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A Block to Cross-Fertilization Located in the Egg Jelly of the Frog Rana Clamitans

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A Block to Cross-Fertilization Located in the Egg Jelly of the Frog Rana Clamitans /. Embryol. exp. Morph. Vol. 32, 2, pp. 325-335, 1974 325 Printed in Great Britain A block to cross-fertilization located in the egg jelly of the frog Rana clamitans By RICHARD P. ELINSON1 From the Department of Zoology, University of Toronto SUMMARY The egg of Rana clamitans enrobed in its native jelly was not fertilized by sperm of R. pipiens. However, when R. clamitans eggs were enrobed by R. pipiens jelly, they were fertilized by R. pipiens sperm. Male pronuclei were found in the eggs, and most of the resulting embryos were diploid. The embryos gastrulated, but most arrested at mid- to late gastrula stages. Some begun neurulation, but none survived longer than 4 days. When R. clamitans eggs in R. pipiens jelly were fertilized by R. clamitans sperm, the embryos developed normally except that they failed to hatch. In the reciprocal experiment, R. pipiens eggs were enrobed in R. clamitans jelly. The eggs were not fertilized by R. pipiens sperm but were fertilized by R. clamitans sperm. Therefore, the R. clamitans jelly plays a major role in preventing fertilization by sperm of other species. The if?, clamitans jelly's block to cross-fertilization was not a block to sperm migration. Further, the R. clamitans jelly contained factors which permitted R. pipiens sperm to fertilize dejellied R. pipiens eggs. Dejellied eggs are usually not fertilizable in the absence of jelly factors. INTRODUCTION It is clear, from the results of a variety of experimental approaches, that the jelly which surrounds the amphibian egg plays an important role in fertilization (see Shaver, 1966; Elinson, 1973b, for references). Species-specific differences have been described in the morphology of the jelly coat (Salthe, 1963) as well as in the antigens of the jelly coat (Shivers, 1965; Katagiri, 1967; Shaver, Barch & Umpierre, 1970). The question arises as to whether the jelly coat plays any role in the species-specificity of fertilization. There is evidence that in many cases there is a lack of species-specificity in amphibian fertilization, and that there is a lack of species-specific action by the jelly coat in particular. For instance, a large number of inter-specific and inter- generic fertilizations are possible among amphibians (Montalenti, 1938; Moore, 1955). Moore (1941), in describing a series of interspecific inseminations, men­ tioned that there did not appear to be differences in fertilization frequencies between homospecific and heterospecific combinations. Experiments on de­ jellied eggs have shown that jelly preparations from one species can support fertilization of dejellied eggs of a second species even though, in some cases, 1 Author's address: Department of Zoology, University of Toronto, Toronto, Ontario M5S1A1, Canada. 326 R. P. ELINSON cross-fertilization does not occur between the species in question (Katagiri, 1966, 1967; Elinson, 1971a). On the other hand, there are two reports of the jelly coat preventing fertiliza­ tion by foreign sperm. Katagiri (1966) demonstrated that the failure of fertiliza­ tion involving Hyla arborea japonica eggs and Rana chensinensis sperm was du e to the failure of the Rana sperm to penetrate into the Hyla jelly coat. Experi- ments by Blackler & Gecking (1972) indicated that the jelly coat of Xenopus mulleri eggs allowed few fertilizations by Xenopus laevis sperm. They hypothe­ sized that the X. laevis sperm were unable to attach to the X. mulleri jelly coat and hence few sperm reached the egg. In addition, Shivers (1962) and Shaver et al. (1970) showed, using antisera against jelly, that there are species-specific jelly antigens which play some role in fertilization. It is likely that each cross-fertilization combination would have to be analysed individually to see whether there is species-specificity for that combination. The specificity may reside at the egg cell membrane, at the vitelline coat, or in the jelly. If the specificity resides in the jelly, then a comparison of the jelly properties in the two species would help to elucidate the mechanisms of jelly action in fertilization. All inseminations reported between species of North American Rana lead to fertilization, with one exception : foreign sperm are unable to fertilize the eggs of Rana clamitans. The experiments reported here demonstrate that this failure of cross-fertilization is due to the jelly surrounding the R. clamitans egg. The experiments delineate a set of possible roles that the jelly could be playing in preventing cross-fertilization. MATERIALS AND METHODS Sexually mature R. pipiens were obtained from J. M. Hazen and Co., Alburg, Vermont, and the J. R. Schettle Frog Farm, Stillwater, Minnesota, during the fall or winter. Sexually mature R. clamitans were collected near Snelgrove and Milton, Ontario, in the late spring, and