As summarized in the introductory section, well-validated CRISPR/Cas9 genome editing tools now make it routine to generate genome alterations via random mutagenesis near guide RNA (gRNA) cleavage sites (via the NHEJ pathway) or to create targeted edits based on templates carrying homologous sequences on either side of the gRNA-directed cut site (via homology direct repair - HDR). We employed the latter precise approach to generate a series of deletions of the endogenous L2-CRM. We utilized two gRNAs directing Cas9 cleavage on each side of a specific region to be deleted, as well as single-stranded oligonucleotides donors (ssODN) with 60 bp of homology to either DNA end flanking the region targeted for deletion (see Materials and Methods). We first used two gRNAs (gRNA-A and gRNA-B) to delete a 2023 bp regulatory region encompassing the previously identified 1.4 kb minimal L2-CRM. The choice of this region was based primarily on significant sequence conservation among closely related flies which extended beyond both borders of the fragment (Figure 1A ) analyzed in our original L2-CRM reporter studies (Lunde et al., 2003). When rendered homozygous, the kni^ri-AB allele resulted in complete elimination of the L2 vein (Figure 1C) suggesting that it represents a true, null allele of the L2-CRM function. Consistent with this phenotypic assessment, Kni protein expression was undetectable in the L2 primordium of ri-AB homozygous third larval instar wing imaginal discs (Figure 2B) or early pre-pupal wings (Figure 2—figure supplement 1).

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Figure 4

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Figure 4—source data 1

Raw Data for Figure 4.

Raw data of phenotypical scoring of female progeny of yellow MCR / riAB-CC heterozygous female crossed to wild type males. Analyzing inheritance of copycat construct by following the GFP marker; while the MCR activity was inferred by scoring yellow body phenotypes in F2 females.

https://doi.org/10.7554/eLife.30281.011

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Figure 5—source data 1

Raw data for quantifications of L2 vein phenotypes in CopyCat transformants.

Raw data of measurements and quanitifcations of relative L2 vein length to L3 vein length in 20 individual riCC-Mdom, wild type, and riCC-Ccap males; quantification of riCC-Mdom L2 vein shift by counting bristle numbers and measuring the angle of L2 to L3, and measurement of distance of L2 vein in riCC-Ccap from the margin of the wing.

https://doi.org/10.7554/eLife.30281.013

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As diagrammed in Figure 1A, we also generated strains of flies carrying a series of four smaller deletions (riΔ1-riΔ4) spanning the ~2 kb AB interval, each eliminating contiguous regions of sequence conservation. We also indicate three short regions that display a particularly high degree of sequence conservation across Drosophilids and other Schizophora flies, that we refer to as hyper-conserved identity islands (labeled ID1, ID2 and ID3, respectively, magenta, red and green boxes in Figure 1A also indicated in the multispecies alignment depicted in Figure 5A, and at the DNA sequence level in Figure 5—figure supplement 1). The generated deletions were tested for phenotypes in trans-heterozygosity with the riAB deletion allele and in the homozygous condition. Consistent with prior studies that identified a central region required for CRM activity (i.e. a 252 bp region deleted in ri1 (Figure 1D) that contains essential Scalloped activator binding sites [ Lunde et al., 2003]), the riΔ2 and riΔ3 deletions, which both partially overlap with the region deleted in ri1, produced strong vein-loss phenotypes similar to those of ri1 when placed in trans to riAB (Figure 1J,K). Also, consistent with the prior mapping of a repressor domain to distal sequences missing in the EC fragment (Figure 1A: blue box), the homozygous riΔ4 deletion produced a mild ectopic vein phenotype in peripheral regions of the wing (Figure 1I). The homozygous riΔ1 deletion produced a similar mild ectopic vein phenotype (Figure 1H) lending support to our suspicion mentioned above that conserved sequences that were not included in our previous reporter analysis, contribute to the overall CRM function. Since both the riΔ1 and riΔ4 deletions produced mild ectopic vein phenotypes, we generated a composite riΔ1+Δ4 deletion (by generating the riΔ1 deletion in the background of riΔ4) to determine whether these separated domains of the L2 CRM might act in concert. We found that this was indeed the case, as the homozygous combined riΔ1+Δ4 deletion produced a much more pronounced phenotype in which large thickened veins formed around L2 and L5 (Figure 1M), a phenotype that was similar albeit less pronounced in trans-heterozygotes over the AB deletion (riΔ1+Δ4/riAB Figure 2L). Consistent with this phenotype, significant levels of ectopic Kni expression were observed in the third instar imaginal discs of riΔ1+Δ4 homozygous mutants in both anterior and posterior domains of the wing pouch (Figure 2L), in a pattern similar to that driven by the EC-lacZ reporter construct analyzed previously (Lunde et al., 2003). The domains of ectopic Kni expression in riΔ1+Δ4 homozygotes are largely complementary to those of the Spalt transcription factor (Figure 2P–R) as is the case for wild-type Kni expression (Figure 2M–O), indicating that binding sites for Spalt to repress Kni expression in central regions of the wing, are likely retained in the remaining CRM sequences. Interestingly, the pattern of ectopic Kni expression in homozygous riΔ4 single mutant wing discs (Figure 2H) was similar in both pattern and level to that in riΔ1+Δ4 double mutant discs (Figure 2L), and only mild ectopic Kni expression was observed in riΔ1 mutant discs (Figure 2G). Slightly later in development during pre-pupal stages, however, narrower domains of high level Kni expression are observed in riΔ1+Δ4 double mutant (Figure 2—figure supplement 1R) in comparison to the persisting broader domains of Kni expression present in the riΔ1 single mutant (Figure 2—figure supplement 1O). These later differences in Kni expression may help account for differences in the severity of ectopic wing vein phenotypes in the riΔ4 single mutant versus the double riΔ1+Δ4 mutant(see Discussion). Thus, a combination of the Kni expression patterns and resulting adult morphological phenotypes provide more informative distinctions between these various mutants than can be ascertained by either measure alone.

