5.7 Nuclear genetics

The basic genetic architecture of fungi is fairly typical of the eukaryotes. Chromosomal structure and the nuclear division process are defining characteristics of eukaryotes, and all the major principles of genetics apply in fungi, namely gene structure and organisation, Mendelian segregations, recombination, and the rest (Moore & Novak Frazer, 2002). Nevertheless, there are some differences between most fungi and other eukaryotes which we will highlight here.

Fungi have a generally smaller genome size than other eukaryotes (Table 2). Remember, though, that the higher organisms have much more non-coding DNA; for example in Homo sapiens, it is estimated that only 3% of the genome codes for protein.

Table 2. Approximate genome sizes of representative eukaryotes.
Organism
Genome size (Mb*)
Rhizopus oryzae
35
Saccharomyces cerevisiae
12
Aspergillus nidulans
31

Neurospora crassa

39

Coprinopsis cinerea

36

Ustilago maydis

20

Drosophila melanogaster

122

Sea Urchin

814

Human

3 300

*Mb = megabases (106 base pairs). Genomic data is held on open databases on the Internet which are freely available. For the most up to date details visit the following URLs (needs Internet connection):

If you want to learn how to search and analyse genomes for yourself, there are two very helpful (free!) resources on the Internet:

  • Visit the excellent Exploring Genomes Bioinformatics Interactive Tutorial published by W. H. Freeman online at (needs Internet connection): http://bcs.whfreeman.com/mga2e/bioinformatics/
  • Visit the European Biocomputing Educational Resource (EMBER) Interactive Tutorial in Bioinformatics at (needs Internet connection): http://www.ember.man.ac.uk/login.php (you need to register for this tutorial module, but at the time of writing there is no cost).

The Japanese pufferfish (Fugu rubripes) has the shortest genome known for any vertebrate species, being only one-tenth the size of the human genome, but the size difference between these two genomes can be explained by differences in intron sizes. In contrast, analysis of genomes of grasses reveals that differences in sizes (up to 40-times) can be explained by extensive regions between genes filled with repetitive DNA. This leads to the conclusions that:

  • in animals most repeats integrate into intron DNA,
  • but in plants most repeats integrate into intergenic DNA (Wong et al., 2000).

So we might reasonably ask what are introns doing in fungi? No-one can answer that question yet, unless they are left over from the universal ancestor (see the section The tree of life has three domains in Chapter 2; CLICK HERE to view it now). The next Resources Box suggests a few articles that will give you a little more information and discussion.

Resources Box

What are introns all about?

You can access the Wikipedia page about introns at this URL (needs Internet connection):
http://en.wikipedia.org/wiki/Intron

CLICK HERE to visit our page that provides access to references about introns.

The genomes are more similar when we compare the numbers of protein coding genes which the genomes specify. For example the Puffer fish and human genomes contain about 30,000 genes (although estimates vary enormously), Drosophila melanogaster contains about 14 000, Neurospoa crassa about 10 000, and yeast about 6 000.

The karyotype of most fungi, that is the profile of the chromosome set, can be resolved by electrophoresis (Wieloch, 2006). The technique reveals that chromosome length polymorphisms are widespread in both sexual and asexual species of fungi, revealing general genome plasticity. Tandem repeats, for example repeats of rRNA genes, frequently vary in length, and dispensable supernumerary chromosomes, which are usually less than one million base pairs in size, as well as dispensable chromosome regions also occur in fungal karyotypes. Many karyotype changes are genetically neutral; others may be advantageous in allowing strains to adapt to new environments.

Updated January 18, 2017