This journey that started in Tucson many years ago finally made its tortuous way into the peer-reviewed system and got published recently here . We also received some nice news coverage here!
One of my childhood cartoon heroes was Lucky Luke. For those who are not well versed in the franco-belgian universe of cartoons, Lucky Luke is a solitary cowboy living circa 1885 in the American Southwest "who shoots faster than his shadow", and whose life consisted of the recurring task of capturing the Dalton brothers, a pair of gangsters.
When I was a little older, I dove into the universe of Sergio Leone and watched Once Upon a Time in The West more times than I can remember, imagining myself being Claudia Cardinale captured by the villain Hendry Fonda. I imagined immense hot plains of red dust, harmonicas, horses, dust balls... This was my sole and idyllic knowledge of the American Southwest when I abruptly landed alone in Tucson in August 2007 with my giant field bag as a fresh PhD student.
We also found that gene flow within a mountain range is much greater between similar habitats than between different habitats, regardless of the geographic distance between them, pointing towards a mechanism of isolation-by-environment. Furthermore, the Sky Island M. emersoni populations revealed an interesting demographic history: the Northern and Southern Sky Islands each form a distinct group within which contact was more extensive in the past (Fig 6). Hold onto that, we’ll come back to it in a minute.
Workers in ants are universally wingless, while most reproductive castes have wings. But it is especially common for Monomorium species to have the unusual peculiarity of having some queens born without any wings. Some species within this genus have winged queens, while other species have wingless queens, and some have both.
Interestingly, we found that for some genes that are expressed in vestigial discs, there were differences in their expression in wingless-queen larvae coming from different Sky Islands. For example, we observed that the expression of mef2 – a key gene involved in wing muscle development – differs among wingless-queen larvae from different populations and shows a signature of the ancient demographic split: Mef2 shows expression differences between wingless queens from the northern and the southern Sky Islands.
Once upon a time, about 20,000 years BP when glaciers covered most of North America, the climate was much cooler. Forests surrounded the Arizona Sky Islands, ecologically connecting each mountain range with the others. One can imagine the hundreds of streams coming from the glaciated mountain tops, the luxurious trees, ferns, and flowers… infinite possibilities for animals to move freely from one Sky Island to another, according to their dispersal abilities. It would have been a perfect habitat for a winged queen, continuous and of low elevation (since the mountaintops were covered with snow!).
This independent evolution of wingless queens on each Sky Island occurred through recurrent interruptions of similar wing-patterning genes, and at the same time, induced the release of the unexpressed differences in some genes, such as mef-2, that accumulated in populations during or after the demographic split between the northern and southern Sky Islands ~80,000 years BP.
We also tried to highlight another phenomenon that hasn’t received much attention: the consequences attributable to organismal development during environmental changes. Scientists have built forecasting models about how species will react under climate change scenarios, which are necessary to predict how our landscapes, our agriculture, and our exposure to diseases will be influenced by global warming. However, most of these models are based on the current phenotypes and abilities of the species under study.
As in the case of Monomorium, developmental outcomes can be modified by a change in the environment and affect, for example here, the dispersal abilities of the individuals, which would greatly affect the predictions of such forecasting models. Our beautiful little Monomorium are probably facing a not-so-beautiful future, as their forested habitats will shrink even further under as global warming progresses in the American Southwest . This habitat loss would likely select for more and more wingless queens, completely eliminating their capacity as a species for any long-range dispersal to more hospitable habitats.
 Favé, Marie-Julie, et al. "Past climate change on Sky Islands drives novelty in a core developmental gene network and its phenotype." BMC evolutionary biology 15.1 (2015): 183.