Parthenogenesis

PARRHO BOJlFSHb, see Osteochondritis. ' PARTHENOGENESIS (from parthenos-girl and genesis-birth), one of the forms of sexual reproduction, characterized by the fact that the female gamete (egg) is capable of developing without fertilization. The first observations of P. in the last century (Siebold and others) were met with skepticism, however at the present time extremely numerous cases of P. in animals and plants are known. P. is observed in worms (rotifers, free-living nematodes), in insects (aphids, gall-forming insects, silkworm, wasps, bees), in crustaceans (daphnia, cyclops, artemia), in algae (chara, spirogyra) and in higher plants (hawkweed-Hieracium). P. can be of a random nature (facultative P.) or constant (obligate P.), alternating with fertilization. In the latter case, parthenogenetic eggs often lose the ability to be fertilized, differing from normal eggs also in the nature of the maturation process and the number of chromosomes (see below). Under the influence of external conditions (nutrition, temperature) the parthenogenetic cycle can be shortened or lengthened. An example of the influence of environmental conditions can be seasonal P. of daphnia, rotifers, aphids, phylloxera, etc. Thus, in aphids during the summer months there is exclusively P. and only in the fall with the onset of unfavorable living conditions, males hatch from eggs along with females. Fertilized eggs, differing from parthenogenetic eggs by the presence of a protective shell, overwinter without developing, and only in the spring parthenogenetic females hatch from them. Such cycles can be significantly more complex (phylloxera), but the eggs may differ from those capable of fertilization in size, and the females of both generations - in their structure. The influence of the external environment can be so great that the same species, living in different places, can reproduce either parthenogenetically or by means of fertilization. Thus, Artemia salina in Trieste lays parthenogenetic eggs with a diploid number of chromosomes (84), while the same species, living in Sardinia, lays haploid, fertilizable eggs with 42 chromosomes. By artificially changing the conditions of nutrition and temperature, it is possible to significantly lengthen or shorten the parthenogenetic cycle. Thus, under favorable conditions, aphids can reproduce parthenogenetically for many years in a row. The difference between parthenogenetic and fertilizable eggs can go so far that individuals of different sexes develop from them. A classic example-bees, ants, etc. Thus, in bees, haploid parthenogenetic eggs produce males (drones), and from diploid fertilizable eggs-diploid females and worker bees, which are underdeveloped in terms of sex females. With appropriate abundant nutrition, these latter can develop their rudimentary sexual organs. The determination of sex is connected with the more general question of the chromosomal composition of parthenogenetic and fertilizable eggs. P. can be both haploid and diploid. Thus, in rotifers, aphids, etc., during egg maturation, only one polar body is extruded, and the eggs therefore have a complete (diploid) set of chromosomes. On the other hand, sooner or later comes the laying of eggs that extrude both polar bodies (haploid eggs). The latter can develop parthenogenetically (haploid P.), but are also capable of fertilization ('winter' eggs). Of much greater fundamental interest is artificial P., which allows one to study not only the connection between sex determination and fertilization, but also the mechanism of the latter itself. The first accidental observations were made long ago. Thus, it was noticed that in bright sunlight, unfertilized eggs of certain butterflies begin to develop, just like the egg of the silkworm (Bombyx mori) when treated with strong sulfuric acid (Tikhomirov). Individual attempts at artificial activation of the egg belong to the Hertwig brothers and other researchers (Morgan, Dewitz), who treated eggs with salts and other substances. However, systematic study of this problem began only at the end of the 19th century (Loeb) and in the last 30 years has advanced significantly. Many agents are capable of activating an unfertilized egg. These include chemical substances (acids, narcotics, tannin, etc.), hypertonic environment, mechanical factors (prick, shaking), electric current, radiant energy. In application to the eggs of different animals, the methods of artificial P. vary widely, nevertheless to a certain extent it has been possible to uncover the basic principle of their action. The classic method of artificial P. belongs to Loeb. The material in his experiments consisted mainly of eggs of echinoderms (sea urchins and stars). Eggs are treated for several minutes with a solution of butyric acid (or any other fatty acid or benzene, chloroform, etc.), after which they are washed with sea water, and then transferred for 1-2 hours to hypertonic sea water (sea water to which a certain amount of NaCl or HgCl₂ or evaporated sea water is added). Even after treatment with butyric acid, fertilization membranes appear on many eggs, often having a normal appearance, sometimes however located somewhat eccentrically not in the form of an even layer, but in the form of separate, unfused bubbles with each other. A 1-2 hour exposure in hypertonic sea water is sufficient for cleavage and subsequent development to begin in most eggs when transferred to normal