In addition to phenotypes described above that were consistent with and extended previous observations, we also encountered unexpected results when testing the effect of homozygous riΔ2 and riΔ3 deletions. In contrast to the strong vein-loss phenotypes associated with these deletions in trans to the complete riAB deletion described above (Figure 1J,K), the phenotypes of these lesions were greatly reduced when homozygous (Figure 1F,G). This disparity was particularly striking for the homozygous riΔ3 deletion, which generated an almost wild-type phenotype, recognizable only by a subtle meandering or less pigmented L2 vein (Figure 1G). This very weak phenotype, which was observed in four independently derived riΔ3 mutant strains, is particularly surprising given that a single base pair change within that region associated with the ri53j allele leads to a substantial vein loss phenotype when homozygous ([Lunde et al., 2003]; Figure 1E). The recovery of these two deletions that produced surprisingly mild homozygous phenotypes suggested the possibility that a pairing-dependent process might contribute to sustaining CRM function. Consistent with this hypothesis, trans-heterozygotes of the riΔ3 deletion over the point mutant ri53j allele (Figure 3J), - a configuration predicted to disrupt precise alignment of homologous chromosomes - resulted in a pronounced vein-loss phenotype. We tested the pairing hypothesis more directly using the classic chromosomal inversion comparison by placing the riΔ3 deletion in-trans to the ri1 mutant either in the context of a native third chromosome or in a multiply inverted balancer chromosome (TM3, ri1) that greatly reduces pairing-dependent phenomena such as transvection (i.e. wherein the CRM on one chromosome acts to promote transcription from the homologous chromosome [Lewis, 1954]). Consistent with the pairing-based mechanism, the riΔ3/ri1 phenotype (Figure 3B) was significantly weaker than that observed in riΔ3/TM3, ri1 individuals (Figure 3F), an effect that was highly consistent (see Figure 3—figure supplement 1 for comparative L2 vein arrays of these two genotypes). A similar, albeit less dramatic, difference was also observed between the riΔ2/ri1 (Figure 3A) and riΔ2/TM3ri1 (Figure 3E) phenotypes. Reflective of the adult wing phenotypes, Kni expression in larval wing was only modestly reduced in riΔ3 homozygotes (Figure 2F), was further reduced in riΔ3/ri1 discs (Figure 2I) and virtually undetectable in either riΔ3/TM3ri1 (Figure 2J) or riΔ3/riAB trans-heterozygous discs (Figure 2K). As a final test of the CRM-pairing hypothesis we crossed the riΔ2 and riΔ3 mutants to each other and observed a substantial vein-loss phenotype in the riΔ2/riΔ3 trans-heterozygotes, again consistent with a disruption of local chromosomal pairing of the remaining activating elements. Cumulatively, these data provide evidence for a pairing-dependent interaction between CRM sequences on homologous chromosomes that promotes CRM activity (see Discussion; Figure 3M–R)

In addition to recovering strains of flies carrying the targeted deletions mentioned above, we also identified an ample collection of imprecise or fortuitously generated mutations that most likely resulted from DNA repair by the NHEJ or other break-repair pathways responding to single or double cleavage events at various locations (Figure 1—figure supplement 1A). For example, a deletion (riΔ3.35) that spans sequences removed in riΔ4 and includes small indels results in a variable loss of vein phenotype when placed in trans to riAB (Figure 1—figure supplement 1B,C) but has a wild-type phenotype when homozygous (Figure 1—figure supplement 1D) in contrast to the extra vein phenotype manifest by riΔ4 homozygotes (Figure 1I). Additionally, we recovered mutants which were generated with the use of a single gRNA, gRNA-C, targeting a conserved region (see Figure 1A and Figure 1—figure supplement 1A). They included deletions (EV 5–1 and EV 7–1) that have very similar breakpoints but result in differing degrees of vein-loss (Figure 1—figure supplement 1E,F), and the addition of a single nucleotide at the edge of the ID3 conserved region (EV 9–2) which has a phenotype that varies from wild type to an ri-like vein phenotype (Figure 1—figure supplement 1G). In another case (riΔ1.1), a 65 bp sequence that has been copied and inserted upstream causes a mild ectopic vein phenotype (Figure 1—figure supplement 1H). Perhaps not surprisingly, a deletion removing sequences spanning riΔ1 to a portion of riΔ4 (riΔ1.6) gives a composite partial vein loss and ectopic vein phenotypes (Figure 1—figure supplement 1I,J), while a mutant with a deletion spanning riΔ3-Δ4 plus a 25 bp insertion (riΔ3.38) causes an extreme vein-loss phenotype similar to that of riAB (Figure 1—figure supplement 1K,L). Although further analysis of several of these mutants will be necessary to fully understand the basis for associated venation phenotypes, we note two salient features of this mutant class. First, a variety of mutations preferentially affect portions of the L2 vein along the proximo-distal (PD) axis, revealing a previously unappreciated role of the PD patterning system in contributing to L2 morphogenesis. Second, in several cases, apparently minor variations in the positions of chromosomal breakpoints or small indels, result in markedly altered final vein patterns that may reveal important effects of specific sequences or requirements for precise spacing between functional motifs. Our recovery of several mutants with novel unanticipated phenotypes highlights the strength of an in locus analysis as all these mutations, even those resulting in very mild phenotypes, were readily identified phenotypically.

