title: Retinoic acid-independent expression of Meis2 during autopod patterning in the developing bat and mouse limb.
abstract: BACKGROUND
  The bat has strikingly divergent forelimbs (long digits supporting wing membranes) and hindlimbs (short, typically free digits) due to the distinct requirements of both aerial and terrestrial locomotion. During embryonic development, the morphology of the bat forelimb deviates dramatically from the mouse and chick, offering an alternative paradigm for identifying genes that play an important role in limb patterning.
  
  
  RESULTS
  Using transcriptome analysis of developing Natal long-fingered bat (Miniopterus natalensis) fore- and hindlimbs, we demonstrate that the transcription factor Meis2 has a significantly higher expression in bat forelimb autopods compared to hindlimbs. Validation by reverse transcriptase and quantitative polymerase chain reaction (RT-qPCR) and whole mount in situ hybridisation shows that Meis2, conventionally known as a marker of the early proximal limb bud, is upregulated in the bat forelimb autopod from CS16. Meis2 expression is localised to the expanding interdigital webbing and the membranes linking the wing to the hindlimb and tail. In mice, Meis2 is also expressed in the interdigital region prior to tissue regression. This interdigital Meis2 expression is not activated by retinoic acid (RA) signalling as it is present in the retained interdigital tissue of Rdh10 (trex/trex) mice, which lack RA. Additionally, genes encoding RA-synthesising enzymes, Rdh10 and Aldh1a2, and the RA nuclear receptor Rarβ are robustly expressed in bat fore- and hindlimb interdigital tissues indicating that the mechanism that retains interdigital tissue in bats also occurs independently of RA signalling.
  
  
  CONCLUSIONS
  Mammalian interdigital Meis2 expression, and upregulation in the interdigital webbing of bat wings, suggests an important role for Meis2 in autopod development. Interdigital Meis2 expression is RA-independent, and retention of interdigital webbing in bat wings is not due to the suppression of RA-induced cell death. Rather, RA signalling may play a role in the thinning (rather than complete loss) of the interdigital tissue in the bat forelimb, while Meis2 may interact with other factors during both bat and mouse autopod development to maintain a pool of interdigital cells that contribute to digit patterning and growth.
authors: Department of Molecular and Cell Biology, University of Cape Town, Rondebosch, 7701 South Africa.
authors: Mandy K
authors: MK
authors: Mason
authors: Department of Molecular and Cell Biology, University of Cape Town, Rondebosch, 7701 South Africa ; Present address: Weatherall Institute of Molecular Medicine, University of Oxford, Oxford, OX3 9DS UK.
authors: Dorit
authors: D
authors: Hockman
authors: Department of Molecular and Cell Biology, University of Cape Town, Rondebosch, 7701 South Africa.
authors: Lyle
authors: L
authors: Curry
authors: Sanford-Burnham Medical Research Institute, Development, Aging, and Regeneration Program, La Jolla, 92037 California USA.
authors: Thomas J
authors: TJ
authors: Cunningham
authors: Sanford-Burnham Medical Research Institute, Development, Aging, and Regeneration Program, La Jolla, 92037 California USA.
authors: Gregg
authors: G
authors: Duester
authors: Randall Division, King's College London, London, SE1 1UL UK.
authors: Malcolm
authors: M
authors: Logan
authors: Department of Biological Sciences, University of Cape Town, Rondebosch, 7701 South Africa.
authors: David S
authors: DS
authors: Jacobs
authors: Department of Molecular and Cell Biology, University of Cape Town, Rondebosch, 7701 South Africa.
authors: Nicola
authors: N
authors: Illing
keywords: Bat wing; Evo-devo; Interdigital webbing; Limb development; Meis2; Retinoic acid signalling