9
A Quick History of Xenopus
fate maps using more advanced methods are now commonly used by the community to target microinjections
(Dale and Slack, 1987; Moody, 1987), but these—and also a
largely forgotten fate map by Nakamura’s colleague Hiroko
Takasaki (1987 )—proved Nakamura’s early conclusions
mostly correct.
Nakamura also carried out a remarkable series of studies on the effects of removing specifc blastomeres from
the early embryo. These studies helped to precisely def ne
the inductive interactions that pattern the early embryo
(Nakamura et al., 1970, 1971) and later inf uenced some
the earliest molecular studies of vertebrate axial patterning
(e.g. Gurdon et al., 1984; Rosa et al., 1988; Yisraeli et al.,
1990). Finally, in a little-noticed paper from 1971, he used
electron microscopy to explore the developmental biology
of nucleoli in the early embryos (Nakamura and Yamada,
1971), extending prior work done by Elizabeth Hay and John
Gurdon (Hay and Gurdon, 1967). Professor Nakamura went
on to become president of two universities and was widely
celebrated in Japan.
1.5. THE NINTH DAY OF CREATION: XENOPUS
AT THE DAWN OF MOLECULAR BIOLOGY
As the 20th century passed into its second half, the advent
of molecular analysis revolutionized biology. It may seem
hard to imagine now, but before the invention of recombinant DNA technologies, isolating genes was the domain of
biochemists (Birnstiel, 2002; Brown, 1994). Given the massive amounts of material that can be obtained from Xenopus,
the frog played a crucial role. In fact, the sheer scale of discovery using Xenopus precludes a comprehensive discussion
here, so I direct the reader to a great review by one of the
principal fgures (Brown, 2004). A brief summary of the key
landmarks might look like this:
Owing to its very high CG content, ribosomal DNA (rDNA)
could be separated on cesium chloride gradients, and thus
the rDNA of Xenopus became the frst eukaryotic gene ever
isolated, by Max Birnstiel (Birnstiel et al., 1966). He further
demonstrated that the anucleolate mutants of Xenopus isolated by Fischberg lacked rDNA, providing a glimpse of the
future in which genetics and biochemistry would work hand
in hand (Wallace and Birnstiel, 1966). Shortly thereafter,
Don Brown isolated the second family of genes: that encoding the 5S rRNA in Xenopus (Brown et al., 1971). As a result,
the Xenopus 5S rDNA was the frst eukaryotic gene ever to
be fully sequenced (Fedoroff and Brown, 1978; Miller et al.,
1978). (For modern molecular biologists hoping to grasp the
massive effort required here, note that the sequencing of this
one gene warranted back-to-back papers in Cell.)
As if that weren’t enough, this string of exceptional studies
was paralleled by several other contemporaneous breakthroughs in other realms of molecular biology, starting
with a paper titled simply “Deoxyribonucleic Acid in
Amphibian Eggs,” in which Igor Dawid discovered mitochondrial DNA and demonstrated its maternal inheritance
in Xenopus ( Dawid, 1965 , 1966 ). Moreover, studying
lampbrush chromosomes in Xenopus, Joe Gall demonstrated that chromosomes of higher eukaryotes (with their
much larger genomes) consisted of single DNA strands
( Gall, 1963 ), as Meselson and Stahl had shown for prokaryotes. Together with Mary Lou Pardue, Joe Gall also
used Xenopus for the invention of in situ hybridization
( Gall and Pardue, 1969 ).
Xenopus also played a role in early explorations of the
central dogma, providing an exceptional system for studies of transcription and translation. The rabbit globin gene
became the frst eukaryotic mRNA to be isolated in 1969,
and mammalian cell-free lysates were used to translate
this into Globin protein shortly thereafter, but only vanishingly small amounts of protein could be made (Lockard and
Lingrel, 1969). In 1971, John Gurdon (again) showed that the
Xenopus oocyte could produce enormous amounts of protein when injected with mRNA (Gurdon et al., 1971), leading to its widespread use for this purpose. Around the same
time, Bob Roeder discovered that Xenopus oocytes were
loaded with RNA polymerases (Roeder, 1974), and Gurdon
(yet again) showed that purifed DNA could be transcribed
when injected in Xenopus oocytes (Mertz and Gurdon,
1977), making this system an effective platform for studies
of transcription. Indeed, the Xenopus 5S gene would have
yet another star turn, when Roeder and colleagues isolated
the frst eukaryotic transcription factor, TFIIIA, and showed
that it bound internal control regions in this gene (Engelke
et al., 1980; Sakonju et al., 1981).
