Differentiation across the Podisma pedestris hybrid zone inferred from high-throughput sequencing data
Abstract
Hybrid zones are regions where genetically differentiated forms come together
and exchange genes through hybrid offspring. The study of characters gradually
changing across such zones (clines) can give insight into evolutionary processes,
providing exceptionally sensitive estimates of the intensity of selection, and
allowing the detection of loci that might be involved in reproductive isolation
and speciation. The Alpine grasshopper Podisma pedestris has a hybrid zone
in Southern France where two populations meet. They differ in their sex
chromosome system, and strong selection against hybrids is observed. These
distinct populations likely have split and re-joined several times during the
Quaternary glacial cycles. A model explaining the selection observed against
hybrids postulates hundreds of loci of small effect spread over two differentiated
genomes meeting in secondary contact. Yet, over 50 years of study to-date non
have been discovered. However, so far the study of P. pedestris has not made
use of high-throughput sequencing data which provides an unprecedented
resolution of molecular markers.
I am aiming to close the gap with this thesis. I assemble the grasshopper’s
mitochondrial genome sequence and infer what proportion of its genome is
made up by mitochondrial inserts (Numts). Using transcriptome data from two
individuals, I then go on to fit demographic models, finding the populations
split approximately 400 000 years ago and that the current-day population sizes
are considerably smaller than the ancestral one. The final data chapter explores
the genetic architecture of the hybrid zone using data from a targeted sequence
capture of hundreds of loci covering some 10 000 polymorphic sites. Only two
loci under selection are identified, which is surprising given the power of the
analysis. Both loci are located on the X chromosome and are subject to weak
selection (0.3% and 0.03%). This shows the power of hybrid zone analysis
to infer targets of selection. The results are discussed in light of a theoretical
chapter on the ‘inexorable spread’ phenomenon and lead to the proposal for
further research into the causes of the reproductive isolation observed between
the grasshopper populations.
Authors
Becher, HannesCollections
- Theses [3593]