Contact:

Cornell University

Department of Neurobiology and Behavior

Mudd hall, 215 Tower Rd, Ithaca, NY

thomasblankers@gmail.com

I use a variety of approaches to unravel the evolutionary history of this fascinating system. Previously, populations of Laupala cerasina were sampled across the big island of Hawaii, passing through several volcanic regions that differ in geological age – between 500,000 and 100,000 years old. These populations are potentially in the process of becoming different species: they are geographically isolated and have diverged in the cricket's signature behavioral phenotype: chirping, that is, the production of a mate calling song, which is a highly heritable sexual communication signal and strongly contributes to isolation of species (and potentially also populations within species). The L. cerasina populations can provide important insight into the effects of geography, gene flow, and selection on divergence through time. Using next-generation sequencing technologies, we obtain genome-wide genetic data for individual crickets across the geographic distribution. The combined genomic data of all the individuals can then reveal to what degree these geographically separated populations still exchange genes. This is important to help understand the extent to which populations are evolving independently or whether some gene flow homogenizes population-wide genomic variation – except for those genomic regions that diverge under the influence of selection. We can also relate this to levels of gene flow among different species. Importantly, these data are expected to reveal how and where in the genome selection is driving genetic divergence and whether this divergence can be coupled to to the earliest phases of speciation.

 

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Genomic divergence along a spatio-temporal cline across Hawaiian volcanoes

 

Our ability to cope with many of the major problems mankind will face in the coming decades – whether extinction of pollinators, evolution of resistance in microbiota, or exhaustion of natural (food) resources – depends strongly on our understanding of the processes that affect natural variation. However, the dynamic and complex nature of variation in organismal morphology, ecology, and behavior as well as variation in the genomic regions underlying these traits is difficult to unravel. This is in part because of the immense difference in time scales of typical research projects (on the order of a couple of years) and the time scales of evolution (on the order of hundreds or thousands of generations). Luckily, there are different ways to overcome this problem, one of which is found in populations of crickets on Hawaii. The sequential and recent origin of the Hawaiian islands and the limited dispersal of the endemic and flightless swordtail crickets (genus Laupala) provide a unique opportunity to track the progress of evolution through time by observing variation in space instead. 

PhD project: Acoustic communication, sexual selection, and speciation in field crickets

 

My PhD thesis broadly separates into two parts: (1) variation, evolutionary lability, and genetic architecture of cricket songs and female preferences for song traits and (2) the contribution of gene flow and selection during speciation in a system that is strongly driven by sexual selection. Under supervision by Matthias Hennig (Humboldt University Berlin) and Frieder Mayer (Naturkundemuseum Berlin), we elucidated how variation in female preferences for the rhythm and acoustic energy in the male song drives divergence in the stereotyped sexual signal, which forms a strong barrier to gene flow.

Using a combination of  quantitative genetic approaches we showed that both song and preference are polygenic and partially non-additive traits, with substantial phenotypic correlations among the different traits comprising the multivariate phenotype. We are still in the progress of following up these initial experiments with a QTL scan to further illuminate the genetic architecture of sexual communication in this system. A population genomic scan (currently in review) showed that a pair of recently diverged sister species are currently reproductively isolated but a long history of ancestral gene flow as well as more recent demographic events have strongly affected the patterns of genetic divergence in the transcriptome. Strikingly, despite this complex history, scans for selection highlighted transcriptomic regions that are potentially related to both acoustic and chemical sexual communication behaviour.

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Population level phylogeny for Laupala cerasina

Wordcloud based on my entire PhD thesis

The genomic architecture of rapid speciation in sexually divergent Laupala crickets

 

The 38 morphologically cryptic species of Laupala show among the highest speciation rates known for animals (Mendelson & Shaw, 2005, Nat.). Closely related species pairs differ conspicuously in the sexual signal - the male song - and corresponding female preferences. Sexual divergence has resulted in reproductive isolation in many species pairs independently. This raises the question if the number, distribution, and effect size of the genetic loci underlying song differentiation is similar across the phylogeny, i.e. whether there is parallel genetic differentiation.

 

In the Shaw lab, there is a strong interest in the quantitative genetics of song and preference variation in Laupala. I am leading two projects that address questions related to repeated divergence in the male song: (i) comparing structural variation in closely related species to see how chromosomal rearrangements and variation in rates of crossing over constrain recombination and genomic divergence, (ii) determining the extent to which QTL - quantitative trait loci, i.e. genetic markers that correlate with phenotypic variation - are on the same location and of the same magnitude in different species pairs. 

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