Ultrastructure of the frontal organ in Convoluta and Macrostomum spp.: significance for models of the turbellarian archetype

Present models of turbellarian evolution depict the organism with a frontal organ — a complex of glands whose necks emerge at the anterior tip of the body — and therefore imply that this organ is homologous throughout the Turbellaria. However, comparisons of representatives of the Acoela and Macrostomida, two putatively primitive orders of the Turbellaria, show that frontal organs in these two are not similar in ultrastructure or histochemistry. The acoel Convoluta pulchra had a prominent cluster of frontal mucous glands whose necks emerged together in a frontal pore at the exact apical pole of the organism, and an array of smaller glands of at least five other types opened at the anterior end, separately from and ventral to this pore. The frontal organs (Stirndrüsen) of two species of Macrostomum on the other hand, comprised an array of discretely emerging necks of at least two gland types including one with rhabdiform (rhammite) and one with globular mucous secretion granules neither of which emerge at the apical pole. In neither species did the organ appear to be sensory. Our findings indicate a low probability of homology between the frontal glands of the Acoela and Macrostomida.     

中国大口涡虫属一新种记述(大口虫目,大口虫科)

记述大口涡虫属1新种,即厦门大口涡虫Macrostomum xiamensis sp.nov..新种的主要鉴别特征是几丁质交接刺光滑、较细长,表面无几丁质瓣膜.其远端呈螺旋状弯曲,弯曲部的平面上夹角达60度,压片后的夹角达100度,螺旋部超过半圈,交接刺末端呈注射器针的末端,开口于螺旋状弯曲面的外侧.标本采自厦门市区菩陀寺的淡水荷花池塘内,水质属富营养型.所有标本保存在深圳大学生命科学学院实验室.     

管大口涡虫生物学特性的观察

报道了分布于安徽、广东和福建一带的大口涡虫属管大口涡虫(Macrostomum tubo)的生物学特性。该涡虫生活于淡水池塘、小河的水生植物的叶子反面。在实验室观察了该涡虫的生活习性。介绍了常年饲养和繁殖管大口涡虫的方法。通过不同方向的连续切片,描述了该涡虫的形态结构。研究结果提示：大口涡虫是涡虫纲教学和动物进化研究的理想实验材料。     

Cell migration and differentiation during wound healing and regeneration in Microstomum lineare (Turbellaria)

Using transmission electron microscopy and serial sections with light-microscopic autoradiography, I have investigated the ultrastructure of wound healing, the distribution of cells preparing for proliferation, and the fates of cells labelled with exogenous tritiated thymidine ([³H]T) in Microstomum lineare undergoing wound healing and regeneration. Immediately after decapitation the open wound was reduced to a minimum by strong contraction of circular muscle fibers. The wound epidermis was cellular, consisting of thin parts of epidermal cells from the epidermis around the wound. These epidermal cells maintained close adhesive contact with one another through zonulae adherentes and septate junctions. No proliferating cells were found in the old epidermis. The only cells taking up [³H]T were mesenchymal and gastrodermal neoblasts which proliferated and migrated towards the surface. The final epidermis was formed by conjunction of the wound epidermis and newly differentiated epidermal cells. Regeneration in Microstomum, in contrast to that of planarians, occurs mainly by morphallaxis, without the formation of a regeneration blastema, but also through continuous cell proliferation, migration, and differentiation.     

Contents Volume 14, 1988

Summary

Within the unpaired testis, spermatogonia, spermatocytes, spermatids and spermatozoa were found. In early spermatids, mitochondria take perinuclear positions and centrioles a diplosomal arrangement. Rootlet-like striated differentiations occur in slightly more advanced stages. Then a conical cytoplasmic projection develops, supported by a single row of closely spaced microtubules. At this stage of maturation, giant Golgi stacks occur within the cytoplasm of the cytophore which is rich in different elongate structures and oval dense bodies. With progressive differentiation, the nucleus elongates and its chromatin condenses into twisted lamellae. Two centrioles, which change their diplosomal configuration and come to lie in line to each other, and rootlet-like structures remain near the tip of the median cytoplasmic outgrowth. Mitochondria start to fuse into a single long cylindrical mitochondrial rod extending beside the lengthening nucleus. Bone-shaped rods, smaller dense sticks and dense bodies migrate into the outgrowth. Spermatozoa are totally ensheathed by cortical microtubules. These tubules show different arrangements along the cell body. The thread-like nucleus extends along the cell, the first quarter excepted, whereas the single mitochondrion extends over two thirds of the cell. Two strings with linearly arranged oval dense bodies run in the median to post-median cell segment; four rows of bone-shaped rods and two rows of smaller electron-dense sticks extend from the frontal end up to the beginning of the last third of the cell. All the different longitudinal cords run in the gaps between 4 sets of microtubules. Ciliary axonemes or lateral bristles were not observed. The present findings substantiate the hypotheses, that spermatozoa in the Macrostomida are aciliate and that Myozona takes an isolated position within the Macrostomidae. The occurrence of two centrioles, which come to lie in line to each other and which stay in the tip of the cytoplasmic outgrowth in spermatids, may indicate that biciliate spermatozoa are characteristic for the Rhabditophora (= Macrostomorpha+Trepaxonemata) and not an evolutionary novelty of the Trepaxonemata.     