We recently demonstrated an active genetic process, which we refer to as the mutagenic chain reaction (MCR), to generate both somatic mutations and meiotic gene-drive (Gantz and Bier, 2015). The key feature of this process is the integration of a vector containing a gRNA and a Cas9 transgene at the precise genomic location of the gRNA-directed cleavage site. We also have proposed a split-drive configuration we refer to as CopyCat elements (Gantz and Bier, 2016), in which the Cas9 source is provided in-trans in a standard Mendelian fashion and the CopyCat element carries one gRNA to target insertion of a gene cassette or two gRNAs to simultaneously trigger cassette insertion and deletion of the region in between the two gRNA cleavage sites. CopyCat cloning vectors could, in principle, be used as versatile elements for germline transformation, which once inserted, are passed down to the progeny in a Super-Mendelian fashion (>>50% inheritance) in the presence of a separate Cas9 source that can be later segregated away. This conditional control of Super-Mendelian inheritance can facilitate combinatorial genetic schemes by circumventing constraints imposed by Mendelian inheritance (i.e. random chromosomal segregation and linkage) (Gantz and Bier, 2016).

We tested the concept of a two gRNA CopyCat element by generating an active riAB allele. A CopyCat riAB vector carrying the two gRNA-expressing genes required for generating the AB deletion, an eGFP marker gene, flanked by 1 kb homology arms corresponding to the kni locus abutting the two gRNA cut sites was inserted into the genome by co-injection with a Cas9-producing plasmid into embryonic polar plasm (Figure 4A). Transformant lines carrying this and other similar CopyCat constructs described below were recovered at frequencies similar to those typical of germline transformation using either P-element vectors or the ɸC31 recombinase system, exemplifying how Cas9-mediated site-directed transgenesis can serve as a viable alternative to traditional methods of germline transformation. We combined the riCC-AB allele with the previously characterized y1-MCR element, which contained a germline-expressed Cas9 gene, a gRNA targeting the yellow locus, and homology arms that precisely flank the genomic cleavage site. Following the cross of riCC-AB males to y1-MCR females (Figure 4B), resulting female progeny were crossed to w- males to evaluate the transmission of the riCC-AB allele in their progeny, as well as the generation of somatic phenotypes in female progeny. The results of nine such crosses carried out in parallel are shown in Figure 4C,D, all of which demonstrated a high rate of somatic mutagenesis of the yellow locus, indicating the efficient activity of the y-1MCR element. In four crosses, the riCC-AB allele was copied with 100% efficiency to F2 progeny, in three crosses the efficiency averaged 89%, and in two crosses the element was inherited at Mendelian frequencies. Regardless of the rate of germline transmission noted in these various crosses, the observable L2 truncation frequencies was indicative of somatic activity of the riCC-AB CopyCat and Cas9 alleles which were very similar and approximated that observed in their F1 female parent (~70%). We conclude that in this system the riCC-AB double-cut CopyCat element typically propagates via the germline with high efficiency and that the somatic and germline activities of the riCC-AB element seem to be separable events. This effect is probably due to high levels of Cas9 protein produced by the vasa driver in the egg, that can freely diffuse prior to cellularization, causing mutations affecting large regions of the organism, consistent with observations reported by Port et al. (Port et al., 2014).

As mentioned above, we identified three highly conserved sequence islands (termed ‘ID’ for identity) within the L2 CRM that displayed virtually no nucleotide variation across the entire Drosophilid clan (Figure 1A magenta, red, and green blocks ID1, ID2, ID3; their location in the CRM in Figure 5A, and IDs sequence alignment in Figure 5—figure supplement 1). We used these short conserved ID sequences alone or in combination to identify homologous regulatory regions from yet more distantly related flies in the broader Schizophora group (Wiegmann et al., 2011) including the housefly (Musca domestica) and the Medfly (Ceratitis capitata). Notably in these later species, sequence conservation in the candidate L2-CRMs compared to D. mel. was restricted to the vicinity of the ID islands (Figure 5A, Figure 5—figure supplement 1). In lieu of the guidance that multi-sequence alignments provided within the Drosophilid clade to identify the likely bounds of the L2-CRM in other species, we included ~200–400 bp before the ID-1 sequence and ~600–1000 bp after the ID-3 sequence for M. domestica and C. capitata, respectively, with a goal of also retaining sequences conserved within the various clades. We inserted candidate orthologous genomic fragments from donor species into a slightly modified riCC-AB CopyCat vector (Figure 5B). This arrangement permits cargo insertion and, if desired, CRE-mediated excision of non-cargo sequences, which also results in loss of the active genetic potential of the transgenic element (see Figure 5—figure supplement 2).

