Distinct Development of the Trigeminal Sensory Nuclei in Platypus and Echidna

Abstract

Both lineages of the modern monotremes have been reported to be capable of electroreception using the trigeminal pathways and it has been argued that electroreception arose in an aquatic platypus-like ancestor of both modern monotreme groups. On the other hand, the trigeminal sensory nuclear complex of the platypus is highly modified for processing tactile and electrosensory information from the bill, whereas the trigeminal sensory nuclear complex of the short-beaked echidna (Tachyglossus aculeatus) is not particularly specialized. If the common ancestor for both platypus and echidna were an electroreceptively and trigeminally specialized aquatic feeder, one would expect the early stages of development of the trigeminal sensory nuclei in both species to show evidence of structural specialization from the outset. To determine whether this is the case, we examined the development of the trigeminal sensory nuclei in the platypus and short-beaked echidna using the Hill and Hubrecht embryological collections. We found that the highly specialized features of the platypus trigeminal sensory nuclei (i.e. the large size of the principal nucleus and oral part of the spinal trigeminal nuclear complex, and the presence of a dorsolateral parvicellular segment in the principal nucleus) appear around the time of hatching in the platypus, but are never seen at any stage in the echidna. Our findings support the proposition that the modern echidna and platypus are derived from a common ancestor with only minimal trigeminal specialization and that the peculiar anatomy of the trigeminal sensory nuclei in the modern platypus emerged in the ornithorhynchids after divergence from the tachyglossids.

Quantitative Analysis of the Maturation of the Main and Accessory Olfactory Systems in Monotremes and Metatherians in Comparison to Rodents and Humans

“…The 31 monotreme embryos, hatchlings and juveniles (21 platypuses, Ornithorhynchus anatinus ; 10 short‐beaked echidnas, Tachyglossus aculeatus ) are held at the MfN or the AMNH and have been used in previous studies of the development of the monotreme nervous system and brainstem by our group (Ashwell and Hardman, ; Ashwell et al, ). The AMNH specimens had been kindly made available by Professor U Zeller, formerly of MfN.…”

Ecophysiology of mammals

“…Therefore, sensory physiology remains of interest, and recent research in this subdiscipline covers vision ( (2017) expanded our understanding of koala (Phascolarctos cinereus) vocalizations. In addition, research on tactile abilities (Pacheco-Cobos et al 2003;Gaspard et al 2017), electroreception (Ashwell and Hardman 2012;Czech-Damal et al 2013), and echolocation (Gonzalez-Terrazas et al 2016;Luís et al 2016) are also producing new insights.…”