The system of epidermal ciliary rootlets in Turbellaria

Summary The rootlets of the kinetic cilia form patterns of different types in the different turbellarian subgroups (cf. Rieger 1981). In the Acoela a rather complex system of ciliary rootlets is found in the epidermis (Dorey 1965; Hendelberg & Hedlund 1973; Bedini & Papi 1974). In the acoel Childia groenlandica (Levinsen) the four rootlets of each cilium make contact with those of adjacent cilia at two levels (Hendelberg & Hedlund 1974). Distinct granules are found in the interior of the main rootlets (Hendelberg & Hedlund 1974; Bedini & Papi 1974, Fig. 16) and basal bodies (Silveira 1972; Hendelberg & Hedlund 1974) of the epidermal kinetic cilia of acoels. Similar granules, probably of identical structure, can be seen in nemertodermatids, in the same positions (Tyler & Rieger 1977, Figs. 3 & 6). Such granules were studied in C. groenlandica with histochemical methods adapted for electron microscopy. Like Silveira (1972) I found the granules of the basal bodies to be Thiéry-positive, and thus evidently to be made up of or at least to contain polysaccharide material. The granules of the main rootlets were also found to be Thiéry-positive (Hendelberg 1976). Digestion experiments (Hendelberg & Hellmén 1978 and unpublished results) strongly support the concept that the granules are glycogen beta-particles.We know that cilia can function as kinetic organelles without any rootlets. But we are still uncertain about the function of the rootlets when occurring. Most probably they form an anchorage, a function which may be favoured by branching rootlets making contact with each other. Another function which has been discussed is the transmittance of impulses regulating the ciliary beat. Glycogen granules represent an energy deposit. The functional implication of these granules in the interior of the ciliary rootlets and basal bodies is not clear. However, the observations raise the question of how energy is transmitted to the cilia. Are the ciliary rootlets, when occurring, involved? This question will be further discussed, with references, in a future full report on the digestion experiments (to be published elsewhere).     

Ultrastructural and immunocytochemical investigation of acoel sperms with 9 + 1 axoneme structure: new sperm characters for unraveling phylogeny in Acoela

Acoel sperm characters proved useful in deciphering acoel taxonomy. The phylogenetic value of sperm characters in closely related sub-groups or in a monophyletic taxon has not yet been assessed. We have investigated sperm ultrastructure in seven members of the monophyletic taxon Childia sensu (Tekle et al. J Zool Sys Evol Res 43(1):72–90, 2005) and in their closest relatives, the Mecynostomidae (four taxa). All members of Childia examined show little variation in their sperm ultrastructure. The common characters of Childia taxa are: 9 + 1 axoneme structure, the presence of six distal cytoplasmic microtubules in the absence of axial or cortical ones, long nucleus and extensive nucleus–flagella overlap. We have identified a new set of cytoplasmic microtubules lying in the centriolar end of the sperm cell, distal microtubules. The origin and phylogenetic significance of this character is discussed. The types and arrangement of cytoplasmic granules could be used as phylogenetic characters at a low taxonomic level. A loose membrane amorphous core type of granule was found to be a synapomorphy for the following clade within the taxon Childia: C. crassum + C. groenlandica + C. vivipara + C. brachyposthium + C. macroposthium. Sausage shaped granules are plesiomorphic among the taxa examined. The rest of the granule characters were found to be homoplasious. Sperm ultrastructural characters have again proven their concordance with molecular phylogeny. The only morphological synapomorphies known for the sister taxa Childia–Mecynostomidae, in the molecular phylogeny, are characters derived from sperm ultrastructure: distal microtubules arranged in two groups of three microtubules each and a 9 + 1 axoneme structure. The spermatozoa of Childia and Mecynostomidae show 9 + 1 axoneme configuration, seemingly similar to the 9 + ‘1’ axoneme pattern of the Platyhelminthes—Trepaxonemata. Using electron-microscope immunocytochemistry, we have demonstrated that, unlike the central cylinder of trepaxonematans, the central cylinder of the 9 + 1 axonemal pattern in acoels is immunoreactive to tubulin and contains a single central microtubule. Therefore, the 9 + 1 patterns in acoels and trepaxonematans are homoplasious.     

