The effect of particle size on the in vivo degradation of poly(d,l-lactide-co-glycolide)/α-tricalcium phosphate micro- and nanocomposites.

This paper reports the effect of particle size within a resorbable composite on the in vivo degradation rate and host response. Resorbable composites based on poly(d,l-lactide-co-glycolide) (PLGA) reinforced with tricalcium phosphate (TCP) have shown suitable degradation, biological and mechanical properties for bone repair. Composites with nano-sized TCP particles degrade more homogenously in vitro than equivalent composites with micro-sized particles. In this study, PLGA and PLGA/TCP composites containing micro- or nano-sized α-TCP particles were implanted into an ovine distal femoral condyle defect and harvested at 6, 12, 18 and 24weeks. An intimate interface was observed between the new bone tissue and degrading implants. Visual scoring of histological images and semi-automated segmentation of X-ray images were used to quantify implant degradation and the growth of new bone tissue in the implant site. Bone growth into the implant site occurred at a similar rate for both composites and the PLGA control. However, the in vivo degradation rate of the nanocomposite was slower than that of the microcomposite and consequently more closely matched the rate of bone growth. For the first 6weeks, the rate of in vivo degradation matched that of in vitro degradation, but lagged significantly at longer time points. These results point to the potential use of ceramic particle size in controlling composite degradation whilst maintaining good bone formation. STATEMENT OF SIGNIFICANCE: This paper concerns degradable composites for orthopaedic application. The effect of particle size on implant degradation in vivo is not yet well characterised and these results give the first opportunity to directly compare in vitro and in vivo degradation rates for composites with micro- and nano-sized particles. This type of data is vital for the validation of models of composite degradation behaviour, which will lead to the design and manufacture of composites with a tailored, predictable degradation profile. The trainable segmentation tool can be used for future studies where X-rays of partially degraded implants (which have complicated greyscales and morphologies) need to be quantified without bias.

Epistasis among Drosophila persimilis factors conferring hybrid male sterility with D. pseudoobscura bogotana.

The Bateson-Dobzhansky-Muller model posits that hybrid incompatibilities result from genetic changes that accumulate during population divergence. Indeed, much effort in recent years has been devoted to identifying genes associated with hybrid incompatibilities, often with limited success, suggesting that hybrid sterility and inviability are frequently caused by complex interactions between multiple loci and not by single or a small number of gene pairs. Our previous study showed that the nature of epistasis between sterility-conferring QTL in the Drosophila persimilis-D. pseudoobscura bogotana species pair is highly specific. Here, we further dissect one of the three QTL underlying hybrid male sterility between these species and provide evidence for multiple factors within this QTL. This result indicates that the number of loci thought to contribute to hybrid dysfunction may have been underestimated, and we discuss how linkage and complex epistasis may be characteristic of the genetics of hybrid incompatibilities. We further pinpoint the location of one locus that confers hybrid male sterility when homozygous, dubbed "mule-like", to roughly 250 kilobases.

Slip-sliding away: serial changes and homoplasy in repeat number in the Drosophila yakuba homolog of human cancer susceptibility gene BRCA2.

Several recent studies have examined the function and evolution of a Drosophila homolog to the human breast cancer susceptibility gene BRCA2, named dmbrca2. We previously identified what appeared to be a recent expansion in the RAD51-binding BRC-repeat array in the ancestor of Drosophila yakuba. In this study, we examine patterns of variation and evolution of the dmbrca2 BRC-repeat array within D. yakuba and its close relatives. We develop a model of how unequal crossing over may have produced the expanded form, but we also observe short repeat forms, typical of other species in the D. melanogaster group, segregating within D. yakuba and D. santomea. These short forms do not appear to be identical-by-descent, suggesting that the history of dmbrca2 in the D. melanogaster subgroup has involved repeat unit contractions resulting in homoplasious forms. We conclude that the evolutionary history of dmbrca2 in D. yakuba and perhaps in other Drosophila species may be more complicated than can be inferred from examination of the published single genome sequences per species.

Molecular evolution of a Drosophila homolog of human BRCA2.

The human cancer susceptibility gene, BRCA2, functions in double-strand break repair by homologous recombination, and it appears to function via interaction of a repetitive region ("BRC repeats") with RAD-51. A putatively simpler homolog, dmbrca2, was identified in Drosophila melanogaster recently and also affects mitotic and meiotic double-strand break repair. In this study, we examined patterns of repeat variation both within Drosophila pseudoobscura and among available Drosophila genome sequences. We identified extensive variation within and among closely related Drosophila species in BRC repeat number, to the extent that variation within this genus recapitulates the extent of variation found across the entire animal kingdom. We describe patterns of evolution across species by documenting recent repeat expansions (sometimes in tandem arrays) and homogenizations within available genome sequences. Overall, we have documented patterns and modes of evolution in a new model system of a gene which is important to human health.

Fine-scale mapping of recombination rate in Drosophila refines its correlation to diversity and divergence.

Regional rates of recombination often correlate with levels of nucleotide diversity, and either selective or neutral hypotheses can explain this relationship. Regional recombination rates also correlate with nucleotide differences between human and chimpanzee, consistent with models where recombination is mutagenic; however, a lack of correlation is observed in the Drosophila melanogaster group, consistent with models invoking natural selection. Here, we revisit the relationship among recombination, diversity, and interspecies difference by generating empirical estimates of these parameters in Drosophila pseudoobscura. To measure recombination rate, we genotyped 1,294 backcross hybrids at 50 markers across the largest assembled linkage group in this species. Genome-wide diversity was estimated by sequencing a second isolate of D. pseudoobscura at shallow coverage. Alignment to the sequenced genome of the closely related species, Drosophila persimilis, provided nucleotide site orthology. Our findings demonstrate that scale is critical in determining correlates to recombination rate: fine-scale cross-over rate estimates are far stronger predictors of both diversity and interspecies difference than broad-scale estimates. The correlation of fine-scale recombination rate to diversity and interspecies difference appears to be genome-wide, evidenced by examination of an X-linked region in greater detail. Because we observe a strong correlation of cross-over rate with interspecies difference, even after correcting for segregating ancestral variation, we suggest that both mutagenic and selective forces generate these correlations, the latter in regions of low crossing over. We propose that it is not cross-overs per se that are mutagenic, but rather repair of DNA double-strand break precursors via crossing over and gene conversion.