Developmental ultrastructure of the male gamete of Selaginella

To evaluate structural homology among streamlined, biflagellated male gamet es of Selaginella and bryophytes, we undertook a thorough ultrastructural s tudy of spermatogenesis in Selaginella kraussiana (Kunze) A. Br. As in Lyco podium and bryophytes, centrioles originate as bicentrioles in the spermati d mother cell. In the nascent spermatid, a fibrous connective band is invol ved in the separation and rotation of the two centriolar constituents of th e bicentriole. The locomotory apparatus consists of two staggered monomorph ic basal bodies that originate from the centrioles and an underlying multil ayered structure (MLS), including a spline of parallel microtubules and sub tending elongated lamellar strip. Beneath the MLS lies the anterior mitocho ndrion, which elongates during development to a length of over 1.5 cellular revolutions by progressive fusion of mitochondria. Spline microtubules (MT s) 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 degrees to 40 degrees. In the late midstage s permatid, the rounded nucleus compacts and elongates in register with splin e growth. Individualization of cellular coils begins at the cell anterior a nd 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 mitochondri on and plastid, containing two starch grains, are positioned alongside the spline at the posterior of the cell. In development, the spermatozoid of Se laginella more closely resembles those of pteridophytes than bryophytes. Co mmon developmental and structural features of the locomotory apparatus amon g Selaginella and members of the Lycopodiaceae are consistent with a monoph yletic lycopsid assemblage.