254
JEAN BRÄCHET
often stops at the late blastula or early gastrula stage. It is known that,
in many cases, grafting of a fragment of the hybrid in a normal host
(even if it belongs to anunrelated species) is followed by a 'revitalization'
of the lethal cells, which partially recover their morphogenetic potentialities (cf. Moore, 1955).
The biochemical reasons for this morphogenetic block remain obscure.
Although it is clear that carbohydrate metabolism is abnormal in the
lethal hybrids, the exact metabolic step which is affected remains
unknown (see review by Gregg, 1957).
Nucleic acid synthesis, as one might expect, is also affected by the
introduction of a foreign nucleus into an egg. It has been suggested by
J. A. Moore (1955, 1958) that the block in morphogenesis might be due
to the exhaustion, at the blastula stage, of the cytoplasmic store of
DNA which has just been discussed. Such an explanation, as we have
seen, meets with some difficulties even in the case of normal embryos.
That things are more complex in the hybrids is indicated by two facts :
first, DNA synthesis goes on for a considerable time in the arrested
hybrid gastrulae (Gregg and Lovtrup, 1955, 1960b; Chen, 1954) ; second,
as shown by B. C. Moore (1959) and by Tencer (1961a) labelled
thymidine is incorporated into DNA or morula as well as blastula cells
of the lethal hybrids. Moreover, 24 hours after development stops, the
incorporation still proceeds in the nuclei, a fact which suggests that the
block in development is not due to the arrest of DNA synthesis.
I Very recently, we have found (Brächet et ah, 1962) that in the Rana
temporaria
$ χ Rana esculenta $ combination, the incorporation of
uridine into DNA is normal during cleavage. But, when the block in
development occurs, this incorporation is very greatly reduced.
Incorporation of the precursor into nuclear RNA continues, but at a
continually decreasing rate.
Cytochemical observations have shown that these hybrids are a
mosaic of nuclei containing different proportions of DNA and RNA
(Brächet, 1954) ; nuclei containing a large excess of RNA or no RNA at
all can be found side by side. We have now found that it is possible to
modify the proportion of nuclei rich in DNA or of nuclei rich in RNA by
the addition of substrates of the tricarboxylic acid cycle to the lethal
hybrids. Pyruvate, citrate and oxaloacetate inhibit the production of
nuclear RNA ; on the other hand, glucose, succinate and especially ATP
produce a striking accumulation of RNA in the nuclei of the blocked
hybrids (Brächet et al., 1962).
Taken together, the experimental results obtained thus far can be
explained by the assumption that the lethal embryos, in contrast with
normal ones, are incapable of maintaining their TPN+/TPNH equilibrium in the presence of added substrates (Brächet et al., 1962). Since it
JEAN BRÄCHET
often stops at the late blastula or early gastrula stage. It is known that,
in many cases, grafting of a fragment of the hybrid in a normal host
(even if it belongs to anunrelated species) is followed by a 'revitalization'
of the lethal cells, which partially recover their morphogenetic potentialities (cf. Moore, 1955).
The biochemical reasons for this morphogenetic block remain obscure.
Although it is clear that carbohydrate metabolism is abnormal in the
lethal hybrids, the exact metabolic step which is affected remains
unknown (see review by Gregg, 1957).
Nucleic acid synthesis, as one might expect, is also affected by the
introduction of a foreign nucleus into an egg. It has been suggested by
J. A. Moore (1955, 1958) that the block in morphogenesis might be due
to the exhaustion, at the blastula stage, of the cytoplasmic store of
DNA which has just been discussed. Such an explanation, as we have
seen, meets with some difficulties even in the case of normal embryos.
That things are more complex in the hybrids is indicated by two facts :
first, DNA synthesis goes on for a considerable time in the arrested
hybrid gastrulae (Gregg and Lovtrup, 1955, 1960b; Chen, 1954) ; second,
as shown by B. C. Moore (1959) and by Tencer (1961a) labelled
thymidine is incorporated into DNA or morula as well as blastula cells
of the lethal hybrids. Moreover, 24 hours after development stops, the
incorporation still proceeds in the nuclei, a fact which suggests that the
block in development is not due to the arrest of DNA synthesis.
I Very recently, we have found (Brächet et ah, 1962) that in the Rana
temporaria
$ χ Rana esculenta $ combination, the incorporation of
uridine into DNA is normal during cleavage. But, when the block in
development occurs, this incorporation is very greatly reduced.
Incorporation of the precursor into nuclear RNA continues, but at a
continually decreasing rate.
Cytochemical observations have shown that these hybrids are a
mosaic of nuclei containing different proportions of DNA and RNA
(Brächet, 1954) ; nuclei containing a large excess of RNA or no RNA at
all can be found side by side. We have now found that it is possible to
modify the proportion of nuclei rich in DNA or of nuclei rich in RNA by
the addition of substrates of the tricarboxylic acid cycle to the lethal
hybrids. Pyruvate, citrate and oxaloacetate inhibit the production of
nuclear RNA ; on the other hand, glucose, succinate and especially ATP
produce a striking accumulation of RNA in the nuclei of the blocked
hybrids (Brächet et al., 1962).
Taken together, the experimental results obtained thus far can be
explained by the assumption that the lethal embryos, in contrast with
normal ones, are incapable of maintaining their TPN+/TPNH equilibrium in the presence of added substrates (Brächet et al., 1962). Since it