Spatial growth and pattern formation in the small intestine microvascular bed from larval to adult Xenopus laevis: a scanning electron microscope study of microvascular corrosion casts

The microvascular anatomy of the small intestine of metamorphosing tadpoles of the South African Clawed Toad, Xenopus laevis (Daudin) is studied from developmental stages 55 to 65 and in adults by scanning electron microscopy (SEM) of vascular corrosion casts (VCCs) and light microscopy. Up to stage 62, VCCs reveal a dense two-dimensional vascular network ensheating the intestinal tube, whose proximal portion forms a clockwise spiralling outer and its distal portion an anti-clockwise spiralling inner coil. Vessels of the intestinal network impose flat and run circularly to slightly obliquely. Locally, dense capillary plexus with small “holes” indicating ongoing intussusceptive microvascular growth (IMG) and vessel maturation, are present. The typhlosole, an invagination along the proximal portion of the small intestine, reveals a dense capillary bed with locally ongoing IMG. VCCs of stages 62/63 for the first time reveal a three-dimensional vascular bed with longitudinal intestinal folds of varying size and heights greatly enlarging the luminal exchange area of the intestinal tube. From stage 65 onwards, longitudinal intestinal folds undulate and, though smaller in size and less mature as indicated in VCCs by the presence of wider, sinus-like vessels with small “holes” interposed between, closely resemble the intestinal folds present in the small intestine of adult Xenopus. Our data suggest that maturation of the vascular pattern in the small intestine of X. laevis tadpoles takes place successively after stages 62–63, and growth during this period is preferentially by intussusception.     

The effect of anaerobiosis on adenosine triphosphate levels in larvalNippostrongylus brasiliensis andHaemonchus contortus

The adenosine triphosphate (ATP) content of the pre-parasitic stages ofNippostrongylus brasiliensis, Haemonchus contortus (L₁, L₂ and L₃) and the adults of the free-living nematode,Panagrellus redivivus, have been measured by bioluminescent photometry in aerated or nearanoxic conditions. The ATP content of the L₁ and L₂ stages of both parasitic species was unaltered by a lack of oxygen over a 90-min period. However, the L₃ stage of both species and the adults ofP. redivivus showed a significant fall in the level of ATP within 10 min of nearanoxia. This lower level of ATP was maintained during oxygen lack but the initial content was restored on return of the nematodes to aerobic conditions. The results suggest that measurement of ATP by bioluminescent photometry offers a readily measured and sensitive indicator of the capacity of a nematode to cope with transient changes in oxygen supply without undue metabolic stress.     

Effect of different amino acids on the phosphatase system of an ectoparasite: Poecilobdella granulosa

Summary Inhibitory effect of different amino acids (L-phenylalanine, L-tryptophan, L-tyrosine, L-histidine-monochloride and L-arginine) on the phosphatase system of the brain, ventral nerve cord, stomach and intestine of the Indian medicinal leech Poecilobdella granulosa, was observed to be substrate, tissue, and inhibitor-specific. Most fascinating observation recorded was the activation of alkaline phosphatase of stomach and intestine by certain amino acids at low concentrations. This has been correlated with the sanguivorous habit of leeches.     

Mechanisms of Hyperglycemic Effect of Calcitonin

We studied the effect of calcitonin on blood glucose level, total calcium content, and the main stages of carbohydrate metabolism: absorption in the intestine, transport from the blood to tissues, tissue sensitivity to insulin in the whole organism, glycogen content, and activity of glucose-6-phosphate dehydrogenase and lactate dehydrogenase in the liver. A hyperglycemic effect of calcitonin was demonstrated, a close negative correlation was found between glucose level and total calcium content (r=-0.834, p<0.02). Calcitonin had no effect on glucose absorption in the small intestine and its transport from the blood to tissues (glucose consumption by peripheral tissues at the organism level), but reduced glycogen content in the liver and increased activities of glucose-6-phosphate dehydrogenase and lactate dehydrogenase.     

