The Mite Genome

 

The genome of the two-spotted spider mite, Tetranychus urticae has been sequenced and the results were published in Nature last month (Grbic et al. 2011).

Spider mites eat plants. They produce silk-like webs and that's why they're called "spider mites". They belong to the class Arachnida, which is the same group that contains spiders. The Arachnids are in the subphylum Chelicerata, a large group of arthropods distantly related to the insects and crustaceans. This is the first genome sequence of a chelicerate and that's why it's important.

Genome Size

The genome is only 90 Mb in size. It's the smallest arthropod genome that has been sequenced so far. Contrast this size with the human genome at 3,200 Mb or the genome of another tick, Ixodes scapularis, estimated to be 2,100 Mb. (Honeybee = 236 Mb, Drosophila = 140 Mb.) According to Ryan Gregory's animal genome size database this is the smallest known arachnid genome and the smallest known arthropod genome.

The authors estimate that there are 18,414 protein-encoding genes in the mite genome. This is about the same number of genes as most insects whose genomes have been sequenced and only slightly less than the number of genes in the human genome.

About 41% of the mite genome consists of exons (protein-encoding). Recall that less than 2% of our genome encodes proteins and in most insects the exon sequences make up less than 10% of the genome. (Honeybees and Drosophila also have smaller than average genome sizes.)

Introns

As you might imagine, the mite genome has a lot less junk DNA than other animals. This is partially reflected in the number and size of the introns. The average protein-encoding gene has less than three introns and the ones that are present are a lot smaller than the introns in species with larger genomes.

The figure on the right is a truncated version of a figure that appears in the supplemental information. It shows that the smallest introns are 40 bp and 70% of all introns are less that 150 bp in length (median = 96 bp). This is close to the smallest possible intron size allowing for slices sites and formation of a loop during splicing.

Transposons and Repetitive Sequences

Transposons (active and degenerate) make up less than 10% of the T. urticae genome and highly repetitive sequences (microsatellites) are almost absent. (The spider mite chromosomes don't have centromeres.)

Transposon sequences and highly repetitive sequences are a major component of the junk DNA found in large genomes so their absence in the mite genome is not a surprise.

Why Is the Mite Genome So Small?

The short answer is, we don't know. The long answer is much more complicated. As Michael Lynch points out (Lynch 2007 p.37), there's a balance between rates of insertion and deletion mutations. In species with small genomes the spontaneous rate of nucleotide deletion exceeds that of insertion so genome sizes shrink over time.

There may not be a selective advantage to having small or large genomes. It may just be that in some species the repair machinery tends to favor deletions while in closely related species the enzymes don't have this bias. Or maybe large genomes are slightly deleterious but the population size isn't large enough to allow natural selection to act. Some lineages may never have encountered significant bottlenecks so they've maintained a huge population size for millions of years allowing natural selection to operate on slightly deleterious mutations. This leads to smaller genomes.

Whatever the explanation, the small genome of mites shows us that most of the junk DNA present in other arthropod genomes is dispensable. That's why it's called "junk."

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Grbic, M. et al. (2011) The genome of Tetranychus urticae reveals herbivorus pest adaptations. Nature 479:487-492. [doi: 10.1038/nature10640] [PubMed]

Lynch, M. (2007) "The Origins of Genome Architecture" Sinauer Associates, Inc. Publishers, Sunderland, Massachusetts, United States