Structure, Synthesis, and Phylogeny of Kisspeptin and its Receptor - Kisspeptin Signaling in Reproductive Biology - page 15

Fig. 2.4 Schematic illustrations for the distribution of kiss1 and kiss2 neurons in vertebrate brains,
including some hypotheses. Because kiss1 and kiss2 are duplicated paralogues, they are considered
to have been co-expressed in the same neurons in the common ancestor of teleosts and tetrapods.
Given that both amphibians and teleosts express kiss2 in POA, the ancestral teleosts and ancestral
tetrapods should have expressed kiss2 . Because Kiss2 was lost in the mammalian lineage, we
hypothesize that Kiss1 began to be expressed where Kiss2 used to be expressed, to compensate for
the loss of Kiss2 during mammalian evolution. Open circles indicate kiss1 , and fi lled triangles
indicate kiss2 neurons. Circles / triangles in red are kiss1 / kiss2 neurons that are steroid sensitive
(fi gure as originally published in Kanda S and Oka Y (2012) Evolutionary insights into the steroid
sensitive kiss1 and kiss2 neurons in the vertebrate brain. Front. Endocrin. 3: doi: 10.3389/
fendo.2012.00028 )
hand, in goldfi sh, which lack kiss1 neurons in NVT, POA kiss2 neurons are the only
population of kisspeptin neurons that shows steroid sensitive kisspeptin mRNA
expression in the brain [ 37 ]. These POA kiss2 neurons are also positively regulated
by gonadal steroids, as in the NVT kiss1 neurons in medaka, and thus there has been
no report of negative regulation of kisspeptin expression in teleost brain so far.
Considering the report that positive or negative steroid feedback regulation can be
rather easily changed by the composition of co-expressing transcription factors
[ 52 ], the important common feature of the vertebrate kisspeptin neurons may be that
steroid sensitive kisspeptin neurons are localized in NVT and POA, which are
anatomically similar to arcuate and POA/AVPV in mammals, respectively. Although
the precise homology of brain nuclei between mammalian and nonmammalian
(especially teleost) kisspeptin neurons should be carefully discussed, the presence
or absence of sex steroid sensitivity in each nucleus may be one of the strongest
pieces of evidence to argue such homology.
An evolutionary working hypothesis of kisspeptin neuronal systems in vertebrates
is shown in Fig. 2.4 . In this hypothesis, kiss1 - and kiss2 -expressing neurons are dif-
ferentially distributed in the brains of mammal and other vertebrates. It is suggested
that the loss of Kiss2 gene during mammalian evolution has been probably been