Ultrastructure of the Mature Egg and Fertilization in the Fern Ceratopteris thalictroides

Abstract

The ultrastructure of the mature egg and fertilization in the fern Ceratopteris thalictroides (L.) Brongn. were observed by transmission electron microscopy. The results revealed that the mature egg possesses an obvious egg membrane at the periphery of the egg. Furthermore, a fertilization pore was identified in the upper egg membrane of the mature egg. The structure of the pore is described for the first time. The fertilization experiment indicated that spermatozoids crowd into the cavity above the egg through the neck canal of the archegonium; however, only one of these can penetrate into the egg through the fertilization pore. Immediately on penetration of the spermatozoid, the egg begins to shrink. The volume of the fertilized egg decreases to almost one-half that of the unfertilized egg. As a result, the protoplasm of the fertilized egg becomes dense and opaque, which may lead to a situation where the organelles of both the egg and the fertilizing spermatozoid become indistinguishable. Simultaneously, abundant vesicles containing concentric membranes or opaque materials appear near the fertilization pore in the cytoplasm of the fertilized egg. These vesicles are considered to act as a barrier that prevents polyspermy. The present study provides a new insight into the ultrastructure of the mature egg and the cytological mechanism of fertilization in ferns.

Cytological features of oogenesis and their evolutionary significance in the fern Osmunda japonica

“…Although we observed an opening on the upper surface of the mature O. japonica egg, it differed from the fertilization pore observed in mature eggs of the ferns Ceratopteris and Adiantum (Cao et al 2009(Cao et al , 2010a. The fertilization pore in the egg cells of Ceratopteris and Adiantum has a fixed location and structure and is located at the central point on the upper surface of the egg and appears to be formed by sheets of ER (Cao et al 2009(Cao et al , 2010a.…”

“…To date, a similar structure has not been found in the other leptosporangiate ferns (Bell and Mühlethaler 1962a, b;Cave and Bell 1975;Bell 1980;Cao et al 2009Cao et al , 2010aDai et al 2010). Periodic acid Schiff staining revealed that the extra wall was composed mainly of polysaccharides, but the specific chemical composition is still uncertain.…”

“…In advanced ferns, this egg envelope, located at the upper surface of the egg, is particularly thick (Bell and Mühlethaler 1962b;Fasciati et al 1994;Bao et al 2005;Cao et al 2008Cao et al , 2009Cao et al , 2010a. In Ceratopteris and Adiantum, the egg envelope appears to be composed of multiple membranes or reticulate structures (Cao et al 2008(Cao et al , 2010b.…”

“…During oogenesis of the advanced leptosporangiate ferns, such as Ceratopteris thalictroides, Adiantum flabellulatum, and Phymatosorus hainanensis, the egg possesses a welldeveloped egg envelope and a fertilization pore (Cao et al 2009(Cao et al , 2010aDai et al 2010). Ferns with a long evolutionary history, including Angiopteris, Macroglossum, Osmunda, Todea, and Dipteris, are considered to be more primitive ferns (Bell 1986).…”

Structure and function of the tentpole in the reproductive process of Ginkgo biloba L.

“…11,12 For example, in Ceratopteris thalictroides, a fertilization pore forms in the upper extra egg membrane that tightly surrounds the egg before insemination, and the spermatozoid subsequently enters through it. 13,14 As another example, the sperm of Ceratopteris richardii extend to their full length once they are released from the antheridium, and their width decreases pollen tubes of Pinus monticola and Picea sitchensis may branch after arriving at the megagametophytes as a strategy to search for archegonia. 16 In G. biloba, just before fertilization, the proximal unbranched portion of the male gametophyte that is anchored to the pollen chamber swells and takes on a saccate appearance, characterizing maturation of the spermatozoid.…”