Geotail Measurements Compared with the Motions of High-Latitude Auroral Boundaries during Two Substorms

Geotail plasma and field measurements at -95 R(sub E) are compared with extensive ground-based, near-Earth, and geosynchronous measurements to study relationships between auroral activity and magnetotail dynamics during the expansion phases of two substorms. The studied intervals are representative of intermittent, moderate activity. The behavior of the aurora and the observed effects at Geotail for both events are harmonized by the concept of the activation of near-Earth X lines (NEXL) after substorm onsets, with subsequent discharges of one or more plasmoids down the magnetotail. The plasmoids must be viewed as three-dimensional structures which are spatially limited in the dawn-dusk direction. Also, reconnection at the NEXL must proceed at variable rates on closed magnetic field lines for significant times before beginning to reconnect lobe flux. This implies that the plasma sheet in the near-Earth magnetotail is relatively thick in comparison with an embedded current sheet and that both the NEXL and distant X line can be active simultaneously. Until reconnection at the NEXL engages lobe flux, the distant X line maintains control of the poleward auroral boundary. If the NEXL remains active after reaching the lobe, the auroral boundary can move poleward explosively. The dynamics of high-latitude aurora in the midnight region thus provides a means for monitoring these processes and indicating when significant lobe flux reconnects at the NEXL

Long-distance cue from emerging dermis stimulates neural crest melanoblast migration

Neural crest melanoblasts display unique navigational abilities enabling them to colonize the dorsal path between ectoderm and somite. One signal shown here to elicit melanoblast migration is a chemotactic cue supplied by the emerging dermis. Until dermis emerges, melanoblasts fail to enter the dorsal path. The dermis emerges from a site that is too distant to stimulate migration by cell contact. Instead, surgeries show that dermis elicits migration from a distance. When dermis is grafted distally, neural crest cells enter the path precociously. Moreover, large grafts recruit melanoblasts from the control sides (without increasing crest cell numbers) as well as a few crest cells from ventral somite. Because other grafted tissues fail to stimulate migration, the dermis stimulus is specific. This report is the first documentation that trunk neural crest cells can be guided chemotactically. It also extends evidence that migration is exquisitely sensitive to temporal–spatial patterns of somite morphogenesis. Developmental Dynamics 229:99–108, 2004. © 2003 Wiley-Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/35174/1/10492_ftp.pd

Considering Intra-individual Genetic Heterogeneity to Understand Biodiversity

In this chapter, I am concerned with the concept of Intra-individual Genetic Hetereogeneity (IGH) and its potential influence on biodiversity estimates. Definitions of biological individuality are often indirectly dependent on genetic sampling -and vice versa. Genetic sampling typically focuses on a particular locus or set of loci, found in the the mitochondrial, chloroplast or nuclear genome. If ecological function or evolutionary individuality can be defined on the level of multiple divergent genomes, as I shall argue is the case in IGH, our current genetic sampling strategies and analytic approaches may miss out on relevant biodiversity. Now that more and more examples of IGH are available, it is becoming possible to investigate the positive and negative effects of IGH on the functioning and evolution of multicellular individuals more systematically. I consider some examples and argue that studying diversity through the lens of IGH facilitates thinking not in terms of units, but in terms of interactions between biological entities. This, in turn, enables a fresh take on the ecological and evolutionary significance of biological diversity