We tested CRM replacements for three fly species in these proof-of-principle experiments (Figure 5C, compare with Figure 5—figure supplement 2 for control experiments in which non-CRM sequences were deleted by CRE-mediated recombination), one distantly related species within the Drosophilid group (Drosophila grimshawi) ~45 Mya, and two Schizophora (M. domestica (Scott et al., 2014) and C. capitata [Papanicolaou et al., 2016]) spanning evolutionary divergence periods of ~60–75 Mya. D. grimshawi is significantly larger than D. mel., but has a very similar vein pattern, while in both M. domestica and C. capitata the position of the L2 vein is shifted anteriorly relative to other elements of the wing (note that M. domestica is also much larger than D. mel.) (Figure 5C). As expected, a control strain homozygous for a CopyCat element that deletes and restores the original D. mel. L2-CRM sequences (riCC-D.mel.) exhibited full rescue of the L2 vein leading to Kni expression in third instar discs that was indistinguishable from wild type (Figure 6C,D; compare to 6A, B). Similarly, flies homozygous for the D. grimshawi L2-CRM (riCC-D.grim.) exhibited a complete L2 vein in approximately the correct position and strong Kni expression in the expected pattern in wing discs (Figure 6E,F), consistent with the similar relative vein spacing patterns within the Drosophilid clade.

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We expected that at greater phylogenetic distances beyond the Drosophilid clade that candidate L2-CRM replacements in D. mel. might fail to support Kni expression or rescue of the L2 vein. What we had not anticipated, however, was that the riCC-M.dom. or riCC-C.cap. L2-CRM replacements for M. domestica and C. capitata would result in shifts of the positions of rescued L2-vein segments. Strains homozygous for the riCC-M.dom. or riCC-C.cap. replacement alleles displayed complete (riCC-M.dom.) or partially (riCC-C.cap.) restored L2 veins that were substantially displaced toward the anterior of the wing relative to the endogenous D. mel. L2 vein (or the control strains homozygous for the riCC-D.mel. element) (Figure 5C). In the case of riCC-M.dom. wings, the full vein coursed in a much sharper anterior angle from its normal point of branching from the L3 primordium, resulting in the rescued vein intersecting the wing margin in a position substantially proximal to that of the wild-type L2 vein (Figure 5C, see figure legend for quantitation). Presaging the anterior displacement of the rescued L2 vein in adults, expression of Kni in third instar wing discs was significantly broadened in riCC-M.dom. discs (Figure 6G,H,N-P) and pre-pupal wings (Figure 2—figure supplement 1S–U), extending further to the anterior than in wild-type wing primordia, but sharing the same posterior limit abutting the Spalt expression domain as in wild-type discs [Figure 6K–M; Figure 2—figure supplement 1G–H]). Kni expression in wild-type M.dom. discs is similarly broadened, suggesting that the pattern observed in the riCC-M.dom. replacement faithfully reproduces the endogenous pattern in its species of origin (Figure 6—figure supplement 1B,C). The exclusion of riCC-M.dom. Kni expression from the central domain of high level Spalt expression suggests that the M.dom. CRM, like that of D.mel., is subject to Spalt-mediated repression. The net anterior displacement of the adult L2 vein is similar to that resulting from low-level ubiquitous expression of kni in an ri1 mutant background (Lunde et al., 1998) and may reflect subsequent lateral inhibitory interactions during pupal stages that restrict vein formation to narrow stripes within a broader pro-vein domain (Biehs et al., 1998; Sturtevant and Bier, 1995). Vein rescue was only partially complete for homozygotes carrying the riCC-C.cap CRM replacement, which consistently had a rescued central L2 vein segment running parallel to the margin at a much reduced distance than observed for the wild-type D. mel. vein (Figure 5C, see figure legend for quantitation). Since little, if any, Kni expression is restored in riCC-C.cap wing discs (Figure 6I,J) or pre-pupal wings (Figure 2—figure supplement 1F), it is likely that the partial vein rescue observed with this construct reflects activity of this CRM at a later developmental stage. This hypothesized kni expression in the L2 primordium is consistent with the observation that the endogenous kni and salm expression patterns in C. capitata (Figure 6—figure supplement 1) and D. mel. are similar. As the observed anterior shifts of L2 veins observed in both the riCC-M.dom. and riCC-C.cap CRM replacements reflect the relative positions of L2 veins in M. domestica and C. capitata, we tentatively conclude that features of the genetic information for proper positioning of the L2 veins in these species reside within kni cis-regulatory sequences themselves and that these L2-CRMs are not merely executing patterning decisions imposed on them by more upstream elements of the wing gene regulatory network.

The ability to generate targeted deletions and combinations of such lesions while recovering a myriad of imprecise lesions as a byproduct of CRISPR mutagenesis strategies, fundamentally transforms the process by which CRM function can be analyzed. In this study, we highlight three fundamentally new insights and attendant questions into the activity of a CRM that we had thought we already understood quite well.

