Distribution, history and evolution of buxus sempervirens L. and buxus balearica Lam

Abstract

The present multidisciplinary study is a first contribution to the knowledge of the genetic structure and a deepening of the post-glacial history of Western Palaearctic Buxus. It also constitute an attempt to depict a detailed distribution of Buxus. Data from vegetation databases, Floras, herbarium specimens and field surveys were collect and used to draw a detailed distribution map of the genus in the Western Palaearctic area. Reviews of Buxus fossil occurrences were undertaken to reconstruct the distribution and population dynamics for the last 30.000 years. Genetic analyses with nuclear markers on plants sampled throughout the distribution range of Buxus in the Western Palaearctic area were performed with the aim of evaluating the current taxonomic framework. Molecular surveys with plastid markers were used to study the genetic structure and the geographical distribution of genealogical lineages to understand the evolutionary history of Buxus. Results from these analyses revealed that Buxus shows a two major distribution ranges, one in the central-western and one in the central-eastern areas of the Western Palaearctic region. In the C-W range, Buxus is more frequent and the populations are abundant and contiguous in Central Europe, whereas they are fragmented in the southern territories. Buxus occurs in Western Mediterranean islands as well (Baleares, Sardinia, Corsica). The C-E range of Buxus consist of several disconnected areas found in Greece and the Middle East (Turkey, Syria, Caucasus, Azerbaijan, Iran). Review of fossil occurrences demonstrate that Buxus survived the last glacial period throughout Central Europe, and that its modern distribution is not the product of postglacial migration from limited southern refugia. The fossil data also indicate that between 30 and 12 ka BP the populations located in the Temperate bioclimatic zone were relatively dense. Their seventy-fold exponential increase in the last 12.000 years resulted in a continuous modern distribution area. By contrast, the Mediterranean bioclimatic zone hosted far less robust populations, which increased only moderately during the first half of the Holocene and resulted in a modern highly fragmented distribution area. The populations located in Minorca, Ibiza, Sardinia, Sicily, Apulia, Basilicata, Croatia, Thessaly, W Greece, and Peloponnese have been completely extirpated. Buxus population dynamics reflect a natural multiplicative process that was continuous in Central Europe, whereas increasing aridity in the Mediterranean is the most likely explanation for the decline of populations. Data from nuclear markers suggest that B. colchica and B. hyrcana are not different taxonomic entities, and should be included as locally adapted and disjunct population of B. sempervirens. Plastid genome analyses revealed that the whole diversity of Buxus is constrained to the southern territories of Europe, whereas a single haplotype occurs throughout Central Europe. The organization of plastidial diversity is unusual and shows maximum levels of population differentiation and a lack of intrapopulational variation. These genetic features seem to be a characteristic of the genus Buxus and are linked to historical and ecological processes. The extreme disjunct distribution of Buxus in the Mediterranean basin is reconciled by the lack of a phylogeographic break between the Western and Eastern domains. Haplotype sharing between parapatric populations of B. sempervirens and B. balearica and some morphological insights suggests that hybridization has occurred during the history of Buxus in this area. This event has occurred twice, the first to the east and a second in the west. Comparing the fossil and genetic data in the light of the modern distribution of Buxus suggest that diversity occurs where the populations are fragmented and experienced a severe reduction. By contrast, genetic evenness is detected where the populations are continuous and experienced an exponential growth throughout the Holocene. These findings question the role of Mediterranean refugia for Buxus and indicate that genetic differentiation occurs where the populations are declining, whereas genetic evenness characterize expanding populations. The above results are used to discuss conservation action for Buxus in the light of ongoing climate change and increasing human impact.