Introduction to Fungi, Third Edition

36 INTRODUCTION
Fig1.24 The spatial arrangement of a nuclear rRNA gene repeat unit. Each haploid fungal genome contains about 50 250 copies
of this repeat, depending on the species (Vilgalys & Gonzalez,1990).The three structural rRNA genes encoded by one repeat unit,
i.e.18S, 5.8S and 28S, are separated by internal and external transcribed spacers (ITS and ETS, respectively). Adjacent copies of
the repeat unit are separated by a short non-transcribed spacer (NTS).The whole unit is transcribed into a 45S precursor RNA in
one piece, followed by excision of the three structural RNA molecules from the spacers which are not used.The 5S rRNA gene
is encoded at a separate locus.The18S rRNA molecule is part of the small ribosomal subunit, whereas the other three contribute
to the large subunit.
Fig1.25 The phylogenetic relationships of Fungi and fungus-like organisms studied by mycologists (printed in bold), with other
groups of Eukaryota.The analysis is based on comparisons of18SrDNA sequences.Modified andredrawn from Brunset al.(1991)and
Berbee and Taylor (1999).