Amphioxus Research Papers - Academia.edu (original) (raw)
Cephalochordates, the sister group of tunicates plus vertebrates, have been called “living fossils” due to their resemblance to fossil chordates from Cambrian strata. The genome of the cephalochordate Branchiostoma floridae shares... more
Cephalochordates, the sister group of tunicates plus vertebrates, have been called “living fossils” due to their resemblance to fossil chordates from Cambrian strata. The genome of the cephalochordate Branchiostoma floridae shares remarkable synteny with vertebrates and is free from whole-genome duplication. We performed RNA sequencing from larvae and adults of Asymmetron lucayanum, a cephalochordate distantly related to B. floridae. Comparisons of about 430 orthologous gene groups among both cephalochordates and 10 vertebrates using an echinoderm, a hemichordate, and a mollusk as outgroups showed that cephalochordates are evolving more slowly than the slowest evolving vertebrate known (the elephant shark), with A. lucayanum evolving even more slowly than B. floridae. Against this background of slow evolution, some genes, notably several involved in innate immunity, stand out as evolving relatively quickly. This may be due to the lack of an adaptive immune system and the relatively high levels of bacteria in the inshore waters cephalochordates inhabit. Molecular dating analysis including several time constraints revealed a divergence time of ∼120 Ma for A. lucayanum and B. floridae. The divisions between cephalochordates and vertebrates, and that between chordates and the hemichordate plus echinoderm clade likely occurred before the Cambrian.
Recent sequencing of amphioxus and sea urchin genomes has provided important data for understanding the origins of enzymes that synthesize adrenal and sex steroids and the receptors that mediate physiological response to these vertebrate... more
Recent sequencing of amphioxus and sea urchin genomes has provided important data for understanding the origins of enzymes that synthesize adrenal and sex steroids and the receptors that mediate physiological response to these vertebrate steroids. Phylogenetic analyses reveal that CYP11A and CYP19, which are involved in the synthesis of adrenal and sex steroids, first appear in the common ancestor of amphioxus and vertebrates. This correlates with recent evidence for the first appearance in amphioxus of receptors with close similarity to vertebrate steroid receptors. Other CYP450 enzymes involved in steroid synthesis can be traced back to invertebrates, in which they have at least two functions: detoxifying xenobiotics and catalyzing the synthesis of sterols that activate nuclear receptors. CYP450 metabolism of hydrophobic xenobiotics may have been a key event in the origin of ligand-activated steroid receptors from constitutively active nuclear receptors.