AOBPreview originally published online on May 11, 2007
Annals of Botany 2007 99(6):1183-1193; doi:10.1093/aob/mcm060
INVITED REVIEW |
Morphological Variation among Betula nana (diploid), B. pubescens (tetraploid) and their Triploid Hybrids in Iceland
1 Institute of Biology, University of Iceland, Askja–Sturlugata 7, 101 Reykjavík, Iceland
2 Icelandic Forest Research (Research Division of the Icelandic Forest Service), Mógilsá, 116 Reykjavík, Iceland
* For correspondence. E-mail aethth{at}hi.is
Received: 6 October 2006 Returned for revision: 18 January 2007 Accepted: 15 February 2007 Published electronically: 11 April 2007
Background and Aims: 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.
Methods: 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.
Key Results: 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.
Conclusions: 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.
Key words: Birch, Betula nana, Betula pubescens, diploid, triploid, tetraploid, morphological variation, morphology index, multivariate data analysis, interspecific hybrid, introgression, introgressive hybridization