Interdigital webbing retention in bat wings illustrates genetic changes underlying amniote limb diversification

Abstract

Developmentally regulated programmed cell death sculpts the limbs and other embryonic organs in vertebrates. One intriguing example of species-specific differences in apoptotic extent is observed in the tissue between the digits. In chicks and mice, bone morphogenetic proteins (Bmps) trigger apoptosis of the interdigital mesenchyme, leading to freed digits, whereas in ducks, Bmp antagonists inhibit the apoptotic program, resulting in webbed feet. Here, we show that the phyllostomid bat Carollia perspicillata utilizes a distinct mechanism for maintaining interdigit tissue. We find that bat forelimb and hindlimb interdigital tissues express Bmp signaling components but that only bat hindlimbs undergo interdigital apoptosis. Strikingly, the retention of interdigital webbing in the bat forelimb is correlated with a unique pattern of Fgf8 expression in addition to the Bmp inhibitor Gremlin. By using a functional assay, we show that maintenance of interdigit tissue in the bat wing depends on the combined effects of high levels of Fgf signaling and inhibition of Bmp signaling. Our data also indicate that although there is not a conserved mechanism for maintaining interdigit tissue across amniotes, the expression in the bat forelimb interdigits of Gremlin and Fgf8 suggests that these key molecular changes contributed to the evolution of the bat wing.bone morphogenetic protein ͉ Carollia perspicillata ͉ Fgf ͉ Gremlin

The developmental basis of bat wing muscle

“…To date, the molecular mechanism underlying the development of the bat wing membrane has not been understood, with the exception of one report that the modulation of Bmp and Fgf signalling has a role in blocking apoptosis of interdigital tissue to make the interdigital wing membrane (chiropatagium) 16 . Through comparative expression analysis of the genes that are known to regulate limb morphogenesis, we found Fgf10 signalling is likely involved in wing membrane development and patterning of the wing muscles.…”

“…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.…”

Embryonic Staging System for the Black Mastiff Bat,Molossus rufus(Molossidae), Correlated With Structure‐Function Relationships in the Adult

“…Staging series can provide a morphological basis for functional characterization of DNA sequences that control development (Cretekos et al, 2001;Baguna and Garcia-Fernandez, 2003). Gene expression patterns obtained throughout the development of similar structures at similar points of developmental maturity in two different species can be used as a starting point for functional genomic studies (Cretekos et al, 2001;Chen et al, 2005;Sears et al, 2006;Weatherbee et al, 2006;Cretekos et al, 2007). Recently, comparative mammalian embryology emboldened the decision to test the functional relevance of a putatively important genetic sequence during limb development by replacing this sequence in one species, the mouse, Mus musculus, with the orthologous sequence from the bat, C. perspicillata (Cretekos et al, 2008).…”

Between “design” and “bricolage”: Genetic networks, levels of selection, and adaptive evolution

“…Making a bat wing, however, involves more than just exaggerated digit length; it also involves suppression of the waves of apoptosis that eliminate interdigital material in tetrapods with distinct digits. In the case of bats, the maintenance of interdigital webbing, however, is due to the specific inhibition of at least three BMP activities in the post-outgrowth phase of the interdigital regions (44).…”

“…This position is seemingly bolstered by recent success in understanding the genetic foundations of several adaptive traits, work that has underlined the key importance of a small number of specific genes. These cases involve finch beak dimensions in Darwin's finches, characteristics related directly to specific feeding adaptations (41,42); the adaptive evolution of bat wings for flight (43,44); and the adaptive loss of pelvic armature in freshwater sticklebacks (45,46).…”