sea water. For some objects, the method can be simplified (eliminate treatment with butyric acid) or the order of operations reversed. By this method, it was possible to raise sexually mature echinoderms (Delage, Shearer, Lloyd). With certain variations, these methods have been successfully applied to the eggs of various marine animals (worms, mollusks). A completely different method-traumatic-was developed for the eggs of amphibians (toads, frogs-Bataillon and others). A prick of an unfertilized egg is sufficient to cause cleavage in a certain number of eggs, the development of which soon stops. The introduction into the egg of nucleus-containing blood cells or generally nuclear cells significantly increases the percentage of developing eggs. Thus, Loeb by this method managed to raise several dozen sexually mature frogs. Delage proposed a method of treating eggs based, in his opinion, on the coagulating and liquefying effect of substances on the colloids of protoplasm. The first group includes substances such as tannin, the second-weak alkalis. A 5-6 minute treatment of eggs with tannin followed by transfer for 1 hour to sea water often gave 100% development of embryos. The theoretical concepts based on these experiments are not yet sufficiently consistent with each other. The deepest penetration into the essence of the process apparently belongs to Loeb, who distinguishes two stages of egg activation. First of all, the formation of the fertilization membrane occurs, which is associated with partial cytolysis of the egg under the influence of butyric acid and other agents. The fact that in some cases the first stage of treatment can be excluded (see above) does not contradict this assumption, since cytolysis can occur in the egg without reaching such a strong morphological manifestation as the formation of the fertilization membrane. Partial cytolysis of the egg, according to Loeb, stimulates the oxidative processes occurring in it, which lead the egg to death (complete cytolysis) if they are not corrected by subsequent treatment. In this respect, hypertonic sea water is the 'healing' factor, provided that it contains free oxygen. Thus, by adding KCN, the healing effect of hypertonic sea water is nullified. On the other hand, it was directly shown (Warburg) that hypertonic sea water enhances the oxidative processes in the egg. Another healing factor, according to Loeb, can be hydrolytic (correspondingly proteolytic) processes in the egg, which are apparently enhanced in normal sea water to which potassium cyanide is added. Loeb and others actually managed to artificially divide into two phases the natural process of fertilization, by removing the sperm after it had touched the surface of the egg. This contact of the sperm is sufficient to cause the formation of the fertilization membrane, but then the egg dies, just as after treatment with butyric acid. Subsequent treatment of the egg with hypertonic sea water, as in the experiments described above, saved its life. The sperm thus introduces substances that stimulate the chemical processes occurring in the egg. Further analysis of the chemical processes occurring in the egg, however, still awaits its researcher, all the more so since Loeb's assumptions meet objections, not always however sufficiently well-founded, from other authors (Delage, Bataillon and others.).-The connection of artificial P. with sex is insufficiently studied due to the fact that the larvae were for the most part not brought to a sexually mature state. In those cases where this was achieved, the male sex was established for two larvae of the sea urchin that had undergone metamorphosis (Delage).

The number of chromosomes has not been counted, but genetic data indicate that the chromosomal set is haploid. For parthenogenetic frogs, of 14 studied cytologically, one was found to be male and 13 of undetermined sex, and in both cases practically a diploid number of chromosomes was found. Experiments in artificial parthenogenesis show that a mature egg contains all that is necessary to begin development; in this sense it is independent of the spermatozoon. The complex cytological picture accompanying fertilization can be reproduced by artificial agents to the extent necessary for the appearance of cleavage figures. Thus, those theories which linked the essence of fertilization with the known functions of the spermatozoon are refuted. On the other hand, attempts to cause the development of an embryo from a spermatozoon cultivated in a nutrient medium (rooster spermatozoa in egg white) ended in failure. The spermatozoon plays a dual role in fertilization: it activates the development of the egg and introduces into it the male nucleus containing paternal hereditary traits. The latter function of the spermatozoon cannot be replaced by the artificial agents described above, while the activation of the egg can be considered as irritation of the cell, understanding irritation in the broadest sense, and cleavage of the egg—as a response to irritation. In favor of this view speaks the diversity of egg activators with the uniform character of its reaction, as well as a number of physicochemical phenomena accompanying both egg activation and irritation of specialized excitable tissues (muscle). In this formulation, the problem of egg activation opens up extremely broad prospects for the researcher and connects the problem of fertilization with the problem of cell division.