Preferential Occupancy of R2 Retroelements on the B Chromosomes of the Grasshopper Eyprepocnemis plorans

R2 non-LTR retrotransposons exclusively insert into the 28S rRNA genes of their host, and are expressed by co-transcription with the rDNA unit. The grasshopper Eyprepocnemis plorans contains transcribed rDNA clusters on most of its A chromosomes, as well as non-transcribed rDNA clusters on the parasitic B chromosomes found in many populations. Here the structure of the E. plorans R2 element, its abundance relative to the number of rDNA units and its retrotransposition activity were determined. Animals screened from five populations contained on average over 12,000 rDNA units on their A chromosomes, but surprisingly only about 100 R2 elements. Monitoring the patterns of R2 insertions in individuals from these populations revealed only low levels of retrotransposition. The low rates of R2 insertion observed in E. plorans differ from the high levels of R2 insertion previously observed in insect species that have many fewer rDNA units. It is proposed that high levels of R2 are strongly selected against in E. plorans, because the rDNA transcription machinery in this species is unable to differentiate between R2-inserted and uninserted units. The B chromosomes of E. plorans contain an additional 7,000 to 15,000 rDNA units, but in contrast to the A chromosomes, from 150 to over 1,500 R2 elements. The higher concentration of R2 in the inactive B chromosomes rDNA clusters suggests these chromosomes can act as a sink for R2 insertions thus further reducing the level of insertions on the A chromosomes. These studies suggest an interesting evolutionary relationship between the parasitic B chromosomes and R2 elements.This study was supported by grants from the Spanish Ministerio de Ciencia y Tecnología (CGL2009-11917) and Plan Andaluz de Investigacion (CVI-6649), and was partially performed by FEDER funds and a grant from the National Institutes of Health (GM42790)

Rate limited diffusion and dissolution of multi-component non-aqueous phase liquids (NAPLs) in groundwater

Contamination of soil and groundwater by nonaqueous phase liquids (NAPLs) poses serious risks to human health and the environment and presents major challenges for cleanup. The presence of complex NAPL mixtures in the subsurface further complicates remediation efforts, transport predictions, and the development of accurate risk assessments. A comprehensive laboratory-scale study was conducted to elucidate the factors affecting dissolution and removal of NAPL including 1) the distribution of NAPL (uniform vs. non-uniform), 2) NAPL-water interfacial area (constant vs. changing), 3) multi-component NAPL systems (composition dependence), and 4) intra-NAPL diffusion. A series of column and time sequential batch experiments were conducted to assess the factors controlling dissolution processes under dynamic flow and equilibrium conditions. For comparison purposes, two independent NAPL systems were established for the series of experiments including single-component trichloroethene (TCE) whereby the NAPL interfacial area decreases as dissolution proceeds, and two-component TCE-hexadecane (HEX) in which the bulk NAPL (comprised primarily of insoluble HEX) interfacial remains constant. The results of this study show that significant dissolution rate and removal limitations during water-flushing exist for systems containing non-uniform NAPL (TCE) distributions, due to less available NAPL-water interfacial area. Effective TCE removal was 2 times longer for the non-uniform NAPL distribution experiment. TCE dissolution in the two-component NAPL systems (TCE and HEX) experienced significantly less rate limitation (absence of concentration tailing) than the single-component TCE systems due to the presence of a constant interfacial area for mass-transfer to occur during flushing. Each column experiment resulted in differing effectiveness with respect to mass removal. The multi-component TCE:HEX system experienced the fastest mass removal time, but was not considered the most efficient. The batch experiments demonstrated that as mole fraction of a particular component of a NAPL (TCE) mixture decreases, greater dissolution nonideality occurs, resulting in greater observed concentrations than those predicted by equilibrium dissolution (i.e. Raoult's Law). Dissolution nonideality, quantified by the NAPL-activity coefficient, increased for the lower TCE mole fraction systems from 1.7 to 6.1 for TCE:HEX mole fractions of 0.2:0.8 to 0.003:0.997, respectively. The results of the batch experiments also indicate that dissolution mass-transfer rates were nearly identical for both the single-component TCE systems and the TCE:HEX systems. This suggests that intra-NAPL diffusion is not a rate-limited process under the conditions of these experiments. Mass flux reduction analyses showed that the two-component (TCE:HEX) NAPL experiment resulted in less efficient removal behavior than the single-component TCE flushing experiments, likely due to the significantly lower TCE mass within the mixed NAPL system. The results from this study improved the understanding of NAPL dissolution and removal processes; most notably for NAPL mixture systems where NAPL-water interfacial area may be maintained during flushing and where significant dissolution nonideality may result from decreasing mole fractions of target contaminants in NAPL. (Published By University of Alabama Libraries