DISCUSSION

The process of spermatogenesis and spermiogenesis and the structure of the spermatozoon in D. wedli follow the same pattern described for other digenean species with some differences: during the spermiogenesis process, the fused spermatids in a common cytoplasmatic mass produce the ZD, and an elongation of the whole cell takes place, including nucleus, mitochondria and axonemes. The ZD and flagella fuse together, producing a mature spermatozoon. The ultrastructure of the spermatozoa of D. wedli presents peculiarities that clearly differentiate it from other digenean spermatozoa.

According to Justine (1998) the incorporation of the axonemes into the cell occurs via proximo-distal fusion, but in D. wedli this fusion was not observed. During the spermiogenesis process of D. wedli, intercentriolar bodies, rootlet fibers, membrane arching and the median cytoplasmatic process are absent. Didymozoon sp. (by Justine & Mattei 1983) also does not present those structures, as well as the ZD, which was observed in the present study. In Gonapodasmius sp. (by Justine & Mattei 1982) only intercentriolar bodies and rootlet fibers are absent during the spermiogenesis.

The majority of the Platyhelminthes display an homogeneous pattern for the spermatozoa with the nucleus, mitochondrion, two axonemes with a 9+1 configuration and cortical microtubules (Justine 1999). The spermatozoa of D. wedli follows this classic pattern, with the exception that the cortical microtubules are absent. Discrepancies occur within the group: the two axonemes of the spermatozoon in Didymozoon sp. (Justine & Mattei 1983) and the single axoneme of Schistosoma mansoni (by Kitajima et al. 1976) present a 9+0 configuration, representing the only exceptions within the Platyhelminthes with this configuration. Another species of Didymozoidae already studied, Gonapodasmius sp., shows the basic pattern for the structure of the spermatozoon. In the digeneans M. monas (by Iomini et al. 1997) and E. caproni (by Iomini & Justine 1997), one of the axonemes is longer than the other. According to these authors, the relative length of axonemes is a useful character for phylogeny of this group of parasites, as is the number of axonemes which have already been used for evolutionary studies of Monogenea (Justine 1991).

The spermatozoa of parasitic platyhelminthes usually presents cortical microtubules, as in Gonapodasmius sp. (by Justine & Mattei 1982), while in Didymozoon sp. (Justine & Mattei 1983) and in D. wedli (present study) there are no microtubules.

The data obtained in the present study may contribute towards elucidating the phylogeny of the family Didymozoidae, given that differences in the processes of spermiogenesis and in ultrastructure of the spermatozoon were observed in members of the same family.

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