Ultrastructure of the epidermis of Meara stichopi (Platyhelminthes,Nemertodermatida) and associated extra-epidermal bacteria

This preliminary mechanistic model of normal swimming and phototactic behaviour in individual Daphnia was constructed using data and assumptions based on experiments and observations. Swimming under constant light intensity is characterized by short periods of upward movements alternating with equal periods of downward movements. Two oscillators are proposed that generate these phases in swimming. Unexpected shifts in depth, as observed in D. magna and D. longispina, are also present in the swimming of the computer daphnid and thus seem to be inherent to the underlying mechanism. As in real daphnids, during relative decreases in light intensity of low velocity, positive phototactic upward swimming is stepwise. With increasing velocity in the change in light, these steps disappear. When the model is triggered by a natural increase in light at dawn, a small downward movement results. Migration distance can be increased to commonly found depths of migrating Daphnia by the introduction of a fish exudate factor into the model, which enhances the phototactic response. Since attenuation of light in the water affects the phototactic swimming response, it also influences migration distance. The results of model calculations agree quite well with an empirical relationship between Secchi disc depth and amplitude of diel vertical migration in a number of lakes.     

The phylogenetic significance of sperm morphology in the Platyhelminthes

The phylogenetic significance of flatworm sperm morphology is discussed against the background of general spermatology. The modified type of spermatozoon of the Nemertodermatida, a group of primitive flatworms, indicates that the Platyhelminthes evolved from forms characterized by the primitive type of metazoan sperm and by the primitive mode of fertilization, implying the release of sperm freely into sea water.The occurrence of aberrant types of spermatozoa in most platyhelminths is obviously a consequence of early evolution of the internal mode of fertilization, which characterizes all true members of this group. It can be concluded, from the ultrastructure of these aberrant spermatozoa that higher metazoans cannot have evolved from seriated flatworms related to the recent Seriata (Proseriata and Tricladida). Even the seemingly primitive Acoela have such aberrant spermatozoa that evolution of higher metazoans from acoels related to the recent Acoela seems highly improbable.The ultrastructure of the spermatozoa of the parasitic groups of flatworms (Monogenea, Digenea, Cestoda) is very similar to that found in the Kalyptorhynchia, a further indication that the parasitic groups are related to the rhabdocoel turbellarians.     

Fission in Convolutriloba longifissura: asexual reproduction in acoelous turbellarians revisited

Studies of the asexual reproduction of Convolutriloba longifissura (Acoela, Acoelomorpha) revealed that there is no longitudinal fission of the whole animal as has been described by Bartolomaeus and Balzer (1997) . Instead, the first step is a transverse fission. This results in the detachment of the caudal fourth of the mother animal. The detached part forms what we call the butterfly stage, which initially has no mouth and no eye fields. This stage gives rise to two new individuals by a longitudinal fission. Within 2–3 days the eye fields and a mouth develop in each of the two progenies formed in this way. In the meantime the mother individual grows and develops the three typical caudal lobes. The mother animal can repeat this process resulting in three individuals every fourth day. The finding of this new pattern of reproduction in the Acoela has prompted us to review the various ways by which asexual reproduction occurs in the group. The peculiar combination of few cases but high diversity of asexual reproduction in the Acoela is discussed from an evolutionary point of view.     

Is the sperm type of the Nemertodermatida close to that of the ancestral Platyhelminthes?

Results from a transmission electron microscope study of the spermiogenesis and spermatozoon of Meara stichopi (Nemertodermatida, Platyhelminthes) indicate that the sperm type of the Nemertodermatida has evolved from the primitive metazoan sperm type rather than from an aberrant biflagellar sperm type as found in many other flatworms. The spirally coiled mitochondrial derivative in the mature spermatozoon develops from two large oval mitochondria in the early spermatid stages. A single flagellum grows out from a peripheral basal body adjacent to a perpendicularly placed accessory centriole. The basal body moves to a distal depression of the nucleus, and becomes equipped with an anchoring fibre apparatus. Most of the flagellum becomes axially incorporated into the developing spermatid. No trace of a second flagellum was found in any stage of the spermiogenesis. Rounded vesicles appear around the proximal, tapering end of the elongating nucleus. Most probably these vesicles form a thin acrosomal structure in the mature spermatozoon. No dense bodies, characteristic of many other ‘turbellarian’ flatworm sperm types, were found. This revised version was published online in July 2006 with corrections to the Cover Date.