Development of <Emphasis Type="Italic">Angiostrongylus costaricensis</Emphasis> Morera and Céspedes 1971 (Nematoda: Angiostrongylidae) larvae in the intermediate host <Emphasis Type="Italic">Sarasinula marginata</Emphasis> (Semper 1885) (Mollusca: Soleolifera)

In life cycle of Angiostrongylus costaricensis, veronicellidae mollusks participate as the invertebrate host while rodents as the main vertebrate host. The current work shows a sequential larval development of A. costaricensis in Sarasinula marginata, individually killed and digested from day 1 to 43, post infection. Some larvae, recovered from sedimentation, were submitted to selective staining after paraffin embedded or inclusion in JB-4 to study inner structures. As control, four slugs were used, two killed at the beginning of infection and the others at the end of the experiment. At day 2 post infection, larvae were motionless and thick, presenting initial retention of granules. At day 4, L₂ were detected, persisting until 43 days post infection. Larvae L₂ displayed a large amount of granules rich in lipids and carbohydrates through its overall body, with more accumulation at the medial third corresponding to the esophagus–intestine transition site. Lipid granules, the main energetic source, were located at the basal and apical regions of intestinal cells. Both L₁ and L₃ presented bilateral alae, which is also common in other nematodes. Transition forms between L₂ to L₃ molts were also observed.     

Further indication of lowered activity of ivermectin on immature small strongyles in the intestinal lumen of horses on a farm in Central Kentucky

Critical tests were performed in 2011 in four weanling horses (L-1, L-2, L-29, and L-30) treated with ivermectin paste at 200 μg/kg. They were born in 2011 and raised together on a farm (MC) in Central Kentucky. The horses had not been treated previously with an antiparasitic drug. However, ivermectin had been administered repeatedly to the horse herd for several years and strongyle eggs per gram of feces (EPGs) returned sooner posttreatment than after initial usage. Critical tests in a recent previous study in this horse herd indicated that the reason for the early return of strongyle EPGs after ivermectin treatment probably was because of lowered drug activity on immature (L₄) small strongyles in the lumen of the large intestine. Therefore, the life cycle was shortened. The main purpose of the present study was to obtain further data on the activity of ivermectin on small strongyle immature stages, in addition to adults, in the intestinal lumen. Twelve species of small strongyles were present. Combined data for immature and adult small strongyles for the four ivermectin-treated horses demonstrated efficacy of 68 to 83 %. Removal of adults was 100 % for all four horses, and on immatures, it ranged from 0 to 16 %. Efficacy on immature small strongyles was even lower than in the previous study. This supported the earlier finding of apparent reduced time for maturation of the parasites because of incomplete elimination of immature small strongyles in the intestinal lumen of horses after ivermectin treatment. Data on five other helminth species were recorded.     

A study on the establishment of Oesophagostomum dentatum in pigs following percutaneous exposure to third-stage larvae

We examined the possibility of establishment of patent infections after percutaneous exposure of parasite-naive pigs to 10,000 Oesophagostomum dentatum infective larvae (L₃). At 5 weeks after percutaneous exposure the pigs were slaughtered and low numbers of adult O. dentatum were recovered from the large intestine. In addition, exsheathed L₃ were intravenously injected into pigs, which resulted in the recovery of fourth-stage larvae (L₄) from nodules in the lungs as well as of L₄ and adult worms from the large intestine. This study demonstrates the ability of porcine nodular worms to establish in the large intestine when pigs have been exposed percutaneously to infective larvae. It is possible that the larvae reach the large intestine by invading blood veins or the lymphatic system at the site of penetration. Received: 24 February 1998 / Accepted: 13 May 1998     

Oswaldocruzia filiformis (Nematoda: Trichostrongylidae): Morphology of developmental stages,parasitic development and some pathological aspects of the infection in amphibians

Differences between the developmental stages ofOswaldocruzia filiformis are described as a result of morphological studies on microscopical sections and intact stages of the parasite. Infective third stage larvae have double lateral alae on sheath and cuticle. The sheath has ten longitudinal lines and a fine cross striation. Parasitic third stage larvae have double lateral alae and a characteristically formed head end. Fourth stage larvae have single lateral alae, while adults have cuticular ridges, approximately 58 in males and 81 in females at the midlevel of the intestine. Detailed measurements and drawings are presented. Early parasitic development occurs in the mucosa of the stomach following oral infection. Third stage larvae return to the lumen of the gastro-intestinal tract and the next two parasitic moults occur in the lumen of the intestine. The prepatent period was 29 days inBufo bufo. Third stage larvae caused atrophy and necrosis in the mucosa of the stomach. The presence of fourth stage larvae and (sub)adults resulted in flattening of the epithelial cells of the intestine. Young fourth stage larvae, deeply burrowed between the villi of the intestine, can cause tunnel-like holes in the epithelium.     

