Characterization of the sleepy sperm mutant in the fern Ceratopteris richardii: A new model for the study of axonemal function

Abstract

Structural and motility characteristics of the zzzI "sleepy sperm" mutant of Ceratopteris richardii Brongn. are described using scanning electron, transmission electron, light, and fluorescence microscopy. Although the zzzI 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 zzzI 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 zzzI sperm, which appears to be age dependent.