Molecular phylogeny of the protochordates: chordate evolution | Billie Swalla - japancarnews.info
Molecular phylogeny of the protochordates: Chordate evolution Phylogenetic relationships of the invertebrate deuterostome classes. In addition, as the term 'protochordate' has often been used, . Figure 1. Phylogenic relationships of deuterostomes and evolution of chordates. (a) Schematic. As far as the chordates are concerned, fast-evolving sequences of On the one hand, a Hox paralog group with no evident link to any.
If you look at the tadpole larval stage instead of the adult, you will see many more characteristics of the Chordates. Subphylum Cephalochordata is represented by the little sand lance, Branchiostoma. This marine animal looks very fish-like with fins, a tail, muscle blocks called myomeres and pharyngeal gill slits for filter feeding.
Study their anatomy and note their chordate characteristics. The larva looks like a miniature adult without gonads. This is the one group that retains the Chordate characteristics throughout its life. Branchiostoma Branchiostoma cross section through the oral hood and through gonads.
Beside this cross-section, draw a Branchiostoma cross-section through the pharynx or gut, showing the gonads. Subphylum Vertebrata is extremely diverse. The notochord may be replaced partially or entirely by vertebrae, which protect the dorsal nerve cord. They have a three part brain in a brain case with specialized sense organs, a closed circulatory system, a chambered and muscular heart, red blood cells, and glomerular kidneys.
This phylum consists of eight classes: We will study the lamprey as a typical vertebrate in the next section. Mucous glands produce large quantities of slime, released through pores in the skin, to deter predators.
Hagfish have soft, scaleless skin, commercially called eel skin. Inside they have a cartilaginous skeleton. Modern forms lack vertebrae although they are present in fossil forms. They have a muscular body divided into myotomes. What distinguishes the modern day Myxinoidea hagfish from other members of the Vertebrata? Class Chondrichthyes This group consists of the sharks and rays Elasmobranchii and the chimeras, or ratfish, Holocephali.
Examine some of the examples on display of the animals in this class. Note the numerous gill slits, the anteriormost of which is sometimes reduced to a spiracle.
The tail of sharks and rays is unequal or heterocercal with the upper lobe larger than the lower. Scales are placoid, and oils in the liver keep them afloat.
Internally the intestine has a unique way of increasing digestive area. Instead of folding, it has an internal spiral valve through which the digested food slowly descends. Class Osteichthyes These are the bony fish, although a few have reverted to cartilage.
Most bony fish have gas filled swim bladders. Most of these fish belong to the Actinopterygii or ray-finned fish. Some are primitive, for example the paddlefish and sturgeon. Somewhat more advanced are the gars and bowfin, Amia.
Recent ray-finned fish belong to the Teleosts and this is the largest group of fish. Their skulls have mobile jaws, their gills are protected by a large flap, the operculum, and their tails are homocercal even sized lobesand their scales do not have ganoin. The second group of bony fish is the Sarcopterygii and they have fleshy fins with a basal projecting appendage containing bone and muscle. This fin type is thought to give rise to the first terrestrial vertebrates. Here we find the lungfish and the coelacanth.
In this group, scales tend to be covered with cosmine and nostrils open internally to the mouth through holes called choanae. Class Amphibia The development of a muscular limb with defined joints and digits tetrapod is first seen in the amphibians. Also, for the first time an atlas allows the head can move up and down separately from the trunk. Modern amphibia have eggs with no shells or membranes which are laid in water, usually have paired lungs, have mucous glands to keep them moist and poison glands for protection.
Amphibia tend to reduce bones, lose scales and are small. They can be divided into three groups, Urodela salamanders and newtsAnura frogs and toads which lack an adult tail and undergo the most dramatic metamorphosis from aquatic tadpoles, and the Gymnophiona caecilians or apodans which have no trace of limbs or girdles and are burrowers.
Class Reptilia Reptiles are the first amniotes, having eggs with extra embryonic membranes, which allow the embryo to develop on land. As we will see in the skull lab, a lot of the relationships are based on the holes in the skull called fenestrae, which are filled with jaw muscles and on the position of the temporal arches.
