Sonoran Desert Research

Continental biotas are evolving assemblages whose present day distribution and diversity reflects a long history of responses (e.g., range shifts, speciation, or adaptation) to habitat change; changes produced by geological activity over the past several million years (deep time) as well as glacial-interglacial cycles over the most recent hundreds of thousands of years (shallow time). Particularly informative for identifying patterns of geological vicariance, glacial refugia, and range shifting are comparisons of the intraspecific phylogenies of unrelated taxa across the same geographic areas. This comparative approach is providing valuable insight into the influence of shared historical events on the genetic architectures of vertebrates, for example in the southeastern US. To date, however, there are relatively few such studies of plants and to my knowledge none involving plants and their associated insect herbivores and seed parasites. This is surprising given that that accounting for the diversity of plants (250,000 sp.) and their phytophagous (plant eating) insects (1-5 million sp.) is one of the most fundamental problems in the study of biodiversity. One candidate mechanism for this astounding diversity is that rapid diversification is driven, at least in part, by the effect on phytophagous insects and their hosts of common geological and climatic histories.

Building upon our past work (21,25,28,29,32,36), we are now exploring the causal linkages between historical processes and patterns of organismal diversification using Sonoran Desert plants and insects as a model system. Based on plate tectonics, geological evidence, and taxonomy, an especially well-developed set of biogeographic and phylogenetic hypotheses have been proposed for the Sonoran Desert. Recent mitochondrial DNA and allozyme studies of a number of vertebrate species (birds, lizards, and rodents) reveal coincident continental, peninsular Baja, and Cape Region phylogroups despite paleoecological evidence of profound changes in the region’s climate during the past 40,000 years.

In contrast to the strong vicariance revealed in vertebrate phylogenies, in recent work we have shown the historical distribution and genetic architecture of senita (Lophocereus schottii), a Sonoran Desert columnar cactus, to be profoundly influenced by climatic environmental change, as well as geological vicariance (32,36). Most plant species characteristic of the Sonoran Desert, including senita, are of tropical origin and have paleoecological distributions showing Pleistocene range contraction followed by northward range expansion from southern refugia only within the last 4-8,000 years. We are currently testing the hypothesis that range shifts in associated insects are geographically and temporally concordant with their host plants by studying the molecular phylogeography of the senita moth, Upiga virescens (Pyralidae), the obligate pollinator and seed parasite of L. schottii (Nason et al. in prep.). In contrast to vertebrates, a shared history of range contraction and expansion in senita and the senita moth appears to have resulted in geographically concordant phylogroups with contemporary distributions in Baja that are not delimited by ancient geotectonic sources of vicariance.

Are L. schottii and U. virescens aberrations? Can we generalize from the significant correlations in population genetic and phylogeographic structure in these species to other Sonoran Desert plant and phytophagous insect species pairs? These questions are the focus of new research that we are currently developing with NSF collaborator Dr. Rodney Dyer and his postdoc Dr. Ryan Garrick (both at Virginia Commonwealth University). The interesting plant and insect species interactions we are studying include the Sonoran Desert rock fig (Ficus petiolaris) and its pollinating (Pegoscapus sp.; Agaonidae) and non-pollinating (Idarnes; Sycophaginae) fig wasps, a succulent euphorb (Pedilanthus macrocarpa) and bark beetle (Araptus attenuatus; Scolytidae), and a wild cotton (Gossypium davidsonii) and associated boll weevil (Anthonomus grandis; Curculionidae). We are now in the process of developing informative genetic markers for these plant and insect species (AFLPs, cpDNA, and single-copy nuclear genes for the former and AFLPs and mtDNA for the latter). These genetic data will be analyzed, in conjunction with the well-developed biogeographic and phylogentic hypotheses for the Sonoran Desert, to ask two important questions linking historical processes to patterns of organismal diversification:

1. Do shared histories of geological vicariance and climatic fluctuation generate geographically concordant patterns of intraspecific genetic variation in plants and associated phytophagous insects?

• Does the symmetry of geographical patterns of gene flow differ in predictable ways between mutualistic and antagonistic plant-insect relationships? Does the localization of this symmetry, when it occurs, underlie geographical hotspots in the coevolutionary interactions between plant and insect taxa?
  

2. Alternatively, do geographic and evolutionary scales of differentiation vary between interactants?

• Can geological and climatic processes promote diversification (and ultimately speciation) of phytophagous insects within the range of a single host species? Or, do individual insect phylogroups (lineages) span multiple host phylogroups (i.e., races, subspecies, or species), a pattern unlikely to promote the diversification of phytophagous insects?