(1) Do CRMs on homologous chromosomes cooperate in a pairing-dependent fashion? The vastly different phenotypes of the riΔ3 deletion (and to a lesser extent riΔ2 as well) when homozygous versus in-trans to the larger full CRM riAB deletion was the first major surprise of this current study. In the case of other previously identified partial CRM loss-of-function mutations such as ri1 or ri53j, the phenotypes of homozygotes are only modestly increased when placed in trans to riAB as is typical when a hypermorphic allele is placed over a deletion. The riΔ3/riΔ3 phenotype, however, is nearly wild-type, while that of riΔ3/riAB trans-heterozygotes results in significant vein loss. This type of trans-chromosomal interaction, which we refer to as CRM-synergy, is reminiscent of, the phenomenon of transvection as described originally by Ed Lewis (Lewis, 1954). In the case of transvection, a regulatory entity (CRM in modern parlance) on one chromosome promotes expression of a trait (activates transcription) on the other chromosome (Figure 3N), and this trans-interaction between the CRM and basal promoter is abrogated by chromosomal inversions that interfere with chromosomal pairing (Figure 3O).

In contrast to transvection, for CRM-synergy we hypothesize that there are interactions between the two CRMs of different chromosomes that mutually reinforce the abilities of both CRM elements to engage either basal promoter. One possible explanation for CRM-synergy in the case of the riΔ2 or riΔ3 deletions is that two mutually reinforcing CRM sub-complexes could form on sequences present in either region deleted in riΔ2 or riΔ3, and that these complexes normally act in concert to provide full CRM activity (Figure 3M). Perhaps when either of the riΔ2 or riΔ3 deletions are homozygous, the remaining complexes present on the other remaining submodule can sustain CRM function as long as those elements on the two homologous chromosomes are well paired and can benefit from the trans-CRM interaction. When they are placed over the larger deletion, however, this pairing breaks down since the one remaining submodule can no longer interact productively with its homologous partner. This hypothesis is further supported by the observation that the phenotype of riΔ3/ri1 (Figure 3B, modeled in Figure 3P) is aggravated by interruption of chromosome pairing by the rearranged TM3, ri1 balancer chromosome (Figure 3F, modeled in Figure 3R) and that riΔ2/riΔ3 trans-heterozygotes display a substantial vein-loss phenotype (Figure 3C, modeled in Figure 3Q). It is also noteworthy that there is only a modest vein loss phenotype manifested in riΔ3/ri1 flies despite the fact that the riΔ3/riAB and ri1/riAB phenotypes are comparable. This effect may arise from the fact that deleted sequences in ri1 overlap with those missing in riΔ3. Consistent with this hypothesis, the riΔ3/ri53j phenotype is substantially stronger than that of riΔ3/ri1 despite the fact that the ri53j/ri53j phenotype is typically less severe than that of ri1/ri1. Since ri53j is a point mutant allele that is covered by the riΔ3 deletion, alignment of homologous chromosomes in riΔ3/ri53j trans-heterozygotes would likely be disrupted as compared to riΔ3/riΔ3 homozygotes that entirely lack the mutated nucleotide in ri53j and surrounding sequence. Further analysis will be required to determine the nature of the hypothesized pairing-dependent interaction and to exclude other potential explanations such as 1) the potential creation of novel transcription factor binding sites at the junction points of particular deletions which could then alter vein phenotypes by a neomorphic mechanism or 2) the ri53j mutation disrupts binding of an activator that normally acts by overcoming the formation of an inhibitory complex on adjacent sequences that are deleted in the riΔ3 mutant (these most obvious alternative explanations, however, do not readily account for the inversion-dependent differences in the riΔ3/ri1 phenotype nor the failure of riΔ3 and riΔ2 deletions to complement). Another question that remains to be addressed is why the ri53j point mutant allele has a stronger homozygous phenotype than the riΔ3 deletion that covers it. One possibility to explore is that the riΔ3 deletes both necessary activator sites (e.g. the ri53j site) and repressor sites leading to a weaker homozygous phenotype than deletion of just the activator sites.

