Effets de faibles abaissements de température sur la différenciation de l'oosphère et sur les processus de fécondation chez le Marsilea vestita (Ptéridophytes)

Male and female gametes of Marsilea vestita were subjected to weak cooling periods (24° C/16° C or 24° C/12° C) of 1–4 h during the ultimate stage of gametogenesis. Effects of these suboptimal temperatures were observed on the organization of the oosphere on fertilization, and further development of the embryos. The changes observed varied with cold intensity and were proportional to the exposure time. The last steps of archegonogenesis were delayed (16° C) or even totally inhibited (12° C). The female gamete structure generally exhibited the following changes: the fertilization cone was enlarged, the cytoplasm more or less vacuolated, and organelles redistributed towards peripheral regions. Furthermore, the presence of chromatic fragments within the cytoplasm (12° C) suggests that the last division of female gametogenesis was strongly affected. In these gametes, fertilization was always reduced. When it occurred, the treatment considerably slowed down the different stages, allowing a better understanding of the male nucleus reactions during its migration. The percentage of embryos able to grow decreased proportionally to the length of cold exposure (16° C). After exposure of the gametes to 12° C, most embryos did not survive.

Observations on the male nucleus during fertilization in the fern Pteridium aquilinum

Fertilization of eggs of Pteridium under controlled conditions has revealed that archegonia containing viable eggs require more than 30 min flooding before opening. Syngamy appears to take place by an engulfing action, and no evidence was found of a ‘membrane-fusion’ kind of conjugation. Although the gametic nucleus lacks a recognizable envelope, as soon as it comes into contact with the female cytoplasm an envelope is reconstituted from an opaque layer at the surface of its chromatin. This membrane also undergoes growth, the outer layer extending into the cytoplasm and surrounding spermatozoid mitochondria. The male envelope remains distinct from the female, and lacks evaginations and well defined pores. The male nucleus lies in irregular loops within a funnel, filled with egg cytoplasm, formed by the microtubular ribbon of the gamete. The mouth of the funnel is bordered by the multi-layered structure. Karyogamy is initiated at the anterior end of the male nucleus, the orifice connecting the 2 nuclei widening from about 0-1 to 2 mum in diameter during fusion. The contents of the male nucleus flow through the mouth of the funnel into the female, the chromatin relaxing locally, but retaining a rod-like form. The envelope of the male nucleus is probably resorbed at the site of junction. There is no evidence that it contributes significantly to the envelope of the zygotic nucleus.

Gametophytic development and fertilization in Macrocystis integrifolia

A cytological investigation of gametophytic development and fertilization was conducted on cultured gametophytes of Macrocystis integrifolia. Most developmental characteristics are similar to those previously reported for other members of the order Laminariales, although some modifications were noted in the production of antheridia and spermatozoids. One of the most significant contributions of this study concerned the nuclear cytology of the egg during fertilization. There appear to be two fertilization phases: sperm penetration and nuclear activity within the egg. After penetration of the egg by one spermatozoid, the male nucleus remains in the peripheral cytoplasm while the egg nucleus undergoes mitosis, producing two daughter nuclei, one of which degenerates. The male nucleus then enters prophase and migrates to the remaining female nucleus, which is also in prophase. Karyogamy occurs while both nuclei are in prophase.

X-ray Inactivation of Nuclei as a Method for Studying their Function in the Early Development of Fishes

It is generally accepted at present that during cleavage in echinoderms, amphibians, and fishes, the nuclei do not have specific functions in regulating development, their role being at this time restricted to participation in the processes of cleavage (Schechtman & Nishihara, 1955). Eggs devoid of nuclei sometimes begin cleavage which may proceed up to the stage of the late blastula. Extirpation or inactivation of nuclei may be achieved through the separation of the nuclear region of the egg by means of centrifugation (Harvey, 1940); through extirpation of the female nucleus followed by fertilization with sperm inactivated by a heavy dose of radiation (Briggs, Green, & King, 1951); through spontaneous degeneration of the male nucleus during artificial androgenesis (Stauffer, 1945); and by means of other techniques. Exposure of early cleavage stages in amphibians (Mangold & Peters, 1956; Sanides, 1956) and fishes (Neyfakh, 1956a) to heavy doses of ionizing radiation also leads to arrest of development at the late blastula stage.