News from IBE (CSIC-UPF)
For some time sientists have been fascinated by the biology of a group of American leaf beetles which seemed prone to give up males and sexual reproduction altogether. It was thought that this was maybe a byproduct of a higher than usual resilience to the harmful effects of interspecific hybridisation and that unisexual species of these beetles were hybrid in origin. Phylogenetic approaches seemed to point in that direction.
Some later studies have confirmed that hypothesis investigating the genomic makeup of unisexual species by characterising allele diversity (and origin) for several unlinked nuclear loci. They have produced convincing proof that each unisexual species had genomic contributions from at least two, but up to three parental bisexual species. Being indeed evolutionary descendants of interspecific crosses.
An intriguing aspect of this system is that it was discovered that while all unisexual lineages had independent hybrid origins from different combinations of parentals, they actually shared the same mtDNA variant. This mtDNA was introgressed in these parental lineages from a possibly extinct ancestor (again, as a result of interspecific hybridisation). All in all, the evidence gathered for this system compelled us to propose that hybridisation is a necessary but not sufficient condition in the transition to unisexuality, which is a humble but important enough conceptual advance in our understanding of this evolutionary transition. This may very well be generalised to most other animals with similar life-histories.
Montelongo T, Gómez-Zurita J. 2015. Nonrandom patterns of genetic admixture expose the complex historical hybrid origin of unisexual leaf beetle species in the genus Calligrapha. American Naturalist 185(1): 113-134.