植物核DNA含量在全球尺度上的纬度变异式样及其气候适应意义——以菊科植物为例

核DNA含量是重要的生物学概念,涉及DNA C-值和基因组大小。前人有关植物核DNA含量在纬度梯度上的变异规律存在着矛盾的报道,而且多数将核DNA含量与纬度、海拔、气候等因素之间的关系描述成线性关系。核DNA含量是否具有环境适应上的意义,也还存在争议。先前有关核DNA含量与环境因素间关系的矛盾性报道可能与取样过小、地理范围过窄、研究对象遗传背景差异过大有关。如果对一个遗传背景相近的类群在全球范围内进行取样,核DNA含量会呈现有规律的纬度梯度变化,可能与大的气候因素之间存在非线性关系。菊科(Asteraceae)是被子植物的最大科,是一个广泛认可的自然分类群。为揭示全球空间尺度上植物核DNA含量在纬度梯度上的变异规律,以及这种变异是否具有环境适应意义,以菊科为对象开展了核DNA含量与纬度、生物气候因素关系的统计分析。从"植物DNA C-值数据库"检索到822种菊科植物的核DNA含量数据;在全球范围内,沿经度方向上设立10条样带,每条样带横跨15个经度,每条样带又均分成22个样块,每个样块纵跨7.5个纬度;其次,从"世界气候数据网站"下载1950—2000年时间段14个生物气候因子数据,应用Arc GIS 9.3获得每个样块14个生物气候因子的平均值;根据"全球生物多样性信息网站"记录,计算每个样块菊科植物平均的核DNA含量数据。为避免气候变量之间的多重共线性对数据分析的影响,应用主成分分析对数据进行了降维,发现最冷季度平均温度、最干季度雨量分别是第一、二主成分上荷载最大的因子,去除与它们相关性在-0.7至+0.7之间的其他气候因子后获得了最冷季度平均温度、最干季度雨量和最湿月份雨量三个变量用于进一步数据分析。结果发现,菊科植物在全球10个样带上的核DNA含量与纬度关系密切,与最冷季度平均温度、最干季度雨量和最湿月份雨量呈现极显著的单峰型的非线性关系,可以用二项式进行拟合。因此,全球空间尺度上植物核DNA含量沿着纬度梯度有规律性的非线性变化,这种变化具有很强的气候适应意义。

Latitudinal variation patterns of plant nuclear DNA amount on a global scale and their environmental adaptation significance: a case study with Asteraceae

Nuclear DNA amount is an important biological concept that includes DNA C-value and genome size. There are conflicting reports about latitudinal variation patterns of plant nuclear DNA amount. Relationships of plant nuclear DNA amount with latitude, altitude, and climatic variables have been reported as linear. Disputes also exist as to whether nuclear DNA amount of plants is of environmental adaptation significance. We speculated that the conflicting reports were due to insufficient sampling of the taxa, limitations in the geographical range, and different genetic backgrounds of the samples. Asteraceae is not only the largest family within angiosperms, but also a widely accepted natural taxa. To clarify the latitudinal variation patterns of plant nuclear DNA amount on a global scale, and to explore the possible climatic adaptation significance of these patterns, we conducted a study with Asteraceae to analyze the relationships of nuclear DNA amount with latitude and bioclimatic variables. First, we obtained the data of nuclear DNA amounts of 822 species of Asteraceae from the Plant DNA C-value database. We selected ten global longitudinal transects, each with a span of 15 longitudinal degrees, and evenly divided each transect(from 82.5°N to 82.5°S) into 22 blocks. We obtained geographical records of the 822 species from the Global Biodiversity Information Faculty, together with fourteen bioclimatic factors within these blocks from the Worldclim Global Climate Database. We calculated the average DNA 1C-values and genome sizes of each species in each block. To avoid multicollinearity among the fourteen climatic variables, we performed principal component analysis(PCA) to reduce the dimensionality of the variables. We found that the mean temperature of the coldest quarter and the precipitation of the driest quarter had the highest loads in the first two principal components. The climatic variables with low correlation coefficients(-0.7 to 0.7) with the above two variables were included in the analyses. We found that the nuclear DNA amounts of Asteraceae had a regular latitudinal variation that could be expressed by polynomial functions. The relationships of nuclear DNA amount with the mean temperature of the coldest quarter, the precipitation of the driest quarter, and the wettest month were typically nonlinear with a unimodal pattern, and could be expressed by binominal equations. Therefore, on the global scale, plant nuclear DNA amount changes regularly on latitudinal gradients, which has distinct climatic adaptation significance.