Gene flow - dependent introgression and species delimitation: evidence from mtDNA & cpDNA variation in spruce
Résumé
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 ones. 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 should be more efficient when using molecular markers experiencing high rates of gene flow. The present thesis was designed to test the hypotheses of this model. The biological model used was conifers, a group in which introgression and hybridization are common because of incomplete reproductive isolation. The species investigated belong to the genus Picea (spruce). We focused on two species complexes, represented by monographic clades in a phylogenetic study using the chloroplast gene matK. All species studied occur in the Qinghai-Tibetan Plateau (QTP) and adjacent highlands. The phylogeography of these species complexes was reconstructed using organelle markers (mitochondrial DNA, mtDNA and chloroplast DNA, cpDNA). In conifers, mtDNA and cpDNA have contrasted modes of inheritance. The former is maternally inherited, transmitted by seeds experiencing little gene flow while the latter is paternally inherited, transmitted by both pollen and seeds experiencing high levels of gene flow. Therefore, uniparentally inherited mtDNA and cpDNA markers experience different rates of gene flow in such a group, providing an ideal model to 3 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；trnS-trnG) were sequenced for 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 variants are often shared among related conifer species, whereas cpDNA variants are 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 differentiation 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 and P. purpurea were found to share the same mtDNA variant specific to P. likiangensis in their sympatric range. Assuming that P. purpurea had recently expanded its distribution range towards the south, the maternally inherited mtDNA has introgressed in the expected direction from the resident to the invading species (from P. likiangensis to P. purpurea). In contrast, the paternally inherited cpDNA has introgressed only in parts of the contact area and in both directions (i.e. there were also some introgression from P. purpurea to P. likiangensis). I also explored the relationship between seed mass, wing loading and altitude in the two species complex. We discovered that seed mass in these species complex decreases within increasing altitudes. However, seed dispersal ability, as assessed by wing loading, increases first but then decreases with altitude. This suggests a complicated scenario for seed adaptation in the harsh environment of the QTP. I also conducted a literature survey of the relation between cpDNA and mtDNA genetic differentiation indexes (GST) and altitude in conifers. The results show that at cpDNA markers, GST increases with altitude, suggesting that gene flow by pollen is reduced at high altitude, possibly because of the harsh environments. In contras, at mtDNA markers, GST is always very high and presents no significant trend with altitude. In conclusion, although smaller seeds seem to have evolved in high altitudes, the primary selective pressure for this reduced seed mass is not likely to be dispersal ability.