Gene flow and the maintenance of species boundaries

Hybrid zones are regions where individuals from genetically differentiated populations meet and mate, resulting in at least some offspring of mixed ancestry. Patterns of gene flow (introgression) in hybrid zones vary across the genome, allowing assessment of the role of individual genes or genome regions in reproductive isolation. Here, we document patterns of introgression between two recently diverged species of field crickets. We sampled at a very fine spatial scale and genotyped crickets for 110 highly differentiated single nucleotide polymorphisms (SNPs) identified through transcriptome scans. Using both genomic and geographic cline analysis, we document remarkably abrupt transitions (<100 m) in allele frequencies for 50 loci, despite high levels of gene flow at other loci. These are among the steepest clines documented for any hybridizing taxa. Furthermore, the cricket hybrid zone provides one of the clearest examples of the semi-permeability of species boundaries. Comparisons between data from the fine-scale transect and data (for the same set of markers) from sampling a much larger area in a different region of the cricket hybrid zone reveal consistent patterns of introgression for individual loci. The consistency in patterns of introgression between these two distant and distinct regions of the hybrid zone suggests that strong selection is acting to maintain abrupt discontinuities within the hybrid zone and that genomic regions with restricted introgression likely include genes that contribute to nonecological prezygotic barriers.

Revised depth scale, splice tie points and chron boundaries of ODP Hole 151-907A and Site 162-907

The magnetic polarity stratigraphy at Site 907 obtained from the shipboard pass-through magnetometer and from discrete samples is readily interpretable back to the onset of the Gilbert Chron (5.89 Ma). From this level to the base of the section at ~14 Ma, the interpretation is corroborated by silicoflagellate datums with predictable correlation to polarity chrons. The resulting magnetostratigraphic interpretation differs from those proposed in the Leg 151 (Hole 907A) and 162 (Holes 907B and 907C) Initial Reports volumes. An important hiatus in the 7-10 Ma interval at Site 907 caused sedimentation to slow or cease for ~2.7 m.y. We have revised the shipboard correlation among the three holes at Site 907, resulting in a new composite section splice and recalculation of composite depths. For Site 985, magnetostratigraphic interpretation is possible down to ~150 meters below seafloor (mbsf) (C3An/C3Ar) at ~6 Ma. There are no useful biostratigraphic datums from Site 985 to support this interpretation; however, the interpretation is supported by the correlation of Sites 985 and 907 using natural gamma data from the shipboard multisensor track. Below ~150 mbsf at Site 985, drilling-related deformation at the onset of extended core barrel drilling precluded magnetostratigraphic interpretation.

(Table 1) Magnetozone boundaries at DSDP Site 68-502

Obtaining long, continuous, and undisturbed sections of unconsolidated Neogene deep sea sedimentary sections has been limited by (1) practical length of piston cores to about 30 meters and (2) disturbance of sediment by rotary drilling with Glomar Challenger. The relatively high deposition rates of late Neogene sediments in the North Atlantic and in the Caribbean in particular has limited penetration, with conventional piston coring, to sediments not much older than late Pliocene in the Atlantic and not even through the late Pleistocene in the Caribbean. Rotary drilling has penetrated much older sediments in both areas, but the cores suffered extensive drilling disturbance that seriously degrades the Paleomagnetism of the material. Utilization of the hydraulic piston corer on the Challenger combines the advantage of a generally undisturbed recovery and great penetration to produce long, relatively undisturbed sections of late Neogene and Quaternary sediments suitable for paleomagnetic studies. In this chapter we present paleomagnetic data from Site 502. We tried to determine relative azimuthal orientation of successive cores (see Introduction for details). Because the low latitude of the site meant a small (inclination of about 22°) vertical component of magnetization, reversals of magnetization could easily be detected only in changes in the horizontal component, as 180° shifts in the declination direction of magnetization. Based on information from the core orienting device, a fiducial line was drawn the length of each core prior to cutting it into the standard 1.5 meter sections.

Depths and ages of seismic sequence boundaries in ODP Leg 166 holes

High-resolution, multichannel seismic data collected across the Great Bahama Bank margin and the adjacent Straits of Florida indicate that the deposition of Neogene-Quaternary strata in this transect are controlled by two sedimentation mechanisms: (1) west-dipping layers of the platform margin, which are a product of sea-level-controlled, platform-derived downslope sedimentation; and (2) east- or north-dipping drift deposits in the basinal areas, which are deposited by ocean currents. These two sediment systems are active simultaneously and interfinger at the toe-of-slope. The prograding system consists of sigmoidal clinoforms that advanced the margin some 25 km into the Straits of Florida. The foresets of the clinoforms are approximately 600 m high with variable slope angles that steepen significantly in the Pleistocene section. The seismic facies of the prograding clinoforms on the slope is characterized by dominant, partly chaotic, cut-and-fill geometries caused by submarine canyons that are oriented downslope. In the basin axis, seismic geometries and facies document deposition from and by currents. Most impressive is an 800-m-thick drift deposit at the confluence of the Santaren Channel and the Straits of Florida. This "Santaren Drift" is slightly asymmetric, thinning to the north. The drift displays a highly coherent seismic facies characterized by a continuous succession of reflections, indicating very regular sedimentation. Leg 166 of the Ocean Drilling Program (ODP) drilled a transect of five deep holes between 2 and 30 km from the modern platform margin and retrieved the sediments from both the slope and basin systems. The Neogene slope sediments consist of peri-platform oozes intercalated with turbidites, whereas the basinal drift deposits consist of more homogeneous, fine-grained carbonates that were deposited without major hiatuses by the Florida Current starting at approximately 12.4 Ma. Sea-level fluctuations, which controlled the carbonate production on Great Bahama Bank by repeated exposure of the platform top, controlled lithologic alternations and hiatuses in sedimentation across the transect. Both sedimentary systems are contained in 17 seismic sequences that were identified in the Neogene-Quaternary section. Seismic sequence boundaries were identified based on geometric unconformities beneath the Great Bahama Bank. All the sequence boundaries could be traced across the entire transect into the Straits of Florida. Biostratigraphic age determinations of seismic reflections indicate that the seismic reflections of sequence boundaries have chronostratigraphic significance across both depositional environments.

Subgrain boundaries and related microstructural features in EDML (Antarctica) deep ice core

Subgrain boundaries revealed as shallow sublimation grooves on ice sample surfaces are a direct and easily observable feature of intracrystalline deformation and recrystallization. Statistical data obtained from the EPICA Dronning Maud Land (EDML) deep ice core drilled in East Antarctica cannot detect a depth region of increased subgrain-boundary formation. Grain-boundary morphologies show a strong influence of internal strain energy on the microstructure at all depths. The data do not support the classical view of a change of dominating recrystallization regimes with depth. Three major types of subgrain boundaries, reflecting high mechanical anisotropy, are specified in combination with crystal-orientation analysis.