Phylogeography of the Cactophilic Drosophila and Other 4 Arthropods Associated with Cactus Necroses in the 5 Sonoran Desert 6

Studies on the population genetics, phylogenetic relationships, systematics and 16 evolution of arthropods that inhabit necrotic tissue of cacti in the Sonoran Desert of North 17 America are reviewed. These studies have focused upon several species of insects (orders 18 Diptera and Coleoptera) and arachnids (order Pseudoscorpiones). For most taxa studied, 19 little genetic structure and high dispersal ability are found in populations inhabiting the 20 mainland and Baja California peninsula regions of the Sonoran Desert, consistent with the 21 availability of the rotting cactus microhabitat which is patchily distributed and ephemeral. 22 There is evidence, however, that the Gulf of California, which bisects the Sonoran Desert, 23 has played a role in limiting gene flow and promoting speciation in several taxa, including 24 histerid beetles, whereas other taxa, especially Drosophila nigrospiracula and D. mettleri, 25 apparently are able to freely cross the Gulf, probably by taking advantage of the Midriff 26 Islands in the northern Gulf as dispersal “stepping stones”. Genetic evidence has also been 27 found for historical population expansions dating to the Pleistocene and late Pliocene in 28 several taxa. Overall, these studies have provided important insights into how arthropods 29 with different life history traits, but generally restricted to a necrotic cactus microhabitat, 30 have evolved in an environmentally harsh and tectonically active region. In addition, they 31 suggest some taxa for further, and more detailed, hypothesis driven studies of speciation. 32 OPEN ACCESS