Temporal and spatial diversification of the shrub Justicia areysiana Deflers (Acanthaceae) endemic to the monsoon affected coastal mountains of the southern Arabian Peninsula

Justicia areysiana Deflers (Acanthaceae) is an endemic shrub of the monsoon affected coastal mountains of the southern Arabian Peninsula with ballochorous seed dispersal. Its occurrence in a vegetation belt of relics of the (semi)-evergreen Afro-montane woodland characterised by Olea europaea subsp. cuspidata and Juniperus procera above an elevation of c. 750-800 m is strongly dependent on the activity of the Indian Ocean southwest monsoon. Therefore, the species offers a good model to study the palaeo-climate induced habitat fragmentation of the once continuous distribution range of the Afro-montane woodland belt on the southern Arabian Peninsula. Thirteen populations from the total distribution range of the species were analysed using AFLP fingerprinting and show a distinct geographic distribution pattern with a main split between eastern (Hawf Mountains/Dhofar) and western/central populations (Jabal Urays, Jabal Gedu, Kor Seiban). Our results on the genetic differentiation of populations clearly demonstrate the strong isolation among populations and confirm geographical patterns found in former studies based on chloroplast haplotypes. The dating of the main regional split between eastern and western/central populations based on nrDNA ITS sequence variation, is dated to 0.8–1.8 Myr before present. This indicates drastic habitat fragmentation processes during the Pleistocene which are compatible with periods of aridisation assumed for the African continent at around 1.7 Myr and 1.0 Myr before present.