Genetic structure of the abd-A gene of Drosophila

Mutation of Bdpaired induces embryo lethality in the oriental fruit fly, Bactrocera dorsalis

BACKGROUND

Pair-rule genes were identified and named for their role in segmentation in animal embryos. Paired, a homolog of mammalian PAX3, acts as one of several pair-rule genes and is key in defining the boundaries of future parasegments and segments during insect embryogenesis.

RESULTS

We cloned the paired gene from the oriental fruit fly, Bactrocera dorsalis, and then applied CRISPR/Cas9-mediated genome editing to investigate its physiological function in the embryonic stage of this pest. We identified one transcript for a paired homolog in B. dorsalis, which encodes a protein containing a Paired Box domain and a Homeobox domain. Phylogenetic analysis confirmed that the paired gene is highly conserved and the gene was highly expressed at the 12-14 h-old embryonic stage. Knock-out of Bdpaired led to lack of segment boundaries, cuticular deficiency, and embryonic lethality. Sequence analysis of the CRISPR/Cas9 mutants exhibited different insertion and deletions in the Bdpaired locus. In addition, the relative expression of Wingless (Wg) and Abdominal A (Abd-A) genes were significantly down-regulated in the Bdpaired mutant embryos.

CONCLUSION

These results indicate that Bdpaired gene is critical for the embryonic development of B. dorsalis, and could be a novel molecular target for genetic-based pest management practices to combat this serious invasive pest. © 2019 Society of Chemical Industry.

CRISPR/Cas9 mediated knockout of the abdominal-A homeotic gene in fall armyworm moth (Spodoptera frugiperda)

The fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith), is an important pest of maize in the Americas and has recently been introduced into Africa. Fall armyworm populations have developed resistance to control strategies that depend on insecticides and transgenic plants expressing Bacillus thuringiensis toxins. The study of various resistance mechanisms at the molecular level and the development novel control strategies have been hampered by a lack of functional genomic tools such as gene editing in this pest. In the current study, we explored the possibility of using the CRISPR/Cas9 system to modify the genome of FAW. We first identified and characterized the abdominal-A (Sfabd-A) gene of FAW. Sfabd-A single guide RNA (sgRNA) and Cas9 protein were then injected into 244 embryos of FAW. Sixty-two embryos injected with Sfabd-A sgRNA hatched. Of these hatched embryos, twelve developed into larvae that displayed typical aba-A mutant phenotypes such as fused segments. Of the twelve mutant larvae, three and five eventually developed into female and male moths, respectively. Most mutant moths were sterile, and one female produced a few unviable eggs when it was outcrossed to a wild-type male. Genotyping of 20 unhatched Sfabd-A sgRNA-injected embryos and 42 moths that developed from Sfabd-A sgRNA-injected embryos showed that 100% of the unhatched embryos and 50% of the moths contained indel mutations at the Sfabd-A genomic locus near the guide RNA target site. These results suggest that the CRISPR/Cas9 system is highly efficient in editing FAW genome. Importantly, this gene editing technology can be used to validate gene function to facilitate an understanding of the resistance mechanism and lead to the development of novel pest management approaches.

CRISPR/Cas9 mediated knockout of the abdominal-A homeotic gene in the global pest, diamondback moth (Plutella xylostella)

The diamondback moth, Plutella xylostella (L.), is a worldwide agricultural pest that has developed resistance to multiple classes of insecticides. Genetics-based approaches show promise as alternative pest management approaches but require functional studies to identify suitable gene targets. Here we use the CRISPR/Cas9 system to target a gene, abdominal-A, which has an important role in determining the identity and functionality of abdominal segments. We report that P. xylostella abdominal-A (Pxabd-A) has two structurally-similar splice isoforms (A and B) that differ only in the length of exon II, with 15 additional nucleotides in isoform A. Pxabd-A transcripts were detected in all developmental stages, and particularly in pupae and adults. CRISPR/Cas9-based mutagenesis of Pxabd-A exon I produced 91% chimeric mutants following injection of 448 eggs. Phenotypes with abnormal prolegs and malformed segments were visible in hatched larvae and unhatched embryos, and various defects were inherited by the next generation (G1). Genotyping of mutants demonstrated several mutations at the Pxabd-A genomic locus. The results indicate that a series of insertions and deletions were induced in the Pxabd-A locus, not only in G0 survivors but also in G1 individuals, and this provides a foundation for genome editing. Our study demonstrates the utility of the CRISPR/Cas9 system for targeting genes in an agricultural pest and therefore provides a foundation the development of novel pest management tools.

Functional characterization of the dRYBP gene in Drosophila

The Drosophila dRYBP gene has been described to function as a Polycomb-dependent transcriptional repressor. To determine the in vivo function of the dRYBP gene, we have generated mutations and analyzed the associated phenotypes. Homozygous null mutants die progressively throughout development and present phenotypes variable both in their penetrance and in their expressivity, including disrupted oogenesis, a disorganized pattern of the syncytial nuclear divisions, defects in pattern formation, and decreased wing size. Although dRYBP mutations do not show the homeotic-like phenotypes typical of mutations in the PcG and trxG genes, they enhance the phenotypes of mutations of either the Sex comb extra gene (PcG) or the trithorax gene (trxG). Finally, the dRYBP protein interacts physically with the Sex comb extra and the Pleiohomeotic proteins, and the homeotic-like phenotypes produced by the high levels of the dRYBP protein are mediated through its C-terminal domain. Our results indicate that the dRYBP gene functions in the control of cell identity together with the PcG/trxG proteins. Furthermore, they also indicate that dRYBP participates in the control of cell proliferation and cell differentiation and we propose that its functional requirement may well depend on the robustness of the animal.

