LEAF FLUCTUATING ASYMMETRY INCREASES WITH HYBRIDIZATION AND ELEVATIONIN TREE-LINE BIRCHES

Fluctuating asymmetry (FA), or random non-directional deviations from perfect symmetry in bilaterally or radially symmetrical traits, has re cently been proposed to be a useful indicator of genetic and environme ntal stress. However, the factors underlying fluctuating asymmetry, es pecially in plants, remain poorly understood. We sampled leaf fluctuat ing asymmetry among and within three species of even-aged birch (Betul a pubescens, B. nana, and B. pendula) and their interspecific hybrids growing in common gardens of northern Finland to determine whether hyb ridization and environmental stress are associated with increased deve lopmental instability. Our predictions were that: (1) interspecific hy brids 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 sap. tortuosa and B. pubescens ssp. pubescens would have a slight decrease or no difference in leaf FA compared to the parent group; and (3) flu ctuating asymmetry would increase between a low-elevation and a tree-l ine-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 be tween B. pubescens and B. nana. Leaf asymmetry was also higher in hybr ids than in parental taxa, and this response was fairly consistent amo ng the three crosses. This suggests that interspecific hybridization r esulted in higher developmental instability. Furthermore, in the intra specific 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 asym metry observed from hybridization will be a function of how closely re lated parental taxa are. Since leaf FA in Betula pubescens increased w ith elevation and hybridization, and both are important in tree-line d ynamics, FA might be a useful indicator of stress and distributional l imits in this and possibly other plant species.