A potentially powerful model system
One potential model system for elucidating the evolution of sex comes from soil-living animals, where asexuals are frequent, wide-spread and often co-exist with closely related ecologically similar sexual taxa (Bell 1982; Maraun et al. 2012). The tiny (< 1 mm) oribatid mites (Acariformes, Oribatida) are the animal group comprising most eukaryotic parthenogenetic species (10% of the group), are species-rich (> 10,000 species) and highly abundant (up to 350,000 ind./m²) (Maraun and Scheu 2000; Heethoff et al. 2009). Several families are exclusively parthenogenetic (Norton and Palmer 1991; Norton et al. 1993). Parthenogenesis has been a successful strategy for many oribatid mite species as many parthenogenetic lineages are species-rich and evolved and radiated over long periods of time (Maraun et al. 2003; Maraun et al. 2004; Heethoff et al. 2009; Schaefer et al. 2010). These taxa likely reproduce via thelytoky with terminal fusion automixis (Taberly 1987), potentially with an inverted sequence of meiosis. Asexual oribatid mites are obligate parthenogens and geographic parthenogenesis has not been described for this animal group.
Fossils of oribatid mites are known since the Devonian (∼390 mya; Norton et al. 1988), but molecular data suggest that oribatid mites likely were among the earliest colonizers of land and originated in the Cambrian to Precambrian era (571 ± 37 mya; Schaefer et al. 2010). Sex determination mechanisms are unknown and diploid chromosome number is mostly 18 (Heethoff et al. 2006; Heethoff et al. 2009).
The most important benefit of this system is presence of evolutionary replicates of different phylogenetic relatedness and age. Oribatid mites comprise congeneric sexual and asexual groups and whole clades of solely asexual taxa. The occurrence of young, old and ancient lineages and the availability of extensive ecological data renders oribatid mites a well suited animal group for investigating mechanisms for the maintenance of (a)sexual reproduction.
Caveats for the study oribatid mites.
As oribatid mites are very specious, the mite phylogeny is non-exhaustive, which can introduce some problems. Further, the inability to rear them in the lab (some are possible though) together with their small size (i.e. small amount of DNA extracts per individual) pose problems for the some prerequisites of genome sequencing. However, sequencing technologies are ever advancing and some currently available methods (e.g. 10xgenomics) can make it possible to generate reference genomes without the need of biased whole genome amplification. With these techniques, it is even possible to phase genomes into haplotypes and call structural variants. Thus, given some time and the vary unique evolutionary patterns of oribatid mites, these animals can be raised to the state of "model-organisms" for evolutionary investigations.
Timema stick insects
One of the best model systems so far and the lab pet of Tanja Schwander's group.
Timema stick insects are endemic to California in the USA and a particularly nice system for research, because sex was lost several times independently at different time scales (up to 2 my ago). Thus, the system offers ecological replicates of asexuals and sexual sister species. Moreover, data indicates that asexual Timema species are not of hybrid origin and not influenced by endosymbionts, which is very neat, because this excludes confounding effects.
The stick insects are not particularly big, but big enough for SMART sequencing and to sequence different tissues. Genome sizes are about 1.3 Gb, which is big, but still feasible. However, also stick insects are very hard to rear in the lab for several generations.