Hybridization and Speciation | Natural History Museum Stuttgart
A grasshopper hybrid zone as a window into the two rules of speciation
Background
Questions
Implications
Team
What can hybrid zones tell us about genetic barriers between species?
The observation of pervasive patterns of reproductive isolation across species pairs suggests that generalities, or ‘rules’, underlie speciation in all animals and plants. The first is Haldane’s rule, or the preferential sterility of hybrid males; and the second is the large X-effect, or the disproportional role of sex chromosomes in species differences. Understanding how these rules first evolve before speciation is complete remains a challenging task because they are often observed between taxa that no longer hybridize in nature. We address this using two recently diverged subspecies of grasshoppers, Pseudochorthippus parallelus parallelus, and P. p. erythropus, that follow the ‘two rules of speciation’ while interacting in a hybrid zone.
We answer:
1. Which demographic history underlying the evolution of reproductive isolation?
2. Which genes and pathways are associated with hybrid male sterility?
3. Do those genes effectively act as long-standing barriers to gene flow between hybridizing taxa?
The combination of targeted sequencing, investigating hybridization under controlled laboratory conditions, and in nature, will provide novel insights on how the genetic patterns commonly observed across species first arise at early stages of species formation. Moreover, we will establish a new genomic approach transferable to other understudied organisms with large genomes that remain unstudied.
Fig. 1. Post-glacial expansion and the origin of the hybrid zone in Pseudochorthippus parallelus. Using transcriptomic data from populations collected throughout the species range (circles), we showed that glacial periods caused allopatric divergence of multiple lineages of grasshoppers (different colors), and that interglacial periods caused secondary contact among them.
Fig. 2. Using the hybrid zone to identify the genes involved in hybrid male sterility. We have sampled hundreds of individuals in independent transects perpendicular to the Pyrenean hybrid zone. Using a target-capture approach, we are able to measure introgression across the giant genome of this species, and identify the genes that cannot introgress across the hybrid zone, such as those that cause male sterility in experimental hybrids.