Abstract
Studies on the population genetics, phylogenetic relationships, systematics and evolution of arthropods that inhabit necrotic tissue of cacti in the Sonoran Desert of North America are reviewed. These studies have focused upon several species of insects (orders Diptera and Coleoptera) and arachnids (order Pseudoscorpiones). For most taxa studied, little genetic structure and high dispersal ability are found in populations inhabiting the mainland and Baja California peninsula regions of the Sonoran Desert, consistent with the availability of the rotting cactus microhabitat which is patchily distributed and ephemeral. There is evidence, however, that the Gulf of California, which bisects the Sonoran Desert, has played a role in limiting gene flow and promoting speciation in several taxa, including histerid beetles, whereas other taxa, especially Drosophila nigrospiracula and D. mettleri, apparently are able to freely cross the Gulf, probably by taking advantage of the Midriff Islands in the northern Gulf as dispersal “stepping stones”. Genetic evidence has also been found for historical population expansions dating to the Pleistocene and late Pliocene in several taxa. Overall, these studies have provided important insights into how arthropods with different life history traits, but generally restricted to a necrotic cactus microhabitat, have evolved in an environmentally harsh and tectonically active region. In addition, they suggest some taxa for further, and more detailed, hypothesis driven studies of speciation.
Highlights
In both D. pachea and D. mojavensis, a common pattern of reduced gene flow between the mainland and peninsula compared with D. nigrospiracula and D. mettleri is found, host plants for all four species of Drosophila are present on the Midriff Islands
We present a brief summary of our preliminary findings on the population genetics of this group, based on analysis of COI sequences [37], which complement the findings on the other species of cactophilic arthropods described above
Tectonic events resulting in the separation of the Pacific Plate from North American Plate, and giving rise to the Gulf of California, have had a wide-ranging impact on the evolution and speciation of animal taxa inhabiting the Sonoran Desert
Summary
The Sonoran Desert of North America encompasses approximately 260,000 km of ecologically diverse habitat in southwestern USA and northwestern Mexico [1], including the southeastern portion of California and most of southern Arizona in the USA, and the states of Baja California, Baja. California peninsula separated from the mainland, represents a potential dispersal barrier for insects and other terrestrial organisms. Another potential dispersal barrier is the Gran Desierto de Altar at the head of the Gulf (Figure 1), the largest active sand dune region in North America. The width of the Gulf varies from ~80–200 km, certain landscape features, the Midriff Islands in the northern Gulf (Angel de la Guarda, Tiburón, San Esteben, San Lorenzo, and several smaller islands; Figure 1), provide potential dispersal “stepping stones” between the mainland and the peninsula, especially for highly vagile insects. Columnar cacti utilized as host plants by a variety of cactophilic insects discussed below are found on many of these islands [5], increasing their potential to facilitate dispersal and gene flow across the Gulf
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