Physiological changes throughout an insect ear due to age and noise - a longitudinal study

Hearing loss is not unique to humans and is experienced by all animals in the face of wild and eclectic differences in ear morphology. Here we exploited the high throughput and accessible tympanal ear of the desert locust, Schistocerca gregaria to rigorously quantify changes in the auditory system due to noise exposure and age. In this exploratory study we analysed tympanal displacements, morphology of the auditory Müller’s organ and measured activity of the auditory nerve, the transduction current and electrophysiological properties of individual auditory receptors. This work shows that hearing loss manifests as a complex disorder due to differential effects of age and noise of several processes and cell types within the ear. The ‘middle-aged deafness’ pattern of hearing loss found in locusts mirrors that found for humans exposed to noise early in their life suggesting a fundamental interaction of the use of an auditory system (noise) and its aging

Investigating the Role of the Meiotic Chromosome Axes in Mediating Crossover Designation in Wheat and Barley

Meiotic crossovers are skewed towards the chromosome ends in wheat and barley. As a result, ~30% of genes rarely recombine in the ‘cold regions’, resulting in linkage-drag, which can be problematic for plant breeders as beneficial traits co-segregate with undesirable traits. An increase in the number of COs, or a shift in their positions, may disrupt linkage blocks and consequently benefit breeders. Pachytene Checkpoint Protein 2 (PCH2) is a conserved AAA+ ATPase, required to remodel the chromosome axes during meiosis and ensure accurate crossover formation. Previous work in Arabidopsis thaliana pch2 mutants has shown that crossovers are more likely to cluster together on short regions of the synaptonemal complex, as well as lose the obligate crossover, in the absence of PCH2. By using different methods of gene interference (CRISPR/Cas9, TILLING and VIGS), pch2 knockouts have been generated in diploid barley and tetraploid wheat and hypomorphs have been produced in hexaploid wheat. A cytological analysis has shown that PCH2 is involved in the timing of meiosis in all three ploidies and is required for the formation of the obligate CO in tetraploid wheat, but not in diploid barley. Full synapsis is achieved in barley pch2 mutants, but not tetraploid wheat pch2 mutants, where only short ZYP1 stretches are observed, localising with class I crossover marker HEI10. In addition, ectopic recombination is observed at meiotic metaphase I in tetraploid wheat pch2 mutants, indicating loss of crossover control and possible homoeologous recombination. Immunolocalisation studies indicate that PCH2 is involved in maintaining positive interference in wheat and barley, as a shift from positive to negative interference is observed in the absence of PCH2. This thesis offers a substantial contribution of knowledge to the meiosis community, providing further understanding of the role of PCH2 in crops and, importantly, in PCH2-mediated interference in plants

MutS homologue 4 and MutS homologue 5 maintain the obligate crossover in wheat despite stepwise gene loss following polyploidization

Crossovers (COs) ensure accurate chromosome segregation during meiosis whilst creating novel allelic combinations. Here we show that allotetraploid (AABB) durum wheat (Triticum turgidum subsp. durum), utilises two pathways of meiotic recombination. The class I pathway requires MSH4 and MSH5 (MutSγ) to maintain the obligate CO/chiasma and accounts for ~85% of meiotic COs, whereas the residual ~15% are consistent with the class II CO pathway. Class I and class II chiasmata are skewed towards the chromosome ends, but class II chiasmata are significantly more distal than class I chiasmata. Chiasma distribution does not reflect the abundance of double strand breaks, detected by proxy as RAD51 foci at leptotene, but only ~2.3% of these sites mature into chiasmata. MutSγ maintains the obligate chiasma despite a 5.4-kb deletion in MSH5B rendering it non-functional that occurred early in the evolution of tetraploid wheat and was then domesticated into hexaploid (AABBDD) common wheat (Triticum aestivum), as well as an 8-kb deletion in MSH4D in hexaploid wheat, predicted to create a non-functional pseudogene. Stepwise loss of MSH5B and MSH4D following hybridization and whole-genome duplication may have occurred due to gene redundancy (as functional copies of MSH5A, MSH4A, and MSH4B are still present in the tetraploid and MSH5A, MSH5D, MSH4A, and MSH4B are present in the hexaploid), or as an adaptation to modulate recombination in allopolyploid wheat