Doublecortin is expressed in trigeminal motoneurons that innervate the velar musculature of lampreys: considerations on the evolution and development of the trigeminal system

Abstract

Studies in lampreys have revealed interesting aspects of the evolution of the trigeminal system and the jaw. In the present study, we found a marker that distinguishes subpopulations of trigeminal motoneurons innervating two different kinds of oropharyngeal muscles. Immunofluorescence with an antibody against doublecortin (DCX; a neuron-specific phosphoprotein) enabled identification of the trigeminal motoneurons that innervate the velar musculature of larval and recently transformed sea lampreys. DCX-immunoreactive (-ir) motoneurons were observed in the rostro-lateral part of the trigeminal motor nucleus of these animals, but not in lampreys 1 month or more after metamorphosis. Combined double DCX/tubulin and serotonin/tubulin immunofluorescence and tract-tracing experiments with neurobiotin (NB) were also performed in larvae for further characterization of this system. Rich innervation by DCX-ir fibers was observed on the muscle fibers of the velum but not on the upper lip or lower lip muscles, which were innervated by tubulin-ir/DCX-negative fibers. No double-labelled DCX-ir motoneurons were observed in experiments in which the tracer NB was applied to the upper lip. Innervation of velar muscles by serotonergic fibers is also reported. The present results indicate that development of the trigeminal motoneurons innervating the velum differs from that of the trigeminal motoneurons innervating the lips, which is probably related to the dramatic regression of the velum during metamorphosis. The absence of data on a similar subsystem in the trigeminal motor nucleus of gnathostomes suggests that they may be lamprey-specific motoneurons. These results provide support for the "heterotopic theory" of jaw evolution and are inconsistent with the theories of a velar origin for the gnathostome jaw.

Evolution of the vertebrate jaw from developmental perspectives

“…The heterotopy theory and the neoclassical theory of Mallatt (2008) share some similarities, especially in that both assume the involvement of premandibular structures in the ancestral oral apparatus, both propose a posteriorly oriented reduction of the oral domain or oral cavity during the establishment of the gnathostome jaw, and both recognize the premandibular origin of the lamprey upper lip. The two theories also agree with each other in how they interpret the innervation patterns of the trigeminal nerves and the distribution of the mandibular arch-derived muscles (Mallatt 1996(Mallatt , 2008Kuratani et al 1997b;Murakami et al 2004; also see Hatschek 1892;Alcock 1898;von Kupffer 1900;Johnston 1905;Damas 1944;Lindström 1949;Whiting 1977;Barreiro-Iglesias et al 2011 for cranial nerves in cyclostomes). However, based only on the anatomy of larval and adult animals, the neoclassical theory does not homologize the lamprey upper lip with the gnathostome trabecula (as the heterotopy theory does), nor does it explain the difference in positions of the nasal and adenohypophyseal placodes, which are only clarified by observing the embryonic distribution patterns of ectomesenchymes at earlier stages, when the overall morphological plans of the lamprey and gnathostomes are most alike (Figs.…”

Spatiotemporal Pattern of Doublecortin Expression in the Retina of the Sea Lamprey

“…In lamprey, molecular weights of RP1 and RP1L1 proteins are about 240 and 200 kDa, respectively (Yamashita et al, 2009 ). However, our previous western blot results in lamprey with the present DCX antibody (Barreiro-Iglesias et al, 2011 ) indicate that no protein of these molecular weights is recognized by the anti-DCX antibody used in this study. The expression of DCX in photoreceptors could be a specialization of lampreys or the ancestral character that was lost in jawed vertebrates.…”

“…Two bands of about around 45 kDa (DCX molecular weight) were observed, which were interpreted as a posttranscriptional modification of DCX (Francis et al, 1999 ). Western blots carried out with brain protein extracts of sea lampreys, have also shown two bands around 45 kDa (Barreiro-Iglesias et al, 2011 ).…”

The sea lamprey UNC5 receptors: cDNA cloning, phylogenetic analysis and expression in reticulospinal neurons at larval and adult stages of development

“…The evolution of UNC5 receptors is poorly understood, because, whereas at least four members of the UNC5 family are found in jawed vertebrate species, only one UNC5 receptor has been found in invertebrates (see above). Lampreys, together with mixynes, belong to the oldest branch of extant vertebrates, and they are thought to be the closest living forms to early vertebrates, which makes them key animals in comparative and evolutionary studies of the nervous system (Barreiro‐Iglesias et al,2009, 2010b, 2011). The identification and cloning of the UNC5 receptors in lampreys might provide valuable information for our understanding of the early evolution of this axonal guidance system in vertebrates and to know when the duplication/s of the ancestral UNC5 gene took place.…”