Ultrastructural studies of mutant spermatozoids in ferns. I. The mature nonmotile spermatozoid of mutation 230X inCeratopteris thalictroides(L.)Brongn

Phylogenetic distribution and expression pattern analyses identified a divergent basal body assembly protein involved in land plant spermatogenesis

Land plant spermatozoids commonly possess characteristic structures such as the spline, which consists of a microtubule array, the multilayered structure (MLS) in which the uppermost layer is a continuum of the spline, and multiple flagella. However, the molecular mechanisms underpinning spermatogenesis remain to be elucidated. We successfully identified candidate genes involved in spermatogenesis, deeply divergent BLD10s, by computational analyses combining multiple methods and omics data. We then examined the functions of BLD10s in the liverwort Marchantia polymorpha and the moss Physcomitrium patens. MpBLD10 and PpBLD10 are required for normal basal body (BB) and flagella formation. Mpbld10 mutants exhibited defects in remodeling of the cytoplasm and nucleus during spermatozoid formation, and thus MpBLD10 should be involved in chromatin reorganization and elimination of the cytoplasm during spermiogenesis. We identified orthologs of MpBLD10 and PpBLD10 in diverse Streptophyta and found that MpBLD10 and PpBLD10 are orthologous to BLD10/CEP135 family proteins, which function in BB assembly. However, BLD10s evolved especially quickly in land plants and MpBLD10 might have acquired additional functions in spermatozoid formation through rapid molecular evolution.

The evolution of land plant cilia

Eukaryotic cilia/flagella are ancient organelles with motility and sensory functions. Cilia display significant ultrastructural conservation where present across the eukaryotic phylogeny; however, diversity in ciliary biology exists and the ability to produce cilia has been lost independently on a number of occasions. Land plants provide an excellent system for the investigation of cilia evolution and loss across a broad phylogeny, because early divergent land plant lineages produce cilia, whereas most seed plants do not. This review highlights the differences in cilia form and function across land plants and discusses how recent advances in genomics are providing novel insights into the evolutionary trajectory of ciliary proteins. We propose a renewed effort to adopt ciliated land plants as models to investigate the mechanisms underpinning complex ciliary processes, such as number control, the coordination of basal body placement and the regulation of beat patterns.

Ultrastructure of mature spermatozoids in the fern Asplenium onopteris L

Asplenium onopteris L. spermatozoids are 8-8.5 microm long in the form of spirals with 4.5 turns. They have about 50 flagella. The nucleus occupies the last three posterior turns. Chromatin is partly honeycomb-shaped and partly highly condensed. An electron transparent space crossed by dense fibers delimits the condensed chromatin. Here, the nuclear membranes are closely apposed without any space between them and the plasmalemma often invaginates among elements of the microtubular ribbon, connecting with the outer nuclear membrane. An electron opaque body apparently links and anchors all anterior spermatozoid components. The cytoplasm contains plastids with starch grains, (lipid) bodies and different membrane systems, which are presumably plasmalemma derivatives involved in a process of cytoplasmic reduction.

Characterization of thesleepy spermmutant in the fernCeratopteris richardii: A new model for the study of axonemal function

Structural and motility characteristics of the zzz1 "sleepy sperm" mutant of Ceratopteris richardii Brongn. are described using scanning electron, transmission electron, light, and fluorescence microscopy. Although the zzz1 phenotype segregates as the product of a single gene mutation, the expression of the mutation varies within a single haploid gametophyte. The majority of mutant sperm cells are slow to initiate motility and typically swim in a slow, spiraling pattern. However, motility phenotypes range from immotile to wild-type (normal). This variable phenotypic expression is associated with a wide range of defects in the microtubule systems, especially the flagellar axonemes and the spline, a structure that provides a structural backbone for the cell. Defects in the spline microtubule array are associated with atypical cell shape and organellar positioning. Axonemal aberrations include an absence of the central pair complex and clumped flagella. We hypothesize that the gene product encoded by the zzz1 locus is not required for the establishment of the cytoskeletal elements necessary for sperm motility but rather is needed for stability and (or) repair (recycling) of these structures. This interpretation is consistent with the variable expression of zzz1 sperm, which appears to be age dependent.Key words: axoneme, microtubule, motility mutant, sperm cell, ultrastructure.

GAMETOPHYTE DEVELOPMENT IN FERNS

▪ Abstract  The fern gametophyte has interested plant biologists for the past century because its structure and development is simple and amenable to investigation. Past studies have described many aspects of its development, including germination of the spore, patterns of cell division and differentiation, photomorphogenic or light-regulated responses, sex determination and differentiation of gametangia, hormone and pheromone responses, and fertilization. Several genes that are predicted to regulate some of these processes have been recently cloned, making it possible to analyze how these processes are controlled at a molecular level. The emergence of the fern Ceratopteris richardii as a model organism for readily identifying and characterizing mutations that affect key developmental processes in gametophytes makes it a powerful tool for dissecting the molecular mechanisms underlying these processes. If advances in gene cloning techniques and transformation are forthcoming in Ceratopteris, it is likely that the study of developmental processes in ferns will significantly contribute to our understanding of plant development and evolution beyond that which can be learned solely from studying angiosperms.

The immotile cilia syndrome. Mice versus man

When homozygous the recessive, pleiotropic, mutation hpy (hydrocephalic-polydactyl) produces post-natal hydrocephalus, complete sterility in males, and reduced reproductive performance in females. Because the fertility problems and the development of hydrocephalus could arise as consequences of defective flagella and ciliary axonemes, this mutant type might serve as a useful animal model for the immotile cilia syndrome. Ultrastructural defects seen in axonemes of flagella, and of cilia from the trachea, oviduct, and ependyma included: a deficiency of inner dynein arms (the most frequent defect); an absence of one or both central-pair tubules; extra central tubules; a displacement of one outer doublet and/or the central-pair tubules. Some axonemes showed more than one of these defects. The frequency of dynein-deficient axonemes in all three tissues was similar (about 35%) and fell within the range reported for human patients with the immotile cilia syndrome. On this basis, this mutant type might be considered as a useful animal model for such studies. There were no indications of situs inversus, nor was there a marked increase in respiratory problems. So hpy/hpy mice do not exhibit all of the clinical symptoms characteristic of the human condition.