Homeotic transformation of tails into limbs in anurans

Abstract

Anuran tadpoles can regenerate their tails after amputation. However, they occasionally form ectopic limbs instead of the lost tail part after vitamin A treatment. This is regarded as an example of a homeotic transformation. In this phenomenon, the developmental fate of the tail blastema is apparently altered from that of a tail to that of limbs, indicating a realignment of positional information in the blastema. Morphological observations and analyses of the development of skeletal elements during the process suggest that positional information in the blastema is rewritten from tail to trunk specification under the influence of vitamin A, resulting in limb formation. Despite the extensive information gained from morphological observations, a comprehensive understanding of this phenomenon also requires molecular data. We review previous studies related to anuran homeotic transformation. The findings of these studies provide a basis for evaluating major hypotheses and identifying molecular data that should be prioritized in future studies. Finally, we argue that positional information for the tail blastema changes to that for a part of the trunk, leading to homeotic transformations. To suggest this hypothesis, we present published data that favor the rewriting of positional information.

Cranial or postcranial—Dual origin of the pectoral appendage of vertebrates combining the fin‐fold and gill‐arch theories?

“…Interestingly, they proposed that the ontogeny of the bifurcated median fin fold in twin‐tail goldfish might involve an increase of the size of the ventral tissues and field, caused by the chdA mutation, changing the originally midline‐located competent stripe into duplicated paired stripes at pre‐ and post‐cloacal regions, similar to what is seen within the trunk bilateral competent stripes of the paired fins. In fact, it should be noted that there are cases of what clearly seem to be “homeotic transformations” leading to paired hindlimbs in the region of the tail of frogs, 58,59 in a way somewhat resembling what Abe and Ota 4 argue about the twin‐tail goldfish. Another interesting point, which will be further discussed in “A dual origin of the pectoral appendage?” section, is that the pelvic appendage, more than the pectoral one, is particularly similar to the median fins.…”

“…However, the reality is that within all the cases given in the literature to support so‐called “experimental homeotic transformations” of pelvic appendages into pectoral appendages, or vice‐versa, a detailed anatomical analysis clearly reveals that there are huge morphological differences between the “produced” and normal phenotypes. Moreover, as noted in the section above, similar cases of so‐called “homeotic transformations” have been also described between for instance the tail and the hindlimb of frogs, but the authors of those descriptions did obviously not argue that such cases show that the tail and hindlimb are necessarily direct serial homologues or that there is a “default tail/hindlimb ancestral appendage.” 57,58 …”

“…Therefore, here I will just provide a few examples of such differences, without of course wanting to criticize any specific study or authors, particularly because I understand that most authors of such papers are not anatomists. For instance, in Ouimette et al 58 paper “Divergent transcriptional activities determine limb identity,” it is stated that introduction of Tbx5 in Pitx1 −/− mice hindlimbs produced one novel skeletal transformation reminiscent of a forelimb‐specific feature. Namely, they argue that the angle of the pelvic girdle relative to the vertebral column (spine) in Tbx5 mutants is greater than normal pelvic girdles, being reminiscent of the scapula/spine angle.…”

Wound repair, remodeling, and regeneration

“…() and Shibata, Kawasaki, Sakai, and Kawakami () report cell signaling and positional information in planarian regeneration and zebrafish fin regeneration, respectively. In addition, Morioka, Mohanty‐Hejmadi, Yaoita, and Tazawa () review homeotic transformation of tails into limbs in frogs. It is our great pleasure that Mohanty Hejmadi, a pioneer who discovered this startling phenomenon (Mohanty‐Hejmadi, Dutta, & Mahapatra, ), contributes to this review.…”