Quiescent memory B cells in human peripheral blood co-express bcl-2 and bcl-xL anti-apoptotic proteins at high levels

The anti-apoptotic proteins bcl-2 and bcl-x_L seem to exhibit strictly opposite expression patterns in normal lymphoid cell differentiation stages, with bcl-2 low and bxl-x_L high in immature and mature proliferating cells, the reverse being the case in recirculating quiescent cells. However, it is in fact not known whether recirculating memory cells are bcl-x_L low or high. We analyzed memory (immunoglobulin isotype-switched) B cells in human peripheral blood, which were small lymphocytes in the G0 phase of the cell cycle, but proliferated better than naive B cells in response to Staphylococcus aureus Cowan I. Ex vivo these cells co-expressed bcl-2 together with bcl-x_L mRNA and protein at high levels. The mcl-1 mRNA level was low. The bcl-x_L mRNA level decreased during culture in medium containing fetal calf serum, which implies that it is maintained in vivo by continuous or frequent, non-mitogenic signal(s). The high bcl-x_L expression of memory B cells may be relevant with regard to their longevity and/or their capacity to undergo an accelerated secondary type immune response.     

Metabolite accumulation increases adenine nucleotide degradation and decreases glycogenolysis in ischaemic rat skeletal muscle

Adenine nucleotides and glycogen are degraded in skeletal muscle during no-flow ischaemia. Past investigations have ascribed these metabolic changes to the severe energetic stress which arises with the removal of exogenous substrates (principally oxygen). We tested this hypothesis by measuring the high-energy phosphagen and glycogen contents of stimulated rat hindlimb muscles (1 twitch s^?1) prior to and following 40 min of no-flow ischaemia or hypoxic perfusion without glucose (P_ao₂ = 4.6 ± 0.1 torr, plasma glucose = 0.3 ± 0.1 mmol L^?1). Both experimental protocols eliminated exogenous substrate supply; however, the maintenance of flow during hypoxic perfusion ensured the removal of metabolic by-products. A period of forty minutes of skeletal muscle ischaemia was characterized by reductions in the total adenine nucleotide pool, phosphocreatine and glycogen in the slow oxidative soleus, fast oxidative-glycolytic plantaris and the fast glycolytic white gastrocnemius. Compared to ischaemia, the total adenine nucleotide pool was higher (by 7.2–13.3 μmol g^?1 dry wt) and the glycogen content lower (by 10.0–16.6 μmol g^?1 dry wt) in skeletal muscle exposed to hypoxic perfusion without glucose. The ability of hypoxic perfusion to attenuate TAN degradation and augment glycogenolysis can be attributed to metabolic by-product removal. By limiting muscle lactate and PCO ₂ accumulation, hypoxic perfusion without glucose attenuates cellular acidification; this could in turn limit AMP deaminase activation and glycogen phosphorylase inhibition. We conclude that the ischaemia-induced alterations in adenine nucleotide and glycogen metabolism arise in response to the elimination of exogenous substrates and to the accumulation of metabolic by-products.     

Light and Scanning Electron Microscope Examination of the Digestive Tract in Peppered Moray Eel,Gymnothorax pictus (Elopomorpha)

The morphology of the digestive tract of the peppered moray eel, Gymnothorax pictus (G. pictus) (Elopomorpha: Anguilliformes) was examined using both light and scanning electron microscopy. The digestive tract is composed of the esophagus, the stomach, and the intestines; pyloric caeca were absent. The stomach was divided into a cardiac region that was continuous with the esophagus, a body which terminated in a long blind sac, and a pyloric region that was continuous with the intestine. The short intestine possessed several partitions that were created by the mucosal folds within the posterior region. The terminal region of the stomach was characterized by the thick longitudinal muscularis and subserosa, and the gastric glands and microvilli were absent. Ciliary tufts of ciliated cells were observed on the surface of the partition‐like mucosal folds within the intestinal wall. Acidic mucus was secreted throughout the digestive tract. It was suggested that the terminal region of the stomach is specialized for storage of large food items. In addition, it is possible that the partition‐like mucosal folds within the intestine perform a function similar to that of the spiral valve and, and along with ciliated cells, facilitated digestion and absorption. The acidic mucus likely maintained surface epithelium pH and protease activity. Within a phylogenetic context, the absence of a pyloric caeca in G. pictus while possessing an intestine implies that this species is affiliated to groups that had branched off earlier than basal teleosts. Anat Rec, 296:443–451, 2013. © 2013 Wiley Periodicals, Inc.     