(2) Split, cooperative, negative regulatory elements in the L2-CRM. In-locus deletion of sequential regions of the full L2-CRM suggested that two separate regions of the enhancer might contribute to repression of kni expression in peripheral regions of the wing primordium. Remarkably, when these two deletions were combined, a highly synergistic ectopic vein phenotype was observed. The ability to assay the effects of these deletions directly on gene function was a key feature of the experiments that reveal this strong cooperative phenotypic interaction. This observation motivates future studies to understand the mechanism by which the separated CRM sequences might interact. Do they assemble similar protein complexes that act in a redundant fashion? Might the two domains bind different transcription factor complexes but loop in some way to compete with activators binding to central regions of the CRM or to block a pairing-dependent form of trans-chromosomal cooperative activation? Another hypothesis to consider is that sequences in the riΔ1 region include not only sites for repressors, but also for inputs that limit anterior kni expression during early prepupal stages. Recall that the double riΔ1+Δ4 and single riΔ4 mutants exhibit comparable levels of ectopic Kni expression in larval stages (Figure 5H,L), but that the zone of strong ectopic Kni expression seems narrowed during pre-pupal stages in riΔ1+Δ4 relative to riΔ4 (Figure 2—figure supplement 1O,R). These observations might help explain why the riΔ+,Δ4 double mutant has a more extreme ectopic vein phenotype than the riΔ4 single mutant since previous studies have shown that high-level ubiquitous kni expression eliminates expression of both vein and intervein markers, possibly via overly exuberant induction of lateral inhibitory processes. According to this seemingly paradoxical hypothesis, the narrower zone of high level Kni expression observed the riΔ1+Δ4 double mutant may be less subject to vein elimination by such lateral inhibitory influences than the riΔ4 single mutant with persistent high level broad ectopic Kni expression throughout the anterior domain. Further analysis will be required to test this hypothesis. Also, one puzzle that remains unanswered by the current analysis is how the Spalt locus might negatively regulate CRM function in central regions of the wing as we failed to recover any mutations that induced ectopic veins in this territory. Unfortunately, the DNA-binding sites for Spalt proteins (Salm and Salr), if any, in the L2-CRM, are among the few transcription factors for which a well-defined binding site consensus remains to be determined (de Celis and Ostale, 2017). Perhaps, the failure to recover mutants in all our different experiments that induce ectopic veins in central regions of the wing reflects an interspersion of Sal repressor sites with activator sequences (e.g. known functionally important Scalloped sites)? Further refined deletion analysis of activator domains should help address this question as well as studies to identify bona fide Spalt protein binding sites. Sequence comparisons with the highly diverged L2-CRMs from M. domestica and C. capitata may prove helpful in this effort, as the exclusion of Kni expression from the central Spalt expression domain suggests that at least the riCC-M.dom. CRM retains sensitivity to Spalt-mediated repression, although this effect could be indirect. We note that these new features and models of kni regulation derive from a combination of morphological and in locus gene expression data that were not revealed previously based solely on analysis of traditional L2-CRM-reporter gene fusion constructs (e.g. [Martín et al., 2017; Lunde et al., 2003]).

(3) Novel unexpected phenotypes. Approximately half of CRISPR-induced mutations we recovered were the targeted mutations we set out to isolate. The remaining fortuitously generated lesions included imprecise indels at one or both of the targeted cleavage sites or, in some cases, deletions or duplications of short sequences at a distance from the cleavage sites that may reflect an error-prone form of HDR repair based on the presence of micro-homologies (sometimes referred to as micro-homology-mediated end joining or MMEJ). In some cases, these unanticipated mutations generated surprising phenotypes, including L2 deletions biased to proximal or distal regions of the vein, dramatically different phenotypes of deletions resulting from minor differences in endpoints, and in one case the addition of a single base in a non-conserved region of the CRM resulting in a vein-loss phenotype. While the basis for these various unexpected phenotypes remains to be determined, the recovery of a diverse array of ancillary mutations accompanying targeted CRISPR-based mutagenesis offers a potential treasure trove for gaining unbiased insights into CRM function.

We previously demonstrated a new CRISPR-based method referred to as the Mutagenic Chain Reaction (MCR) that results in efficient copying of genetic elements to the homologous chromosome during meiosis resulting in strong gene drive (Gantz and Bier, 2015). Similar gene-drive systems have also been shown to transmit efficiently to offspring in mosquitoes (Gantz et al., 2015; Hammond et al., 2016) and to copy efficiently in diploid yeast where such mutations can be transmitted subsequently during meiosis (DiCarlo et al., 2015). Based on these initial studies, we proposed the design of a new type of transgenesis vector that we refer to as CopyCat elements (Gantz and Bier, 2016) that could be copied efficiently to the homologous chromosome during meiosis in a Cas9-dependent fashion thereby bypassing traditional constraints of Mendelian inheritance, such as those associated with independent chromosomal assortment or genetic linkage. CopyCat elements can contain either a single gRNA that cuts at the site of vector insertion into the genome or two gRNAs that generate a self-propagating deletion of sequences lying between the two gRNA cut sites. In this study, we demonstrate that double cut CopyCat cloning vectors function as efficiently as conceived. In addition, CopyCat vectors can be used routinely to recover site directed genomic insertion making them an attractive alternative vehicle for transgenesis. Furthermore, when CopyCat elements are in presence of a Cas9 source, they can be passed on via the germline with super-Mendelian efficiency, a feature which should substantially simplify and shorten otherwise tortuous genetic schemes needed to assemble complex arrays of transgenes. If active genetics systems can also be developed in vertebrate and plant systems, they could revolutionize genetic manipulations in an array of organisms, in which standard Mendelian constraints pose a yet greater impediment than in fruit flies.

