2. Modelos para un duelo en Cobro revertido
2.3 Una canción de gesta malograda
1. The concept of model organisms
In trying to understand the many biological phenomena that we humans are confronted with, we have a long-standing tradition of using so called “model organisms”. The central idea is that the insights gained through examining a specific phenomenon in one particular ‘model’ organism can, to some extent, be generalised for other organisms (with as main interest: us humans). This is possible because of the common descent of all living organisms, hence the many features that are to a bigger or lesser extent shared by them (development, physiology, genome, etc.). The higher the relatedness among organisms, the more features they are expected to share (e.g. many features are shared among mammals, but even more among apes and humans), hence the more confidently we can generalise findings found for one representative of the group to the whole of the group (e.g. generalise results based on rats for all mammals). Through this use of model organisms, we can gain important ‘general’ knowledge, focussing only on a tiny subset of all living species. As mentioned above, the main use of model organisms is for human-oriented research. Some typical examples of model organisms are baker’s yeast (Saccharomyces cerevisiae, used to study cell division), the fruit fly Drosophila melanogaster (for the study of development), the nematode Caenorhabditis elegans (e.g. for the study of aging), mice (Mus musculus, for medicinal studies) and Arabidopsis (Arabidopsis thaliana, used for the study of plant physiology and development).
2. My ‘model’ organism: the two-spotted spider mite
The arthropod Tetranychus urticae Koch (the two-spotted spider mite) is not a model organism sensu stricto, but shares many of the typical advantages of such model organisms: it is easy to breed, easy to manipulate and intensely studied (e.g. its whole genome has already been sequenced, see Van Leeuwen et al. 2013). There are, of course, also some specific drawbacks, like difficulties for the transformation of mites, difficulty of tissue dissections due the mite’s small size, and the need to further develop tools for reverse genetics (see Van Leeuwen et al. 2013; Van Leeuwen & Dermauw 2016). Nevertheless, T. urticae is suitable for a very diverse set of approaches, ranging from field and common garden studies to artificial selection and experimental evolution (Belliure et al. 2010). More importantly for the current PhD thesis, this herbivorous pest species (see Kennedy & Storer 2000; Hill 2008) shifted its European range from the Mediterranean up to (at least) Northern Denmark during the last decades.
Reproduction and life cycle
The two-spotted spider mite is a highly fecund (Krainacker & Carey 1989) haplodiploid species that reproduces through arrhenotokous parthenogenesis (Helle 1967b). This implies that unfertilised females (diploid) can produce eggs, though only male ones (haploid). A fertilised female, in contrast, can produce both diploid (female) and haploid (male) offspring (see figure 26). Secondary (i.e. population-level) sex ratios are usually female-biased (3:1, see Krainacker & Carey 1990), but a mother can adjust her primary (i.e. individual-level) sex ratio according to the local circumstances (Young et al. 1986; Macke et al. 2011a).
Under optimal conditions (around 30°C; see Sabelis 1981), T. urticae completes its life cycle in approximately 8 days. This life cycle includes eight distinct stages: egg, larvae, nymfochrysalis, protonymph, deutochrysalis, deutonymph, teleiochrysalis, adult (see figure 27). The stages can relatively easily be distinguished visually since the larval stage has only six legs (instead of eight), since the –chrysalis stages are quiescent (immobile), since each stage is slightly bigger than the previous
Figure 26: The arrhenotokous parthenogenesis of the two-spotted spider mite Tetranychus urticae. Each coloured line represents one chromosome. Females are diploid (6 chromosomes) and males are haploid (3 chromosomes). Virgin females produce all male offspring. Fertilised
one (with females bigger than males), and since last moulting-stage females (teleiochrysalis) are often guarded by a male. Males typically develop on average 0.5 day faster than females (Sabelis 1981), which allows fresh males to locate a teleiochrysalis female to then guard her and be the first one to inseminate her upon emergence (Potter et al. 1976; Satoh et al. 2001). With increasing temperatures (with a maximum tolerable temperature of 35°C), development typically accelerates (Sabelis 1981), though not necessarily to the same degree for males and females.
Figure 27: Development cycle of Tetranychus urticae. The development from egg to adult includes eight distinct stages, of which four (including the egg) are immobile (dotted) and four (including the adult) are mobile (white). Percentages indicate the relative length of the different stages. These are rather constant (i.e. more or less irrespective of the temperature) (percentages
Distribution and dispersal behaviour
As described in detail in Carbonnelle et al. (2007), T. urticae expanded its European range from the Mediterranean to more northern areas. Based on my own field campaigns (where I sampled natural mite populations), the species at least reached northern Denmark by 2011. This spread is probably a combined result of global warming allowing a gradual spread to higher latitudes and the trans-European transport of plants (mostly for greenhouses –from which mites can escape to form local, isolated populations).
T. urticae can disperse both through crawling (ambulatory dispersal) and by making use of aerial currents (aerial dispersal). In order to be able to take advantage of such currents, the species exhibits a specific pre-dispersal behaviour whereby it uplifts its forebody and front legs to increase drag (i.e. the aerial take-off posture, see figure 28) (see Li & Margolies 1994). As ‘aerial plankton’, the species can then be carried along for relatively long distances (from neighbouring plant to several kilometres, see Smitley & Kennedy 1985). This behaviour is evoked by starvation and desiccation and allows the species to reach new food sources (though most individuals will face death) (Boykin & Campbell 1984; Hoy et al. 1984; Smitley & Kennedy 1985).
Figure 28: The aerial take-off posture