Stick insects: parthenogenesis, polyploidy and beyond

The most common way animals reproduce is bisexuality, namely through the mixis of male and female gametes. Bisexual reproduction relies on the Mendelian mechanism, although several additional modes also occur. In turn, the Mendelian inheritance of genetic variability stands on the meiotic process, which entrains recombination and chromosome reshuffling within the frame of a balanced segregation: any departure from these standard features will of necessity have a bearing on the genetic structure at both individual and population level. Sexual systems are typically eu-Mendelian when an equal complement of chromosomes is inherited from both parents and it is also likely to be transmitted to the following generations the same way (Normark 2006). Some genetic systems clearly derive from the sexual one but do not completely maintain the above defined symmetries: if they skip any of them, they are termed asymmetric and typically give rise to thelytoky, haplodiploidy and/or parent-specific gene expression (Normark 2006). Collectively, those sex-derived asymmetric reproductive systems have been defined as metasexual, leaving the term asexual to those genetic systems/reproductive modes which did not evolve from the Mendelian mechanism and do not make use of gametes (Scali et al. 2003 and quotations therein). The widespread identification of asexuality with the lack of recombination and/or fertilization and also the assumption that unisexuals invariably lack both of them, thus being therefore barred from any genetic variance and evolutionary potential, is ill-adviced (e.g., Bell 1982; Baxevanis et al. 2006): as a matter of fact, it is increasingly clear that many all-female taxa are not invariant genetic replicas as they should be if really asexual (McKinnel 1985; Hughes 1989), since most of the truly investigated thelytokous animals have shown a variety of genetic mechanisms producing new clones. This is obtained either by real recombination, as in automicts and even in some apomicts, or by mutation and new genome