Strain-Dependent Relationship Between Growth Rate and Hyphal Branching in Neurospora Crassa

In a previous study of branch frequency in Neurospora crassa focused on the wild-type, no relationship between growth rate and the frequency of hyphal branching was observed. In subsequent experiments, it became clear that while this independence is valid for the wild type and most mutant strains, it fails to hold for a subset of morphological mutants. This study distinguishes a subset of Neurospora morphological mutants for their morphological response to altered growth rate. Growth rates are altered using two different methods: reduced temperature and nutrient-deficient media. This should assure that the observed effect is not due to simple conditional mutations in the mutant strains examined. The observed effect provides an additional method for characterizing morphological mutants. It also provides support for models of branching in which control of branching is tightly linked to mechanisms of tip growth

\u3ci\u3eDrosophila\u3c/i\u3e Muller F Elements Maintain a Distinct Set of Genomic Properties Over 40 Million Years of Evolution

The Muller F element (4.2 Mb, ~80 protein-coding genes) is an unusual autosome of Drosophila melanogaster; it is mostly heterochromatic with a low recombination rate. To investigate how these properties impact the evolution of repeats and genes, we manually improved the sequence and annotated the genes on the D. erecta, D. mojavensis, and D. grimshawi F elements and euchromatic domains from the Muller D element. We find that F elements have greater transposon density (25–50%) than euchromatic reference regions (3–11%). Among the F elements, D. grimshawi has the lowest transposon density (particularly DINE-1: 2% vs. 11–27%). F element genes have larger coding spans, more coding exons, larger introns, and lower codon bias. Comparison of the Effective Number of Codons with the Codon Adaptation Index shows that, in contrast to the other species, codon bias in D. grimshawi F element genes can be attributed primarily to selection instead of mutational biases, suggesting that density and types of transposons affect the degree of local heterochromatin formation. F element genes have lower estimated DNA melting temperatures than D element genes, potentially facilitating transcription through heterochromatin. Most F element genes (~90%) have remained on that element, but the F element has smaller syntenic blocks than genome averages (3.4–3.6 vs. 8.4–8.8 genes per block), indicating greater rates of inversion despite lower rates of recombination. Overall, the F element has maintained characteristics that are distinct from other autosomes in the Drosophila lineage, illuminating the constraints imposed by a heterochromatic milieu