In this article we will discuss about the involvement of parental genes in the development of control of embryos.
Apart from the foregoing data, which characterize the activity of the nuclear DNA in general terms, there are many indirect indications that nuclear genetic factors (the genes), after having been dormant throughout the period of cleavage, begin to manifest themselves during gastrulation and in ever-increasing measure control the processes of development from this stage onward.
Evidence to this effect may be derived from certain hybridization experiments. These must be considered here in greater detail. If the egg of one species is fertilized by a spermatozoon of another species, the nuclei of the two gametes may fuse and together may participate in cleavage and development.
Each cleavage cell would then consist of maternal cytoplasm (because the contribution of the spermatozoon to the cytoplasm of the fertilized egg is negligible), the maternal nuclear half, and the paternal nuclear half. Embryos of this composition are true hybrids. The eggs, however, may be treated just after fertilization, before the nuclei of the egg and the sperm have time to fuse, in order to remove the egg nucleus.
This can be achieved, for instance, by puncturing the egg and allowing a small quantity of cytoplasm together with the egg nucleus to flow out of the egg. If the egg is fertilized with sperm of another species, an embryo will develop in which the cytoplasm is maternal, but the nucleus is paternal.
Such embryos will be referred to as hybrid andromerogones. (The term merogony is applicable to the fertilization of a part or fragment of the egg; androgenesis is the development of an embryo having only paternal nuclei.) Andromerogones are embryos in which an enucleated egg or egg fragment is fertilized by sperm of the same species. As a rule, andromerogones are haploid.
The results of hybridization depend primarily on the relationship between the two parents. The penetration of the egg by foreign sperm is much more easily achieved than the subsequent cooperation with the maternal cytoplasm. Cases are known in which eggs were “fertilized” by sperm belonging to animals of a different class and even of a different phylum (e.g., fertilization of the sea urchin Sphaerechinus with sperm of the sea lily Antedon; fertilization of the sea urchin Strongylocentrotus with sperm of the mollusc Mytilus).
In these cases, the sperm penetrates the egg and activates it but does not play any part in the subsequent development. This is tantamount to a parthenogenetic development of the egg, caused by the penetration of foreign sperm. If the animals are more closely related, as for instance in the case of a toad’s eggs being fertilized by frog’s sperm (Bufo vulgaris × Rana temporaria), the sperm nucleus fuses with the nucleus of the egg.
The development (cleavage) begins, but the embryos die before the beginning of gastrulation owing to incompatibility between the sperm nucleus and the egg nucleus and cytoplasm. If the animals are still more closely related, such as species of the same genus, then the hybrids may be fully viable throughout their entire development.
In the cases of rather closely related species, producing viable hybrids, the influence of the nucleus can best be analyzed. Much of the work has been done on sea urchin development. If two species of sea urchin, differing in rate of cleavage, are crossed, the cleavage of the hybrid is always the same as the cleavage of the maternal species.
In the sea urchin Dendraster eccentricus, the egg cleaves at the rate of 29 to 30 minutes between successive divisions. In Strongylocentrotus franciscanus, the interval between successive cleavages is 47 minutes. The hybrid has the maternal rate of cleavage. Also, if fragments of the egg devoid of a nucleus are fertilized, the resulting hybrid andromerogones cleave with a rhythm that is the same as in the maternal species.
The rate of cleavage cannot be influenced by the sperm nucleus. On the other hand, the characters of the larva—the pluteus—are clearly intermediate between the two parents; at the larval stage the sperm nucleus has been able to exert its influence.
In a special case, the influence of the sperm nucleus has been discovered as early as the beginning of gastrulation. In the sea urchin Lytechinus, the mesenchyme cells produced from the micromeres migrate into the blastocoele slightly earlier than the beginning of invagination of the archenteron.
In the sea urchin Cidaris, the mesenchyme cells do not separate from the epithelium until after the archenteron begins to invaginate; they are consequently given off from the inner end of the archenteron. Furthermore, the cleavage of Lytechinus proceeds at a much greater pace than in Cidaris; the blastula stage is reached after 5.5 hours in the Lytechinus embryos, but only after 16 hours in Cidaris embryos.
The beginning of gastrulation is correspondingly delayed in the second species. When the Cidaris eggs were fertilized by Lytechinus sperm, the early stages of development up to the gastrula stage proceeded exactly as in the maternal species, Cidaris. The rate of cleavage corresponded to that typical for Cidaris.
As the gastrulation approached, the mesenchyme cells began to be separated from the blastoderm just as the invagination of the archenteron first became visible. The stage at which the mesenchyme cells migrated into the blastocoele was therefore intermediate between the two species, thus proving that at the beginning of gastrulation the paternal nucleus was already able to manifest itself.
