Decompression during Late Proterozoic Al2SiO5 Triple-Point Metamorphism at Cerro Colorado, New Mexico

An outstanding problem in understanding the late Proterozoic tectonic assembly of the southwest is identifying the tectonic setting associated with regional metamorphism at 1.4 Ga. Both isobaric heating and cooling, and counter-clockwise looping PT paths are proposed for this time. We present a study of the Proterozoic metamorphic and deformation history of the Cerro Colorado area, southern Tusas Mountains, New Mexico, which shows that the metamorphism in this area records near-isothermal decompression from 6 to 4 kbar at ca. 1.4 Ga. We do not see evidence for isobaric heating at this time. Decompression from peak pressures is recorded by the reaction Ms + Grt = St + Bt, with a negative slope in PT space; the reaction Ms + Grt = Sil + Bt, which is nearly horizontal in PT space; and partial to total pseudomorphing of kyanite by sillimanite during the main phase of deformation. The clearest reaction texture indicating decompression near peak metamorphic temperature is the replacement of garnet by clots of sillimanite, which are surrounded by halos of biotite. The sillimanite clots, most without relict garnet in the cores and with highly variable aspect ratios, are aligned. They define a lineation that formed with the dominant foliation. An inverted metamorphic gradient is locally defined by sillimanite-garnet schists (625 degrees C) structurally above staurolite-garnet schists (550 degrees C) and implies ductile thrusting during the main phase of deformation. The exhumation that led to the recorded decompression was likely in response to crustal thickening due to ductile thrusting and subsequent denudation.

The role of olfactory cues in the sequential radiation of a gall-boring beetle, Mordellistena convicta

1. Herbivorous insects often have close associations with specific host plants, and their preferences for mating and ovipositing on a specific host-plant species can reproductively isolate populations, facilitating ecological speciation. Volatile emissions from host plants can play a major role in assisting herbivores to locate their natal host plants and thus facilitate assortative mating and host-specific oviposition. 2. The present study investigated the role of host-plant volatiles in host fidelity and oviposition preference of the gall-boring, inquiline beetle, Mordellistena convicta LeConte (Coleoptera: Mordellidae), using Y-tube olfactometers. Previous studies suggest that the gall-boring beetle is undergoing sequential host-associated divergence by utilising the resources that are created by the diverging populations of the gall fly, Eurosta solidaginis Fitch (Diptera: Tephritidae), which induces galls on the stems of goldenrods including Solidago altissima L. (Asteraceae) and Solidago gigantea Ait. 3. Our results show that M. convicta adults are attracted to galls on their natal host plant, avoid the alternate host galls, and do not respond to volatile emissions from their host-plant stems. 4. These findings suggest that the gall-boring beetles can orient to the volatile chemicals from host galls, and that beetles can use them to identify suitable sites for mating and/or oviposition. Host-associated mating and oviposition likely play a role in the sequential radiation of the gall-boring beetle.

The Role of Olfactory Cues in the Sequential Radiation of a Gall-boring Beetle, Mordellistena convicta

1. Herbivorous insects often have close associations with specific host plants, and their preferences for mating and ovipositing on a specific host-plant species can reproductively isolate populations, facilitating ecological speciation. Volatile emissions from host plants can play a major role in assisting herbivores to locate their natal host plants and thus facilitate assortative mating and host-specific oviposition. 2. The present study investigated the role of host-plant volatiles in host fidelity and oviposition preference of the gall-boring, inquiline beetle, Mordellistena convicta LeConte (Coleoptera: Mordellidae), using Y-tube olfactometers. Previous studies suggest that the gall-boring beetle is undergoing sequential host-associated divergence by utilising the resources that are created by the diverging populations of the gall fly, Eurosta solidaginis Fitch (Diptera: Tephritidae), which induces galls on the stems of goldenrods including Solidago altissima L. (Asteraceae) and Solidago gigantea Ait. 3. Our results show that M. convicta adults are attracted to galls on their natal host plant, avoid the alternate host galls, and do not respond to volatile emissions from their host-plant stems. 4. These findings suggest that the gall-boring beetles can orient to the volatile chemicals from host galls, and that beetles can use them to identify suitable sites for mating and/or oviposition. Host-associated mating and oviposition likely play a role in the sequential radiation of the gall-boring beetle.

The role of olfactory cues in the sequential radiation of a gall-boring beetle

Species diversity itself may cause additional species diversity. According to recent findings, some species modify their environment in such a way that they facilitate the creation of new niches for other species to evolve to fill. Given the vast speciesdiversity of insects, the occurrence of such sequential radiation of species is likely common among herbivorous insects and the species that depend on them, many of them being insects as well. Herbivorous insects often have close associations with specific host plants and their preferences for mating and ovipositing on a specific host-plant species can reproductively isolate host-specific populations, facilitating speciation. Previous research by our laboratory has established that there are two distinct populations of thegall fly, Eurosta solidaginis (Tephritidae), which attack different species of goldenrods, Solidago altissima (Asteraceae) and S. gigantea. The gall fly’s host-associated differentiation is facilitating the divergence and potential speciation of twosubpopulations of the gall-boring beetle Mordellistena convicta (Mordellidae) by providing new resources (galls on stems of the galdenrods) for the gall-boring beetles. These beetles exist as two host-plant associated populations of inquilines that inhabit the galls induced by the gall fly. While our previous research has provided genetic and behavioral evidence for host-race formation, little is known about the role of their host plants in assortative mating and oviposition-site selection of the gall-boring beetles’ hostassociated populations. Volatile emissions from host plants can play a major role in assisting herbivores to locate their natal host plants and thus facilitate assortative mating and host-specific oviposition. The present study investigated the role of host-plant volatiles in host fidelity (mating on the host plant) and oviposition preference of M. convicta by measuring its behavioral responses to the host-plant volatile emissions using Y-tube olfactometers. In total, we tested behavioral responses of 615 beetles. Our resultsshow that M. convicta adults are attracted to their natal host galls (67% of S. altissima-emerging beetles and 70% of S. gigantea-emerging beetles) and avoid the alternate host galls (75% of S. altissima-emerging beetles and 66% of S. gigantea-emerging beetles),while showing no preference for, or avoidance of, ungalled plants from either species. This suggests that the gall beetles can orient to the volatile chemicals emitted by the galls and can potentially use them to identify suitable sites for mating and/or oviposition. Thus, host-associated mating and oviposition may play a role in the sequential speciation of the gall-boring beetle.