Sensory Systems in Amphioxus: A Window on the Ancestral Chordate Condition

Abstract

Amphioxus has an assortment of cells and organs for sensing light and mechanical stimuli. Vertebrate counterparts of these structures are not always apparent, and a strong case can be made for homology in only a few instances. For example, amphioxus has anatomically simple but plausible homologs of both the pineal and paired eyes of vertebrates. Placodal and neural crest derivatives are, however, more problematic: the evidence for an olfactory system in amphioxus is only circumstantial and, despite the variety of secondary sensory cell types that occur on the body surface in amphioxus, none are obvious homologs of vertebrate taste buds, neuromasts or acoustic hair cells. A useful perspective can nevertheless be gained by examining differences in amphioxus and vertebrate development, specifically how each specifies and positions sensory precursors, controls their proliferation, and deploys them through the body. The much larger size of vertebrate embryos and the need to cope developmentally with increased scale and cell numbers may account for some key vertebrate innovations, including placodes and neural crest. The presence or absence of specific structural adaptations, like the latter, is therefore less useful for judging homology between amphioxus and vertebrates than shared features of specific cell types. It is also clear that the duration of embryogenesis in vertebrates has been significantly extended in comparison with ancestral chordates so as to incorporate events that would originally have occurred during the post-embryonic growth period, including events of neurogenesis. Consequently, no scenario for the origin of vertebrates can be considered complete unless it deals explicitly with the whole of the life history and changes to it.

The opsin repertoire of the European lancelet: a window into light detection in a basal chordate

“…amphioxus or lancelet, consists of approximately 29 extant species, with a worldwide distribution (Poss and Boschung, 1996). Amphioxus possesses four types of photoreceptive systems -the dorsal ocelli and the Joseph cells are rhabdomeric receptors, while the frontal eye and the lamellar body contain ciliary photoreceptors (Lacalli, 2004). Function of the first two receptors is still not clear, nevertheless, rhabdomeric receptors similar to Joseph cells are present in the cerebral eyes of tunicates (salps).…”

Unique system of photoreceptors in sea urchin tube feet

“…Instead, these PRCs mediate the pupillar reflex and entrain the circadian clock. The few known examples of deuterostome animals possessing microvillar PRCs comprise a tornaria larva of an enteropneust (45,46), the lancelet Joseph cells and Hesse eyecups (47)(48)(49)(50), a tunicate salp (51), and sea stars with their optic cushions (27,52). With the single exception of sea star optic cushions, none of the other microvillar PRCs has been shown to serve vision.…”

Early crest animals and the insight they provide into the evolutionary origin of craniates

“…Amphioxus, however, lacks an identifiable olfactory organ, and its sensitivity to chemical stimuli remains unknown (Lacalli, 2004). Based on the fact that Pax6, a marker for olfactory epithelium, is expressed only in the developing rostrum ectoderm (Glardon et al, 1998) and dopaminergic neurons that are present in the vertebrate olfactory bulb occur in the anterior nerve cord of amphioxus (Moret et al, 2004), the rostral pathway in amphioxus may be homologous to the olfactory nerve (Lacalli, 2004).…”

“…A cluster of cells with club-shaped cilia in the cerebral vesicles is the only possible balance organ known in amphioxus (Lacalli, 2004). Molecular markers suggest that atria in urochordates represents potential candidate of mechanosensory organs orthologous to the otic placode (Bassham and Postlethwait, 2005).…”

“…Molecular markers suggest that atria in urochordates represents potential candidate of mechanosensory organs orthologous to the otic placode (Bassham and Postlethwait, 2005). The homology of atria with otic placodes remains questionable because their sensory cells are different from those in vertebrates (Lacalli, 2004).…”