Widespread, but less than assumed: Populations of Euphorbia amygdaloides (Euphorbiaceae) from western Asia represent two new cryptic species

Abstract

The Hyrcanian forests positioned along the southern and south-western shores of the Caspian Sea and the Euxine-Colchic forests along the southern and eastern shores of the Black Sea (western Asia) are renowned as hotspots of biodiversity and represent major refugia of Eurasian Tertiary relict forest species. One of them was considered to be Euphorbia amygdaloides (Euphorbia sect. Patellares), a forest species with wide distribution from northern Africa across Europe to western Asia. Using nuclear ribosomal ITS and plastid ndhF–trnL sequences we here show that the western Asian populations previously treated as E. amygdaloides are clearly divergent from European E. amygdaloides and actually pertain to two new species, which are also morphologically different. The newly described E. sylvicola is widespread in the Hyrcanian and Euxine-Colchic forests and is morphologically most similar to E. amygdaloides, with which it also shares its genome size. On the other hand, E. caspica is endemic to the Hyrcanian forests in Iran and is most closely related to another western Asian species, E. macroceras, with which it partly overlaps in distribution. Both species have also similar genome size, slightly lower from that of E. amygdaloides and E. sylvicola, but morphologically E. caspica resembles more E. amygdaloides than E. macroceras. Our study uncovered cryptic diversity in the forests of western Asia that had remained hidden due to slow rates of morphological evolution, i.e. morphological stasis, commonly observed in other Tertiary forest relict species, and highlights the Euxine-Colchic and especially the Hyrcanian forests as important centres of biodiversity and endemism.