Dk. Aanen et al., A widely distributed ITS polymorphism within a biological species of the ectomycorrhizal fungus Hebeloma velutipes, MYCOL RES, 105, 2001, pp. 284-290
The ectomycorrhizal fungus Hebeloma velutipes consists of two biological sp
ecies (BSP 16 and 17). Within BSP 17 a dikaryon was found with two divergen
t types of the ribosomal Internal Transcribed Spacer (ITS1 and 2). The two
ITS types segregated in monokaryotic progeny of that dikaryon, showing that
these different ITS types represent different alleles at homologous rDNA l
oci in the two nuclei. RFLP analysis of a number of strains of BSP 17 showe
d that the polymorphism is widespread in Europe. There was no deficiency of
the heterokaryotic type, demonstrating that ITS divergence in this species
is not correlated with reduced intercompatibility. A strain from North Ame
rica, not assigned to a biological species, showed the same polymorphism. C
ladistic analysis of the two ITS sequences showed that they were not sister
groups. One of the ITS types formed a monophyletic group together with the
ITS type of BSP 16, the other type formed a clade with the ITS type of H.
incarnatulum (BSP 18). BSP 16 and 17 showed partial intercompatibility. How
ever, several lines of evidence suggest that the polymorphism of BSP 17 is
not the result of frequent and continuing hybridisation with BSP 16. Instea
d, we give arguments for the hypothesis that the polymorphism evolved in al
lopatry and that the two types have come together relatively recently. The
results of the polymorphism indicate a potential problem for molecular iden
tification of fungal species based on ITS fingerprinting. The results also
show that no generalisations are possible about the relation of speciation
(the formation of BSP) and nuclear ITS divergence.