Incipient speciation in the meadow grasshopper, Chorthippus parallelus (Orthoptera: Acrididae).
This thesis examines the evolutionary divergence between northern European and
Italian populations of Chorthippus parallelus. Several differing approaches were
taken, which identified the inception of various components of the speciation
process between these parapatric populations which meet in the Alps.
Firstly, partial post-zygotic reproductive isolation was demonstrated using hybrid
crosses. The male hybrid offspring of both reciprocal crosses were sterile,
displaying severe testicular dysfunction, while the female hybrids showed no
deleterious effects of hybridisation. In this grasshopper the males are the
heterogametic sex, possessing a single X chromosome, and so this pattern of hybrid
sterility conforms to Haldane's rule. Secondly, investigation of the calling song of
the male grasshopper, a component of the mate recognition system, suggested the
presence of pre-mating reproductive isolation. Males from the different races were
found to sing calling song of a significantly different structure. Finally, examination
of DNA sequence divergence in a mitochondrial DNA marker demonstrated
Significant levels of genetic differentiation between the races. This population
divergence and incipient reproductive isolation parallels that found between the
north European and Iberian populations of this grasshopper, and provides further
evidence that the divergent geographical races have resulted from allopatric
divergence while in isolated refugial populations during the glacial periods of the
These approaches were repeated to investigate genetic divergence between
localised populations within the Italian peninsula. No hybrid dysfunction was
observed between these populations, suggesting that they are recently derived
from one continuous population. This was probably the refugial population of the
last ice-age. Additionally, investigation with the mtDNA marker gave preliminary
evidence for population expansion from the south to the north of Italy.
Interestingly, the male calling song was Significantly different in populations from
the north and the south of Italy, suggesting that a component of pre-mating
reproductive isolation may have evolved prior to post-mating isolation in allopatric
populations of C. parallelus.