We investigated the locomotory apparatus of the mid-stage spermatid of Aneura pinguis in order to gain a better understanding of the evolution and development of gamete ultrastructure in liverworts. The locomotory apparatus consists of two staggered but overlapping basal bodies and a multilayered structure made up of a spline and lamellar strip. The basal bodies include a transition region between the basal body and flagellum containing a short stellate pattern and dense matrix. The spline's maximum width is 20 microtubules and reduces in number at the posterior of the multilayered structure; however, this number appears variable at the mid-stage of development and so it is recommended that the mature spermatozoid be investigated for this feature. The anterior basal body is situated over the spline aperture, which measures 2 microtubules in width and 0.85 µm in length. The lamellar strip measures 4.06 µm in length and 0.60 µm in width, and is spatulate in shape with a right lateral offset or keel. When compared with Riccardia multifida, the two locomotory apparatuses share an overall similarity of features, such as a closed spline aperture, similar lamellar strip shapes, and similar stellate pattern lengths. However, A. pinguis has several distinctive features, such a wider spline and a longer posterior basal body that may be adaptations associated with a rapid increase in DNA from a polyploidization event. These results are fully consistent with an 85 year old light microscope study where illustrations depict a similar posterior flagellum distance from the front of the cell. Observations of male gametes of liverworts are imperative to illuminating details of character evolution within this lineage and could provide a model for further understanding of evolutionary modifications in the development of complex cells.
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