Different genetic paths lead to the same phenotypic destination

Male field crickets (Teleogryllus oceanicus) on the Hawaiian archipelago sing to attract mates using acoustic structures on their wings. While singing makes the ladies swoon, it also gives away the male cricket’s location, making it vulnerable to fatal attacks by a parasitoid fly (Ormia ochracea). In the last decade, male crickets that have lost the ability to sing have appeared, first on Kauai and then later on Oahu.
In their 2014 Current Biology paper, Pascoal et al. tested whether the presence of the silent, ‘flatwing’ phenotype on Oahu resulted from migration and introgression from the Kauai population or independent evolution of the trait on each island. Morphometric analyses showed Kauai and Oahu flatwing phenotypes were four times more different from one another than any two normal-winged phenotypes were from one another. Interestingly, this means different wing shapes achieve the same result- silence. Breeding crosses between normal and flatwing crickets showed flatwing is inherited as a Mendelian, sex-linked mutation on both islands. The shared mode of inheritance presents the possibility that a single mutation could produce the different flatwing phenotypes due to the different genetic backgrounds of the Kauai and Oahu crickets. The authors collected RADseq data and used a bulk segregant analysis (BSA) to identify single nucleotide polymorphisms (SNPs) in linkage disequilibrium with the flatwing phenotype in each population.

 If the observed population-level morphological differences are caused by expression of the same sequence variant in different genomic backgrounds after introgression, then the majority of linked SNPs should be shared between the two populations. In contrast, if the two wing-silencing phenotypes are caused by sequence variants that affect genetically distinct regions of the X chromosome, then the BSA should recover nonoverlapping sets of linked SNPs for each island.

And what did they find? The genome-wide scans showed a distinct set of SNPs were linked with flatwing in each island population, indicating different genomic architectures are responsible for the silent phenotypes in Hawaiian crickets. Essentially, this silent, flatwing trait has evolved independently on each island.

Divergent wing morphologies linked to different loci thus cause identical behavioral outcomes—silence—illustrating the power of selection to rapidly shape convergent adaptations from distinct genomic starting points.

Pascoal S, Cezard T, Eik-Nes A, et al. (2014) Rapid convergent evolution in wild crickets. Current Biology 24, 1369-1374.

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