Hybrid origin of an annual leafy spurge (<i>Euphorbia</i> subgen. <i>Esula</i>) from the Alborz Mountains (Iran) in the Irano-Turanian floristic region

The Alborz Mountains represent an area of endemism within the Irano-Turanian floristic region. They are particularly rich in the number of species belonging to <i>Euphorbia</i> subgenus <i>Esula</i> (leafy spurges). Here, we present a new annual leafy spurge discovered in the area. Morphologically and ecologically, it resembles annual <i>Euphorbia szovitsii</i>, the sole representative of <i>Euphorbia</i> sect. <i>Szovitsiae</i>, and <i>Euphorbia arvalis</i> from <i>Euphorbia</i> sect. <i>Arvales</i>. Phylogenetic analyses of the nuclear internal transcribed spacer sequences inferred its sister position to <i>Euphorbia</i> sect. <i>Patellares</i>, which includes perennials growing mostly in more mesic habitats; interestingly, the new species shares partly connate raylet-leaves with this section. Conversely, the plastid sequences indicated that it was most closely related to <i>E. szovitsii</i>. This suggests its hybrid origin, possibly including an allopolyploidisation event in its evolutionary history, indicated by an increased relative genome size compared to <i>E. szovitsii</i>. Based on our integrative approach we describe the new Alborz endemic as <i>Euphorbia alborzensis</i> and include it in <i>E.</i> sect. <i>Szovitsiae</i>. We provide a taxonomic treatment and discuss its morphological affinities with close relatives. Our study highlights the Alborz Mountains as an important area of endemism in the Irano-Turanian region and indicates that further in-depth studies are needed in this region.