Lecture 23 Invertebrates 4

Lecture 23 Invertebrates 4 - Invertebrates 4: Ecdysozoans &...

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1 Invertebrates 4: Ecdysozoans & Deuterostomes Sydney Brenner Introduction The primary evidence for defining the clade Ecdysozoa is molecular data However, all members of this group share the phenomenon of ecdysis , the shedding of an external coat (exoskeleton / cuticle) as they grow Species-rich clade - Ecdysozoa consists of 8 phyla that contain more species than all protist, fungus, plant and animal groups combined. The Ecdysozoa Phylum Nematoda : Roundworms are non-segmented pseudocoelomates covered with tough cuticles Phylum Arthropoda : segmented coelomates with exoskeletons and jointed appendages Other Phyla : Loricifera, Priapula, Tardigrades, Onychophora Phylum Nematoda Roundworms, pseudocoelomates, only recently classed as ecdysozoans Cylindrical bodies covered with a tough outer collagenous cuticle • To grow, shed old cuticle & secrete larger one
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2 Nematode Features Complete digestive tract but no circulatory system; fluid in pseudocoelom transports nutrients Nematode Features Sexual reproduction usual – Separate sexes, hermaphrodites or both, with internal fertilization. Can self fertilize. – Females may lay 100,000 fertilized eggs per day that produce resistant zygotes More Nematode Features Locomotion caused by contraction of longitudinal muscles Abundant in most aquatic habitats and moist soil & decomposing organic matter on the bottom of lakes & oceans. Eggs and some larval stages resistant to stress.
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3 Caenorhabitis elegans Shares many of the same biological structures and processes with more complex organisms Short time to reproductive maturity (2-3 days), a two-week life span Detailed knowledge of its genetics and the function of each of its 959 cells Model for aging to workings of the nervous system 100um Caenorhabditis elegans We think we have a good candidate in the form of a small nematode worm, Caenorhabditis briggsae , which has the following properties. It is a self-fertilizing hermaphrodite , and sexual propagation is therefore independent of population size. Males are also found (0.1%), which can fertilize the hermaphrodites, allowing stocks to be constructed by genetic crosses . Each worm lays up to 200 eggs which hatch in buffer in twelve hours , producing larvae 80 microns in length. These larvae grow to a length of 1 mm in three and a half days , and reach sexual maturity. However, there is no increase in cell number, only in cell mass . The number of nuclei becomes constant at a late stage in development, and divisions occur only in the germ line. Although the total number of cells is only about a thousand , the organism is differentiated and has an epidermis, intestine, excretory system, nerve and muscle cells. Reports in the literature describe the approximate number of cells as follows: 200 cells in the gut, 200 epidermal cells, 60 muscle cells, 200 nerve cells. The organism normally feeds on bacteria, but can also be grown in large quantities in liver extract broth. It has not yet been grown in a defined synthetic
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This note was uploaded on 10/03/2009 for the course BISC 120 at USC.

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Lecture 23 Invertebrates 4 - Invertebrates 4: Ecdysozoans &...

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