Heteromorphic sex-determining regions or mating-type loci can contain large regions of non-recombining sequence where selection operates under different constraints than in freely recombining autosomal regions. Detailed studies of these non-recombining regions can provide insights into how genes are gained and lost, and how genetic isolation is maintained between mating haplotypes or sex chromosomes. The Chlamydomonas reinhardtii mating-type locus (MT) is a complex polygenic region characterized by sequence rearrangements and suppressed recombination between its two haplotypes, MT+ and MT−. We used new sequence information to redefine the genetic contents of MT and found repeated translocations from autosomes as well as sexually controlled expression patterns for several newly identified genes. We examined sequence diversity of MT genes from wild isolates of C. reinhardtii to investigate the impacts of recombination suppression. Our population data revealed two previously unreported types of genetic exchange in Chlamydomonas MT—gene conversion in the rearranged domains, and crossover exchanges in flanking domains—both of which contribute to maintenance of genetic homogeneity between haplotypes. To investigate the cause of blocked recombination in MT we assessed recombination rates in crosses where the parents were homozygous at MT. While normal recombination was restored in MT+×MT+ crosses, it was still suppressed in MT−×MT− crosses. These data revealed an underlying asymmetry in the two MT haplotypes and suggest that sequence rearrangements are insufficient to fully account for recombination suppression. Together our findings reveal new evolutionary dynamics for mating loci and have implications for the evolution of heteromorphic sex chromosomes and other non-recombining genomic regions.

Volvocine algae constitute a unique comparative model for investigating the evolution of oogamy from isogamous mating types. The sex- or mating type-determining gene encodes a conserved RWP-RK transcription factor found in either the or male mating locus of dioecious volvocine species. We previously found that from the isogamous species () could not induce ectopic spermatogenesis when expressed heterologously in females, suggesting coevolution of Mid function with gamete dimorphism. Here we found that ectopic expression of from the anisogamous species () could efficiently induce spermatogenesis when expressed in females and, unexpectedly, that from the isogamous species was also able to induce spermatogenesis. Neither nor could complement a mutant, at least partly owing to instability of heterologous Mid proteins. Our data show that Mid divergence was not a major contributor to the transition between isogamy and anisogamy/oogamy in volvocine algae, and instead implicate changes in cis-regulatory interactions and/or trans-acting factors of the Mid network in the evolution of sexual dimorphism.

Biologic research is experiencing a transformation brought about by the ability of programmable nucleases to manipulate the genome. In the recently developed CRISPR/ Cas system, short RNA sequences guide the endonuclease Cas9 to any location in the genome, causing a DNA double-strand break (DSB). Repair of DSBs allows the introduction of targeted genetic manipulations with high precision. Cas9-mediated gene editing is simple, scalable, and rapid, and it can be applied to virtually any organism. Here, we summarize the development of modern gene editing techniques and the biology of DSB repair on which these techniques are based. We discuss technical points in applying this technology and review its use in model organisms. Finally, we describe prospects for the use of gene editing to treat human genetic diseases. This technology offers tremendous promise for equipping the nephrology research community to better model and ultimately, treat kidney diseases.

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