Role of Serotonin in the Recovery of Electrical Activity in the Stomach and Small Intestine of Rats during the Early Postsurgery Period

The effects of serotonin and 5-HT₄ receptor agonist cisapride on electrical activity of the stomach and small intestine were studied in rats with postoperative ileus. Postoperative ileus was accompanied by the absence of the migrating myoelectric complex in the stomach and small intestine. All phases of the migrating myoelectric complex were successively recovered in the jejunum, duodenum, and stomach. Administration of serotonin and 5-HT₄ receptor agonist cisapride into the jejunum was followed by the appearance of spike activity, which spread from the stomach to the jejunum. Intraintestinal treatment with cisapride and serotonin shortened the period to recovery of the migrating myoelectric complex to 3 and 4 days, respectively. Our results suggest that serotonin plays a role in the regulation of the migrating myoelectric complex at the early postoperation period. __________ Translated from Byulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 140, No. 11, pp. 511–514, November, 2005     

Probable reason why small strongyle EPG counts are returning “early” after ivermectin treatment of horses on a farm in Central Kentucky

Critical tests were carried out in 2008 in four yearling horses (H-2, H-4, H-10, and H-11) born and raised together on a farm (MC) in Central Kentucky. These horses were treated intraorally with ivermectin paste at the dose rate of 200 μg/kg. The main interest was to try and determine more precisely, from posttreatment (PT) worm count data, the current activity of ivermectin against small strongyles in a horse herd. These horses had been treated repeatedly with this compound and counts of small strongyle eggs per gram of feces (EPGs) of these parasites have been returning sooner than previously in field tests (Lyons et al. Parasitol Res 103:209–215, 2008a). Data from the four horses revealed that a total of 3,237 (nonfactored number) specimens of small strongyles was recovered from aliquot samples of feces passed PT and in the large intestinal contents at necropsy; all specimens were examined and identified. Thirteen species of adult small strongyles were recorded. Cylicocyclus (Cyc.) insigne was the predominant species. Three of the yearlings (H-2, H-4, and H-11) were necropsied at 6 days PT with ivermectin, and removals of small strongyles were: 50%, 80%, and 36% of fourth stages (L₄), respectively, and 100%, 99%, and 100% of adults, respectively. As indicated, the only incomplete removal of adults from the three horses was for H-4. They consisted of two species: (1) young Cyc. insigne (those passed in the feces were fully developed); removal of this species was 89% and (2) fully developed Cylicostephanus longibursatus; 99% were removed. The fourth yearling (H-10), necropsied at 25 days PT, harbored 19,150 adult small strongyles in the large intestinal contents. Most of the species were Cyc. insigne; all were fifth stage but not sexually mature. Comparing the percentage of adult small strongyles found at necropsy relative to the total number present (those passed in the feces and at necropsy), only 0% to 1% were in the contents of the large intestines of the three horses at 6 days PT but in 26% for horse H-10 at 25 days PT. As mentioned earlier, only a few adults were found in one horse and several L₄ in the three horses at necropsy at 6 days PT. Therefore, in horse H-10, most adults found at 25 days PT presumably developed from “young” specimens not removed by ivermectin. Thus, data from the present critical tests indicate the probable cause of the “early” return of small strongyle EPG values after ivermectin treatment in the horses in field tests on Farm MC. It seems this was the result of incomplete removal of luminal specimens (L₄ and possibly young adults), some of which matured and began laying eggs by about 4 weeks PT (Lyons et al. Parasitol Res 103:209–215, 2008a). The research also showed that ivermectin was highly effective on adult small strongyles. At necropsy, the following other species of parasites (adult) were found, but none was recovered from the feces. These were (n = number of horses infected): (1) ascarids (Parascaris equorum—n = 1), (2) tapeworms (Anoplocephala perfoliata—n = 4), and (3) pinworms (Oxyuris equi—n = 3). Immature (L₄) O. equi were present in two horses and removals were 0% in one horse and 39% in the other. Eyeworms (Thelazia lacrymalis) were found in one horse at necropsy. Even though a small number of horses were used in the present research, the commonality of their background made them ideal candidates as a group for this study. This aspect helps strengthen the validity of the interpretation of the findings.     

Human and mouse perforin are processed in part through cleavage by the lysosomal cysteine proteinase cathepsin L

Summary Infection of mice with the gastrointestinal nematode Trichuris muris represents a valuable tool to investigate and dissect intestinal immune responses. Resistant mouse strains respond to T. muris infection by mounting a T helper type 2 response. Previous results have shown that CD4⁺ T cells play a critical role in protective immunity, and that CD4⁺ T cells localize to the infected large intestinal mucosa to confer protection. Further, transfer of CD4⁺ T cells from immune mice to immunodeficient SCID mice can prevent the development of a chronic infection. In the current study, we characterize the protective CD4⁺ T cells, describe their chemokine receptor expression and explore the functional significance of these receptors in recruitment to the large intestinal mucosa post‐T. muris infection. We show that the ability to mediate expulsion resides within a subpopulation of CD4⁺ T cells marked by down‐regulation of CD62L. These cells can be isolated from intestine‐draining mesenteric lymph nodes (MLN) from day 14 post‐infection, but are rare or absent in MLN before this and in spleen at all times post‐infection. Among CD4⁺ CD62L^low MLN cells, the two most abundantly expressed chemokine receptors were CCR6 and CXCR3. We demonstrate for the first time that CD4⁺ CD62L^low T‐cell migration to the large intestinal mucosa is dependent on the family of Gα_i‐coupled receptors, to which chemokine receptors belong. CCR6 and CXCR3 were however dispensable for this process because neutralization of CCR6 and CXCR3 did not prevent CD4⁺ CD62L^low cell migration to the large intestinal mucosa during T. muris infection.     

