Phylogeography and population genetics of the riparian relict tree Pterocarya fraxinifolia (Juglandaceae) in the South Caucasus

Abstract

We aimed to (i) assess the extant genetic diversity of the riparian relict tree Pterocarya fraxinifolia across its current distribution range in the South Caucasus, including the past refugial areas Colchis and Hyrcan, and (ii) test if a separation of these areas is reflected in its phylogeographic history. Genetic diversity of natural populations was examined using nuclear microsatellite and plastid DNA markers. Spatial genetic structure was evaluated using Bayesian clustering methods and the reconstruction of plastid DNA networks. Divergence times of Colchic and Hyrcanian populations were estimated via divergence dating using a relaxed molecular clock. Allelic richness, private allelic richness, and expected heterozygosity were significantly higher in Hyrcan than in Colchis and the Greater Caucasus, and significant genetic differentiation was revealed between the two groups. Whereas only two plastid haplotypes were detected for the Colchic and Caucasian populations, the Hyrcanian populations displayed 11 different haplotypes. Significant isolation by distance was detected in Hyrcan. The most recent common ancestor of all P. fraxinifolia haplotypes was dated to a time well before a suggested glaciation period in the Caucasus during the late Pliocene (5.98 Ma [11.3–2.48 Ma HPD]). The widespread Colchic haplotype that also occurs along the southern slope of the Greater Caucasus and reaches south-eastern Azerbaijan has appeared more recently (0.24 Ma [1.41–0 Ma HPD]). This diversification pattern of Colchic haplotypes from ancient Hyrcanian haplotypes suggests a colonization of the region from south-east to north-west that predates the last glacial maximum (LGM). Natural populations of P. fraxinifolia show low-to-intermediate levels of genetic diversity and a significant decrease of diversity from Hyrcan to Colchis. However, the genetic differentiation between Colchic-Caucasian and Hyrcanian populations for nuclear markers suggests that independent gene pools existed in both areas at least since the LGM. Particular attention to conservation seems justified for the more diverse Hyrcanian populations.

Key words:

• Caucasus ecoregion
• plastid DNA haplotypes
• climate refugia
• Colchis
• genetic diversity
• Hyrcan
• Juglandaceae
• mesophytic forests
• nuclear microsatellites
• nature conservation

Acknowledgements

We thank V. Alizade and E. Alirzayeva (Institute of Botany, Azerbaijan National Academy of Sciences, Baku) and N. Korotkova (BGBM, Germany) for the organization of the fieldwork. We especially thank M. Jafari (University of Tehran, Iran), G. Parolly (BGBM, Germany), H. Safarov (Hirkan NP, Azerbaijan National Academy of Sciences, Baku) and A. Mutallimov (Institute of Botany, Azerbaijan) for help with fieldwork in Iran, Georgia and Azerbaijan, and B. Giesicke for her support in the lab. For insightful discussions on palaeovegetation of the Caucasus we thank D. Tarkhnishvili (Ilia State University, Tbilisi, Georgia). Research was conducted at Botanischer Garten und Botanisches Museum Berlin-Dahlem, Freie Universität Berlin, Königin-Luise-Str. 6-8, 14195 Berlin, Germany. The study was performed as part of the project “Developing tools for conserving the plant diversity of the Transcaucasus” (AZ85021) funded by the Volkswagen Foundation.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental data

Supplemental data for this article can be accessed here: https://doi.org/10.1080/14772000.2017.1333540.

Additional information

Funding

This work was supported by the Volkswagen Foundation [grant number AZ85021].