Unintended side effects of transformation are very rare

Received wisdom in the field of fungal biology holds that the process of editing a genome by transformation and homologous recombination is inherently mutagenic. However, that belief is based only on circumstantial evidence. We provide the first direct measurement of the effects of transformation on the genome by sequencing the genomes of 29 transformants and 30 untransformed controls to high coverage. Contrary to the received wisdom, our results show that transformation of DNA flanked by long targeting sequences, followed by homologous recombination and selection on a drug marker, is extremely safe. Gene deletion may select for a few mutations that occur after transformation, but even then we found fewer than two point mutations per gene deletion strain. We also tested these strains for changes in gene expression and found only a few genes that were consistently differentially expressed between wild types and strains with a drug resistance marker inserted. As part of this process, we provide the first published genome sequence of the commonly used laboratory strain Cryptococcus neoformans var. grubii strain KN99. A total of 39 expression profiles were generated by RNA-seq analysis of wildtype and neutral transformant cells grown in capsule inducing conditions for 90 minutes. Raw sequencing data has already been submitted to the Short Read Archive under SRA study SRP107007 and BioProject PRJNA384617