- Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences
This thesis summarizes and discusses results o f five separate studies in which molecular techniques have been used to study the genetic variability and evolution o f the Abies taxa occurring in the Mediterranean region. In particular, the investigation focused on the rare species Abies nebrodensis (Lojac.) Mattei, endemic to the island of Sicily, and the three neighbouring species A. alba (Mill.), A. cephalonica (Laud.) and A. numidica (De Lann.).
The main aim of the studies was to determine the amount and distribution of the genetic variability within and among Mediterranean taxa of Abies, at both the nuclear and chloroplast levels, in order to elucidate their origin and evolution and to shed light on the taxonomic position o f A. nebrodensis. In studies I, II and V allozyme markers were used to provide information on the level and distribution o f genetic variation among and within natural populations of A. alba, A. cephalonica, A. nebrodensis and A. numidica and to estimate the outcrossing rate within A. alba.
In studies III and IV, DNA markers from the chloroplast genome were developed and employed at the intra- and inter-specific levels to estimate the degree of cpDNA variation in the genus and to derive inferences concerning species relationships. Two different approaches were used: the first involved a comparative restriction-site analysis of ten different amplified chloroplast DNA fragments and the second involved the analysis of six chloroplast hypervariable repetitive simple-sequence repeats (cpSSRs or microsatellites). Analyses of both allozyme and cpDNA data indicated that A. nebrodensis differs from the neighbouring Abies species and justified its classification as a separate species. Based on results from studies III, IV and V we suggested that A. nebrodensis originated during the Messinian crises of the latest part o f the Miocene, through hybridisation between A. alba and A. numidica. During the warmer period of the Holocene A. nebrodensis became more isolated from both A. numidica and A. alba, promoting further divergence.
Results from studies III, IV and V agreed well with the hypothesis that the genus Abies had an Asiatic origin and then differentiated along an east-west axis (beginning in the Miocene) in the Balkan and Middle East regions. Taxa like A. alba, A. bornmuelleriana, A. nordmanniana, A. cephalonica and A. cilicica were the first to differentiate from the common Abies ancestor. Abies pinsapo and A. numidica appear to have differentiated much later, and to have remained isolated for a long time after their settlement in Spain and North Africa. The lower levels o f polymorphism observed in these two species appear to be a consequence of genetic drift due to isolation and small population size.
Abies; allozyme; chloroplast DNA; mating system; variation; migration and evolution
Acta Universitatis Agriculturae Sueciae. Silvestria
2000, number: 148
Publisher: Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences