Species Divergence And Phylogeography Of Abies Chensiensis And A.Fargesii

A fundamental problem in evolutionary biology is to identify the evolutionary processes (such as mutation, random genetic drift, gene flow and natural selection) that influence genetic variation across genomes and populations, and their role in the causal origins of reduced gene flow between incipient species. Here, we report a case study of genetic divergence between two fir species, Abies chensiensis Tieghem and A. fargesii Franchet, on the basis of surveys of two maternally inherited mitochondrial (mt) DNA markers (nad5 intron 4 and nad7 intron 1), two paternally inherited plastid (pt) markers (trnS-trnG and trnL-trnF) and 126 biparentally inherited amplified fragment length polymorphism (AFLP) markers. These two fir species occur exclusively in central China, on the basis of their morphological differences, they are often grouped into different sections of genus Abies. So, in this study, we tried to examine concordance between genetic variation across genomes and traditional morphological boundaries between A. chensiensis and A. fargesii.First, range-wide genetic variation was investigated using mt and pt DNA sequences, which contrast in their rates of gene flow. Four mtDNA haplotypes were recovered, three of which were shared between species and showed no obvious species'bias in terms of relative frequency. However, their geographical distributions indicated past refugia and independent regional range expansions across each species' range. In contrast, a high level of ptDNA variation was recorded in both species with three common ptDNA haplotypes shared between them and 21 rare ptDNA haplotypes specific to one or other species. Despite the occurrence of species-specific ptDNA haplotypes, the two species could not be delimited according to a partitioning of ptDNA variation within and between species. Mismatch analyses of ptDNA haplotype variation also suggested that each species had undergone a recent range expansion, while network analyses indicated that the three shared ptDNA haplotypes showed no geographic bias, were ancestral, and that the rare species-specific ptDNA haplotypes were produced during a period of recent range fragmentation and expansion. The lack of concordance between morphological and molecular variation between the two fir species reflects extensive ancestral polymorphism sharing for both forms of cytoplasmic DNA variation. It is feasible that a fast mutation rate for ptDNA contributed to the production of many species-specific ptDNA haplotypes. but that these haplotypes have remained rare due to insufficient time having passed for them to have spread and become fixed in either species despite high levels of intraspecific gene flow exhibited by ptDNA. The results of our analysis suggest that polymorphisms in both organelle genomes most likely originated during and following glacial intervals preceding the last glacial maximum when species distributions became fragmented into several refugia and then expanded their range across central China.In this study, we also tried to determine the level of nuclear genetic divergence between A. chensiensis and A. fargesii. especially at sites where they are parapatric relative to where they are allopatric. Our analyses demonstrated that AFLP divergence was much greater between the species when comparisons were made between parapatric populations than between allopatric populations. This increased genetic divergence between these two fir species in parapatry is similar to the character displacement reported for various ecological and reproductive traits in previous studies, indicating that natural selection favouring increased divergence between species in parapatry may have acted on the nuclear genome in this case. Our inference is strengthened by employing two genome scan methods (DFDIST and BayeScan). which repeatedly identified 9 outlier loci that departed from the neutral expeciation in populations at parapatric sites. We further discuss the likely causes of these findings in terms of possible niche divergence, increased prezygotic reproductive isolation (reinforcement) and local adaptation to different environments being favoured by selection in paraptry. In general, it is clear from this study that different patterns of genetic variation across the entire genomes can exist within and between closely related species due to different rates of mutation and gene flow, various effects of demography and selection, and the length of time that has passed since speciation occurred.