Identifying flagella from videomicroscopy: exploiting a conserved
morphology

Ubiquitous in eukaryotic microorganisms, the flagellum is a well-studied and
highly-functional organelle that is well-known to be responsible for motility
in a variety of organisms. Pertinent to multiple areas of active scientific
interest is the understanding of their substructures, mechanical regulation,
and beating patterns, with the latter being of particular relevance to the
investigation of motile flagellated microorganisms. Commonly necessitated in
their study is the capability to image and subsequently track the movement of
one or more flagella using videomicroscopy, requiring digital isolation and
location of the flagellum within a sequence of frames. Such a process in
general currently requires some researcher input, providing some manual
estimate or reliance on an experiment-specific heuristic to correctly identify
and track the motion of a flagellum. Here we present a fully-automated method
of flagellum identification from videomicroscopy based on the fact that the
flagella appear to be of approximately constant width when viewed with typical
optics. This approximate morphological characteristic, inherent in the
ultrastructure of the organelle, is conserved across many eukaryotes. We
demonstrate the effectiveness of the algorithm by application to captured
videomicroscopy of Leishmania mexicana, a parasitic monoflagellate of the
family Trypanosomatidae and the cause of a major but neglected human disease,
leishmaniasis. Both high accuracy and remarkable throughput are achieved via
this unsupervised method, obtaining results comparable in quality to previous
studies of closely-related species but achieved without the need for precursory
measurements or the development of a specialised heuristic. Further, we process
a dataset documenting a canonical microswimmer that has accessory flagellar
structures, the motile human spermatozoon, thus...