the slides were prepared by makerere university students doing bachelors of science in fisheries and aquaculture on the origin of fish respiratory gills citing origin in skate fishes

1. origin of respiratory gills
 Group 1 members
 Namugaya Barbra
 Aisu Ronald 19/U/7868/PS
 Sabavuma musa 19/U/7223/PS
 Kivengeri Denis
 Zziwa Micheal
 BUSOBOZI TOBIAS 19/U/22310/PS

2. Objectives
 Definition of respiratory gills
 Origin of respiratory gills derivation from chordates
 Endo-branchial hypothesis
 General evolution of respiratory gills in fishes

3. Introduction
 Respiratory gills are organs that extract dissolved
oxygen from the water and excrete carbon dioxide into
and out of the body tissues respectively. There are two
forms of gills as shown below

4. Origin of respiratory
 The respiratory gills were derived from the pharyngeal
slits which is one of the four basic derived
characteristics of chordates
 Derived characteristics of chordates

5. Origin of respiratory gills ctd
 Pharyngeal gills (slits) are a fundamental feature of the
vertebrate body plan
 This slits allow water entering the mouth to exit the
body without passing through the entire digestive tract
 Pharyngeal slits function as suspension-feeding
devices in many invertebrate chordates.
 In vertebrates (with the exception of tetrapods), these
slits and the structures that support them have been
modified for gas exchange and are known as gill slits.

6. Ecto-Endo branchiat hypothesis
 This hypothesis observes that gills of cyclostomes and
gnathastomes form on different regions of branchial
arches (medial vs lateral respectively)
 And arise from distinct embryonic epithelia ( endo
dermal vs ecto dermal respectively)

8. Gill evolution in fishes
 Gills on fishes develop on pharyngeal arches
 These are paired columns of tissue that are bound by
ectodermal and endodermal epithelia and form from
the walls of the embryonic foregut
 Pharyngeal arch development begins with the iterative
out pocketing of foregut endoderm in a rostral-to-
caudal sequence, giving rise to a series of endodermal
pouches
 These pouches subsequently contact and fuse with
overlying surface ectoderm, resulting in the
perforation of gill slits and the delineation of arches

9. Origin in skates
 In skate embryo, the gene encoding the developmental
signaling molecule Sonic hedgehog (Shh).
 This is expressed in the anterior endodermal domain
of each developing pharyngeal pouch
 Once the pouches have fuse with the overlying
ectoderm,
 The Shh expression occurs at the posterior epithelium
of each pharyngeal arch, where it functions to
establish the anterior-posterior axis of the arch.

10.  The sequence of pharyngeal arch formation may be
captured along the rostrocaudal axis of a single
embryo
 Endodermally derived pharyngeal pouches (pp)
contact surface ectoderm and ultimately fuse with this
ectoderm (black arrow), giving rise to a gill slit (gs)
 The columns of tissue that are isolated by adjacent gill
slits are pharyngeal arches

11. Origin in skates
 Shh is expressed along the anterior wall of each
pharyngeal pouch (pp) and, eventually, along the
posterior wall of each pharyngeal arch
 Early filaments (gf) arise within Shh-expressing
epithelium

12. Origin in skates
 Based on histological and gene expression analyses,
predict that gill filaments (gf) derive from endodermal
epithelium

13. conclusion
 Generally origin of gills varies between the agnatha
(cyclostomes) and gnathastomes basing on the studies
carried out on different embroys of organisms
belonging to two groups above but in both it’s the
modification of pharyngeal slits found in all chordates.

14. References
 Jarvik, E. (1968). Aspects of vertebrate phylogeny. In
Current Problems in Lower Vertebrate Phylogeny, T.
Orvig, ed. (Almqvist & Wiksell), pp. 497–527.
 Heimberg, A.M., Cowper-Sal-lari, R., Semon, M.,
Donoghue, P.C.J., and Peterson, K.J. (2010).
microRNAs reveal the interrelationships of hagfish,
lampreys, and gnathostomes and the nature of the
ancestral vertebrate. Proc. Natl. Acad. Sci. USA 107,
19379–19383
 . Stockard, C.R. (1906). The development of the mouth
and gills in Bdellostoma stouti. Am. J. Anat. 5, 482–517.