Biphasic Hoxd Gene Expression in Shark Paired Fins Reveals an Ancient Origin of the Distal Limb Domain

Abstract

The evolutionary transition of fins to limbs involved development of a new suite of distal skeletal structures, the digits. During tetrapod limb development, genes at the 5′ end of the HoxD cluster are expressed in two spatiotemporally distinct phases. In the first phase, Hoxd9-13 are activated sequentially and form nested domains along the anteroposterior axis of the limb. This initial phase patterns the limb from its proximal limit to the middle of the forearm. Later in development, a second wave of transcription results in 5′ HoxD gene expression along the distal end of the limb bud, which regulates formation of digits. Studies of zebrafish fins showed that the second phase of Hox expression does not occur, leading to the idea that the origin of digits was driven by addition of the distal Hox expression domain in the earliest tetrapods. Here we test this hypothesis by investigating Hoxd gene expression during paired fin development in the shark Scyliorhinus canicula, a member of the most basal lineage of jawed vertebrates. We report that at early stages, 5′Hoxd genes are expressed in anteroposteriorly nested patterns, consistent with the initial wave of Hoxd transcription in teleost and tetrapod paired appendages. Unexpectedly, a second phase of expression occurs at later stages of shark fin development, in which Hoxd12 and Hoxd13 are re-expressed along the distal margin of the fin buds. This second phase is similar to that observed in tetrapod limbs. The results indicate that a second, distal phase of Hoxd gene expression is not uniquely associated with tetrapod digit development, but is more likely a plesiomorphic condition present the common ancestor of chondrichthyans and osteichthyans. We propose that a temporal extension, rather than de novo activation, of Hoxd expression in the distal part of the fin may have led to the evolution of digits.

Copulation in antiarch placoderms and the origin of gnathostome internal fertilization

“…More derived asterolepidoid and bothriolepioid antiarchs lack pelvic girdles and fins, which are thus assumed to be lost (secondarily absent) in these groups 19 . In chondrichthyans, the clasper is attached to the posterior extremity of the pelvic fin metaptyerygium 9,22,23 . In ptyctodont and arthrodire placoderms, the clasper is immediately posterior to the pelvic fin 4 , and has previously been interpreted as an elaboration of the pelvic fin skeleton 2,3 .…”

Evolution of vertebrate appendicular structures: Insight from genetic and palaeontological data

“…The molecular investigations with cartilaginous fish further suggest the acquisition of the expression of Hoxd, Shh, Tbx4, Tbx5, En1, Bmp4, Fgf8, and dHand as early limb patterning regulators deep in vertebrate history at least before the diversification of gnathostomes (Tanaka et al, 2002;Dahn et al, 2007;Freitas et al, 2007).…”

“…Diagram illustrating the approximate history of major molecular and morphological innovations leading to diversification of limb elements during the evolutionary history of vertebrates. Genetic and expression analysis with model and non-traditional model vertebrates suggests the presence of GCR/ZRS in the ancestral lineage leading to chondrichthyans and osteichthyans, whereas Gli3 transcript was shown to be present in the fin field of osteichthyans ancestors Davis et al, 2007;Freitas et al, 2007). However, given the well-developed paired fins (at the pectoral level) of osteostracans (closest jawless relative of jawed vertebrates) and placoderms, containing appendicular endoskeleton (including metapterygium) and a/p polarity (Mabee, 2000;Coates, 2003), here it is hypothesized that Gli3, Shh, and phase II Hoxd expression was present in the fin field at least prior to the divergence of the osteostracans from the lineage leading to jawed vertebrates.…”

“…mtr, metapterygial radials; dr, distal radials; mer, mesopterygial radials; lep, lepidotrichia; Da, digit arch; Ra, radius; d1-5, digit1 to digit5; Ul, ulna; Hu, humerus. Drawings adapted from Hinchliffe (2002), Shubin et al (2006), Davis et al (2007), Freitas et al (2007), Boisvert et al (2008).…”

“…In agreement with the fossil data, recent analysis has revealed a late phase, inverted collinear expression of 5 0 Hoxd genes in fish fin including sarcopterygian, basal actinopterygian, and chondrichthyan. However, spatiotemporal and quantitative aspects of Phase II expression may vary slightly among taxa Freitas et al, 2007;Johanson et al, 2007) (Fig. 3).…”