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TRANSFORMATION OF THE NUCLEUS IN MARCHANTIA SPERMATIDS: MORPHOGENESIS

A B S T R A C T It is proposed that elongation of the nucleus in spermatids of Marchantia results from interaction between its membranous envelope and microtubules of the spermatid's cytoskeleton. The nucleus may be drawn out in two directions along microtubules until forces attracting the nucleus to them are balanced by forces resisting envelope distortion. Condensation of nuclear chromatin into fibrils of uniform diameter and probable shaping of the nucleus by blebbing of its envelope occur together before elongation is complete. The nucleus becomes crescent shaped and it is prolonged distally into a chromatin-free diverticulum. In accord with their distribution along the axis of the nucleus, chromatin fibrils are compacted together forming a cone-like rod of chromatin which narrows anteriorly and extends distally to the tip of the preexisting diverticulum. Elongation and shaping of the nucleus influence the distribution of its chromatin and thus its ultimate morphology. Coiling of the nucleus is related to a reduction of spermatid cytoplasm during maturation. IN CHAROPHYTE algae and in most archegoniate plants transformation of spermatids into motile sperm entails elongation and coiling of the nucleus as well as condensation and compaction of its chromation. During early development spermatids produce a cytoskeleton comprised of a band of parallel, cross-linked microtubules (Carothers and Kreitner, 1967). Nuclei fail to elongate and coil in the absence of the microtubules (Turner, 1970; Myles and Hepler, 1974), although condensation of nuclear chromatin may proceed uninterrupted. The cytoskeleton is thought to be a supporting framework for nuclear