Leghemoglobin is a small nuclear gene family in Leguminosae and other plant families. Of the two subfamilies that are apparently the products of an ancient gene duplication, the best-studied in legumes is the one that encodes proteins involved in symbiotic nitrogen fixation (nodulation). Symbiotic leghemoglobins themselves form a small gene family. Phylogenetic analysis of this subfamily has revealed a complex pattern of paralogy and concerted evolution, in which all of the several genes known from soybean (Glycine max) form a clade. This pattern is most easily explained as the result of concerted evolution. Other legume genera show a similar concerted pattern, although some (e.g., Medicago, Pisum, Vicia) appear to retain paralogous relationships. Because there are no studies of closely related species that would permit a detailed look at the process of the hypothesized concerted evolution in this gene family, we amplified and cloned leghemoglobin genes from members of Glycine subg. Glycine, the wild perennial relatives of soybean, a group for which phylogenetic hypotheses were available from previous work. Our expectation, given the apparent concerted evolution in Glycine max, was that the several leghemoglobin genes in each species would form a clade relative to other species, in the typical concerted evolution pattern. This pattern was not encountered. Instead, clear clades of orthologous sequences were strongly supported in our phylogenetic results; these clades included sequences from G. max and from species of subg. Glycine. We are exploring the possible utility of these paralogous genes for studies of phylogenetic relationships within Glycine. However, the unpredictability of the evolutionary pattern in leghemoglobin should serve as a warning for those interested in using gene families as a source of characters for reconstructing taxic phylogenies.

Key words: leghemoglobin, concerted evolution, paralogy, Glycine, Leguminosae, phylogeny