Sperm morphogenesis in wild-type and fertilization-defective mutants of Caenorhabditis elegans.

Abstract

Taking advantage of conditions that allow spermatogenesis in vitro, the timing and sequence of morphological changes leading from the primary spermatocyte to the spermatozoon is described by light and electron microscopy . Together with previous studies, this allows a detailed description of the nuclear, cytoplasmic, and membrane changes occurring during spermatozoan morphogenesis . By comparison with wild type, abnormalities in spermatogenesis leading to aberrant infertile spermatozoa are found in six fertilization-defective ( fer) mutants . In fer-1 mutant males, spermatids appear normal, but during spermiogenesis membranous organelles (MO) fail to fuse with the sperm plasma membrane and a short, though motile, pseudopod is formed . In fer-2, fer-3, and fer-4 mutants, spermatids accumulate 48-nm tubules around their nuclei where the centriole and an RNA containing perinuclear halo would normally be. In all three mutants, spermatids still activate to spermatozoa with normal fusion of their MOs, but the pseudopods formed are aberrant in most fer-2 and fer-4 spermatozoa and in some fer-3 spermatozoa . In fer-5 mutant males, spermatozoa do not form. Instead, defective spermatids with crystalline inclusions and abnormal internal laminar membranes accumulate. In fer-6 mutant males, only a few spermatozoa form and these have defective pseudopods. These spermatozoa retain their fibrous bodies, a structure which normally disassembles in the spermatid . The time of appearance of developmental abnormalities in all of these mutants correlates with the temperature-sensitive periods for development of infertility .The observation that each of these mutants has a different and discreet set of morphological defects shows that the strict sequence of morphogenetic events that occurs during wild-type spermatogenesis cannot arise because each event is dependent on previous events . Instead, spermatozoa, like bacteriophages, must be formed by multiple independent pathways of morphogenesis .The development of spermatozoa is among the most intricate examples of cellular differentiation . Detailed morphological studies in many organisms, especially mammals (reviewed in references 13, 33) have raised many questions about the molecular mechanisms controlling cellular morphogenesis . Studies of spermatogenesis in nematodes, which have nonflagellated amoeboid sperm, are less detailed (1,8,14,16,17,24,26,27,32,38), but they raise the same basic problems : how is the morphogenesis of a highly specialized cell controlled and, especially, how is this control specified genetically?To address these problems, our laboratory has been isolating and analyzing fertilization-defective (fer) mutants of the nematode Caenorhabditis elegans that have infertile sperm (3, 36) . C. elegans is an excellent organism for isolation and characterization of developmental mutants (7, 19, reviewed in reference 34) . Mutants altered in spermatogenesis or spermiogenesis can be recognized by their sterile phenotype and more than 15 genes that can mutate to a...

Membrane flow during nematode spermiogenesis.

“…The morphological defects in sperm of both strains are the same: the pseudopods are shorter than normal and MO's do not fuse with the surface membrane (29). Although short, these pseudopods bear pseudopodial projections that undergo the same movements as those on wild-type cells (19).…”

“…Activating spermatozoa were recognized by several morphological criteria : few fused MO's, incomplete accumulation of cytoplasmic laminar membranes at the pseudopod-cell-body junction and short pseudopods without projections (18,29) . Electron micrographs of these immature spermatozoa revealed a near uniform distribution of ferritin on the surface of both the cell body and the pseudopod (Fig.…”

“…Unbound avidin was removed by washing and the spermatids were either fixed immediately in 1 .3% glutaraldehyde in 0 .95x SM or were activated with monensin and fixed at various times afterward . These preparations were postfixed in Os0, and prepared for TEM as described (29). Electron micrographs were taken on a JEOL 1005 microscope operated at 80 kV.…”

“…This event can be induced in vitro with the monovalent ion ionophore, monensin (18). It requires extensive cytoplasmic rearrangements : laminar membranes underlying plasma membrane in spermatids accumulate at the base of the pseudopod; membranous organelles (MO) fuse with the plasma membrane; and a pseudopod, which is devoid of organelles but filled with amorphous cytoplasm, extends 3-4 lam from the hemispherical cell body (18,29) .…”

Actin in the Drosophila embryo: Is there a relationship to developmental cue localization?

“…60 and references therein). Or perhaps there are other proteins not yet identified that create a novel cytoplasmic structure, such as the major sperm protein of nematode sperm (61) , which appears to be entirely different from actin, yet plays a very similar function in the motility of these migratory sperm (62) .…”