The relationship between dung quality and oocyte resorption in laboratory and field populations of Lucilia cuprina

Oocyte resorption in Lucilia cuprina (Wiedemann) (Diptera: Calliphoridae) is indicative of protein shortages during ovarian development. It is commonplace in field populations and is thought to reflect the species’ dependence on sheep dung as its primary source of protein. To test this proposition, resorption was monitored in flies fed with sheep dung collected when pastures varied from lush to drought affected. Laboratory assays showed that the proportion of females that became gravid, their rate of ovarian development, and the size and number of oocytes matured all decreased significantly as the protein content of the food source decreased. Conversely, the number of oocytes resorbed increased as dung protein decreased. However, estimates of dung quality (DQ), as derived from a pasture growth–DQ model, were not informative in explaining the seasonal or regional prevalence of resorption in flies collected over several years in three widely disparate sheep‐grazing areas. Recalibration of the DQ model to provide daily estimates of dung protein, which were then used to predict the incidence of oocyte resorption at different times of the year, also failed to show any systematic effects. There was, however, a clear tendency for this model to underestimate the extent of resorption in field populations. Because wild flies would have ready access to a wide variety of food sources, many of which would have been more nutritious than sheep dung, this finding is contrary to expectation and hence the nutritional role of sheep dung merits further study.     

Relationships between nitrogen cycling microbial community abundance and composition reveal the indirect effect of soil pH on oak decline

Tree decline is a global concern and the primary cause is often unknown. Complex interactions between fluctuations in nitrogen (N) and acidifying compounds have been proposed as factors causing nutrient imbalances and decreasing stress tolerance of oak trees. Microorganisms are crucial in regulating soil N available to plants, yet little is known about the relationships between soil N-cycling and tree health. Here, we combined high-throughput sequencing and qPCR analysis of key nitrification and denitrification genes with soil chemical analyses to characterise ammonia-oxidising bacteria (AOB), archaea (AOA) and denitrifying communities in soils associated with symptomatic (declining) and asymptomatic (apparently healthy) oak trees (Quercus robur and Q. petraea) in the United Kingdom. Asymptomatic trees were associated with a higher abundance of AOB that is driven positively by soil pH. No relationship was found between AOA abundance and tree health. However, AOA abundance was driven by lower concentrations of NH₄⁺, further supporting the idea of AOA favouring lower soil NH₄⁺ concentrations. Denitrifier abundance was influenced primarily by soil C:N ratio, and correlations with AOB regardless of tree health. These findings indicate that amelioration of soil acidification by balancing C:N may affect AOB abundance driving N transformations, reducing stress on declining oak trees.Subject terms: Biogeochemistry, Soil microbiology, Microbial ecology     

Branch migration of Holliday junctions: identification of RecG protein as a junction specific DNA helicase.

The product of the recG gene of Escherichia coli is needed for normal recombination and DNA repair in E. coli and has been shown to help process Holliday junction intermediates to mature products by catalysing branch migration. The 76 kDa RecG protein contains sequence motifs conserved in the DExH family of helicases, suggesting that it promotes branch migration by unwinding DNA. We show that RecG does not unwind blunt ended duplex DNA or forked duplexes with short unpaired single-strand ends. It also fails to unwind a partial duplex (52 bp) classical helicase substrate containing a short oligonucleotide annealed to circular single-stranded DNA. However, unwinding activity is detected when the duplex region is reduced to 26 bp or less, although this requires high levels of protein. The unwinding proceeds with a clear 3' to 5' polarity with respect to the single strand bound by RecG. Substantially higher levels of unwinding are observed with substrates containing a three-way duplex branch. This is attributed to RecG's particular affinity for junction DNA which we demonstrate would be heightened by single-stranded DNA binding protein in vivo. Reaction requirements for unwinding are the same as for branch migration of Holliday junctions, with a strict dependence on hydrolysis of ATP. These results define RecG as a new class of helicase that has evolved to catalyse the branch migration of Holliday junctions.