Genes and gene families - Human Gene Evolution - page 141

19q13.1-qter; FUT3 , FUT5 , FUT6 , 19p13.3; FUT4 , 11q21; FUT7 , 9; FUT8 ,
14q23) is thought to have emerged during this period by a process of gene dupli-
cation, translocation and divergence (Costache et al ., 1997).
α

-Lactalbumin is a mammary gland-expressed calcium-binding protein which
interacts with galactosyl transferase to promote lactose synthesis. Although this
function is clearly confined to mammals, the -lactalbumin gene ( LALBA ;

12q13) is thought to have arisen by duplication of the gene encoding lysozyme c
which encodes a bacteriolytic enzyme present in the tissues and secretions of
mammals, birds, reptiles and even insects. This gene duplication probably
occurred 300-400 Myrs ago (Nitta and Sugai, 1989; Prager, 1996; Prager and
Wilson, 1988; Qasba and Kumar, 1997) well before the evolutionary emergence of
the mammals and the process of lactation.

4.1.4 Human genes whose origin preceded the divergence of mammals

Other genes were specifically vertebrate creations, their emergence through
duplication and divergence having perhaps been made possible by the genome
duplications thought to have occurred prior to the adaptive radiation of this sub-
phylum (Chapter 2, section 2.1). A new wave of gene creation may have accompa-
nied the emergence of the amphibians and early reptiles thereby equipping them
for terrrestrial life.
One example of a vertebrate creation appears to be the vitamin K-dependent
serine proteases of blood coagulation (factors VII, IX, X, protein C and pro-
thrombin). Prothrombin is present in bony fish (trout), cartilaginous fish (dog-
fish) and the hagfish, one of the modern representatives of the jawless Agnatha
(Banfield and MacGillivray, 1992; Doolittle, 1993). There is no evidence, how-
ever, for the existence of thrombin or a thrombin-like protein in either proto-
chordates or echinoderms. Whether the other four vitamin K-dependent factors
of coagulation are present in fish is as yet unclear (Doolittle, 1993). If they are, the
adaptive radiation of the vitamin K-dependent factors of hemostasis must have
occurred during the space of some 50 Myrs between the divergence of the proto-
chordates and the appearance of the Agnatha , some 450 Myrs ago (Doolittle,
1993). The subsequent evolution of the vitamin K-dependent factors of coagula-
tion is explored in Section 4.2.3, Serine protease genes .
Another specifically vertebrate invention was pulmonary surfactant which
comprises a series of proteins which serve to reduce surface tension at the air-
liquid interface in the lung. This was probably a prerequisite for air breathing.
The human genome contains a number of clustered pulmonary surfactant genes
(Kölble et al ., 1993), among them SFTPA1 (10q21-q24) encoding pulmonary sur-
factant protein A. Orthologues of this gene have been found in airbreathing lung-
fish and surfactant protein has been detected both in the swimbladder of goldfish
and in the lungs of lungfish (Sullivan et al ., 1998).
Transthyretin is a thyroid-binding protein which in human is synthesized
mainly in the liver but also in the choroid plexus and retina. The transthyretin
gene ( TTR ; 18q11-q12) is a vertebrate invention having made its first appearance
in the stem reptiles some 300 Myrs ago (Schreiber and Richardson, 1997).
Transthyretin is expressed in the choroid plexus in reptiles. Liver synthesis of the