Hoxgene expression in the specialized limbs of the Iberian mole (Talpa occidentalis)

Unique bone microanatomy reveals ancestry of subterranean specializations in mammals

Acquiring a subterranean lifestyle entails a substantial shift for many aspects of terrestrial vertebrates' biology. Although this lifestyle is associated with multiple instances of convergent evolution, the relative success of some subterranean lineages largely remains unexplained. Here, we focus on the mammalian transitions to life underground, quantifying bone microanatomy through high-resolution X-ray tomography. The true moles stand out in this dataset. Examination of this family's bone histology reveals that the highly fossorial moles acquired a unique phenotype involving large amounts of compacted coarse cancellous bone. This phenotype exceeds the adaptive optimum seemingly shared by several other subterranean mammals and can be traced back to some of the first known members of the family. This remarkable microanatomy was acquired early in the history of the group and evolved faster than the gross morphology innovations of true moles' forelimb. This echoes the pattern described for other lifestyle transitions, such as the acquisition of bone mass specializations in secondarily aquatic tetrapods. Highly plastic traits-such as those pertaining to bone structure-are hence involved in the early stages of different types of lifestyle transitions.

Non-model systems in mammalian forelimb evo-devo

Mammal forelimbs are highly diverse, ranging from the elongated wing of a bat to the stout limb of the mole. The mammal forelimb has been a long-standing system for the study of early developmental patterning, proportional variation, shape change, and the reduction of elements. However, most of this work has been performed in mice, which neglects the wide variation present across mammal forelimbs. This review emphasizes the critical role of non-model systems in limb evo-devo and highlights new emerging models and their potential. We discuss the role of gene networks in limb evolution, and touch on functional analyses that lay the groundwork for further developmental studies. Mammal limb evo-devo is a rich field, and here we aim to synthesize the findings of key recent works and the questions to which they lead.

Accelerated Evolution of Limb-Related Gene Hoxd11 in the Common Ancestor of Cetaceans and Ruminants (Cetruminantia)

Reduced numbers of carpal and tarsal bones (wrist and ankle joints) are extensively observed in the clade of Cetacea and Ruminantia (Cetruminantia). Homebox D11 (Hoxd11) is one of the important genes required for limb development in mammals. Mutations in Hoxd11 can lead to defects in particular bones of limbs, including carpus and tarsus. To test whether evolutionary changes in Hoxd11 underlie the loss of these bones in Cetruminantia, we sequenced and analyzed Hoxd11 coding sequences and compared them with other 5' HoxA and HoxD genes in a taxonomic coverage of Cetacea, Ruminantia and other mammalian relatives. Statistical tests on the Hoxd11 sequences found an accelerated evolution in the common ancestor of cetaceans and ruminants, which coincided with the reduction of carpal and tarsal bones in this clade. Five amino acid substitutions (G222S, G227A, G229S, A240T and G261V) and one amino acid deletion (G254Del) occurred in this lineage. In contrast, other 5' HoxA and HoxD genes do not show this same evolutionary pattern, but instead display a highly conserved pattern of evolution in this lineage. Accelerated evolution of Hoxd11, but not other 5' HoxA and HoxD genes, is probably related to the reduction of the carpal and tarsal bones in Cetruminantia. Moreover, we found two amino acid substitutions (G110S and D223N) in Hoxd11 that are unique to the lineage of Cetacea, which coincided with hindlimb loss in the common ancestor of cetaceans. Our results give molecular evidence of Hoxd11 adaptive evolution in cetaceans and ruminants, which could be correlated with limb morphological adaptation.

Complete mitochondrial genome of the Iberian Mole Talpa occidentalis (Talpidae, Insectivora) and comparison with Talpa europaea

The complete mitogenome of Talpa occidentalis, the Iberian mole, was sequenced using a combination of the Illumina and Sanger methods. The 16,962 bp genome obtained contains 13 protein-coding genes, 22 transfer RNAs, 2 ribosomal RNAs, and a control region. Thirty-seven identical repetitions of a 10-nucleotide (CACACGTACG) repeat element were identified in the non-coding control region (D-loop). The number, order, and orientation of the mitochondrial genes are the same as in T. europaea, the only mitogenome published so far for this genus. These two mitogenomes differ only at the repeat element included in the control region. The phylogeny obtained for the Talpidae species using the protein-coding genes of these mitogenomes agrees with the current classification of this family.

Noncanonical Hox, Etv4, and Gli3 gene activities give insight into unique limb patterning in salamanders

Limb development in salamanders is unique among tetrapods in significant ways. Not only can salamanders regenerate lost limbs repeatedly and throughout their lives, but also the preaxial zeugopodial element and digits form before the postaxial ones and, hence, with a reversed polarity compared to all other tetrapods. Moreover, in salamanders with free-swimming larval stages, as exemplified by the axolotl (Ambystoma mexicanum), each digit buds independently, instead of undergoing a paddle stage. Here, we report gene expression patterns of Hoxa and d clusters, and other crucial transcription factors during axolotl limb development. During early phases of limb development, expression patterns are mostly similar to those reported for amniotes and frogs. Likewise, Hoxd and Shh regulatory landscapes are largely conserved. However, during late digit-budding phases, remarkable differences are present: (i) the Hoxd13 expression domain excludes developing digits I and IV, (ii) we expand upon previous observation that Hoxa11 expression, which traditionally marks the zeugopodium, extends distally into the developing digits, and (iii) Gli3 and Etv4 show prolonged expression in developing digits. Our findings identify derived patterns in the expression of key transcription factors during late phases of salamander limb development, and provide the basis for a better understanding of the unique patterning of salamander limbs.