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Abstract The characteristic tadpole shape of sperm is formed from round spermatids via spermiogenesis, a process which results in dramatic morphological changes in the final stage of spermatogenesis in the testis. Volume 19 , Issue 11 June Related Information. Close Figure Viewer. Browse All Figures Return to Figure. Previous Figure Next Figure.
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Returning user. A-I, transverse sections, in antero- posterior sequence. A-C, section with two axonemes, mitochondrion, nucleus with various degrees of dilatation, and peripheral microtubules. The nuclear envelope often has additional membranes. D-F, sections with a single axoneme, mitochondrion, nucleus, and peripheral microtubules. G, single axoneme, section of nucleus with dense chromatin and small diameter, mitochondrion with very small diameter, and peripheral microtubules.
H, I, extremity with section of nucleus and no mitochondrion. A few microtubules present. Spermiogenesis was not observed in this species. The anterior extremity of the spermatozoon is thin; it contains only a few microtubules Figure 11 A-C. More posteriorly, the first axoneme appears Figure 11 D, E followed by the second axoneme Figure 11 G.
At the level of the first centriole and slightly posterior to it, two dense zones are visible within the cytoplasm, outside the microtubule row Figure 11 C-F.
The anterior region of the spermatozoon contains the two axonemes and numerous microtubules, organized at two rows of dorsal and ventral microtubules, each doubled by a more internal row of microtubules Figure 11 H-K ; no cortical microtubules are present on the sides. This region does not contain a section of the mitochondrion nor of the nucleus. Progressively, this region is transformed in a region with two sets of microtubules, one ventral and one dorsal, devoid of the internal row of microtubules; a section of the mitochondrion and a section of the nucleus appear Figure 11 L-P ; intermediary sections Figure 11 L-P show that the second, internal row of microtubules is progressively lost.
More posteriorly, one of the axonemes disappears and sections contain only a single axoneme, a section of mitochondrion and nucleus, and dorsal and ventral peripheral microtubules; the nucleus has a wide section with fibrous chromatin Figure 12 A-C. The next region is quite particular: the shape of transverse sections progressively flattens, with the axoneme on one side, the section of the nucleus on the other side, and a thin layer of cytoplasm containing only the two rows of peripheral microtubules, now close one to the other, between the axoneme and the nucleus Figure 12 D-I.
Most posterior sections show the axoneme ending as singlets Figure 12 K,L. The posterior extremity shows only a comma-shape section with peripheral microtubules Figure 12 M-O , then a small structure with only a few microtubules Figure 12 P. Figure 15 is a schematic drawing of our interpretation of the structure of the mature spermatozoon of Callorhynchocotyle callorhynchi. Spermatozoon of Callorhynchocotyle callorhynchi , anterior part. A-T, transverse sections, in antero-posterior sequence.
A-C, anteriormost sections, showing a few microtubules. D, E, centriole and first axoneme. F, two sections with single axoneme surrounding a section with two axonemes. G, one fully formed axoneme and centriole of other axoneme. H-L, sections with two axonemes, peripheral microtubules on dorsal and ventral part, and additional microtubules.
M-P, sections with peripheral microtubules on dorsal and ventral part, progressive disappearance of additional microtubules, appearance of sections of mitochondrion and nucleus. Spermatozoon of Callorhynchocotyle callorhynchi , posterior part.
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A-P, transverse sections, in antero-posterior sequence. The nucleus chromatin is not electron-dense. D, intermediary region in which the section is not flattened in the middle; one axoneme, ventral and dorsal peripheral microtubules, section of mitochondrion with very small diameter, and section of nucleus with electron-dense content. E-I, various sections in the flattened region, with single axoneme, ventral and dorsal peripheral microtubules, no mitochondrion, nucleus as small diameter electron-dense section.
J-K, flattened region, distal extremity of axoneme; the section of the nucleus has a very small diameter or almost disappeared K. L, side-by-side section in flattened region and distal comma-shaped section with axoneme reduced to a circle of single microtubules. M-O, distal comma-shaped section, no axoneme. P, distalmost section. The ultrastructure of spermatozoa has been considered as relatively homogeneous among polyopisthocotyleans [ 35 , 46 ], and sperm structure did not provide any character useful for understanding the relationships between the various families of polyopisthocotylean monogeneans [ 34 , 42 ].
This is a contrasting situation to what was found in the monopisthocotyleans, in which the structure is highly variable [ 16 , 17 , 24 , 39 , 44 , 46 , 49 — 53 , 57 , 59 — 63 , 65 , 66 , 71 , 72 , 76 , 77 , 93 , 98 , — , — ] and makes it possible to distinguish synapomorphies for certain families or groups of families [ 34 , 35 , 38 , 39 , 42 , 43 , 46 , 57 , 61 , 66 ].
In most polyopisthocotylean monogeneans, the principal region of the spermatozoon i. A single significant divergence from this pattern was found in the Diplozoidae [ 47 , 48 ]. Diplozoids are unique in the monogeneans and even in the animal world in that the two hermaphrodite members of a pair are united for life, with the genital ducts in permanent communication [ 15 ].
Sperm morphology is known to be driven by constraints of sperm function and competition [ 25 ]; it was hypothesized [ 39 , 47 ] that the aberrant sperm structure in diplozoids was the result of the absence of sperm competition, and thus more representative of a variation of fertilization biology than the mark of a distinctive diplozoid branch in the polyopisthocotylean tree.
A parallel situation is also encountered in the schistosomes Digenea [ 31 , 36 , 39 , 45 ]. A special structure found in rare polyopisthocotyleans is an undulating membrane. A typical undulating membrane was described in Gotocotyla sp. This undulating membrane is composed of a lateral flange containing more than one hundred parallel microtubules, and observations of living spermatozoa showed that the undulating membrane was functional, i.
This was considered an autapomorphy of Gotocotyla [ 34 ]. Other structures superficially looking like this undulating membrane have been mentioned in various polyopisthocotyleans, generally in the form of a lateral flange with longitudinal peripheral microtubules. Examples, listed by Quilichini et al.
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A single flange is found in P. We have no evidence that these various lateral flanges are homologous between themselves, and they should probably be considered as autapomorphies of the various taxa in which they were found.
This structure is different from all other cases found in monogeneans since it includes the nucleus and a single axoneme, with a flattened part between them with longitudinal microtubules; we have no information whether this structure is functional or not, i. We consider it to be an autapomorphy of Callorhynchocotyle , and we point out that, given the current state of knowledge on hexabothriid sperm, it is not a synapomorphy of the Hexabothriidae since it was not found in Rajonchocotyle emarginata.
We also point out that it does not resemble and is not homologous to the undulating membrane of Gotocotyla and the lateral flanges of certain other polyopisthocotyleans. Various modifications of the nucleus shape, especially in cross sections, have been described. These include a crescent-shaped nucleus partially surrounding the axonemes, in Discocotyle sagittata , an annular nucleus completely surrounding the axonemes in Concinnocotyla australensis , and a special polygonally shaped nucleus in Atriaster spp.