A second wave of Sonic hedgehog expression during the development of the bat limb

Abstract

Sonic hedgehog (Shh) plays an integral role in both the anteriorposterior (A-P) patterning and expansion of developing vertebrate limbs through a feedback loop involving Fgfs, Bmps, and Gremlin. In bat limbs A-P patterning and the size of the digital field are unique. The posterior digits of the forelimb are elongated and joined by tissue, whereas the thumb is short. The hindlimb digits often are uniform in length. Here, we reveal novel expression patterns for Shh and its target, Patched 1 (Ptc1), during limb development in two bat species. Early Shh expression in the zone of polarizing activity is wider in the bat forelimb than in the mouse forelimb, correlating with the reported expansion of Fgf8 expression in the apical ectodermal ridge and the early loss of symmetry in the bat forelimb. Later in limb development, Shh and Ptc1 expression is reinitiated in the interdigital tissue. Shh is graded along the A-P axis in forelimb and is expressed uniformly at a lower level across the hindlimb interdigital tissue. We also show that the reported Fgf8 expression in the interdigital tissue precedes the expression of Shh. We propose that the reinitiation of Shh and Fgf8 expression in bat limbs reactivates the Shh-Fgf feedback loop in the interdigital tissue of stage 16 bat embryos. The cell survival and proliferation signals provided by the Shh-Fgf signaling loop probably contribute to the lengthening of the posterior forelimb digits, the survival of the forelimb interdigital webbing, and the extension of the hindlimb digits to a uniform length.Miniopterus natalensis ͉ Carollia perspicillata ͉ Patched1 ͉ Fgf8 ͉ evo-devo

The role of early development in mammalian limb diversification: A descriptive comparison of early limb development between the natal long‐fingered bat (Miniopterus natalensis) and the mouse (Mus musculus)

“…The resulting hand plate at CS 16 is asymmetrical across the A-P axis and is relatively larger than that of an E13.0 mouse. Cartilage staining emphasises this asymmetry, as the primoridia of digits two to five of the bat hand plate are considerably longer than the thumb (Hockman et al, 2008). This is in contrast to the symmetrical pattern of cartilage staining in the mouse paw, where digits one and five are shorter than two and four, which are in turn shorter than digit 3 (Wanek et al, 1989).…”

“…This symmetry is maintained as the digital condensations begin to form between E12.5 and E13.0. The appearance of the cartilage digit progenitors at E12.5 in the mouse and their sequence of formation (Martin, 1990) correspond to their initiation in both M. natalensis and C. perspicillata at CS 15 (Hockman et al, 2008).…”

“…Studies on postnatal growth of the hand and foot of the bat propose that this may be achieved through both a higher cell turnover during chondrogenesis and increased cell expansion during hypertrophy (Farnum et al, 2008a,b). Recent developmental studies have shifted the focus to pre-natal limb development, comparing the expression patterns of many important limb patterning genes and growth factors in developing bat and mouse limbs prior to and during bone formation (Cretekos et al, 2001(Cretekos et al, , 2007Sears et al, 2006;Weatherbee et al, 2006;Hockman et al, 2008;Ray and Capecchi, 2008). These studies have revealed differences in gene expression both between developing bat forelimbs and hindlimbs and between bat limbs and those of the mouse, exposing the importance of early genetic patterning in laying the blueprint for the unique bat limb.…”

The developmental basis of bat wing muscle

“…To detect expression of Lbx1 and Pax3 genes in mouse tissues, antisense RNA riboprobes constructed from complementary DNA sequences of bats were used. To detect expression of Fgf8 genes in bat tissues, RNA probe of mouse was used following the method of Weatherbee et al 16 Generally, hetero-specific RNA probes easily hybridize with the transcripts among mammalian lineages, because of the low level of nucleotide sequence variation in the protein-coding regions of genes 57,59 . To confirm the expression pattern of each gene, two to four individual bat embryos were analysed for each stage.…”

Unique expression patterns of multiple key genes associated with the evolution of mammalian flight

“…However, so far, targeted candidate gene approaches have identified only seven genes with differences in expression in bat forelimb compared with hindlimb and mouse limbs during development. Only two of these are transcription factors that may play an upstream and regulatory role for bat wing development [4][5][6][7][8][9]. Thus, the full complexity of the mechanism of bat wing evolution remains to be shown, and the key differentially expressed genes contributing to this unique morphology remain to be identified.…”