Many reptile groups, for example the dinosaurs, are extinct. The Sauropsids, which includes the dinosaurs, contains the modern day reptiles and led to the birds. Another group, the Synapsids, produced the extinct therapsid reptiles and led to modern mammals. Modern reptiles with no temporal fossa anapsid are the Parareptilia turtles. Most modern reptiles have two temporal fossae diapsid and include snakes, lizards, legless lizards, crocodiles, alligators and the birds.
A second neck bone, the axis, allows greater mobility of the head and most reptiles have long necks with many cervical vertebrae. Class Aves Birds are specialized diapsid reptiles, which are second only to fish in number of vertebrate species. Their closest relatives are the crocodiles; both lay shelled eggs and have similar musculature and bones, including a wishbone. Birds are warm blooded, have feathers and complex lungs.
Birds usually have wings for flying and hollow, air-filled bones which keep them light. They also have beaks and walk on two legs bipedal.
Class Mammalia The mammalian skull, like that of the therapsid reptiles has only the lower temporal fossa and thus is a synapsid skull. Mammals are warm-blooded, have hair and mammary glands.
They also have sebaceous oil glands and sweat glands. They have anucleate red blood cells. Mammals have three bones in the middle ear and most mammals have large brains. The lower jaw is composed of a singe bone, the dentary, which forms a joint with the squamosal bone.
Although the dentaries are paired structures, they are often fused into a single structure by the mandibular symphysis. There are three groups of living mammals. The monotremes include the duckbill platypus and spiny anteater. They are amniotes with a primitive, shelled egg.
Another group of mammals, represented by the opossum and best known by the kangaroo, are the Monotheria marsupials. These animals have a simple placenta and young born at a very early stage, which crawl into a pouch and suckle there until they are larger. Most modern mammals are Eutheria placentalswith the fetus connected to a complex placenta in the uterus enabling it to develop considerably before it is born.
Since blood is easy to digest, they are all gut no stomachwhich exits in a common cloaca with the kidney and gonads. Like hagfish, lamprey have a single nostril, a cartilaginous skeleton with vertebraeand a muscular body divided into myotomes. There are whole mounts of lampreys and cross sections on display.
There are a series of lamprey cross sections of the trunk region cut posterior to the heart in plastic mounts and bowls on the bench. View using a dissecting microscope. The liver is anterior to the gonads.
These animals are dioecious separate sexes and the gonads are small in the males testesbut large and pebbly in the females ovaries. The gut is a tiny hollow tube with endoderm inside and mesoderm outside. The kidneys are paired and large, blue on the top because of the injected blood supply.
Therefore, we suggest that the chordate, but unfortunately several years later Yarrell deuterostome ancestor, and thus the chordate ancestor, was a placed the English lanceolatus in a new genus, Amphioxus, benthic worm that filter-fed through a pharyngeal pouch de- that he coined. Yarrell published this name in a popular rived from the endoderm. Attempts to encourage researchers to use the proper world, but we have yet to understand how and when these Branchiostoma have not been successful Gansper- tissues evolved.
There Acknowledgements are two recognized genera of lancelets, Branchiostoma and Epigonichthys, which are identified by the location of their Billie J. Swalla thanks, first and foremost, Charlie and gonads and the terminal metapleural folds on the body Poss Gretchen Lambert for their support and many delicious and Boschung Gretchen has been invaluable to these studies and and the morphology of the dorsal and anal fin chambers and in training our laboratory in the fine art of identifying ascid- pharyngeal slits Poss and Boschung Billie also thanks the many undergraduates, graduate has 23 recognized species and Epigonichthys contains 7 rec- students, and postdoctoral associates that have worked in her ognized species Poss and Boschung Although onomic terminology and references.
The Systematics, Phylo- some of the chapters are summarized here, it is impossible genetics and Molecular Evolution Discussion group at the to cover all of the topics in this review. The bibliography it- University of Washington were instrumental in offering ad- self contains over references! One of the major ways vice and discussions at various stages of this study.
Also, on- that lancelets differ from vertebrates is in lacking a head, but going research and discussions with Nori Satoh, Peter some nervous-system structures can still be homologized Holland, Ed Ruppert, and Hiroshi Wada have been very Fritzsch ; Lacalli This research was supported by a Seaver Foundation Grant to B. The Ray Society, Lon- The topic of chordate origins has been intriguing scien- don.
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