Ober Lab Research

Research in Insect Evolution

My primary interest is exploring molecular and morphological diversity in insects. Evolutionary history, molecular evolution, and developmental biology are critical in investigating the patterns and processes of morphological change and adaptations in groups of insects. Fundamental to my investigation of diversity and morphological and ecological change is seeking the evolutionary history of organisms. Beetles, in particular, offer a spectacular example of evolutionary success and diversification. They are a particularly good group of insects to examine the patterns and processes of morphological, ecological, and molecular change. My research interests center on understanding evolution and relationships of beetles in the family Carabidae and understanding processes producing patterns of morphological and taxonomic diversity in insects. Carabids are incredibly diverse and offer good models to study evolution and morphological adaptation.

Carabid Beetle Molecular Systematics and Evolution

Phylogeography of carabids in isolated habitats

Caves are considered to be extreme and isolated habitats because they are characterized by complete darkness and highly variable food resources. Nonetheless caves are home to a unique and diverse community of species that complete their life cycles within caves and are never found on the surface. Biologists are interested in cave fauna because they are often characterized by the reduction of eyes and pigmentation, and they represent some striking examples of morphological convergence. Cave species are often considered “super-specialists” that cannot survive outside the cave and have a very limited potential to disperse. As a result, the geographical range of cave species is usually very restricted, in many cases to a single cave. Caves species are well suited for evolutionary and biogeographic studies.

Isolated cave systems effectively function as subterranean islands for terrestrial cave specialists. At least for terrestrial animals, colonization and speciation across isolated caves is analogous in many ways to classic examples of island biogeography. Trechine cave beetles are important components of terrestrial cave communities throughout North America. Eastern North America supports an impressive diversity of cave-specialized trechine beetles with over 250 species. The exceptional species diversity of North American cave beetles makes their lineage uniquely valuable to the study of speciation processes in cave insects and other terrestrial cave organisms. Highly endemic cave faunas are relatively poorly studied, and it is likely that more species remain undiscovered. Many of species have very small ranges restricted to a single cave system or karst area and are Candidate Species for listing under the U.S. Endangered Species Act.

Scaphinotus petersi and Arizona Sky Islands

How populations become separate species is central to the study of evolution. Speciation stands at the interface between genetic changes and variation within and among populations and the diversity of life we see today. The Sky Islands ground beetle project sought to understand the evolutionary and biogeographic history along with the patterns of variation (genetic and morphological) in populations of beetles on the cusp of becoming separate species.

The Sky Islands of Arizona are a unique complex of high elevation mountain ranges in southeastern Arizona that are isolated from each other by intervening valleys of hot, dry grassland or desert. Like their oceanic counterparts, the Sky Islands are generators of diversity over multiple spatial and temporal scales and offer considerable potential for investigating how different evolutionary processes such as natural selection and genetic drift lead to species formation. Low-elevation habitat acts as a barrier to dispersal on Sky Islands, facilitating divergence of isolated populations.

Scaphinotus petersi is a large, flightless ground beetle confined to the cool, alpine, coniferous forest of mountaintops in Arizona. Currently, six subspecies of S. petersi are isolated from each other on separate mountain peaks in Arizona. It has been hypothesized that the forest habitat was connected at lower elevations during ice ages. The populations of beetles were thought to have diverged when the climate became warmer and dryer after the last glacial maximum (LGM) as the forests moved upslope and caused habitat fragmentation. We found that most mountain top forests were not connected during the LGM and much cooler and wetter conditions would be required to connect all the forests where S. petersi are found. We found that only some of the ranges hold genetically distinct populations, and the timing of separation among the populations does not appear to coincide with specific climatic events such as warming trends. In addition, we showed that predicted changes to the climate of the Sky Islands may result in the extirpation of S. petersi from some of the lower mountain ranges by the end of this century. Climate change poses a unique threat to Sky Islands. Temperature increases of as little as a few degrees could push Sky Island habitats to higher elevations, reducing their area and potentially causing local extinction of endemic taxa and divergent populations harboring unique genetic and phenotypic diversity.

We characterized the morphological variation and differentiation of S. petersi subspecies using morphometrics and measurements of several characters of the head, “neck,” and legs (among others). We found that most populations in different mountain ranges have significantly distinctive prontonum (“neck”) shapes and there is a geographical pattern to the differences in morphological shape. There are two genetically (Mitchell & Ober 2013) morphologically distinct groups of S. petersi that have been evolving independently for about 60,000 years. Some populations have evolved significant morphological differences in less than 10,000 years.

Evolution and Development of Insects

I have been building on my interests in insect diversity and morphological innovations that have made this group so successful. I am used a comparative approach to uncover how developmental pathways and genetic mechanisms influence morphological evolution. I studied developmental evolution of insect appendages and genes controlling body axis patterning to explore the genetic basis of morphological diversity. Diversification of insect segments and appendage structure and function were essential features of the evolutionary radiation of insects. Body segments and the appendages they bear have become specialized for feeding, walking, swimming, flying, and mating. How are developmental regulatory networks known to pattern particular aspects of morphology, such as appendages and body axes in Drosophila, modified in a related organism, Tribolium casteneum, an insect with a very different mode of development? And what is the role of positional information from genes in anterioposterior and dorsoventral axis formation? I studied whether morphological evolution involves regulatory changes in otherwise conserved gene networks. I asked what role such changes may play in the evolution of morphological diversity and whether developmental systems have properties that constrain or promote phylogenetic change.

Insects, and animals in general, set up the body axes early in development, using mechanisms that are paramount in directing the rest of morphogenesis. In Drosophila, and presumably other insects, morphogenesis results from progressive subdivision of the embryo along both the dorsoventral (D/V) and anterioposterior (A/P) axes. In addition, genes that direct segment boundaries and segment polarity are crucial in the spatial location of the head and thoracic appendages and especially in initiating appendage primordia. Proximodistal (P/D) axis patterning for appendages relies on positional information from the A/P and D/V axes for correct development. Segment polarity genes such as wingless (wg) and Engrailed (En) are involved in establishing the pattern of segments by refining the A/P axis and maintaining segmentation patterns. wg is expressed at the A/P compartment boundary in each segment in cells immediately anterior to the stripe of En gene product. Mutations in these genes lead to defects in A/P compartment boundaries and in downstream gene expression across Drosophila parasegments. Appendage primordia form at discrete A/P and D/V coordinates within particular segments in Drosophila. Unlike most insects, however, Drosophila appendages are formed from imaginal discs that differentiate during larval stages rather than developing appendages during embryogenesis. In addition, there are differences in the spatial and temporal regulation of axis patterning genes. Long germ band insects, like Drosophila, establish segments along the A/P axis in the entire germ band nearly simultaneously. In contrast, short germ band insects, like the red flour beetle Tribolium, add segments sequentially from anterior to posterior as the germ band elongates. Inferences about the evolution of insect axis, appendage development and regulatory networks employed in axis formation require an examination of genes purported to be important in the development of insects other than Drosophila in order to reveal the general developmental mechanisms underlying these phenomena.

Developmental gene expression studies have raised the possibility that despite substantial differences in embryology, many molecular aspects of axis and appendage patterning may be conserved. For example, wg expression in segmentally reiterated stripes is highly conserved across insect species as is wg expression along a ventral stripe in developing appendages. However, other developmental genes involved in axis patterning and appendage morphogenesis, such as decapentaplegic, snail, nubbin, and apterous, show a divergent pattern of gene expression between Tribolium and Drosophila.