Haplotype and nucleotide characteristics of photobiont and mycobiont gene sequences in the lichen Cetraria aculeata

Lichens, symbiotic associations of fungi (mycobionts) and green algae or cyanobacteria (photobionts), are poikilohydric organisms that are particularly well adapted to withstand adverse environmental conditions. Terrestrial ecosystems of the Antarctic are therefore largely dominated by lichens. The effects of global climate change are especially pronounced in the maritime Antarctic and it may be assumed that the lichen vegetation will profoundly change in the future. The genetic diversity of populations is closely correlated to their ability to adapt to changing environmental conditions and to their future evolutionary potential. In this study, we present evidence for low genetic diversity in Antarctic mycobiont and photobiont populations of the widespread lichen Cetraria aculeata. We compared between 110 and 219 DNA sequences from each of three gene loci for each symbiont. A total of 222 individuals from three Antarctic and nine antiboreal, temperate and Arctic populations were investigated. The mycobiont diversity is highest in Arctic populations, while the photobionts are most diverse in temperate regions. Photobiont diversity decreases significantly towards the Antarctic but less markedly towards the Arctic, indicating that ecological factors play a minor role in determining the diversity of Antarctic photobiont populations. Richness estimators calculated for the four geographical regions suggest that the low genetic diversity of Antarctic populations is not a sampling artefact. Cetraria aculeata appears to have diversified in the Arctic and subsequently expanded its range into the Southern Hemisphere. The reduced genetic diversity in the Antarctic is most likely due to founder effects during long-distance colonization.

地衣（Lichens）是由真菌（真菌共生体，mycobiont）与绿藻或蓝细菌（光合共生体，photobiont）构成的共生联合体，属于高度适应极端逆境环境的变水生物体（poikilohydric organisms）。因此，南极陆地生态系统的优势类群以地衣为主。全球气候变化的影响在海洋性南极（maritime Antarctic）地区尤为显著，据此可推测当地地衣植被未来将发生剧烈改变。种群的遗传多样性与其适应环境变化的能力及未来进化潜力密切相关。本研究针对广布地衣刺芽冰岛衣（Cetraria aculeata）的南极共生体种群，证实其真菌共生体与光合共生体均存在低遗传多样性特征。针对每种共生体的3个基因座（gene loci），我们分别比对了110至219条DNA序列，共分析了来自3个南极种群、9个反北极带（antiboreal）、温带及北极种群的总计222个个体样本。真菌共生体的遗传多样性在北极种群中最高，而光合共生体的多样性则在温带区域最为丰富。光合共生体的多样性向南极方向显著降低，但向北极方向的降幅相对较小，这表明生态因子在调控南极光合共生体种群多样性方面仅发挥次要作用。针对四个地理区域计算的物种丰富度估计值显示，南极种群的低遗传多样性并非采样假象（sampling artefact）。刺芽冰岛衣似乎起源于北极并发生分化，随后将分布范围扩张至南半球。南极种群的遗传多样性降低，最可能源于长距离定殖过程中的奠基者效应（founder effects）。