LEAF FLUCTUATING ASYMMETRY INCREASES WITH HYBRIDIZATION AND ELEVATION IN TREE-LINE BIRCHES

Abstract

Fluctuating asymmetry (FA), or random non-directional deviations from perfect symmetry in bilaterally or radially symmetrical traits, has recently been proposed to be a useful indicator of genetic and environmental stress. However, the factors underlying fluctuating asymmetry, especially in plants, remain poorly understood. We sampled leaf fluctuating asymmetry among and within three species of even-aged birch (Betula pubescens, B. nana, and B. pendula) and their interspecific hybrids growing in common gardens of northern Finland to determine whether hybridization and environmental stress are associated with increased developmental instability. Our predictions were that: (1) interspecific hybrids among B. pubescens, B. nana, and B. pendula would have enhanced leaf FA compared to the parent groups because of a possible disruption of coadapted genomes; (2) intermediates between Betula pubescens ssp. tortuosa and B. pubescens ssp. pubescens would have a slight decrease or no difference in leaf FA compared to the parent group; and (3) fluctuating asymmetry would increase between a low-elevation and a tree-line-elevation site, and this increase would be greater in B. pubescens, which forms the major subarctic community below the tree line, than in B. nana, which is common above the tree line. As predicted, leaf FA was higher in the tree-line common garden than in the lower elevation garden, but the amount of increase was not significantly different between B. pubescens and B. nana. Leaf asymmetry was also higher in hybrids than in parental taxa, and this response was fairly consistent among the three crosses. This suggests that interspecific hybridization resulted in higher developmental instability. Furthermore, in the intraspecific studies with B. pubescens, there was no relationship between tree height (an intraspecific hybrid index) and leaf asymmetry. Thus, these data support the ideas of Markow (1995) that the changes in asymmetry observed from hybridization will be a function of how closely related parental taxa are. Since leaf FA in Betula pubescens increased with elevation and hybridization, and both are important in tree-line dynamics, FA might be a useful indicator of stress and distributional limits in this and possibly other plant species.