight new information about the taxonomy of these taxa so as to reduce them into a single specific entity. In fact, the better taxonomic choice is to consider the populations studied as part of a single species, i.e., Limonium pontium. Three subspecies are recognized, i.e., subsp. pontium [= L. circaei = L. amynclaeum; from Circeo to Gianola localities (excluding Terracina) and from islands Ponza, Palmarola, Zannone, and Santo Stefano], subsp. pandatariae comb. et stat. nov. (from island of Ventotene), and subsp. terracinense subsp. nov. (from Terracina).