‘Penis Worm’ Shakes Evolutionary Tree
|October 26, 2012||Posted by News under News|
A study on the development of priapulids or ‘penis’ worms throws doubt on a feature that has been thought for more than 100 years to define the largest branch of the animal tree of life. Members of this branch — the protostomes — have historically been defined by the order in which they develop a mouth and an anus as embryos. But gene-expression data suggest that this definition is incorrect, researchers report this week inCurrent Biology.
Now, using molecular techniques to analyze gene expression, Hejnol and his team have revealed that a primitive protostome, the priapulid Priapus caudatus, develops like a deuterostome. These ‘living fossils’ look nearly identical to their priapulid ancestors, which littered the ocean floor during the Cambrian period, when protostomes originated (see ‘Protostome outlier’). That their development does not follow the protostome pattern suggests that early protostomes might also have developed differently. The order of the origin of the mouth and anus is now uncertain.
“Here is an animal that is the poster child for early protostomes, and it develops just like a deuterostome,” says Mark Martindale, a developmental biologist at the University of Hawaii in Honolulu. “We’ve been using the name protostome for 100 years, and now it’s clear that it doesn’t mean anything.”
Priapulids are not simply placed on the wrong side of the tree. Other similarities, such as DNA sequences, indicate that they are closely related to their protostome kin. Further, this is not the first report of a maverick protostome. Embryologists who have watched invertebrates develop under a microscope have previously noted variations in how protostome mouths develop. Hejnol’s study corroborates the underlying diversity and takes the evidence further by backing it up with molecular techniques and assessing a primitive protostome, says Martindale.