The myth of the common segmented ancestor
Posted by Nathan Shapiro
Before the advancement of mollecular genetics, segmentation was viewed with an emphasis on the functional and phylogenetic advantages of having a segmented body plan. However, recent molecular evidence has shifted the view of segmentation from that of a physiological approach to that of an onto-genetic view. Common subjects for such studies of segmentation include the mollsuks the annelids, arthropods and chordates; all of which exhibit segmentation in one form or another. However, even with recent mollecular evidence, there is still debate as to what exactly constitutes a segmented organism. There is usually a distinction between animals that are “segmented” and those that merely exhibit serial repetition (Seaver,2003). Serial repetition refer to body parts that exhibit repeating themes. An example of such an organims would be branching points in hydrozoa colonies. (Pechenik, 2005). In order for an animal to be considered truly segmented, repeating structures must be present from anterior to posterior ends with each segment consisting of repeated tissues or organs that are ectodermally and mesodermally derived,(Seaver 2003).
Phylogenetic approaches classify animals based on characteristics of not only their morphology but also their embryology, larval state or even their genetic basis. There is a tendency to attempt to group segmented organisms together based on the fact that they merely exhibit segmentation. Phylogenies constructed upon the basis of the ancestry of segmentation would tend to try to prove that each of the major phyla that exhibit repetition would share a common ancestor.
I will argue that although mollusks, annelids , arthropods and chordates all exhibit segmentation of one form or another, they do not share a common segmented ancestor. Evidence supporting lack of a common segmented ancestor can be shown by observing not only anatomy of present day mollusks, annelids arthropods, and chordates but also through studies of developmental biology and phylgenetic analyses. Recent studies on the topic give such developmental evidence as the appearence of para segmentatin in both annelids and arthropods as well as neruogenetic evidence suggesting that mollusks did not arise from a segemented ancestor. Through the use of these studies, as well as a general knowledge of invertebrate zoology I will show that these phyla are each distinctly derived yet share many homologus traits in terms of segmentation and its effects on their evolutionary position as well as their everyday life.
Animals which belong to the phylum Molluska are diverse and unique. Each class differs drastically in their appearnece and lifestyle yet many commonalities between the classes of Molluska can be observed. Several of these include the presence of a radula, bilaterl symmetry, a muscular foot for locomotion and the presence of calcaerous spines or shells. Mollusks are not generally considered to exhibit true segmentation. However, due to lack of a consensus as to the differences between serial repetition, a trait that several molluscan classes do exhibit, and segmentation, there is often confusion as to how to define the molluscan repetition. One such class that exhibits this “segmentation” or serial repetition is the monoplacophora.
Monoplacophoreans are gernally thought to be a fairly ancestral state of molluscan evolution rivaled only by the polyplacophorans and the aplocophorans. Monoplacophorans exhibit a repetition, of gills an metanephridia running down the sides of their bodies (Pechenik 2005). Although such structures exhibit serial repetition, there is little evidence to support that these structures fall under the category of true segmentation. A better class to look at is the polyplacophorans which have a serial repetition of seven to eight plates along their ventral side.
Polyplacophorans, or Chitons as they are commonly referred to, have long been the subject of debate as a model molluskan for the presence of segmentation. Recent ontological studies provide neurogenetic evidence that polyplacophorans and thus all mollusks did not evolve from a segmented ancestor. Out group studies which looked at Chiton developmental stages noted that the nonganglanized chord-like nervous system of the Chiton was in fact a reduced trait and not a primitive characteristic for molluskans. It was also noted that at no time during development did chitons exhibit any segmentation in their nervous system. (Friedrich, 2002) “immunosensitivity of pedal commisures develops from posterior to anterior suggesting independent serial repetition rather than annelid- like conditions.” (Friedrich, 2002) All this evidence together supports the idea mollusks did not stem from a segmented ancestor and merely exhibit serial repetition independently from the segmented characterisitics of their sister taxa Annelida.
The phylum Annelida contains animals which some might consider the epitome of segmentation. There are many diverse clases which each display very unique characteristcs. Currently phylogenists are debating as to the members and phylogenic placement of the phylum Annelida. (Pechenik 2005). This poses some problems when trying to use phylogenic evidence. Embryological data however posseses strong evidence that supports a lack of a common segmented ancestor. Onfe of the major commonalites in the annelid classes is the trait of metameric organization or segmentation. (Pechenik, 2005). The annelids display what we here have termed “true” segmentation and are often refered to as “the segmented worms.” However, unlike other segmented animals the endodermal tissues of adult annelids are also segmented. (Pechenik, 2005).
Traditionally the phylum Annelida has been made up of the class Clitellata which exhibits the trait of possessing a clitellum and Polychaeta as well as several other classes. Polychetes are the largest class of the annelids and although they exhibit a vast array of traits and characteristics they are currently considered to be the most ancestral state thus far of the annelids. (Seaver, 2003). There is an abundance of evidence that supports that annelids are in no way related to arthropods ancestrally. The strong evidence deals with the embryological development of the annelid blastula.