In hybridization experiments performed on amphibians, it is a rule without exception that the cleavage rate is strictly maternal. This also holds true in cases of hybrid andromerogones. Therefore, one may conclude that the cytoplasm alone determines the rate of cleavage.
The pigmentation of the egg and early developmental stages is, of course, always maternal, because the egg pigment is synthesized before maturation. Differences in the pigmentation of the larvae become evident only much later when the melanophores, derived from the neural crest, become differentiated.
During subsequent development the pigmentation depends on the nuclear factors. A hybrid andromerogone was produced by using eggs of the black race of the axolotl (Ambystoma mexicanum) and sperm of the white race. In the embryo, the cytoplasm therefore derived from the black race and the nucleus from the white race. The animals were all white, thus showing that the nucleus dominated over the cytoplasm.
No crosses are known in amphibians in which morphological differences between the two parent species could be discovered in the early stages—the gastrulation or the neurulation stages. It has been found, however, that the end of cleavage and the beginning of gastrulation is the stage when the incompatibility of the sperm nucleus and the egg nucleus and cytoplasm first manifests itself.
In many hybrid combinations the hybrid goes through the early stages of cleavage more or less normally, but the development stops in the blastula or early gastrula stage (Rana esculenta × Bufo vulgaris; R. esculenta × R. temporaria; B. vulgaris × R. temporaria; Triturus palmatus × Salamandra maculosa; R. pipiens × R. sylvatica; R. pipiens × R. clamitans; and others).
If the true hybrid between two species is viable, the hybrid andromerogone may not be. The andromerogone embryos, even without hybridization, are always weaker than normal embryos owing to the haploid state of their nuclei. However, the hybrid andromerogones are much inferior in their capacity for development.
The hybrid combination R. pipiens × R. capito is fully viable up to the adult stage when the chromosomes of both parental species are present. The andromerogone of R. pipiens fertilized with sperm of the same species develops into a tadpole. The hybrid andromerogone (R. pipiens × R. capito) develops normally through cleavage and gastrulation stages but dies in the early neurula stage.
The paternal chromosomes in the presence of maternal chromosomes may function normally and transmit the paternal characters to the diploid hybrid, but left alone with the foreign cytoplasm they prove to be inadequate for supporting development. Further cases are known in which the andromerogonic hybrid dies during gastrulation or shortly after (R. pipiens × R. palustris; Tritunis alpestris × T. palmatus; T. palmatus × T. cristatus).
There is some direct evidence that in hybrids the paternal chromosomes cause the synthesis of proteins of their own species about the time of the beginning of gastrulation.
The evidence was obtained by immunological methods in the following experiment:
Eggs of the sea urchin Paracentrotus lividus were fertilized with sperm of Psammechinus microtuberculatus. Using the blood plasm of animals immunized against proteins of Psammechinus microtuberculatus, it was found that the fertilized eggs showed no trace of these specific antigens.
Twenty-four hours after fertilization, however, when the hybrid embryos were in the mesenchyme blastula stage, antigens of the paternal species, Psammechinus microtuberculatus, could be clearly detected. Since the paternal antigens could not be discovered earlier, it is evident that these antigens (predominantly proteins) had been built up in the cytoplasm, which is derived mainly from the egg, under the influence of the sperm chromosomes.
The death of nonviable hybrids and hybrid andromerogones is preceded by an arrest of at least some of the physiological processes which characterize the onset of gastrulation, such as the increase in oxygen consumption, glycolysis, and the increase in the amount of ribonucleic acid; the latter is especially suggestive, since it involves a synthetic process.
We have seen that besides the synthesis of ribonucleic acids the embryo during gastrulation begins synthesizing new proteins, manifested as new antigens when investigated by immunological methods. It is highly probable that in cases of incompatibility between the foreign paternal nuclei and the maternal cytoplasm these synthetic processes cannot go on, or cannot go on satisfactorily, hence the arrest of development and death.
If, on the other hand, the nuclei and the cytoplasm are compatible, the synthesis of new proteins and nucleic acids (and probably also other cytoplasmic substances) goes on progressively.
Since some of the newly synthesized substances are different from the ones present before, rapid changes in the constitution of the embryo are inaugurated, changes that involve not only the position and arrangement of cells, as in the gastrulation movements, but also their inherent physiological properties. This leads to the next period of development in which the various parts of the embryo become progressively differentiated from one another; the period of organogenesis.
An exception to the late participation of paternal genes in development are the histone genes which, start producing histone mRNA, which becomes immediately transcribed into histone proteins. It has been found that paternal histone genes in hybrids participate in producing histone RNA as early as the 2 cell stage, together with the corresponding maternal genes.