This golden age of molecular biology with Xenopus led
directly to its modern use in cell and developmental biology.
Ultimately, Brown, Gall, and Roeder would each receive a
Lasker Award for their discoveries made in Xenopus, and
each has written entertaining retrospectives on these discoveries (Brown, 2012; Gall, 2006; Roeder, 2019).
1.6. WHERE HISTORY STOPS AND
“THE LITERATURE” BEGINS
This chapter has covered roughly 180 years of the history of
research with Xenopus, and obviously, there is a great deal
more to tell. The 1980s was a decade of explosive growth
for Xenopus research, and the frog would be used for seminal contributions spanning the biochemical analysis of the
cell cycle, to fundamental analyses of replication and transcription, to the hunt for molecular regulators of development. For example, Manfred Lohka and Jim Maller would
develop the Xenopus egg extract system ( 1985 ), building on
Lohka’s work with Lasker Award winner Yoshio Masui with
extracts from Rana ( Lohka and Masui, 1983 ). Such extracts
would prove invaluable for a wide range of studies of the
cell cycle, DNA replication, and the cytoskeleton ( Blow
and Laskey, 2016 ; Maller, 2012 ; Masui, 2001). At this
same time, molecular analysis of development in Xenopus
also exploded, a story that has been insouciantly told by
Jonathan Slack in his book Egg & Ego ( Slack, 1999 ). On a
personal note, the close of the 1980s would see me squeeze
my frst frogs and look at the tadpoles with a microscope.
A Quick History of Xenopus
fate maps using more advanced methods are now commonly used by the community to target microinjections
(Dale and Slack, 1987; Moody, 1987), but these—and also a
largely forgotten fate map by Nakamura’s colleague Hiroko
Takasaki (1987 )—proved Nakamura’s early conclusions
mostly correct.
Nakamura also carried out a remarkable series of studies on the effects of removing specifc blastomeres from
the early embryo. These studies helped to precisely def ne
the inductive interactions that pattern the early embryo
(Nakamura et al., 1970, 1971) and later inf uenced some
the earliest molecular studies of vertebrate axial patterning
(e.g. Gurdon et al., 1984; Rosa et al., 1988; Yisraeli et al.,
1990). Finally, in a little-noticed paper from 1971, he used
electron microscopy to explore the developmental biology
of nucleoli in the early embryos (Nakamura and Yamada,
1971), extending prior work done by Elizabeth Hay and John
Gurdon (Hay and Gurdon, 1967). Professor Nakamura went
on to become president of two universities and was widely
celebrated in Japan.
1.5. THE NINTH DAY OF CREATION: XENOPUS
AT THE DAWN OF MOLECULAR BIOLOGY
As the 20th century passed into its second half, the advent
of molecular analysis revolutionized biology. It may seem
hard to imagine now, but before the invention of recombinant DNA technologies, isolating genes was the domain of
biochemists (Birnstiel, 2002; Brown, 1994). Given the massive amounts of material that can be obtained from Xenopus,
the frog played a crucial role. In fact, the sheer scale of discovery using Xenopus precludes a comprehensive discussion
here, so I direct the reader to a great review by one of the
principal fgures (Brown, 2004). A brief summary of the key
landmarks might look like this:
Owing to its very high CG content, ribosomal DNA (rDNA)
could be separated on cesium chloride gradients, and thus
the rDNA of Xenopus became the frst eukaryotic gene ever
isolated, by Max Birnstiel (Birnstiel et al., 1966). He further
demonstrated that the anucleolate mutants of Xenopus isolated by Fischberg lacked rDNA, providing a glimpse of the
future in which genetics and biochemistry would work hand
in hand (Wallace and Birnstiel, 1966). Shortly thereafter,
Don Brown isolated the second family of genes: that encoding the 5S rRNA in Xenopus (Brown et al., 1971). As a result,
the Xenopus 5S rDNA was the frst eukaryotic gene ever to
be fully sequenced (Fedoroff and Brown, 1978; Miller et al.,
1978). (For modern molecular biologists hoping to grasp the
massive effort required here, note that the sequencing of this
one gene warranted back-to-back papers in Cell.)
