Structure and chromosomal arrangement of leghemoglobin genes in kidney bean suggest divergence in soybean leghemoglobin gene loci following tetraploidization

Abstract

We have determined the structure of one of the leghemoglobin (Lb) genes of Phaseolus vulgaris (kidney bean) and deduced the chromosomal arrangement of leghemoglobin genes by genomic hybridizations with Lb and two other sequences, each specific to the 5' or 3' region of the soybean leghemoglobin loci. By comparing this organization with two other species of legumes, Glycine max (soybean) and G. soja (wild soybean), a phylogeny of leghemoglobin gene loci was traced. The intragenic structure of the kidney bean leghemoglobin gene shows the same intron/exon arrangement as that of soybean leghemoglobin genes and extensive sequence homologies in both coding as well as 5' and 3' non-coding regions. The presence in the kidney bean genome of four leghemoglobin genes suggests that tandem duplications of a single primordial plant globin gene had occurred to generate four leghemoglobin genes in an ;Lb-locus' before Glycine and Phaseolus species diverged. Chromosome duplication by tetraploidization in Glycine generated two loci containing four genes each. A large deletion in one of the two four-gene loci in soybean resulted in the generation of the Lbc(2) locus containing two leghemoglobin genes, one functional and another pseudo (LbPsi(2)). This pseudogene, unlike that present on the main locus, is represented by only two and a half exons and appears to be truncated. The two other truncated genes (LbT(1) and LbT(2)) were probably generated similarly in the genome of Glycine spp. following tetraploidization before the divergence of G. max and G. soja.