One of the most known characterics of the annelids is that they possess spiral cleavage. Segmentation in annelid blastula can be observed during the higlhy asymmetric division of the five teloblasts commonly observed inblastula of the subclass Hirundinida (Shimizu, 2001) Polychetes however do not display the onset of segmentation in their embryonic stages; Instead segmentation in polychetes is first seen during their pelagic larval stage. This embryological development sets annelids aside from both the mollusks and the arthropods.
One of the most fundamental characteristics of the diverse and immense phylum of arthropods is that they all exhibit a chitonous exoskeleton that is segmented. THis differs from the annelids, which are considered t obe soft bodied animals, and instead is more like the mollusks which exhibit a hard shell. Insects and crustaceans tend to be the most diverse group of arthropods and recent trends group these two classes together into one super clade of arthropod known as the Pancrustacean (Seaver 2003).
An advantage arthropods have to their segmentation is the fact that their segments particularly the exctodermally derived portions can be specialized for many purposes. Specialization of segments such as that in arthropods is not observed in the other phylum to the same degree. Segments have evolved into spines, false eyes, claws and even wings. Some segments in arthropods have even fused together to from distinct body regions. This is known as tagmatization. Animals of the pancrustacea typically exhibit segmentation of their head, thoracic and abdominal regions leading to many structures such as mouthparts and modified podia.
This segmentation in the short germ insects as well as the crustaceans tends to proceed in an anterior to posterior direction as in the annelids. (Zravy, 1995) Segments of arthopods, however, unless tagmatized, allow for spaces between each segment unlike the anneldids such that the arthropods can move their “armor plating” yet internally they are still connected. Segmentation in arthropods arises from a set of genes which function in a hierarchy to divide the octoderm of the arthropod embryo into smaller and smaller units. This embryological para-segmentation in arthropods is common to annelids as well. However, ontologically primary segmentation which is observed in annelids is not found in arthropods and although is thought to be evolutionarily prior to the apperance of para-segmentation and the eventual metasegmentation, developmentally actually appears after these two events. (Zravy, 1995) This indicates a separation of the arthropod evolutionary ancestor from that of the annelids ancestor due to the lack of the suggested primary segmentation leading into a tate of secondary segmentation in embryos.
Animals belonging to the phylum chordata although diverse typically exhibit segmentation only during their embryo stage although some repetition in structures such as the back bone of vertebrate animals can be seen. Somitogenesis, which the production of somites that give rise to muscle and axial skeletons in vertebrates, progresses from the typical anterior to posterior direction common to annelids as well as some arthropods. (Davis, 2000) Chordates, unlike the Mollusca, Annelida and Arthropoda which are protostomes belonging to Lophotrochozoa, are deuterosomes belonging to Deuterostomia and by definition they do not undergo spiral cleavage. (Pechenik, 2005) Therefore embryologically the emergence of segmentation in chordates differs significantly from the appearance of segmentation in the annelids and arthropods. Genetically their segmentation does however share some commonality with the annelids and arthropods. Genetically their segmentation does however share some commonality with the lophotrochozoans namely the presence of pair-ruled patterning genes in creating animals that exhibit segmentation in either even or odd numbers of segments. This alone, however, is not sufficient to show the existence of a common segmented ancestor.
Segmentation among animals of the phylum molluska, annelida, arthropoda and chordata, can be observed yet evolutionarily the animals of these phyla did not arise from a common ancestor Neurogenic evidence in molluskans coupled with their lack of true segmentation shows they lacked a segmented ancestor completely thus disproving any possibility that they shared a segmented ancestor with the other phyla. Segmentation of endodermal tissues of annelids, a unique feature to the phyla as well as varying appearance points of segmentation also extinguishes any possibility of commonality between the remaining phyla.
Arthropods although much like the annelids still exhibit differences morphologically in terms of their segments not being fused as well as the fact they lack what is often known as a primary stage of segmentation. Chordates only exhibit segmentation in their embryonic stage and undergo radial cleavage as opposed to the spiral cleavage of the protostomes. Although there is evidence for commonality between the groups such as intermediate phylum which exhibit shared traits among the said phyla as well as genetic similarities such as pair-ruled patterning, this evidence is strongly outweighed by not only the embryological, morphological, molecular and neurogenic evidence but also by the fact that the independent gain of metamerism in each of the phyla is more parsimonious than the loss of this segmentation in all the segmented phyla. Future research into the topic of segmentation along with the eventual stablization of such phyla as Annelida should help to further answer this question.
Davis, G.K., Patel, N.H. 1999 This origin and evolution of segmentation. Trends Cell Biol.; 9 (12) M68-72.
Friedrich, S., Wanniger, A., Bruckner M, Haszprunar, G., 2002 Neurogenesis in mossy Chiton, Mopalia muscosa (Gould) (Polyplyacophora): Evidence against molluscan metamerism. Journal of Morphology. 253: 109:117.
Pechenik, JA 2005 Biology of the Invertebrates. Fifth Edition Mc Graw Hill, New York
Seaver, E.C 2003. Segmentation: mono or polyphyletic? In. J. Dev. Biol 47:583-595.
Shimizu T., Nakamoto A. 2001. Segmentation in Annelids: Cellular and molecular basis for metameric body plan. Zoological Science 18: 285-298
Zravy, J, Pavel, S., 1995. Evolution of Metamerism in Arthropoda: Developmental and Morphological perspectives. The quarterly Review of Biology. 70(3) 279-295.