As if that weren’t enough, this string of exceptional studies
was paralleled by several other contemporaneous breakthroughs in other realms of molecular biology, starting
with a paper titled simply “Deoxyribonucleic Acid in
Amphibian Eggs,” in which Igor Dawid discovered mitochondrial DNA and demonstrated its maternal inheritance
in Xenopus ( Dawid, 1965 , 1966 ). Moreover, studying
lampbrush chromosomes in Xenopus, Joe Gall demonstrated that chromosomes of higher eukaryotes (with their
much larger genomes) consisted of single DNA strands
( Gall, 1963 ), as Meselson and Stahl had shown for prokaryotes. Together with Mary Lou Pardue, Joe Gall also
used Xenopus for the invention of in situ hybridization
( Gall and Pardue, 1969 ).
Xenopus also played a role in early explorations of the
central dogma, providing an exceptional system for studies of transcription and translation. The rabbit globin gene
became the frst eukaryotic mRNA to be isolated in 1969,
and mammalian cell-free lysates were used to translate
this into Globin protein shortly thereafter, but only vanishingly small amounts of protein could be made (Lockard and
Lingrel, 1969). In 1971, John Gurdon (again) showed that the
Xenopus oocyte could produce enormous amounts of protein when injected with mRNA (Gurdon et al., 1971), leading to its widespread use for this purpose. Around the same
time, Bob Roeder discovered that Xenopus oocytes were
loaded with RNA polymerases (Roeder, 1974), and Gurdon
(yet again) showed that purifed DNA could be transcribed
when injected in Xenopus oocytes (Mertz and Gurdon,
1977), making this system an effective platform for studies
of transcription. Indeed, the Xenopus 5S gene would have
yet another star turn, when Roeder and colleagues isolated
the frst eukaryotic transcription factor, TFIIIA, and showed
that it bound internal control regions in this gene (Engelke
et al., 1980; Sakonju et al., 1981).
This golden age of molecular biology with Xenopus led
directly to its modern use in cell and developmental biology.
Ultimately, Brown, Gall, and Roeder would each receive a
Lasker Award for their discoveries made in Xenopus, and
each has written entertaining retrospectives on these discoveries (Brown, 2012; Gall, 2006; Roeder, 2019).
1.6. WHERE HISTORY STOPS AND
“THE LITERATURE” BEGINS
This chapter has covered roughly 180 years of the history of
research with Xenopus, and obviously, there is a great deal
more to tell. The 1980s was a decade of explosive growth
for Xenopus research, and the frog would be used for seminal contributions spanning the biochemical analysis of the
cell cycle, to fundamental analyses of replication and transcription, to the hunt for molecular regulators of development. For example, Manfred Lohka and Jim Maller would
develop the Xenopus egg extract system ( 1985 ), building on
Lohka’s work with Lasker Award winner Yoshio Masui with
extracts from Rana ( Lohka and Masui, 1983 ). Such extracts
would prove invaluable for a wide range of studies of the
cell cycle, DNA replication, and the cytoskeleton ( Blow
and Laskey, 2016 ; Maller, 2012 ; Masui, 2001). At this
same time, molecular analysis of development in Xenopus
also exploded, a story that has been insouciantly told by
Jonathan Slack in his book Egg & Ego ( Slack, 1999 ). On a
personal note, the close of the 1980s would see me squeeze
my frst frogs and look at the tadpoles with a microscope.