Développement d’individus haploïdes adultes élevés en parabiose chez le triton Pleurodeles waltlii Michah.: Syndrome de l’haploïdie et différenciation sexuelle
Authors: Gallien Par L. 
Published: April 2021

The development of adult haploid individuals of the urodele Pleurodele waltlii joined in parabiosis: the haploid syndrome and sexual differentiation

Haploid individuals of the Urodele amphibian Pleurodeles waltlii, joined in parabiosis with diploid embryos of the same species at the tail-bud stage, were raised to adults. This is the first such case among vertebrates. The haploid individuals lived for 2 or 3 years and more. When they were sacrificed, nothing indicated a limit to their life-span.

The hormonal secretions of the gonads are active and the target territories competent, as shown by the reactions of the Müllerian ducts, cloaca and callosities.

The lethal haploid condition is overcome through cross-circulation by which the haploid benefits from the diploid’s blood. The diploid parabiont is not affected in its general structure by the presence of the haploid’s blood. Crosscirculation is demonstrated by the presence of both haploid and diploid erythrocytes in blood smears.

The manifestations of the haploid syndrome obvious at the adult stage are described. The growth of both animals in the pair is greatly retarded; larval life is lengthened. This is not specific to the dual genetic constitutions, but results from the physiological conditions set up through parabiosis. Despite the restricted growth of the diploid partners, their gonads can undergo sexual maturation, i.e. produce spermatozoa and mature ovocytes.

The haploid individuals, obtained by heat shock of the egg, were androgenetic. In the best cases, they differentiate testes having the general structure of an adult gonad. In such cases spermatogenesis begins, but the process fails at the stage of primary spermatocytes, during the first division. Bivalents do not form. The meiotic cycle can proceed up to anaphase of the first division. Afterwards, pycnosis is general in all testicular cysts. A poorly developed glandular tissue is built up in the cysts emptied of germinal elements.

Usually, however, testicular development in the haploids is limited. The gonads are small and remain juvenile, a condition normally observed in young animals after metamorphosis. Sometimes the testes are even vestigial and sterile. These different conditions are best analysed in the pairs (♂ 2n/♂ n) in which antagonistic hormonal effects do not occur.

It seems likely that the hypodevelopment of the haploid gonads is due, at least partly, to the physiological conditions of parabiosis and to endocrine deficiencies associated with the haploid syndrome. In heterosexual parabiosis a new factor intervenes, i.e. the antagonistic hormonal secretions of the gonads themselves, during and after sexual differentiation.

Different reaction patterns displayed in parabiosis (♀ 2n/♂ n) are described: (a) Dominant inhibitory action of the haploid testes on the diploid ovaries. The effect is in accordance with the general case observed in pairs (♀ 2n/♂ 2n). (b) Simultaneous development of the two types of gonads. The ovaries reach the adult stage and activate the Müllerian ducts. The testes can reach a submature state where spermatogenesis begins. In such combinations, the Müllerian anlage of the male partner react to the feminizing effects of the ovaries, (c) Gynogenic dominant effect of the ovaries on the sexual differentiation of the male gonad. The latter is feminized into an ovotestis. In this case, haploid ovocytes with lampbrush chromosomes differentiate. The diploid partner’s ovaries remain immature through the androgenic effect of the testis, though their ovarian nature is not modified. This reaction corresponds to the one already described in heterosexual hetero specific, homoploid (♀ 2n/♂2n) combinations obtained between species in Triturus and Ambystoma.