The ancestors of bony fish evolved many new molecules,
including prolactin which mammals would use to control milk production,
liver cytochrome enzymes, local hormones, and lipoproteins.
The early bony fish developed the hormoneTSH (Hoar, Vol. 2) and possessed
two forms of TRH which have been conserved in their descendants (Harder,
2001). The neurohypophysis and adenohypohysis contacted each other (Gorbman,
1995). The hormone prolactin was adapted to perform a variety of functions,
including parental behavior and even examples of producing nourishment
for young (Hoar, Vol. 2)219-20). N-acetylation of -MSH and -endorphin
occured before these hormones were secreted (Dores, 1994). All of the
major groups of steroid hormone receptors found in mammals had evolved
in ancestral bony fish (Thornton, 2001). Basal actinopterygians, lungfish,
amphibians, marsupials, and primitive placental mammals possess two GnRH
genes (Fernald, 1999; King, 1995).
Ancestral bony fish developed the first true enamel for teeth (Romer,
p. 338). There was a change in the FABPs expressed in the liver (Baba,
1999) and the enzymes sucrase and chymotrypsin were utilized by the digestive
system (Stevens). Seventeen of the eighteen families of P450 enzymes present
in mammals are also represented in fish (the only exception being the
CYP39 family). Of the eighteen families, duplications of the CYP2 family
members have produced the greatest diversity (Nelson, 2003). The CYP2J,
CYP2N, and CYP2P subfamilies of P450 enzymes function in arachidonic acid
metabolism and are members of a clade of P450 enzymes shared between fish
and mammals (Oleksiak, 2003).
Many of the genetic changes of the ancestral bony fish would be conserved
in their tetrapod descendants. Tight junctions are a chordate feature,
although they have also been found in blood-brain and blood-testis barriers
in arthropods. Mammals possess about 20 claudin genes which maintain junctions
such as the tight junctions of the blood-brain and blood-testis barriers.
Many of these genes (and even their intron position) predate the split
of bony fish and tetrapods (Kollmar, 2001).
Ancestral bony fish evolved CC cytokines, CC receptors, and CXC receptors
are known in bony fish (Magor, 2001; Knaut, 2003).
ApoA-I and ApoE are expressed in yolk sac. The duplication which produced
these two genes from an ancestral gene them predates the evolution of
bony fish (Babin, 1997). Cytoglobin is the fourth type of globin known
in humans, mice, and fish. It is expressed in almost all tissues and appears
to be related to vertebrate myoglobin (Hankeln, 2005; DeSantis, 2004).