| Introgression is a widespread phenomenon with potentially profound evolutionary consequences. Recently, significant progress in our understanding of introgression has been made with the development of a neutral model. This model predicts that, when one species invades an area already occupied by a related species, introgression of neutral genes takes place mainly from the local species towards the invading species. In addition, following a contact between two hybridizing species, the model predicts that introgression should be particularly frequent for genome components experiencing little gene flow. However, to date, there was no empirical example available, in which one species expanded into the range of a closely related one and two markers with contrasted rates of gene flow had been studied for both species. Only in such a case could the two predictions outlined above be tested simultaneously. In addition, based on these two predictions, species delimitation will be more marked based on the molecular markers with high rates of gene flow.The present thesis was designed to test this model by comparing phylogeographic patterns of the two species complexes of the genus Picea (spruce) occurring in the Qinghai-Tibetan Plateau (QTP) and adjacent highlands based on DNA variation in the two organelle genomes (mitochondrial DNA, mtDNA and chloroplast DNA, cpDNA). In conifers, hybridization and introgression are common because of incomplete reproductive isolation. Moreover, mtDNA and cpDNA have contrasted modes of inheritance:the former is maternally inherited, transmitted by seeds experiencing comparatively little gene flow, while the latter is paternally inherited, transmitted by both pollen and seeds, and hence experiencing higher levels of gene flow. Therefore, uniparentally inherited mtDNA and cpDNA markers experience different rates of gene flow in This group, providing an ideal model to test the relationship between rates of gene flow, introgression and species delimitation. Two mtDNA fragments (nad1intron b/c; nad5 intron1) and three cpDNA fragments (ndhK-C, trnL-trnF and trnS-trnG) were sequenced in nine species belonging to the Picea asperata and P. likiangensis species complex. (1) Nine mtDNA and nine cpDNA haplotypes were detected in 459 individuals from 46 natural populations in five species of P. asperata complex. As found in most conifer species studied so far, low variation is present in the two mtDNA introns along with a high level of differentiation among populations (GST=0.90). In contrast, higher variation and lower differentiation among populations was found at cpDNA markers (GST=0.56). The cpDNA, although far from being fully diagnostic, is more species-specific than mtDNA:four groups of populations were identified using cpDNA markers, all of them related to species or groups of species, whereas for mtDNA, geographical variation prevails over species differentiation. A literature review shows that mtDNA haplotypes are often shared among related conifer species, whereas cpDNA variation is more species-specific. Hence, increased intraspecific gene flow appears to decrease differentiation within species but not among species.(2) A total of 13 mtDNA haplotypes and eight cpDNA haplotypes were identified from 751 individuals in 75 natural populations of the four species of the P. likiangensis complex. High genetic differentiations was detected in both mtDNA and cpDNA population data. Genetic differentiation at cpDNA markers (GST=0.73) is exceptionally high, compared to all conifers studied so far. The harsh environment (presence of high mountains and fragmented landscape) may have weakened gene flow by seeds and pollen in this area. P. likiangensis to P. purpurea were shown to share the same mtDNA variant also found in allopatric P. likiangensis in their area of sympatry. Because P. purpurea has recently expanded its distribution range, as inferred from nuclear population genetic data, it can be inferred that the maternally inherited mtDNA introgressed in the expected direction, i.e. from the resident to the invading species (from P. likiangensis to P. purpurea), immediately following their contact. In contrast, the paternally inherited cpDNA introgressed only in parts of the contact area and in both directions (i.e. introgression from P. purpurea to P. likiangensis was also detected).Seed mass of all species decreases with altitude, whereas seed dispersal ability, as assessed by the wing loading, increases first (from 1000m to 3100m), but then decreases (from 3200m to 4600m). This suggests a complicated scenario for seed adaptation in the harsh environment of the QTP:when altitude increases, seed dispersal ability would increase to conquer the hard environment and broaden the ecological niche. However, at very high altitudes (more than 3200 m), the seed weight and wing size would decrease because of reduced allocation to reproduction (resulting in decreased dispersal ability, somewhat compensated by stronger winds). Another explanation for such a variation is that the seed wings become smaller in order to avoid being taken too far away by wind and lost in the deep valley where these seeds could not survive. This hypothesis is similar to that of Charles Darwin, which noticed the decreased dispersal ability of animals and plants occurring in oceanic islands. Finally, a literature survey of the covariation between cpDNA and mtDN A genetic differentiation indexes (GST) and altitude shows that the cpDNA GST increases with altitude, suggesting that gene flow by pollen is reduced at high altitudes, possibly because of reduced flowering intensity in the harsh environments. However, the mtDNA GST is always very high and presents no significant trend with altitude, indicating that gene flow by seeds has not changed significantly. In conclusion, although smaller seeds might have evolved in high altitudes to adapt to the harsh environment, dispersal ability does not simply covary with seed mass. |
PDF Full Text Request