Tetranychus urticae

Taxonomy: Eukaryota; Fungi/Metazoa group; Metazoa; Eumetazoa; Bilateria; Coelomata; Protostomia; Panarthropoda; Arthropoda; Chelicerata; Arachnida; Acari; Acariformes; Trombidiformes; Prostigmata; Anystina; Eleutherengona; Raphignathae; Tetranychoidea; Tetranychidae; Tetranychus

Introduction

The Chelicerata are the second largest group of predominantly terrestrial animals. Chelicerates (horseshoe crabs, scorpions, spiders, ticks and mites) are at the root of the arthropod phylum, representing the primitive state of the taxon. The most economically significant chelicerates are spider mites, dust mites and ticks, which belong to the order Acari. Spider mites represent major pests in agriculture, dust mites cause asthma, perennial rhinitis and atopic dermatitis while ticks are vectors of human diseases, including Lyme disease and haemorrhagic fever. Unfortunately, the developmental genetics of chelicerates is poorly understood and a major obstacle for future progress in many aspects of chelicerate biology is the lack of a model organism in this group.

To date, the development of a chelicerate model system has been hampered by their complex ontogeny, long development time and large genomes. In spider-mites known genome size varies from ~723Mbp in Tetragnatha elongata to ~5.5 Gbp in Habronattus decorus suggesting poliploidisation of the genome. So far no chelicerate genome has been sequenced.

Tetranychus urticae: a chelicerate with a potential for genetic studies

Spider mites include assemblages of web-spinning species that, in contrast with their predatory and parasitic relatives, feed on plants. Unlike spiders and scorpions, which have long development times, spider mites undergo very rapid development. One of most polyphagous species (recorded to feed on more that 200 plant species!) and one that undergoes extremely rapid development, is the Two spotted spider mite, Tetranychus urticae. T. urticae completes its embryonic development in 39 hours. Full developmental time from egg to adult can be less than 7 days in favourable temperatures. The spider mite is therefore a classical "R" strategist exhibiting rapid population growth. It produces large numbers of offspring, and can have explosive population growth under conducive conditions. This life history strategy, together with the use of agricultural plants as a food source, has made this species a major pest. Besides rapid development, spider mites have several other advantages for developmental work. They have transparent eggs, 150mm in size, which allow observation of the complete development under the microscope. T. urticae has one of the smallest known genomes in the chelicerates, 75Mbp. This small genome is distributed on three equal-sized holocentric chromosomes. Such a miniature genome (60% Drosophila genome and 80% C. elegans genome) made T. urticae not only a promising species for developmental studies but also opened an avenue for developing it as the chelicerate genetic model organism.

In addition to its location at an important phylogenetic node, T. urticae is an important pest species attacking field crops such as soybean and cotton, horticultural crops including apple, pear, peach and hops and also greenhouse vegetable crops and ornamental plants. The use of chemical pesticides is the predominant method of controlling spider mites, but due to its short generation time and high fecundity this mite has rapidly developed resistance to major pesticide groups. In addition to simple resistance to individual compounds, spider mites are also capable of developing cross-resistance to several different insecticide groups. Control of multi-resistant mites has become increasingly difficult and the genetic basis of such resistance remains poorly understood.

Currently, there is no well-defined system for dissecting the genetics of the plant-pest interaction. Although Arabidopsis makes an excellent genetic model for such studies, a genetically defined plant-eating arthropod is lacking. Thus a genetic dissection of mite-plant interactions would thus provide insights into the signalling and transcriptional basis of plant defences used against herbivores. In addition, genome-wide sequences of pest organisms lend themselves to analysis of the transcriptome's response to host plant defensive compounds, infection by viruses and microbial pathogens, and any number of phenomena affecting the pest status of arthropods. Therefore, the annotation of the T. urticae genome, representing a major plant pest, will be important for plant science and agriculture.

Our involvement

Toward this goal we're developing the spider mite Tetranychus urticae as a plant-eating arthropod model. This project capitalizes on multidisciplinary collaboration between our lab, the laboratory of Dr. Grbic (with expertise in developmental biology and pest control who is leading the sequencing project of the T. urticae genome) and the Joint Genome Institute.

In collaboration with:

• DOE Joint Genome Institute
• Miodrag Grbic Lab, Ontario, Canada