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Ecophysiological Differences In Adaptive Mechanisms Between Diploid Hybrid Species And Its Parental Species

Posted on:2011-02-19Degree:DoctorType:Dissertation
Country:ChinaCandidate:F MaFull Text:PDF
GTID:1100360305965712Subject:Ecology
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Natural hybridization plays a crucial role in plant evolution and diversification, contributing to both gene introgression between species and the establishment of novel lineages through homoploid or polyploidy speciation. In contrast to the numerous records of polyploidy hybridization, the occurrence homoploid hybrid speciation is relatively difficult, because not only does it involve hybridization between two sympatric taxa with the same ploidal level, but it also requires the ecological divergence between the novel species and its parents. Habitat divergence is believed to be essential to homoploid hybrid speciation, which has been evidenced to be the result of transgressive segregation that genes from parental species with opposing effect fixing in the hybrid speices and the transgressive segregation leads to the expression of extreme traits in the hybrid species relation to its parental species. However, the evidence of how widespread of this phenomenon might be in the plant kingdom is lacking, especially the evidence from tree and shrub hybrid species because natural homoploid hybrid speciation appears to be less common in shrubs and trees than in herbs. Therefore, examining the ecophysioloigical differences between diploid hybrid species and its parental species under various environmental conditions is of great importance to improve our understanding on the evolutionary mechanisms by which the homoploid hybrid species colonizes the extreme habitats.1) The homoploid hybrid species Pinus densata is restricted to alpine habitats that exceed the altitude range of its two parental species, Pinus tabulaeformis and Pinus yunnanensis. In alpine regions, low temperature and cold-induced drought stress are two important factors determing the distribution of plants. To understand the ecological differentiation between the hybrid species and its parental species, we examined their physiological responses to drought and low temperature stress, respctively. (i) The different responses of the three species to drought stress indicated that as water stress increased, leaf, stem, root dry mass and total dry mass accumulation, maximal photosynthetic rate, stomatal conductance, transpiration rate, maximal quatum yields, total water use per plant and long-term water use efficiency were found decreased significantly, and simultaneously the root/shoot ratio, carbon isotope composition and instantaneous water use efficiency increased significantly, whereas the responses of nitrogen use efficiency and carbon content to water stress varied between the hybrid species and its parents. However, Pinus densata exhibited higher fitness than both parental species in terms of total dry mass production (TDM) and long-term water use efficiency (WUEL) across all treatments, and had extreme values in LDM, WUEi, C%, NUE,δ13C in at least three treatments and other traits were extreme in at least one treatment. Overall, P. densata had extreme values relative to both parent species for nine out of 15 physiological traits under both low and mild water stress treatments and seven and six in the medium and high stress treatments, respectively, indicating that extreme characters that have become well fixed in P. densata, confer a faster seedling growth rate and more efficient water use, which in turn should confer increased drought tolerance, (ii) The responses of the three species to a period of chilling stress and a period of temperature recovery indicated that significant decreases in Amax, g and E were found in all three species under chilling stress and the depression of Amax for P. densata was mainly due to stomatal closure while that for the two parental species resulted from both stamatal and nonstamatal limitations, which may be reflected by the increase of intercellular CO2 concentration (Ci). Chlorophyll fluorescence parameters of P. densata remained nearly consistent during the whole experiment but those of the two parental species were significantly decreased by chilling stress. Furthermore, remarkably higher Amax and chlorophyll fluorescence parameters were found in P. densata than those in the parental species after 33-day chilling stress. At the end of chilling stress, area-based chlorophyll and carotenoid concentrations decreased significantly, whereas area-based anthocyanin concentration and Chl a/b increased, with significant higher concentration of anthocyanin in P. densata than that in two parental species. As temperature recovered, restorations of all parameters were examined in all species and P. densata obviously exhibited faster recoveries than its parental species in those photosynthetic traits. Overall, eight of fifteen ecophysiological traits were extreme for P. densata and those extreme traits are of great importance to maintain PSII function, implyiing a potentially superior chilling tolerance for P. densata than its parental species. In conclusion, the superior tolerance to drought and low temperature for P. densata likely promoted its ecological separation from its parental species and facilitated its successful colonization and establishment in high-altitude habitats.2) In this study, we also compared the physiological performances of two genotypes with different maternal origins (H. goniocarpa-R and H. goniocarpa-N, mothered respectively by H. rhanmoides ssp. sinensis and H. neurocarpa) and the two parental species. Our results indicated that the two H. goniocarpa genotypes distinctly differed in rates of photosynthesis (Amax), instantaneous water use efficiency (WUEi), quantum efficiencies (QE), carboxylation efficiency (CE), non-photochemical quenching (NPQ), light compensation point (LCP), integrated water use efficiency (δ13C), nitrogen contents per unit area (Narea), mean single leaf area (MSLA), leaf mass per unit area (LMA) and carbon concentration (C%). In addition, H. goniocarpa-R outperformed both parental species in Amax, integreted water use efficiency (δ13C), NPQ, MSLA and LCP. However, Amax and integrated WUE (δ13C) values of H. goniocarpa-N were intermediate between those of the two parental species, and the variations in these traits showed no correlation with those of the maternal species. The WUEi and Narea of both H. goniocarpa genotypes were distinctly higher than those of the two parental species, further suggesting that this recombinant species may be concordantly transgressive in these respects. These consistent performances may provide partly inherent power to combine all individuals of two genotypes as a distinct species unit. In contrast, the MSLA and Nmass of the two genotypes were intermediate between those of their parental species and their C concentrations and QE were distinctly lower. Our results reveal differences in the physiological performances of two genotypes of the same hybrid species with different maternal donors. These findings should help extend our understanding of the habitat preferences of the maternal genotypes within a few hybrid species.
Keywords/Search Tags:Pinus densata, Homoploid hybrid speciation, Ecological differentiation, Transgressive segregation, Drought stress, Chilling stress, H. goniocarpa, Genotype, Physiological fitness
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