Effect of polymorphism in the Drosophila regulatory gene Ultrabithorax on homeotic stability

Development is buffered against unpredictable environmental and genetic effects. Here, a molecular genetic analysis of one type of developmental homeostasis, the establishment of thoracic segmental identity under the control of the Ultrabithorax (Ubx) gene in Drosophila melanogaster, is presented. Flies were artificially selected for differential sensitivity to the induction of bithorax phenocopies by ether vapor. The experiments demonstrated that increased sensitivity to ether correlated with a loss of expression of UBX in the third thoracic imaginal discs and that a significant proportion of the genetic variation for transcriptional stability can be attributed to polymorphism in the Ubx gene.

Genetic factors controlling the expression of the abdominal-A gene of Drosophila within its domain

The homeotic gene abdominal-A (abd-A) is normally expressed in parasegments 7 to 13. We find that the initial distribution of the product is approximately uniform within this domain, but the subsequent elaboration of the expression pattern results in differences between, as well as within, parasegments. We have investigated the possible role of several pair-rule, e.g. fushi tarazu, even-skipped, runt, hairy, paired, and segment polarity e.g. engrailed, wingless, naked, patched and cubitus interruptus genes on the patterning of abd-A expression. We find that the establishment of the original abd-A expression domain is independent of any of these genes, but most of them are required for the subsequent elaboration of abd-A expression within the domain. The genes fushi tarazu, and especially engrailed, appear to act as transcriptional activating factors of abd-A.

The Tribolium homeotic gene Abdominal is homologous to abdominal-A of the Drosophila bithorax complex

The Abdominal gene is a member of the single homeotic complex of the beetle, Tribolium castaneum. An integrated developmental genetic and molecular analysis shows that Abdominal is homologous to the abdominal-A gene of the bithorax complex of Drosophila. abdominal-A mutant embryos display strong homeotic transformations of the anterior abdomen (parasegments 7–9) to PS6, whereas developmental commitments in the posterior abdomen depend primarily on Abdominal-B. In beetle embryos lacking Abdominal function, parasegments throughout the abdomen are transformed to PS6. This observation demonstrates the general functional significance of parasegmental expression among insects and shows that the control of determinative decisions in the posterior abdomen by homeotic selector genes has undergone considerable evolutionary modification.

Suppression of abdominal legs inDrosophila melanogaster

Drosophila melanogaster normally have six thoracic legs and no abdominal legs. However, one or two legs often appear in the first abdominal segment ofbithoraxoid mutants. The extent to which these extra legs develop is determined both by thecis-regulatory action ofbithoraxoid lesions onUltrabithorax and by the number of copies of the adjacent homeotic geneabdominal-A. Thebithoraxoid region does notcis-regulateabdominal-A.

Expression and regulation of the abd-A gene of Drosophila

We have developed a specific polyclonal antibody that recognizes the protein products of the abdominal-A (abdA) gene, a member of the bithorax complex of Drosophila. The normal expression domain extends from parasegments 7 to 13, in good correspondence with previous genetic and molecular results. However, while the anterior border of expression is precisely demarcated by a parasegmental boundary, the posterior border does not coincide with a lineage boundary. Within the normal domain, the expression of abd-A shows intrametameric modulation; the amount of product is higher in posterior compartments and in the most anterior cells of the anterior compartments and then gradually decreases. We have examined the effect on abd-A expression of a number of mutations, some mapping within and others outside the abd-A transcription unit. Those mapping to the transcription unit eliminate or severely reduce the amount of abd-A antigen, while those mapping outside produce an abnormal distribution of abd-A protein. Finally, we show that the abd-A gene is down-regulated in part of the Abdominal-B (Abd-B) domain, precisely in those regions where the Abd-B gene is expressed at high levels.

abdA expression in Drosophila embryos

The abdominal A (abdA) gene is one of three transcription units in the Bithorax Complex of Drosophila encoding a homeo box protein; it is flanked by Ultrabithorax (Ubx) and Abdominal B (AbdB). The abdA gene is required for segmental identity of the second through eighth abdominal segments. The transcription unit of abdA is approximately 20 kb long and encodes a protein of 330 amino acids. The abdA homeo box is almost identical to the homeo box of Ubx but is quite different from the AbdB homeo box. A polyclonal antibody to abdA protein stains embryonic nuclei in segments A1-A7 (parasegments 7-13). The iab-2, 3, and 4 mutant classes define positive cis-regulatory elements that induce expression of abdA in segments A2-A4 (parasegments 7-9), respectively. Once a pattern of abdA expression is turned on in a given parasegment, it remains on in the more posterior parasegments, so that the complex pattern of expression is built up in the successive parasegments. The abdA product appears to repress expression of Ubx whenever they appear in the same cell, but abdA is repressed by AbdB only in the eighth and ninth abdominal segments.