obtained from Connecticut Valley Biological Supply Co., Southampton, Massachusetts, from Mogul-Ed, Oshkosh, Wisconsin, and Nasco-Steinhilber, Fort Atkinson, Wisconsin, also in the late spring. Females were induced to ovulate by injection of one to three female pituitaries intraperitoneally, and 0-25 mg progesterone intramuscularly if required. Body-cavity eggs were transferred between frogs using a previously described technique (Smith, Ecker & Subtelny, 1968). Donor eggs were stained with 0-1 % neutral red in Ringer's solution for 1 min and washed to remove excess stain. Following passage down the oviducts, eggs were inseminated with sperm of R. pipiens (0-4 testes/ml of 10 % Ringer's solution) or of R. clamitans (0-2 testes/ ml of 10% Ringer's solution) following standard procedures. Donor and host eggs were separated on the basis of the neutral red staining. In most cases, Frog jelly and cross-fertilization 327 R. pipiens eggs were much larger than the R. clamitans eggs, and size of the egg served as confirmation to the staining. Eggs were scored as unfertilized or fertilized based on blastula formation. Eggs which had abortive furrows or had puckered surfaces were not included in the data. These had been activated, but it could not be determined whether they had been injured, or had been fertilized but failed to develop. The experiments were carried out at room temperature (21-23 °C). Dejellied eggs were inseminated as described previously (Elinson, 1971 a). The sperm concentrations used were 0-67 testes/ml for R. pipiens and 0-33 testes/ml for JR. clamitans. Control inseminations of jellied eggs were run with each experi­ ment to check for contamination of the sperm suspension by sperm of the other species; no contamination was found. R. clamitans egg-water was prepared similarly to the preparation of R. pipiens egg-water (Elinson, 1971a) except that the former had to be separated from the eggs by filtering it through cheese cloth, due to the nature of the R. clamitans jelly. (The outer jelly layer of the R. clamitans jelly coat expands markedly in water filling the culture dishes, and the eggs do not remain stuck to the dishes as do R. pipiens eggs.) Inseminated eggs were kept overnight and scored for fertilization on the following day. Embryos or eggs were fixed in Smith's fixative and stored in 4% formalin. Eggs were sectioned at 9-10 /mi, stained with Feulgen and counterstained with light green (Moore & Ryan, 1940) for examination of the nucleus. Chromosome preparations of late blastulae or early gastrulae were made by the method of DiBerardino (1962). RESULTS Cross-insemination between R. clamitans and R. pipiens In the course of these experiments, several thousand R. clamitans eggs in their native jelly were inseminated by R. pipiens sperm. With one questionable exception described later, no eggs were fertilized. This was true even when extremely high concentrations of R.pipiens sperm were used. Sperm preparations at concentrations of more than two orders of magnitude greater than that re­ quired to obtain 90 % fertilization of R. pipiens eggs failed to fertilize R. clamitans eggs. The inseminated eggs did not rotate, and were not activated by R. pipiens sperm. When eggs were sectioned 1 h post-insemination, the female nucleus was in metaphase II, the meiotic state of an unactivated egg. Male pronuclei were not seen, although a condensed nucleus which remained in the pigmented cortex and formed no aster would be exceedingly difficult to detect. The failure of JR. pipiens sperm to fertilize R. clamitans eggs was not due to a failure of sperm migration through the jelly to the eggs. It is often difficult to find amphibian sperm near the egg surface using sperm concentrations which are sufficient to ensure fertilization. This is due in part to the large size of the egg. In order to see if R. pipiens sperm migrated through R. clamitans jelly, two 328 R. P. ELINSON Table 1. Fertilization of R. clamitans eggs transferred to R. pipiens body cavities The results are the sum of a number of experiments in which 11 donors, 16 hosts, MR. pipiens sperm suspensions, and 8 R. clamitans sperm suspensions were used in total. Donor's eggs (R. clamitans) Host's eggs (R. pipiens) , " \ i •> No. of No. % No. of No. % Sperm eggs fertilized fertilized eggs fertilized fertilized R. pipiens 3050 517 17 5258 4823 92 R. clamitans 1367 1090 80 2374 2285 96 observations were made. First, the eggs were inseminated with high concentra­ tions of sperm. In these inseminations, a very large number of sperm were seen near the egg surface. Secondly, the application of sperm was localized by inject­ ing sperm into an area of the outer jelly layer. Sperm were observed to move from the injection point through the jelly and to reach the egg surface. Despite the presence of far larger numbers of sperm near the egg surface than seen normally, no fertilizations occurred in these two experiments. It did not appear that the R. pipiens sperm penetrated the egg's vitelline coat.
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