ALBERT

Description: :— Formation of microcrystalline quartz formation has proven to be effective at preserving porosity in deeply buried sandstone petroleum reservoirs, typically cemented by syntaxial quartz cement. There remains much uncertainty about what controls the growth of microcrystalline quartz and how it prevents syntaxial quartz overgrowths. Here, the Cretaceous Heidelberg Formation, Germany, provides a natural laboratory to study silica polymorphs and develop an understanding of their crystallography, paragenetic relationships, and growth mechanisms, leading to a new understanding of the growth mechanisms of porosity-preserving microcrystalline quartz. Data from scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) data illustrate that porosity-preserving microcrystalline quartz cement is misoriented with respect to the host grain upon which it grows. In contrast, ordinary quartz cement grows in the same orientation (epitaxially) as the host quartz sand grain, and typically fills pore spaces. EBSD and TEM observations reveal nanofilms of amorphous silica (~ 50–100 nm in thickness) between the microcrystalline quartz and the host grain. The microcrystalline quartz is interpreted to be misoriented relative to the host grain, because the amorphous silica nanofilm prevents growth of epitaxial quartz cement. Instead, the microcrystalline quartz is similar to chalcedony with [11–20] perpendicular to the growth surface and c axes parallel with, but randomly distributed (rotated) on, the host quartz grain surface. Development of pore-filling quartz growing into the pore (in the fast-growing c- axis direction) is thus inhibited due to the amorphous silica nanofilm initially and, subsequently, the misoriented microcrystalline quartz that grew on the amorphous silica.

Description: Evolutionary changes in genome size result from the combined effects of mutation, natural selection, and genetic drift. Insertion and deletion mutations (indels) directly impact genome size by adding or removing sequences. Most species lose more DNA through small indels (i.e., ~1–30 bp) than they gain, which can result in genome reduction over time. Because this rate of DNA loss varies across species, small indel dynamics have been suggested to contribute to genome size evolution. Species with extremely large genomes provide interesting test cases for exploring the link between small indels and genome size; however, most large genomes remain relatively unexplored. Here, we examine rates of DNA loss in the tetrapods with the largest genomes—the salamanders. We used low-coverage genomic shotgun sequence data from four salamander species to examine patterns of insertion, deletion, and substitution in neutrally evolving non-long terminal repeat (LTR) retrotransposon sequences. For comparison, we estimated genome-wide DNA loss rates in non-LTR retrotransposon sequences from five other vertebrate genomes: Anolis carolinensis , Danio rerio , Gallus gallus , Homo sapiens , and Xenopus tropicalis . Our results show that salamanders have significantly lower rates of DNA loss than do other vertebrates. More specifically, salamanders experience lower numbers of deletions relative to insertions, and both deletions and insertions are skewed toward smaller sizes. On the basis of these patterns, we conclude that slow DNA loss contributes to genomic gigantism in salamanders. We also identify candidate molecular mechanisms underlying these differences and suggest that natural variation in indel dynamics provides a unique opportunity to study the basis of genome stability.