Evolution of the frequency (frq) clock locus in Ascomycete fungi.

The frequency (frq) locus of Neurospora crassa plays a key role in the organization of circadian rhythms. Similar timing systems have been found in nearly all eukaryotes as well as some prokaryotes; thus, frq may be an excellent gene with which to conduct evolutionary studies. To investigate, we used the cloned frq locus from ascomycete fungi representing two classical taxonomic classes and three orders to examine two open questions in ascomycete evolution. Class Pyrenomycetidae is represented by several species of Neurospora, Sordaria fimicola, and Chromocrea spinulosa; class Loculoascomycetidae is represented by the marine fungus Leptosphaeria australiensis. Generation of detailed restriction maps of homologs from the Neurospora species allows analysis of evolutionary relationships among these closely related species. A maximum-parsimony tree based on these restriction data suggests that Neurospora tetrasperma groups more closely with Neurospora sitophila than with Neurospora crassa using the homothallic species Neurospora galapagosensis as an outgroup. A maximum-parsimony tree derived using amino acid sequences from Neurospora crassa, Sordaria fimicola, Chromocrea spinulosa, and Leptosphaeria australiensis surprisingly suggests that Leptosphaeria austral should be classified within Pyrenomycetes rather than in a separate class. This suggestion is based on the observations that Leptosphaeria groups with Chromocrea on an evolutionary tree, is more closely related to Neurospora and Sordaria than is Chromocrea, and shares a conserved intron with Chromocrea. Together, these data show that frq is a useful gene with which to conduct evolutionary studies.