Genetic fine structure, site clustering and marker effect in theftrcistron ofCoprinus

Abstract

SUMMARYMutants in theftrcistron of the BasidiomyceteCoprinus lagopushave a lesion in sugar transport. Fifty-two alleles are placed in an allele map using recombination frequencies obtained from over 400 heteroallelic crosses. The mutant sites in the allele map are distinctly clustered into three approximately equally spaced regions. It is demonstrated that the clustering is not due to any mutational specificity. Evidence is presented which indicates that the clusters are functionally differentiated both within themselves and from one another. Additivity of recombination frequencies was good over the whole of the allele map and there was no overall map expansion. However, specific marker effect sites could be recognized. The data indicate that marker effect may act to enhance or reduce recombination frequency and that enhancement is equal and opposite to reduction. It is also shown that marker effect enhancement is only observed when the interval being mapped exceeds a certain minimum value, but that there was no upper limit to the size of the interval. Expression of marker effect was influenced both by the background genome and by the nature of the mutation at the second site in the heteroallelic cross. It is suggested that current models of recombination do not adequately explain these data and, more importantly, that then-reliance on initial breakage of DNA strands in the formation of hybrid DNA creates grave mechanical difficulties. A model for genetic recombination is proposed in which the sequence of events is: (i) separation of intact double helices into single strands; (ii) illegitimate pairing between single strands from non-sister chromatids; (iii) strand breakage and exchange of covalent links so as to legitimize the illegitimately paired regions. The model allows for the formation of hybrid DNA either with or without coincident chiasmata. It is envisaged that the error recognized by any excision-repair system involved in recombination is the tangled strands at each end of the illegitimately paired region rather than base mismatching; and that the exchange of covalent links in stage (iii) involves the excision and replacement of segments of DNA.