Palæos:		Unit 10: Chordata
The Vertebrates		200: Chordata (2)

Chordata: Chordata (2)

More urochordates, cephalochordates, and Haikouella

Chordata
|--Urochordata
|  |--Larvacea
|  |  |--Oikopleuridae
|  |  `-+--Kowalevskiidae
|  |    `--Fritillaridae
|  `--+--Ascidiacea
|     `--Thaliacea
`--+--Cephalochordata
   `--+--Haikouella 
      `--CRANIATA 

1. Ascidiacia
2. Cephalochordata
3. Haikouella X
4. Thaliacea

DNA and Divergence

In 2002, the ascideacean Ciona intestinalis became one of the first chordates whose DNA was more or less fully sequenced.  The availability of this information is allowing molecular workers to develop new insights into the early evolution of the chordates.  However, one of those insights is that C. intestinalis may not be a single species.  Caputi et al. (2007) have learned that there are at least two major subspecies of C. intestinalis which cannot cross-breed successfully, although there are few obvious morphological differences. It seems likely that the published DNA sequence is entirely from Caputi's "subspecies A", since this variant is the sole type in both California and Japan, the locations where the sequencing was performed.  Other subspecies, or perhaps even intermediate forms may remain to be characterized. 

Recently, several species of the fruit-fly Drosophila, have been fully sequenced in tandem.  Using these sequences, it will be possible to look closely at the types of genetic changes which are involved in the initial divergence and radiation of new species in that model system.  The existence of cryptic speciation in Ciona will be a valuable check on these results.  Earlier work in plants (Arabidopsis) and fungi (Neurospora) suggests that the types of genetic changes involved in speciation may not be universal across phylospace.  That is, at the genetic level, evolution may (or may not) work in quite different ways for widely separate types of organisms.

One early lesson from these comparative studies seems to be that gene rearrangement is at least as important as gene mutation in the formation of new species.  The preliminary findings of Caputi et al.  are consistent with this pattern. However, it remains to be seen whether gene rearrangements simply cause gross errors during chromosomal exchange in meiosis, or whether they also implicate more subtle rules relating to gene regulation.  ATW080123.

Ascidiacea: Ciona, Distaplia, Diplosoma. Classic urochordates with "tadpole" larvae and sessile adult.

Range: From the Jurassic. 

Phylogeny: Urochordata:: Thaliacea + *. 

Characters: Adult has incurrent and excurrent siphons and uses branchial filter feeding; motile larva has notochord, dorsal nerve chord, segmented muscles, and tail; solitary, social & compound forms. 

Links: Ascidian Home Page for the United States; UCSC Biology 150/Urochordata: Ascidiacea (see Image); The Urochordata; Ascidia - Sea squirts (many, many photos); Springer LINK: Development, Genes and Evolution - Abstract Volume 209 Issue 10 (1999) pp 592-600; inventaire (French); ascidian; Filo Ascidiacea; Ascidiacea Characteristics; M20.htm; Woodbridge.  APW040314.

Thaliacea: Salps. Free-living forms morphologically like Ascidiacean adults  

Range: Recent. 

Phylogeny: Urochordata:: Ascidiacea + *.  

Characters: Free-living forms morphologically like Ascidiacean adults.

Links: Thaliacea: brief checklist; Dive Log New Zealand : Phylum Chordata.  APW 040314.

Cephalochordata: Branchiostoma (amphioxus).

Range: from Cambrian?, from Early Permian?, Recent?  

Phylogeny: Chordata:: (Haikouella + Craniata) + *. 

Characters: Fusiform, <5cm. Brain segmented & perhaps functionally homologous to vertebrate brain; segmented muscle (myomeres) pull alternate sides, stiffened by notochord. Pharynx & gills used only for filter feeding. Circulation limited, but ventral pump & major dorsal artery. No hemoglobin; oxygen carried in solution; no kidney, but similar excretory cells. No connection between segmented & unsegmented nerves. Profound left-right asymmetry in development. Semi-sedentary filter feeder, trapping detritus in mucus; water moved by cilia – mouth and gut have little or no musculature. No paired fins, thus little or no control in 3 dimensions. Neural crest cells absent. 

Links: Chordata introduction - Subphylum Cephalochordata; Characteristics; Chordata; Berkeley. APW 040314.

Haikouella: H. lanceolata Chen et al. 1999.

Range: Early Cambrian of South China. 

Phylogeny: Chordata::: Craniata + *. 

Characters: Body 25-30 mm, broadly triangular anteriorly, narrowly triangular posteriorly [C+99]; brain present and bi- or tri-partite, but degree of integration with cranial sensory structures unclear [C+99] (contra [SM03]: brain is absent); rostral "skirt" similar to larval lamprey & also present in Haikouichthyes [M+03]; lateral eyes or eyespots [M+03] (contra [SM03]: eyes are rare artifacts); ventral buccal (?) cavity with tentacular structure [C+99]; probable mineralized pharyngeal teeth [C+99]; 6 pairs of branchial arches [C+99] (compare [SM03]: gills are external); gill slits between arches, covered by folds of body [C+99]; unclear if gills open to exterior or atrial cavity [C+99]; notochord extending well into anterior [C+99]; dorsal nerve cord present [C+99] [SM03]; body segmented dorsally with segmental blocks of muscle fibers (hence myomeres) [M+03] (contra [SM03]: no evidence of "cone-in-cone" structure); muscle fibers & segmentation extend well dorsal to notochord [M+03]; about 24 myomeres not in V- or W-shape [C+99]; postanal tail present [M+03] (compare [SM03]: tail is quite variable and is probably an artifact); heart, plus branchial, ventral & dorsal aortae present [C+99]; gut differentiated into esophagus, spiral mid-gut and straight intestine [C+99]. 

Links: Haikouella and Myllokunmingia; An early Cambrian craniate-like chordate; Titulo (Spanish); Das Kambrium (German); m-hydrate-nov99; ScienceNow; Taipei Times - archives (newspaper article on recent work); Haikouella (Wikipedia article); Chinese Science Bulletin ISSN-1001-6538 2003 Vol.48 No.8 725-735 ... (abstract of article arguing that it's a stem deuterostome).

Reference: Chen et al. (1999) [C+99]; Mallatt et al. (2003) [M+03]; Shu & Morris (2003) [SM03].  

Note: The description given above is only slightly more conservative than given in the reference. Haikouella, unlike most other early Paleozoic species, is known from over 300 well-preserved specimens, and Chen's description therefore has very high credibility, despite the great age and small size of the organism. Chen suggests that Haikouella may actually be a craniate, and this may well be correct in the sense that it has of the features typically listed as synapomorphies of Craniata. However, it seems unlikely that it is a member of the crown group subtended by hagfish and hogs. In any case, with the addition of conodonts to the chordates and the wealth of new information from South China, it is no longer very certain exactly what the synapomorphies of Craniata are going to be. APW040314.

checked APW040314