The journal BioEssays has a lovely series called "My favorite animal", in which biologists get to wax rhapsodic about their favorite creatures. It's a great idea, since not only does it mean we get some enthusiastic writing, but more exposure is given to organismal biology. I read so many papers that go on and on about some specific molecule and in which the only illustrations are photos of gels and blots that it's nice to see whole animals for a change.

This month, Arthur and Chipman wrote about a centipede, Strigamia maritima, and its development—so not only do I get animals, I get embryos. Oh, happy day!

Centipedes exhibit one of the underlying patterns of animal organization most vividly: segmentation. One strategy for making a larger animal in evolution without requiring a major increase in complexity is to build it from modular, repeating blocks, the segments. Arthur points out an interesting contradiction, though: while we argue for variation in segmentation as a feature manipulated by natural selection, most extant arthropod species have tightly fixed numbers of segments with no intraspecific variation. Centipedes are no exception. Three of the four major orders of centipedes have exactly 15 trunk segments, no more and no less. The one exception are the Geophilomorpha, which have secondarily lost segment number constancy and show variability between species, between populations, between sexes, and between individuals.

Relationships of the four major orders of centipedes, showing the main changes that have taken place during the evolution of the lineage leading to the geophilomorphs.

So Arthur is studying the mechanisms of segmentation in wild populations of a centipede species with variable segment numbers. This could be fascinating stuff; unfortunately, they haven't published anything on how segment number is regulated yet, and essentially are documenting the basics and the similarities with other arthropods. And getting pretty pictures of this beautiful beast!

A peripatoid in the process of hatching.

An adult female coiled around an egg brood.

They're also analyzing segmentation in embryogenesis, and finding as expected that many of the same molecules we've found in other arthropods and in vertebrates and every other segmented animal are present here. The embryos are also lovely.

Photos of DAPI-stained intact embryos of Strigamia maritima. A: Embryo at early stage of segmentation, after approximately 12 segments have formed; viewed from posterior-left. B: The same embryo viewed from anterior-left. C: Embryo at the end of the segmentation process, after almost all of the segments have been formed; viewed from above. Anterior is to the left; the ventral side of the embryo faces the outer surface of the egg. D: The same embryo viewed laterally. Anterior is to the left. All embryos are approximately 1 mm in diameter.

One thing they have discovered is that segmentation proceeds from anterior to posterior, and the animal probably uses something like a segmentation clock to tick off segments. One of the surprises in analyzing segmentation in Drosophila was the presence of the pair rule genes—genes that are turned on in every other segment and create a two segment periodicity. Arthur and Chapman have found a pair rule gene in Stragamia, odr-1, that is expressed in a two-segment period, and another gene, caudal that initially overlaps with odr-1 but then is activated in between the stripes as well, to generate a one-segment period.

Pair rule gene homologs in vertebrates, such as hairy, are expressed in a one segment period. One explanation for the different patterns is that the ancestral ur-segment corresponds to a pair of segments in arthropods, and that we vertebrates retain the primitive arrangement. Arthropods evolved a second mechanism (caudal would be part of it) that split the ur-segment in two, effectively doubling the segment number in one stroke.

One thing I will be very interested in seeing in the future is the mechanism for regulating the number of segments. Zebrafish also exhibit a small amount of variability in segment number, and it may be that the same processes are at work in both my favorite vertebrate and Arthur's favorite invertebrate.

Arthur W, Chipman AD (2005) The centipede Strigamia maritima: what it can tell us about the development and evolution of segmentation. BioEssays 27:653-660.