A speciation continuum within repeated origin of hypertrophied lips in
cyprinid fish from East Africa
Abstract
Trophic specialization plays a major role in ecological speciation.
Multiple adaptive radiations among cyprinid fish of the genus
Labeobarbus were recently discovered in riverine environments of the
Ethiopian Highlands, East Africa. These radiations contain parallel
diversifications of trophic morphology, including thick-lipped or
‘rubberlip’ phenotype, whose trophic function remains largely
unexplored. To test the adaptive value of thickened lips, to identify
the ecological niche of the thick-lipped ecomorphs, and to test of these
ecomorphs are the products of speciation we studied six sympatric pairs
of ecomorphs with hypertrophied lips and the normal lip structure from
different riverine basins. Trophic morphology, diet, stable isotope (15N
and 13C) signatures as well as mtDNA markers and genome-wide SNP
variation were analyzed. Our results show that thick-lipped ecomorphs
partition trophic resources with generalized ecomorphs in only one half
of the examined sympatric pairs despite the pronounced divergence in lip
structure. In these thick-lipped ecomorphs that were trophically
diverged, the data on their diet along with the elevated 15N values
suggest an insectivorous specialization different from the basal
omnivorous-detritivouros feeding mode of generalized ecomorphs. Genetic
data confirmed an independent and parallel origin of all six lipped
ecomorphs. Yet, only one of those six thick-lipped ecomorphs had a
notable genetic divergence with sympatric non-lipped ecomorphs based on
nuclear SNPs data (FST = 0.21). Sympatric pairs can be sorted by
combinations of phenotypic, ecological, and genetic divergence within
the speciation continuum from an ecologically non-functional mouth
polymorphism via ecologically functional polymorphism to completed
speciation via divergent evolution.