Leguminosae, Polygalaceae, Quillajaceae and Surianaceae together comprise the order Fabales. Phylogenetic relationships within Fabales remain an unsolved problem even though interfamilial relationships have been examined in a number of studies using different sampling approaches and both molecular and morphological data. In this study, we gather information from the nuclear 26S rDNA region as well as previously published data from the sqd1,matK and rbcL regions. Phylogenetic analyses were performed by maximum parsimony, maximum likelihood and Bayesian inference. Overall, the best-supported topology for the relationships among families within the order places the pair of Leguminosae and Polygalaceae as sister to the pair of Quillajaceae and Surianaceae. However, our approximately unbiased (AU) test of the combined data results has shown that none of the seven different topologies rejected. Furthermore, three topologies were not significantly different from each other. Therefore, similar to the previous studies, this study did not find well-supported dichotomous relationships among the four Fabales families. The Fabales topology was very sensitive to both data choice and the phylogenetic methods used, which may indicate a rapid-near-simultaneous evolution of the four Fabales families. Our results also show that while nuclear sqd1 can be helpful as a complementary region, both the nuclear sqd1 and rDNA 26S regions could be problematic when analyzed individually.
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