Fine Mapping of a Degenerated Abdominal Legs Mutant (Edl) in Silkworm, Bombyx mori

Distinct roles of the Hox genes Ultrabithorax and abdominal-A in scorpionfly embryonic proleg development

The abdominal appendages of larval insects have a complex evolutionary history of gain and loss, but the regulatory mechanisms underlying the abdominal appendage development remain largely unclear. Here, we investigated the embryogenesis of abdominal prolegs in the scorpionfly Panorpa liui Hua (Mecoptera: Panorpidae) using in situ hybridization and parental RNA interference. The results show that RNAi-mediated knockdown of Ultrabithorax (Ubx) led to a homeotic transformation of the first abdominal segment (A1) into the third thoracic segment (T3) and changed the distributions of the downstream target Distal-less (Dll) expression but did not affect the expression levels of Dll. Knockdown of abdominal-A (abd-A) resulted in malformed segments, abnormal prolegs and disrupted Dll expression. The results demonstrate that the gene Ubx maintains an ancestral role of modulating A1 appendage fate without preventing Dll initiation, and a secondary adaptation of abd-A evolves the ability to specify abdominal segments and proleg identity. We conclude that changes in abdominal Hox gene expression and their target genes regulate abdominal appendage morphology during the evolutionary course of holometabolous larvae.

Cis-regulatory modes of Ultrabithorax inactivation in butterfly forewings

Hox gene clusters encode transcription factors that drive regional specialization during animal development: for example the Hox factor Ubx is expressed in the insect metathoracic (T3) wing appendages and differentiates them from T2 mesothoracic identities. Hox transcriptional regulation requires silencing activities that prevent spurious activation and regulatory crosstalks in the wrong tissues, but this has seldom been studied in insects other than Drosophila, which shows a derived Hox dislocation into two genomic clusters that disjoined Antennapedia (Antp) and Ultrabithorax (Ubx). Here, we investigated how Ubx is restricted to the hindwing in butterflies, amidst a contiguous Hox cluster. By analysing Hi-C and ATAC-seq data in the butterfly Junonia coenia, we show that a Topologically Associated Domain (TAD) maintains a hindwing-enriched profile of chromatin opening around Ubx. This TAD is bordered by a Boundary Element (BE) that separates it from a region of joined wing activity around the Antp locus. CRISPR mutational perturbation of this BE releases ectopic Ubx expression in forewings, inducing homeotic clones with hindwing identities. Further mutational interrogation of two non-coding RNA encoding regions and one putative cis-regulatory module within the Ubx TAD cause rare homeotic transformations in both directions, indicating the presence of both activating and repressing chromatin features. We also describe a series of spontaneous forewing homeotic phenotypes obtained in Heliconius butterflies, and discuss their possible mutational basis. By leveraging the extensive wing specialization found in butterflies, our initial exploration of Ubx regulation demonstrates the existence of silencing and insulating sequences that prevent its spurious expression in forewings.

Lepidopteran prolegs are novel traits, not leg homologs

Lepidopteran larvae have both thoracic legs and abdominal prolegs, yet it is unclear whether these are serial homologs. A RNA-seq analysis with various appendages of Bicyclus anynana butterfly larvae indicated that the proleg transcriptome resembles the head-horn transcriptome, a novel trait in the lepidoptera, but not a thoracic leg. Under a partial segment abdominal-A (abd-A) knockout, both thoracic leg homologs (pleuropodia) and prolegs developed in the same segment, arguing that both traits are not serial homologs. Further, three of the four coxal marker genes, Sp5, Sp6-9, and araucan, were absent from prolegs, but two endite marker genes, gooseberry and Distal-less, were expressed in prolegs, suggesting that prolegs may be using a modular endite gene-regulatory network (GRN) for their development. We propose that larval prolegs are novel traits derived from the activation of a pre-existing modular endite GRN in the abdomen using abd-A, the same Hox gene that still represses legs in more lateral positions.

The Hox gene Abdominal-B regulates the appendage development during the embryogenesis of scorpionflies

The Homeotic Complex (Hox) genes encode conserved homeodomain transcription factors that specify segment identity and appendage morphology along the antero-posterior axis in bilaterian animals. The Hox gene Abdominal-B (Abd-B) is mainly expressed in the posterior segments of the abdomen and plays an important role in insect organogenesis. In Mecoptera, the potential function of this gene remains unclear yet. Here, we performed a de novo transcriptome assembly and identified an Abd-B ortholog in the scorpionfly Panorpa liui. Quantitative real-time reverse transcription PCR showed that Abd-B expression increased gradually in embryos 76 h post oviposition, and was mainly present in the more posterior abdominal segments. Embryonic RNA interference of Abd-B resulted in a set of abnormalities, including developmental arrest, malformed suckers and misspecification of posterior segment identity. These results suggest that Abd-B is required for the proper development of the posterior abdomen. Furthermore, in Abd-B RNAi embryos, the expression of the appendage marker Distal-less (Dll) was up-regulated and was additionally present on abdominal segments IX and X compared with wild embryos, suggesting that scorpionfly Abd-B may act to suppress proleg development and has gained the ability to repress Dll expression on the more posterior abdominal segments. This study provides additional information on both the functional and evolutionary roles of Abd-B across different insects.