The Structure, Composition and Origin of the Subcrystalline Layer in Some Species of the Genus Heterodera

Abstract

The sub-crystalline layer from lemon-shaped females of plant parasitic cyst-nematodes belonging to the genus Heterodera was examined by the scanning electron microscope, X-ray diffraction, X-ray fluorescence spectrometry and mass spectrometry. In H. mani the inner zone of the layer, closely adpressed to the female cuticle and replicating the cuticle pattern, consisted almost entirely of n-tetracosanoic acid. The outer thicker zone consisted of a mixture of the acid and its calcium salt. In H. avenae, the layer had up to 25% hexacosanoic acid in addition; another species, H. trifolii had three aliphatic acids, do-, tetra-, and hexa-cosanoic acids. The outer layer, either from intrinsic shrinkage caused by calcium salts forming or from increasing pressure as the female grows, fractures into pyramidal blocks presenting an irregular polygonal appearance. The fatty acid seems to be produced by an unidentified fungus that metabolises products excreted by the nematode. The sedentary female cyst-nematodes feed similarly to aphids and probably excrete a copious sugary waste, which the fungus converts to inert fatty acid thereby preventing local pollution of the rhizosphere. The layer of acid and its calcium salt may also act as a barrier to potential pathogens and predators. The relationship between the nematode and fungus seems to be symbiotic.