How to turn an organism into a model organism in 10 'easy' steps

Abstract

Many of the major biological discoveries of the 20th century were made using just six species: Escherichia coli bacteria, Saccharomyces cerevisiae and Schizosaccharomyces pombe yeast, Caenorhabditis elegans nematodes, Drosophila melanogaster flies and Mus musculus mice. Our molecular understanding of the cell division cycle, embryonic development, biological clocks and metabolism were all obtained through genetic analysis using these species. Yet the 'big 6' did not start out as genetic model organisms (hereafter 'model organisms'), so how did they mature into such powerful systems? First, these model organisms are abundant human commensals: they are the bacteria in our gut, the yeast in our beer and bread, the nematodes in our compost pile, the flies in our kitchen and the mice in our walls. Because of this, they are cheaply, easily and rapidly bred in the laboratory and in addition were amenable to genetic analysis. How and why should we add additional species to this roster? We argue that specialist species will reveal new secrets in important areas of biology and that with modern technological innovations like next-generation sequencing and CRISPR-Cas9 genome editing, the time is ripe to move beyond the big 6. In this review, we chart a 10-step path to this goal, using our own experience with the Aedes aegypti mosquito, which we built into a model organism for neurobiology in one decade. Insights into the biology of this deadly disease vector require that we work with the mosquito itself rather than modeling its biology in another species.

Keywords: Behavior; CRISPR-Cas9; Genetics; Genome; Mosquito; Transgenesis.

Fig. 1.

Genomes, transcriptomes and gene annotations. (A) Comparative size of three insect genomes. (B) Methods for genome sequencing, assembly and annotation. (C) Schematic diagram of tissues used for bulk tissue RNA sequencing for the Aedes aegypti neurotranscriptome (Matthews et al., 2016). (D) Tissue-specific expression of chemosensory receptors as annotated in the updated genome (Matthews et al., 2018). TPM, transcripts per million. Photo credit: André Karwath (fly) and Alex Wild (mosquitoes).

Fig. 2.

Genome editing for cell type-specific labeling. (A) A female Ae. aegypti simultaneously contacting water with her foreleg and laying an egg on a substrate near the water line. Photo credit: Alex Wild. (B) Schematic diagram of a CRISPR-Cas9-generated knock-in/knock-out in which the ppk301 gene is disrupted by the insertion of the QF2 transcription factor. (C) Schematic diagram of a CRISPR-Cas9-generated gene-sparing knock-in in which a T2A ‘ribosomal skipping’ peptide facilitates expression of both the endogenous gene and the QF2 transcription factor. (D) Schematic diagram of the Q system. (E) Image of the 5th tarsomere of the front legs of animals of the indicated genotype expressing dTomato in ppk301-expressing cells. (F) Demonstration of the utility of the T2 ribosome-skipping peptide by co-expression of both dTomato and GCaMP6s in a single ppk301-expressing cell. Figure adapted from Matthews et al. (2019).