Potential caveats for the transmission of active genetic elements: The high frequency of CopyCat transmission in the experiments presented in this study suggest that this should be a generally applicable method. We noted, however, that in two of the nine crosses, transmission of the element was only Mendelian. The basis for the failure of the active copying process in such instances remains to be further evaluated, however, based on previous analysis of MCR transmission in flies (Gantz and Bier, 2015) and mosquitoes (Gantz et al., 2015; Hammond et al., 2016), the most likely explanation is that NHEJ repair can in some instances intervene between the time of egg fertilization and the partitioning of germ cell lineages, which takes place following seven cell divisions in the blastoderm embryo. In Drosophila, the frequency of such NHEJ events seems to be fairly low as revealed by propagation of the y1-MCR occurring at comparable frequencies via eggs that either are preloaded or not with a source of Cas9 (e.g. eggs from females that carry the MCR element versus eggs from wild-type females fertilized by MCR males). In contrast, in mosquitoes there is a large difference in transmission frequency between these two scenarios (98–99% chromosome conversion via males versus only 12–25% conversion via females - [Gantz et al., 2015]). The basis for this difference in male/female propagation between the two Dipteran species remains to be determined. However, it may be that for double cut CopyCat elements, which by their nature are more demanding of the HDR repair process than single cut MCRs, that non-HDR events are more frequent. Alternatively, the nuclease targeting of the two gRNA sites may occur asynchronously or the Cas9 enzyme may preferentially bind one gRNA thereby reducing cleavage directed by the second gRNA. In either of these scenarios, if the gRNA-A cuts before the gRNA-B, it could generate an NHEJ event at the location ‘A’ preventing successful HDR-driven chromosomal conversion of the CopyCat element at a subsequent cell cycle, when the gRNA-B has a chance of successfully guiding Cas9 to generate a dsDNA cleavage. Future experiments should resolve this question. Another factor to be kept in mind when using CopyCat elements as a genetic tool is that DNA repair mechanisms involved in copying these elements may be more error prone than standard cellular DNA replication (Malkova and Haber, 2012).

CopyCat CRM-replacement elements should accelerate evo-devo analysis of gene-regulatory networks: We also present proof-of-principle experiments showing that CopyCat elements can also carry gene sequences of interest that replace those deleted in a single genetic event. Such CopyCat elements should greatly facilitate the assembly of multiplex replacements to help identify genetic sequences from divergent species that underlie the morphological diversification. Indeed, we were surprised that replacement of just the kni L2-CRM with candidate homologous regulatory sequences from the housefly or Medfly led to pronounced anterior shifts of the L2 vein typical of vein placement in those donor species, suggesting that alterations in this single down-stream regulatory element are capable of mediating significant morphological changes. The nature of the salient changes in the L2-CRM remain to be investigated. Candidates for negative regulators in peripheral regions of the wing primordium that may contribute to these anterior vein shifts include the repressors Brinker and Optix/Six3 which have been implicated as part of a cross-inhibitory network involved in L2 development (Al Khatib et al., 2017; Martín et al., 2017). Intriguingly, knock-down of Optix function in anterior regions of the wing results in large anterior shifts of the L2 vein (Al Khatib et al., 2017; Martín et al., 2017), although this phenotype results at least in part from decreased proliferation of cells in the anterior domain where Optix is normally expressed (Martín et al., 2017). It also will be of significant interest to compare such changes that have occurred in different lineages that have yielded similar alterations in vein positions. For example, will similar or different sequences be identified as the key substrates for the anterior vein shifts we observe in M. domestica versus C. capitata? Sequencing and selective functional testing of additional species may provide estimates for how often such alternative evolutionary trajectories are taken. Also, how many CRMs of genes acting in the wing gene regulatory network would need to be replaced to transform the D. mel. wing into one resembling a housefly or Medfly? CopyCat-mediated CRM replacements should greatly enable studies to answer these and other frontier questions.

The studies presented here highlight the phenomenal impact of CRISPR-Cas9 mediated genome editing and active genetics in Drosophila, and by extension, most likely in many other organisms. These methods that efficiently generate targeted mutations, as well as a mix of fortuitous mutations in regulatory regions, provide unprecedented genetic leverage in dissecting non-coding DNA functionality. For example, as described above, in locus analysis of CRM function based on a combination of readout of endogenous gene expression at different developmental stages and on final morphological phenotype, revealed features of kni regulation that were not evident from previous CRM-gene-fusion studies. These new insights include: proposed chromosome pairing dependent CRM interactions, identification and analysis of complex cooperative interactions between CRM sub-domains, isolation of novel CRM mutations with unexpected phenotypes, and subtle phenotypic analysis of divergent CRM function during evolution. In addition, the comparable efficiency of CRISPR-based HDR-mediated transgenesis to that of traditional methods offers the possibility of delivering genetic cargo of interest to any desired chromosomal location. It should also be possible to develop genome-spanning collections of active CopyCat elements capable of delivering transgenic elements to a broad array of chromosomal insertion sites in a variety of sexually reproducing organisms as alternatives to Mendelian recombination-based systems. Multiple CopyCat elements could then be rapidly combined in the presence of an unlinked source of Cas9. Manipulations of this kind should significantly facilitate the import of entire gene regulatory networks from one organism into another to address important questions in evolution and promote the goals of principle-directed synthetic biology.

