Morphometric discrimination among Betula nana, Betula pubescens and their hybrids using the shapeR package
Downy birch (Betula pubescens) and dwarf birch (Betula nana) are natural woody species in Iceland and play important roles in the terrestrial ecosystems. They both belong to the same genus and can hybridize. Since downy birch is tetraploid and dwarf birch is diploid, their hybrids become triploid. These hybrids can then backcross with either one of their parental species and thus transfer genes between the two species. This has resulted in a considerable amount of similar morphological characteristics between the two species, and consequently the hybrids are not clearly distinguishable from their parent species. This study attempts to facilitate discrimination between the two species and their hybrids using the ShapeR package in R, which enables the user to extract outlines and several traditional morphometric parameters from image samples and to conduct statistical testing. Using the ShapeR package in conjunction with other R packages used for statistical analysis, it was possible to discriminate between both species and their hybrids with unprecedented accuracy. Surprisingly, the most effective discriminants were among the simple traditional morphometric parameters that the ShapeR package extracts. These were leaf width, length and ratio of leaf perimeter and leaf area. This approach can be used efficiently and reliably in the species-hybrid differentiation in population-based studies of Betula species.
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