Polymorphism around cog extends into adjacent structural genes

The recombination hotspot cog overlaps a highly polymorphic 950-bp region of linkage group I in Neurospora crassa. The sequence of this region in the four strains, Lindegren 25a, Lindegren A, Emerson A and St. Lawrence 74A, each differs from the others by 1.4% or more. Comparison of the sequence of St. Lawrence 74A and Lindegren 25a each side of cog shows a high level of sequence heterology extending in both directions, including the coding sequences for his-3 and a putative gene lpl with homology to yeast lysophospholipase. The St. Lawrence 74A and Lindegren 25a sequences of his-3, centromere-proximal to cog, differ at 14 nucleotides, resulting in six amino-acid variations between the predicted protein sequences. In lpl, distal from cog, the sequences differ at 19 nucleotides leading to five amino-acid differences between the predicted proteins. Sequence heterology between St. Lawrence 74A and Lindegren 25a peaks either side of cog and then declines with distance. At the am locus on linkage group V, heterology is much less but peaks close to a weak recombination hotspot 5' of the coding sequence. Uneven distribution of polymorphism along chromosomes has been explained by a hitch-hiking hypothesis in which selection for advantageous mutations causes local fixation of unselected variation. We suggest that new mutations arising from errors in recombination also contribute to the uneven distribution of polymorphism.