Young, Rebecca L., and Günter P Wagner. “
Why ontogenetic homology criteria can be misleading: lessons from digit identity transformations..” Journal of experimental zoology. Part B, Molecular and developmental evolution 316B (3): 165 - 70.
AbstractAs the basis for comparative biology, correctly assigning character homology is critical. Yet, identifying homologous characters in practice is often challenging. Among the major roadblocks is that the mechanistic bases of character homology remain in question. Thus, investigators must rely on several independent lines of evidence (e.g., character anatomy, phylogenetic distribution, or embryological position); however, these distinct sources of evidence often lead to conflicting diagnoses of character homology. What is more, there is no consensus regarding the relative importance of distinct lines of evidence for determining character homology. Here, we review the difficulties that have hindered the search for the mechanistic bases of character identity, and relate these issues to a recently proposed mechanistic hypothesis of character identity--the Character Identity Network Hypothesis. Next, using two well-studied cases of homology conflict (i.e., avian and skink digit identity), we assess the utility of different lines of evidence in diagnosing homology. We conclude that, when comparing adult structures, because anatomical characters more closely reflect the actions of the developmental genetic mechanisms of character individuation they are more reliable than embryological homology criteria.
Young, Rebecca L., Gabe S Bever, Zhe Wang, and Günter P Wagner. “
Identity of the avian wing digits: Problems resolved and unsolved.” Developmental Dynamics 240 (5). Wiley-Liss, Inc.: 1042 - 1053.
Abstract
Controversy over bird wing digit identity has been a touchstone for various ideas in the phylogeny of birds, homology, and developmental evolution. This review summarizes the past 10 years of progress toward understanding avian digit identity. We conclude that the sum of evidence supports the Frame Shift Hypothesis, indicating that the avian wing digits have changed anatomical location. Briefly, the derivation of birds from theropod dinosaurs and the positional identities of the avian wing digits as 2, 3, and 41 are no longer in question. Additionally, increasing evidence indicates that the developmental programs for identity of the wing digits are of digits I, II, and III. Therefore, the attention moves from whether the digit identity frame shift occurred, to what the mechanisms of the frame shift were, and when in evolution it happened. There is considerable uncertainty about these issues and we identify exciting new research directions to resolve them. Developmental Dynamics 240:1042–1053, 2011. © 2011 Wiley-Liss, Inc.
Wang, Zhe, Rebecca L. Young, Huiling Xue, and Gunter P Wagner. “
Transcriptomic analysis of avian digits reveals conserved and derived digit identities in birds.” Nature 477 (7366). Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.: 583 - 586.
Abstract
Morphological characters are the result of developmental gene expression. The identity of a character is ultimately grounded in the gene regulatory network directing development and thus whole-genome gene expression data can provide evidence about character identity. This approach has been successfully used to assess cell-type identity1, 2, 3. Here we use transcriptomic data to address a long-standing uncertainty in evolutionary biology, the identity of avian wing digits4, 5. Embryological evidence clearly identifies the three wing digits as developing from digit positions 2, 3 and 4 (ref. 6), whereas palaeontological data suggest that they are digits I, II and III7. We compare the transcriptomes of the wing and foot digits and find a strong signal that unites the first wing digit with the first foot digit, even though the first wing digit develops from embryological position 2. Interestingly, our transcriptomic data of the posterior digits show a higher degree of differentiation among forelimb digits compared with hindlimb digits. These data show that in the stem lineage of birds the first digit underwent a translocation from digit position 1 to position 2, and further indicate that the posterior wing digits have unique identities contrary to any model of avian digit identity proposed so far5, 8.