Speciation in the face of long-range dispersal: population genomic
structure within a species complex of biting midges
Abstract
The level of gene flow between diverging lineages ultimately determines
the outcome of a speciation event. If secondary contact occurs before
this process is complete, reproductive isolation barriers must exist or
evolve to prevent hybridization. The selective pressures facilitating
and maintaining genetic divergence do not always involve an observable
phenotypic response, thus cryptic species form. The inability to
distinguish between sibling species can be a particularly serious
problem in groups responsible for pathogen transmission. Culicoides
biting midges occur almost world-wide and vector many disease-causing
pathogens that affect wildlife and livestock. In North America, the C.
variipennis species complex contains three currently recognized species,
only one of which is a vector, and limited molecular and morphological
differences have hindered vector surveillance. Here, genomic methods
were used to investigate speciation and genetic structure within this
complex. Single nucleotide polymorphism (SNP) data were generated using
ddRAD sequencing for 206 individuals originating from 17 locations
throughout the United States and Canada. Clustering analyses
consistently suggest the occurrence of five putative species with
significant differentiation occurring in both sympatric and allopatric
populations. Evidence of hybridization was detected in three different
species pairings, indicating a lack of pre-zygotic reproductive
isolation within the complex. Mitochondrial genes were used to trace the
hybrid parentage of these individuals, which illuminated discordance
with the SNP data. In this study, we highlight the potential role of
geographic, ecological, and behavioral isolation in speciation and in
maintaining species boundaries, despite hybridization and long range
dispersal.