New study confirming splendid cave beetle isolation for neighborly Ptomaphagus hatchi
Hello there friends of the cave beetle,
This is a note I should have anticipated because I knew the study was in the making. But I didn't make the connection to posting here until now that it is published open access: Balogh et al. (2020): Population genomics in two cave-obligate invertebrates confirms extremely limited dispersal between caves. Scientific Reports 10, 17554.
In a nutshell, cave beetle supporter and researcher Kirk Zigler and colleagues at The University of the South - Sewanee used state of the art population genomic methods to test whether cave beetles travel between caves. Many cave species are not only found in one single cave but in a cluster of neighboring caves. These patterns could be explained by migration along subterranean routes between the existing caves. Alternatively, there might have been a wider distribution of the species at an earlier time, when it was still possible to move through surface soil at a more travel-friendly climate and terrain, but became subsequently "trapped" in the now disconnected caves. This earlier time period may even have differed by the availability of better travel gear such as the possession of functional flight wings which, as you may remember, have been lost in cave beetles like P. hirtus.
Speaking of which: Balogh et al. (2020) studied populations of the closely related P. hatchi, which is well represented in caves of Tennessee as part of the famously cave biodiverse southern Cumberland Plateau. Size, shape, and behavior-wise, P. hatchi and P. hirtus are pretty much indistinguishable. You would need to dissect their sex organs to distinguish the species. However, they have never been encountered in the same cave:
As to the contemporary cave travel vs isolation scenarios: Previous work of the Zigler group supported the latter by finding that the different cave populations of P. hatchi differ very clearly by gene sequence variants. This would not be the case if gene sequences continue to be exchanged through regular travelers (Dixon and Zigler, 2011). While very compelling, these studies were limited in sensitivity, advising caution before drawing final conclusions. The new work used a genome-wide scale approach, i.e. a variant of Restriction site–associated DNA (RAD) tag sequencing technique (Wang et al., 2012), to address the question with the necessary rigor.
To this end, Balogh et al. (2020) explored the genetic makeup of P. hatchi from 4 different caves that are 3-13 km apart from each other, clearly human walking distance! Notwithstanding these proximities, the patterns of genetic diversity between these cave populations were confirmed to be very distinct even at the partial genome-wide level, thus supporting the lack of frequent contemporary travel. As a matter of fact, the researchers were able to estimate that cave to cave travel events may still be happening but at the minuscule rate of 1-2 every 100,000 years. This must make for major welcome parties. And lots of long overdue snail mail...
As always, there is much more intriguing detail in a thorough and fascinating study like this. I will leave it at pointing out that the research group probed the cave hopping vs isolation scenario not only for P. hatchi but also for another arthropod species: the cave spider species Nesticus barri, which populates the same 4 locations and was found to engage in the same cave traveling frequency, thus making a strong point about the generality of low intercave travel activities in subterranean animals.
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