Plasmids were purified using the Qiagen Plasmid Midi kit (#12191). The gRNA plasmids were co-injected with respective 120 nucleotide ssODNs (also synthesized de novo in their entirety) that contained 60 bases homologous to the sequence flanking either side of the targeted deletion. Injection mixes were assembled with two gRNA plasmids (final concentration: 250 ng/µl each) and the donor oligo (final concentration: 100 ng/µl) in a volume of 50 µl. The mixes for riΔ1-riΔ4 were sent to Best Gene Inc. for injection into their Vasa-Cas9(X) stock (BDSC #51323) while CopyCat constructs were injected into the w1118 stock (BDSC #5905) with a transient source of pHsp70-Cas9 (pHsp70-Cas9 was a gift from Melissa Harrison and Kate O'Connor-Giles and Jill Wildonger (Addgene plasmid # 45945). All constructs were fully sequenced prior to injection and after recovery in analyzed transgenic fly stocks.

injected flies were singly crossed to the riAB stock, approximately 20 larvae were collected from each vial, and genomic DNA was prepared. The larvae were rinsed in deionized water and homogenized with a motorized pestle in homogenizing buffer prepared according to protocols by Steller et al. (Steller and Pirrotta, 1986). Genomic DNA from single adult flies were prepared according to protocols by (Gloor et al., 1993).

PCR screening of deletion mutants: Some crosses were screened by phenotype followed by sequencing of the kni CRM, while others were screened by PCR using primers that would anneal to the sequence flanking the target deletion. In the case of deletions that did not display obvious overt phenotypes across the AB deletion, DNA was prepared from 20 third instar larvae from each vial of single injected F0 flies crossed to the riAB deletion as summarized above, PCR amplified with appropriate primers to detect the expected deletion, and analyzed by gel electrophoresis. Crosses giving the expected deletion band were kept and individual F1 progeny from those crosses were crossed again to the riAB deletion and re-tested. F2 flies again testing positive were then crossed to a TM3 balancer stock, and flies lacking an overt L2 phenotype (i.e. the non-AB/TM3 flies) were crossed to each other to establish a stock. Stocks homozygous for the desired deletion were then selected based on the absence of the TM3 balancer. Other primers used to screen for additional deletion events were designed with sequences lying outside of the deletion sequence but inside of the kni enhancer (AB fragment) in order to amplify the chromosome containing the newly generated deletions. PCR reactions were assembled with Phusion High-fidelity polymerase from NEB (#M0530S) and the genomic DNA previously prepared from collected larvae. PCR products were purified using the QIAquick PCR purification kit (#28104) before sequencing.

All crosses using active genetics were performed in accordance to an Institutional Biosafety Committee (IBC) approved protocol in a secure ACL2 insectary as previously described (Gratz et al., 2013) consistent with currently suggested guidelines for laboratory confinement of gene drive systems (Hammond et al., 2016; DiCarlo et al., 2015).

Non-CRM sequences flanked by Lox-P sites carried on the CRM-replacement CopyCat vectors were deleted by crossing the DsRed +CopyCat elements to a ubiquitous source of CRE-recombinase (BDSC #851). The resulting F1 progeny displayed a loss of the DsRed marker as trans-heterozygotes indicating early developmental CRE-mediated deletion of the cassette resulting in somatic as well as germline deletion of the DsRed cassette. These F1 flies were crossed to balancer stocks and isogenic lines were established from individual DsRed- F2 progeny. Molecular analysis of these deletion stocks revealed the clean predicted deletions of the DsRed cassette. Chromosomes carrying these unmarked elements were homozygosed by elimination of balancer chromosomes. Homozygous CRM-only replacements were analyzed for their phenotypes (Figure 5—figure supplement 1), which displayed little if any differences from their DsRed +parent lines (Figure 5).

Drosophila wings were dissected in 90% ethanol and mounted in 100% Canada balsam.

3rd instar larvae were dissected on ice in 0.1% Tween-PBS and fixed with 2% PFA in Brower buffer for 1 hr at 4˚C. Discs were stained in a mix of 1:1000 guinea pig anti-Knirps, 1:200 mouse anti-Engrailed, 1:500 mouse anti-Delta, 1:1000 rabbit anti-Spalt overnight at 4˚C. Samples were mounted in Slowfade diamond anti-fade mountant (#S36963) and imaged on a Leica SP5 confocal microscope.

The funders had no role in study design, data collection, and interpretation, or the decision to submit the work for publication.

We thank Bill McGinnis for the original suggestion of creating active genetic cloning elements, Nico Posnien at the University of Göttingen for kindly pointing us to unpublished data from Natalia Siomava were generated during her PhD graduate studies in the laboratory of Ernst Wimmer at the University of Göttingen, Giuseppe Saccone for providing C. capitata flies for wing mounting preparation, Al Handler for providing C. capitata DNA samples, Alec Gerry for M. domestica DNA samples and flies for wing mounting preparation, Bill McGinnis, Steve Wasserman, Joseph Vinetz, Scott Rifkin, Marty Yanofsky, Kim Cooper, Hannah Grunwald, Omar Akbari, Giuseppe Saccone and members of the Bier lab for help comments on the manuscript. These studies were supported by NIH grant 1R01GM117321 to EB, the Paul G. Allen Frontiers Group Distinguished Investigator Award to EB, and a gift from Sarah Sandell and Michael Marshall to EB, NIH 1DP5OD023098 to VMG, and NIH NS047101 to the UCSD Imaging Core.

© 2017, Xu et al.

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