“…On the other hand, lateral gene transfer of the Sym genes appears to be the most plausible hypothesis to explain cases of phylogenetic incongruence between Sym and housekeeping genes (Martinez-Romero & CaballeroMellado, 1996 ;Young & Haukka, 1996). Indeed, phylogenetic trees based on sequences of nod genes are generally not congruent with those based on 16S rDNA sequences, but the nod trees show some correlation with host plant range (Dobert et al, 1994 ;Lindstro$ m et al, 1995 ;Ueda et al, 1995 ;Haukka et al, 1998 ;Wernegreen & Riley, 1999). By contrast, the phylogeny of nifH genes, which encode the dinitrogenase reductase enzyme, has been reported as closely following that of 16S rRNA genes (Hennecke et al, 1985 ;Young, 1992 ;Dobert et al, 1994), despite some exceptions (Eardly et al, 1992).…”