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Developmental Ultrastructure of the Male Gamete of Selaginella

To evaluate structural homology among streamlined, biflagellated male gametes of Selaginella and bryophytes, we undertook a thorough ultrastructural study of spermatogenesis in Selaginella kraussiana (Kunze) A. Br. As in Lycopodium and bryophytes, centrioles originate as bicentrioles in the spermatid mother cell. In the nascent spermatid, a fibrous connective band is involved in the separation and rotation of the two centriolar constituents of the bicentriole. The locomotory apparatus consists of two staggered monomorphic basal bodies that originate from the centrioles and an underlying multilayered structure (MLS), including a spline of parallel microtubules and subtending elongated lamellar strip. Beneath the MLS lies the anterior mitochondrion, which elongates during development to a length of over 1.5 cellular revolutions by progressive fusion of mitochondria. Spline microtubules (MTs) increase posteriorly from one at the anterior tip to a maximum of 19 at the cell posterior. The angle between lamellar strip plates and spline MTs changes front to back from 85° to 40°. In the late midstage spermatid, the rounded nucleus compacts and elongates in register with spline growth. Individualization of cellular coils begins at the cell anterior and progresses posteriorly leaving a cytoplasmic mass in the region between coils. In the final stages of development, the lamellar strip regresses and the cylindrical nucleus shortens. Concomitantly, the posterior mitochondrion and plastid, containing two starch grains, are positioned alongside the spline at the posterior of the cell. In development, the spermatozoid of Selaginella more closely resembles those of pteridophytes than bryophytes. Common developmental and structural features of the locomotory apparatus among Selaginella and members of the Lycopodiaceae are consistent with a monophyletic lycopsid assemblage.