Spermatogenesis in a Homosporous Fern, Onoclea Sensibilis

Abstract

A detailed study of spermatogenesis in a homosporous fern, Onoclea sensibilis L., is presented from the formation of the first spermatogenous cell to the release of the sperm. Two different walls are deposited around the developing spermatids at specific developmental stages as opposed to one wall reported for other species. Most ultrastructural changes that occur in Onoclea during spermatid differentiation resemble those described in previous studies on other fern species, with the following exceptions: 1) A previously undescribed structure appears during midspermatid stage. This dense layer of amorphous material with a row of evenly spaced light areas occurs between the anterior portion of the mitochondrion associated with the multilayered structure and the anterior plasmalemma of the spermatid. 2) An early stage in blepharoplast formation resembles that which occurs in the heterosporous fern Marsilea, in contrast to that which has been reported in Platyzoma, the only other homosporous fern studied at this stage. 3) The osmiophilic crest does not form as early as reported in other ferns. 4) The cap cell of Onoclea is removed intact, rather than collapsing or forming a pore during sperm release. Observations are reported on the number of sperm per antheridium, the time course of spermatogenous cell mitosis, and of differentiation of spermatids into sperm. In Onoclea, an antheridium may contain either 32 or 64 sperm. Regardless of the final number of sperm, each has approximately the same volume.

Architecture of the sperm cell of Psilotum

“…In Equisetum and Ophioglossum (Renzaglia and Garbary, 2001), this band includes a fibrous as well as a well-developed striated component, while in Psilotum, Angiopteris, Osmunda, Onoclea, Ceratopteris, Platyzoma, and Pteridium cross striations have not been observed in any portion of the band (Duckett, 1975;Doonan, Lloyd, and Duckett, 1986;Kotenko, 1990;Mainwaring, 1997;Renzaglia et al, 2000). Typically, a band of fibrous material runs parallel to the locomotory apparatus and delineates the leading edge of the cell.…”

“…In Angiopteris, no accessory mitochondria exist at the cell anterior while in most Filicopsida, accessory mitochondria are orderly aligned along the spline MTs (Duckett, 1975;Kotenko, 1990;Mainwaring, 1997). The MLS is subtended by a single large anterior mitochondrion but numerous accessory mitochondria are randomly positioned near the nucleus at the cell anterior.…”

“…Cellular coiling has likewise increased from nearly 2.5 revolutions in Angiopteris and Osmunda to 3-6 coils in most taxa (Duckett, 1975;Kotenko, 1990;Mainwaring, 1997). Concomitantly, cell organization has become more precise with organelles aligned along the frame- work of the spline microtubules and adjacent to the nucleus.…”

“…Detailed ultrastructural studies of motile male gametes have been conducted on a wide range of tracheophytes, including homosporous and heterosporous lycopsids (Maden et al, 1996(Maden et al, , 1997Renzaglia, Dengate, and Bernhard, 1998;Renzaglia, Bernhard, and Garbary, 1999), two species of Equisetum (Duckett and Bell, 1977;Renzaglia et al, 2001), Angiopteris (Renzaglia et al, 2000;Renzaglia and Garbary, 2001), a variety of filicopsid ferns (Duckett, 1975;Myles and Hepler, 1977;Kotenko, 1990), Ginkgo (Gifford and Lin, 1975;Gifford and Larson, 1980;Li, Wang, and Knox, 1989) and cycads (Norstog, 1967(Norstog, , 1968(Norstog, , 1977(Norstog, , 1990Norstog and Nicholls, 1997). The present study provides the first detailed description of the multiflagellated male gamete of a member of the Psilotaceae.…”

“…Typically, a band of fibrous material runs parallel to the locomotory apparatus and delineates the leading edge of the cell. In Equisetum and Ophioglossum (Renzaglia and Garbary, 2001), this band includes a fibrous as well as a well-developed striated component, while in Psilotum, Angiopteris, Osmunda, Onoclea, Ceratopteris, Platyzoma, and Pteridium cross striations have not been observed in any portion of the band (Duckett, 1975;Doonan, Lloyd, and Duckett, 1986;Kotenko, 1990;Mainwaring, 1997;Renzaglia et al, 2000). In Botrychium, the anterior band is highly elaborate and composed of electron dense, amorphous material, i.e., fibers are not differentiated.…”