Thórsson Ae Th, Pálsson S, Sigurgeirsson A, Anamthawat-Jónsson K
Institute of Biology, University of Iceland, Askja-Sturlugata 7, 101 Reykjavík, Iceland.
Ann Bot. 2007 Jun;99(6):1183-93. doi: 10.1093/aob/mcm060. Epub 2007 May 11.
Introgressive hybridization between two co-existing Betula species in Iceland, diploid dwarf birch B. nana and tetraploid downy birch B. pubescens, has been well documented. The two species are highly variable morphologically, making taxonomic delineation difficult despite stable ploidy levels. Here an analysis is made of morphological variation within each ploidy group with an aim to establishing a reliable means to distinguish the species.
Plant materials were collected from 14 woodlands in Iceland. The plants were identified based on 2n chromosome numbers. Morphological variation in species-specific characters within each ploidy group was analysed qualitatively and quantitatively. The morphological index was based on eight discrete characters, whereas the multivariate analysis was based on nine leaf variables.
Of the 461 plants examined, 9.5 % were found to be triploid hybrids. The three ploidy groups were morphologically distinguishable but their variation overlapped. The diploid, triploid and tetraploid groups had average scores of 1.3, 4.1 and 8.3, respectively, in the morphology index scale from 0 (B. nana) to 13 (B. pubescens). A linear discriminant analysis also revealed significant separation among the three ploidy groups and the model assigned 96 % and 97 % of the B. nana and B. pubescens individuals correctly. The triploid hybrids were difficult to predict since only half of them could be assigned correctly. Leaf length was the most useful variable identifying triploid hybrids. Geographical patterns within the ploidy groups could partly be explained by differences in mean July temperature.
Hybridization between B. nana and B. pubescens is widespread in Iceland. The species can be distinguished from each other morphologically, and from the triploid hybrids. The overlapping morphological variation indicates bidirectional introgression between the two species via triploid hybrids. Iceland could be considered a birch hybrid zone, harbouring genetic variation which may be advantageous in subarctic regions.
冰岛两种共存的桦木物种，二倍体矮桦（Betula nana）和四倍体毛桦（Betula pubescens）之间的渐渗杂交已有充分记载。这两个物种在形态上高度可变，尽管倍性水平稳定，但分类界定仍很困难。本文对每个倍性组内的形态变异进行分析，旨在建立一种可靠的方法来区分这两个物种。
从冰岛的14个林地收集植物材料。根据2n染色体数对植物进行鉴定。对每个倍性组内物种特异性特征的形态变异进行定性和定量分析。形态学指标基于八个离散特征，而多变量分析基于九个叶片变量。
在检查的461株植物中，发现9.5%为三倍体杂种。三个倍性组在形态上可区分，但它们的变异有重叠。在从0（矮桦）到13（毛桦）的形态学指标量表中，二倍体、三倍体和四倍体组的平均得分分别为1.3、4.1和8.3。线性判别分析也显示三个倍性组之间有显著分离，该模型正确地将96%的矮桦个体和97%的毛桦个体进行了分类。三倍体杂种难以预测，因为只有一半能够被正确分类。叶片长度是鉴定三倍体杂种最有用的变量。倍性组内的地理模式部分可以用7月平均温度的差异来解释。
矮桦和毛桦之间的杂交在冰岛很普遍。这两个物种在形态上可以相互区分，也能与三倍体杂种区分开来。形态变异的重叠表明这两个物种通过三倍体杂种进行双向渐渗。冰岛可被视为一个桦木杂交区，拥有在亚北极地区可能具有优势的遗传变异。
