Credit to Patrick Alexander
The rabbit hole of asexual reproduction literature is full of weird detours in the evolution of life.
There are asexual lineages that facultatively have sex, asexuals that still need sperm from other species, and asexuals that steal sperm from other species, among a whole host of flavors of asexual reproduction. These groups, which can be contemporary to ancient in age, are of significant interest to those studying the origin and maintenance of sex. However, one of the main problems in comparing asexual lineages with closely-related sexual species is that asexuals usually have traits, mainly increased ploidy, that confound inferences about reproductive mode.
An interesting paper in the recent edition of PNAS by Mau et al. investigate one of the only systems where disentangling the effects of reproductive mode and ploidy is even possible, the genus Boechera. These perennial plants include widely-spread sexual diploid species, asexual diploids, and asexual triploids. The authors identify specific alleles associated with the creation of apomictic seeds and use these alleles to distinguish between sexual and asexual specimens from across western North America.
Analyses of this distributional data showed comprehensive niche conservatism between sexual and asexual diploids (“the frozen niche”), but niche differentiation between sexual diploids and polyploids.
Our data provide phylogeographic evidence for multiple origins of apomictic cytotypes in Boechera and support a frozen-niche variation model for diploid apomixis niche evolution. Importantly, we provide statistical evidence that ploidy variation, both within and among species, is a stronger driver of niche evolution than reproductive mode.
This is just one aspect of a more complicated evolutionary history that includes multiple bouts of hybridization and transitions between reproductive modes, but Mau and colleagues provide one of the best recent efforts at understanding the mechanism that makes weird asexuals, well, weird.
Mau M., Lovell J.T., José M. Corral, Christiane Kiefer, Marcus A. Koch, Olawale M. Aliyu & Timothy F. Sharbel (2015). Hybrid apomicts trapped in the ecological niches of their sexual ancestors, Proceedings of the National Academy of Sciences, 201423447. DOI: http://dx.doi.org/10.1073/pnas.1423447112