Heterochronic limb patterning in marsupials reveals flexibility in the processes underlying lateral plate mesoderm morphogenesis.

Marsupial embryos develop under intense functional constraints: neonates are born after a short gestation but must immediately crawl to the teat using precociously developed forelimbs. To meet this demand, marsupials have evolved extreme acceleration of limb morphogenesis, yet the cellular and molecular mechanisms underlying this shift remain unresolved. Using two distantly related marsupials, the fat-tailed dunnart (Sminthopsis crassicaudata) and the gray short-tailed opossum (Monodelphis domestica), we show that this acceleration extends upstream to the earliest stages of lateral plate mesoderm (LPM) formation. The forelimb field is specified in relative isolation from other axial structures, marked by accelerated activation of limb genes prior to neural tube and somite formation. Forelimb outgrowth begins before overt LPM subdivision and epithelial-mesenchymal transitions, with limb mesenchyme arising from an undifferentiated LPM. These findings reveal unexpected flexibility in the temporal relationships between axial morphogenesis and limb initiation, highlighting evolutionary plasticity in the processes that govern vertebrate limb patterning.