Differentiation of the body wall musculature in Macrostomum and Hoploplana (Turbellaria,Platyhelminthes)

Using the Phalloidin-Rhodamine flourescence-labelling technique for F-actin, we have studied the development of the body wall musculature in Macrostomum hystricinum marinum and in thepolyclad Hoploplana inquilina. The structure of the muscle grid in the freshly hatched Macrostomum (see also Rieger & Salvenmoser, 1991) and the young larva of Holplana served as reference systems for the embryonic development of the body wall musculature. In Macrostomum muscle fiber differentiation starts around 60% of developmental time between egg-laying and hatching, and in Hoploplana around 80% of embryonic development.In Macrostomum, early stages show TV-antenna-like arrangements of one longitudinal and several circular fibers. In Hoploplana our preliminary results show a particularly large, longitudinal fiber on either side of the body. These primary longitudinal fibers may serve as a founder cell for other longitudinal fibers and as spatial guides for the circular muscles. Similar founder cells have been reported during early muscle differentiation in leeches (Jellies & Kristan, 1988; Jellies, 1990). In Hoploplana, a special muscle system is present at the outset under the apical organ. It consists of what seems to be a spirally toranged fiber — when seen in head-on view — and of two additional fibers crossing this spiral, from the later developing posterior to the anterior lobe.TEM-studies of embryos of Macrostomum suggest that the longitudinal nerve cords represent an important guide during early differentiation of the pattern within the body wall musculature. Young stages of myoblasts can be identified along the main lateral nerve cord. Commonly, the myoblasts are seen to alternate with young neurons in their position along the nerve cord. Embryonic stages of Macrostomum hystricinum marinum were obtained from our cultures (Rieger et al., 1988). Immediately prior to fixation (Paraformaldehyde, Stephanini's fixative) the eggshells were punctured with tungsten needles. We noted some variability of developmental time for certain embryonic stages, which we cannot explain. Developmental stages of Hoploplana inquilina were collected at the Marine Biological Laboratory, Woods Hole, MA, USA according to the procedure outlined in Boyer (1987) and Boyer (1989). They have been timed in relation to normal developmental time to an early Müller's larva at about 100 hours.     

Catecholamines demonstrated by glyoxylic-acid-induced fluorescence and HPLC in some microturbellarians

The microturbellarians Stenostomum leucops (Catenulida), Microstomum lineare (Macrostomida), Promonotus schultzei (Proseriata) and Gyratrix hermaphroditus (Rhabdocoela) were subjected to the glyoxylic-acid-induced-fluorescence method to reveal catecholaminergic parts of their nervous systems. Histofluorescence indicative of catecholamines was evident in nerve cells and fibers of all investigated species. The patterns of distribution were compared to those of other neuroactive substances. The catecholaminergic fluorescence was at least partly confined to separate neuronal subsets in S. leucops and M. lineare and probably also in P. schultzei. In S. leucops, the presence of dopa and dopamine was demonstrated by high-pressure liquid chromatography. The occurence of catecholamines in these representatives of four orders of microturbellarians suggests that catecholamines arose early in the evolution of flatworms and underscores the importance of dopaminergic substances for the function of early nervous systems.     

中国大口涡虫属三新种(扁形动物门,大口虫目,大口虫科)

报道中国广东地区生活于淡水的大口涡虫属3新种:即中国大口涡虫Macrostomum sinensis sp.nov.;针大口涡虫M.acus sp.nov.;钝大口涡虫M.obtusa sp.nov..对其形态特征作了详细描述,并与近似种进行了比较.所有标本保存在深圳大学生命科学学院实验室.     

Wide distributions and cryptic diversity within a Microstomum (Platyhelminthes) species complex

Microstomum lineare is a common species of fresh and brackish waters found worldwide. Three genes (18S, CO1 and ITS) were sequenced from specimens of M. lineare collected from four countries, and the levels of cryptic diversity and genetic structuring were assessed. Results showed M. lineare has very wide haplotype distributions suggesting higher than expected dispersal capabilities. In addition, three new species were described on the basis of molecular taxonomy: Microstomum artoisi sp. nov., Microstomum tchaikovskyi sp. nov. and Microstomum zicklerorum sp. nov.