JOURNAL OF MORPHOLOGY 23(1):1-3. MARCH 20. 1912
HEREDITY IN HETEROGENEOUS HYBRIDS1
The Rockefeller Institute, New York
for the Whitman Memorial Volume but received too late to be included.
2Loeb, King and Moore, Arch. f. Entwicklungsmechanik, vol. 29, 1910.
1. The study of heredity in embryos offers in one respect a wider field than that in adults inasmuch as heterogeneous hybrids rarely reach the adult stage. Eight years ago I found a method by which the eggs of the sea-urchin can be fertilized by the sperm of starfish, ophiurians and holothurians. The larvae are purely maternal, namely plutei. The results were confirmed by Godlewski for the fertilization of the egg of the sea-urchin by the sperm of the crinoid. It is well known that if we cross two homogeneous forms, e.g., two forms of sea-urchins, the paternal influence can be clearly seen in the pluteus stage. Since I have never published the figures of my experiments on heterogeneous hybridization, I may supplement my former statements with a few drawings. Figs. 1 to 6 are camera drawings of plutei of Strongylocentrotus purpuratus produced by artificial parthenogenesis. The plutei are, of course, in every detail identical with the plutei obtained if these eggs be fertilized with sperm of their own species. Figs. 7 to 9 are drawings of five days old plutei of Strongylocentrotus purpuratus and Strongylocentrotus franciscanus . They differ from the pure breeds of S. purpuratus in several characters of the skeleton which exist in the pluteus of franciscanus but are absent from purpuratus, namely the greater roughness of the skeleton, the presence of cross bars and the greater length of the arms.2 In figs. 10 to 13 are shown the five days old plutei of the egg of S. purpuratus fertilized with the sperm of the starfish (Asterias). It is obvious that the latter plutei are purely maternal. It should, however, be borne in mind, that the objection might be raised that the presence of the skeleton in the sea-urchin pluteus might be dominant over its absence in the starfish larva. I have thus far vainly tried to produce a starfish larva with a pluteus skeleton by the hybridization of the two species.
We are therefore compelled to state that the hybrids between the sea-urchin egg and the starfish sperm represent more closely the purely maternal form than do the hybrids between two seaurchins, which always show paternal characters.
|3Tennent, Publication 132, Carnegie Institution, 1910.|
2. It is well known that Herbst and Tennent have made experiments in which the paternal influence in the hybrid embryo was diminished. Tennent states that in the cross between Hipponoe and Toxopneustes, Hipponoe characters become dominant in sea water of a high OH concentration and Toxopneustes characters in sea water of a low OH concentration.3 The amount of acid or alkali Tennent needed to accomplish his result was very small; namely about 2 cc. N/10 acetic or hydrochloric acid to 500 cc. of sea water. A. R. Moore, W. R. King and myself made a number of experiments in which the hybrids between S. purpuratus and S. franciscanus were raised in sea water to which varying quantities of HCl or acetic acid or NaHO were added (from 0 to 0.4 cc. N/10 acid or NaHO to 50 cc. of sea water). We were able to retard or accelerate the rate of development, but the character of the hybrid remained absolutely unaltered.
I wish to call attention to the necessity of sterilizing the pipettes by boiling them after each experiment, instead of sterilizing them by rinsing in distilled or fresh water as is often done.
|4Moenkhaus, Am. Jour, of Anat., vol. 3, p. 29, 1904.|
3. Moenkhaus measured the rate of segmentation in hybrid fish eggs and found that the rate for the first five cleavages is determined by the egg.4 The egg of Ctenolabrus segments about forty minutes after impregnation with sperm of its own kind, while the egg of Batrachus tau, if fertilized with the sperm of the same species, segments after about eight hours. If the egg of Batrachus be fertilized with the sperm of Ctenolabrus it also does not chromosomes of the sperm are thrown out of the egg or disintegrate. This is not in harmony with the observations of Moenkhaus for the hybrids between Menidia and Fundulus heteroclitus, and with those of Godlewski for the hybrids between sea-urchin and crinoid. I am not in a position to decide the differences in the observations of these authors. The observations mentioned in the preceding paragraph are more in harmony with the observations of Moenkhaus and Godlewski.
The spermatozoon has two distinct effects upon the egg: namely, it causes its development and it transmits certain parental hereditary characters to the offspring. The experiments in heterogeneous hybridization confirm the idea supported by the experiments on artificial parthenogenesis, that the formation of the embryo is purely a matter of the egg and that the main function of the spermatozoon is the causation of the development of the egg. If we may express this statement in the form of a paradox we may say that fertilization is primarily and essentially artificial parthenogenesis. The transmission of hereditary characters through the sperm is in many cases merely an accessory function. It becomes of vital importance only in those forms where the male is heterozygous for sex and where the species can only be propagated through sexual reproduction.
If the sperm nucleus be chemically almost identical with the egg nucleus it is possible for it to force one or a few characters upon the developing embryo. If the difference between sperm and egg nucleus exceed a certain limit—which structural chemistry may one day be able to define—the hereditary influence of the spermatozoon is as a rule completely or almost completely obliterated; and the result is a purely maternal larva, rendered more or less sickly through the presence or formation of foreign or faulty substances.
The camera drawings of the sea-urchin larvae were made by Mr. W. O. R. King, those of the fish embryos by Mr. Bagg. To both gentlemen I wish to express my thanks.