Anastomosis of germ tubes and migration of nuclei in germ tube networks of the soybean rust pathogen, Phakopsora pachyrhizi

An important mechanism for genetic diversity in filamentous fungi is hyphal anastomosis and the formation of heterokaryons. In this study, we observed fusion of germ tubes in germinating urediniospores of Phakopsora pachyrhizi resulting in a complex hyphal network. Staining of the germ tubes derived from P. pachyrhizi urediniospores with 4', 6-diamidino-2-phenylindole (DAPI) showed migration of nuclei through the network resulting in multinucleate hyphae. Short bridges connecting the hyphal network tubes were also observed. Our study provides the first evidence of germ tube and hyphal anastomosis, and nuclear migration in P. pachyrhizi. Considering the lack of a known sexual stage of P. pachyrhizi, this hyphal anastomosis followed by the parasexual cycle may explain the genetic diversity in virulence among populations of P. pachyrhizi.