It's a long way from amphioxus: descendants of the earliest chordate

Abstract

The origin of chordates and the consequent genesis of vertebrates were major events in natural history. The amphioxus (lancelet) is now recognised as the closest extant relative to the stem chordate and is the only living invertebrate that retains a vertebrate-like development and body plan through its lifespan, despite more than 500 million years of independent evolution from the stem vertebrate. The inspiring data coming from its recently sequenced genome confirms that amphioxus has a prototypical chordate genome with respect to gene content and structure, and even chromosomal organisation. Pushed by joint efforts of amphioxus researchers, amphioxus is now entering a new era, namely its maturation as a laboratory model, through the availability of a large amount of molecular data and the advent of experimental manipulation of the embryo. These two facts may well serve to illuminate the hidden secrets of the genetic changes that generated, among other vertebrates, ourselves.

Pikaia gracilensWalcott, a stem‐group chordate from the Middle Cambrian of British Columbia

“…Whilst this is most obvious in the urochordates, where the larval stage provides at best a very rough proxy for the ur‐chordate (ancestral chordate), in the case of amphioxus although it is widely regarded as ‘a chordate in waiting’ (Conway Morris, 2000) a strong argument can be made that although genomically archaic (e.g. Putnam et al , 2008; Garcia‐Fernàndez & Benito‐Gutiérrez, 2009) its bodyplan has undergone radical re‐organization associated with a highly specialized infaunal life. The reliability of amphioxus as any sort of guide to the ur‐chordates is thus open to question.…”

“…Since then this view, albeit without the advantage of a detailed description, has remained the consensus (e.g. Jefferies, 1986; Briggs, Erwin & Collier, 1994; Gee, 1996; Smith, Sansom & Cochrane, 2001; Garcia‐Fernàndez & Benito‐Gutiérrez, 2009), albeit with a few exceptions (e.g. Janvier, 2003; Dzik, 2004; Turner et al , 2010).…”

Origin and diversification of steroids: Co-evolution of enzymes and nuclear receptors

“…Indeed, since the transition from amphioxus to lamprey involves the evolution of the head [29][30][31][32] [ Figure 4], it seemed reasonable to propose a role for the ER in the evolution of a more complex brain in vertebrates [24]. As more nuclear receptor sequences became available, the phylogenetic evidence for the ER as the ancestral steroid receptor became stronger [25].…”

First Record of Amphioxus Branchiostoma californiense (Amphioxiformes: Branchiostomatidae) Adjacent to a Shallow Submarine Hydrothermal System at Banderas Bay (Mexico)

“…These animals are dioecious and without apparent sexual dimorphism [1], and their larvae can be benthic or planktonic [3,6,12]. The adults are occasional swimmers, since they prefer the benthic substrate, where they spend most of their life filtering their food in small particles towards plankton [13].…”