Larval legs of mulberry silkworm Bombyx mori are prototypes for the adult legs

Abstract

Morphological diversity of leg appendages is one of the hallmarks of developmental evolution. Limbs in insects may develop either from their embryonic prototypes or from imaginal discs harbored inside the larva. Bombyx mori (B. mori), a Lepidopteran insect, develops adult wings from larval wing imaginal discs. However, it has been debated whether the adult legs of B. mori arise from imaginal discs or from the larval legs. Here we addressed how the larval legs relate to their adult counterparts. We present the morphological landmarks during early leg development. We used expression of developmental genes like Distalless and extradenticle to mark leg primordia. Finally, we employed classical excision approach to develop a fate map of the adult leg. Excision and ablation of thoracic legs along proximo-distal axis at various times during larval development resulted in the loss of corresponding adult leg segments. Our data suggest that B. mori legs develop from larval appendages rather than leg imaginal discs.

Functional Analyses in the Silkworm,Bombyx mori, Support a Role for Notch Signaling in Appendage Development But Not theGroucho‐Dependent Pair‐Rule Process

“…The Y‐shaped thoracic legs were reminiscent of a similar biramous phenotype observed in delta RNAi embryos of cricket (Kainz et al, ; Mito et al, ). In normal silkworms, thoracic legs consist of three podomeres that further develop into five segments during development (Singh et al, ). In Bmnotch RNAi embryos, individual podomeres could be clearly identified (Fig.…”

“…E). The deformed and lengthened prolegs were dramatically distinct from the normal counterparts, which are small non‐segmented protrusions (Singh et al, ). The unseparated proleg terminated in a central planta that was bordered by a row of sclerotized crochets (Fig.…”

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

“…In addition, BmDll was shown to be present in the proleg primordial tips in silkworm [35]. Since the E dl mutant possesses degenerated prolegs without distal limb structures on A3, we performed whole-mount BmDll antibody staining to detect whether its expression pattern was changed.…”

“…In silkworm, BmDll induces proleg development with the absence of Bmabd-A in the proleg primordia during early embryogenesis, thus it is suggested that BmDll is suppressed by Bmabd-A [16, 35]. However, the interaction between Hox and Dll in distal appendage structures development is not clear.…”

“…But Proboscipedia ( Pb ) and Sex combs reduced ( Scr ) downregulate Dll to give rise to unique distal proboscis structures [43]. Interestingly, the distal structures of prolegs also differ from the thoracic legs, and Bmabd-A expression is found in the distal part of proleg primordia in silkworm [15], and overlaps with the expression of BmDll [35]. In this study, we also found that the up-regulation of BmUbx and down-regulation of Bmabd-A expression may lead to the homeotic transformation of A3 to A2 in the E dl mutant, thus leading to the degenerated abdominal legs phenotype.…”