Targeted CRISPR-Cas9-based gene knockouts in the model brown alga Ectocarpus.

Brown algae are an important group of multicellular eukaryotes, phylogenetically distinct from both the animal and land plant lineages. Ectocarpus has emerged as a model organism to study diverse aspects of brown algal biology, but this system currently lacks an effective reverse genetics methodology to analyse the functions of selected target genes. Here, we report that mutations at specific target sites are generated following the introduction of CRISPR-Cas9 ribonucleoproteins into Ectocarpus cells, using either biolistics or microinjection as the delivery method. Individuals with mutations affecting the ADENINE PHOSPHORIBOSYL TRANSFERASE (APT) gene were isolated following treatment with 2-fluoroadenine, and this selection system was used to isolate individuals in which mutations had been introduced simultaneously at APT and at a second gene. This double mutation approach could potentially be used to isolate mutants affecting any Ectocarpus gene, providing an effective reverse genetics tool for this model organism. The availability of this tool will significantly enhance the utility of Ectocarpus as a model organism for this ecologically and economically important group of marine organisms. Moreover, the methodology described here should be readily transferable to other brown algal species.

Repeated co-option of HMG-box genes for sex determination in brown algae and animals.

In many eukaryotes, genetic sex determination is not governed by XX/XY or ZW/ZZ systems but by a specialized region on the poorly studied U (female) or V (male) sex chromosomes. Previous studies have hinted at the existence of a dominant male-sex factor on the V chromosome in brown algae, a group of multicellular eukaryotes distantly related to animals and plants. The nature of this factor has remained elusive. Here, we demonstrate that an HMG-box gene acts as the male-determining factor in brown algae, mirroring the role HMG-box genes play in sex determination in animals. Over a billion-year evolutionary timeline, these lineages have independently co-opted the HMG box for male determination, representing a paradigm for evolution's ability to recurrently use the same genetic "toolkit" to accomplish similar tasks.