Clinicopathological features of 49 primary gastrointestinal diffuse large B‐cell lymphoma cases; comparison with location,cell‐of‐origin,and frequency of MYD88 L265P

The gastrointestinal (GI) tract is the most common primary site of extranodal diffuse large B‐cell lymphoma (DLBCL), with approximately one‐third of extranodal DLBCL occurring in the GI tract. We investigated the clinicopathological features and immunohistochemically‐assessed cell‐of‐origin of 49 GI DLBCL cases (stomach, 24; small intestine, 10; colon, 15) and also examined the presence of MYD88 L265P as recently this mutation has been frequently identified in ABC‐like DLBCL, particularly in extranodal sites. Small intestinal DLBCL was characterized by the preponderance of women (P = 0.041) and elevated LDH (P = 0.002) and soluble interleukin‐2 receptor (P = 0.033). Small intestinal DLBCL more frequently showed anemia (P = 0.031) and elevated CRP (P = 0.029) than gastric DLBCL. ABC‐like phenotype was seen in 71.4 % cases (stomach, 79 %; small intestine, 70 %; colon, 60 %). MYD88 L265P was detected in 6.1 % cases; all were primary gastric DLBCL with ABC‐like phenotype but had no distinct clinicopathological features. In conclusion, GI DLBCL had different clinicopathological features according to the primary site especially in the small intestine. Also, MYD88 L265P had little involvement in GI DLBCL compared with other extranodal DLBCLs, suggesting that its pathogenesis might be different from that of organs with a high frequency of MYD88 L265P.     

Lipids of fish parasites and their hosts: fatty acid fingerprints of four species of acanthocephalans and of their hosts' intestinal tissues

The fatty acid fingerprints of total lipid extracts from the four fish parasitesAcanthocephalus lucii, Neoechinorhynchus rutili, Paratenuisentis ambiguus, andPomphorhynchus laevis were investigated by capillary gas-liquid chromatography (GLC). The fatty acid patterns of the parasites were compared with those of their respective host tissue, viz., intestinal tract tissue of infected perch, brown trout, eel, and chub. The highly complex gas chromatograms revealed significant differences not only between parasite and host tissue but also between the parasites themselves. For instance, all the parasites contained much more eicosapentaenoic acid (205n-3) than did the respective fish intestinal tissue. Differences were also observed between the presoma and metasoma ofPomphorhynchus laevis.     

Critical tests evaluating efficacy of moxidectin against small strongyles in horses from a herd for which reduced activity had been found in field tests in Central Kentucky

Critical tests were performed in 2009 and 2010 in four 2-year-old horses naturally infected with internal parasites. The horses were from a herd (Farm MC) where reduced activity of ivermectin and moxidectin on small strongyles was demonstrated previously from EPG (eggs/gram of feces) data in field tests. Also, in critical tests in horses from the same herd, ivermectin was less effective on immature small strongyles in the lumen of the large intestine than when the drug was first marketed. The main interest in the present critical tests was to determine the efficacy of moxidectin (400 μg/kg) on small strongyles. This was done to try and find indications of why there has been a return of strongyle EPG counts sooner after treatment in field tests than when moxidectin was first commercially available. Removal of adult small strongyles for the four treated horses was >99% to 100%. Efficacy on immature (L₄) small strongyles was 82%, 96%, 98%, and >99% for the individual horses. Identification of small strongyles recovered from two of the horses revealed that three genera and 11 species were present. Specimens of Cylicocyclus ashworthi are reported for the first time in horses in Kentucky although eggs of this species have been identified. Moxidectin, in the present study, was excellent on removing adult small strongyles but was less effective on immatures (L₄) in the intestinal contents. The question as to why moxidectin efficacy on small strongyles has declined in field tests may have been answered at least to a certain extent. It seems that a significant factor is “quick development” of a few remaining immatures in the gut lumen of horses. Also, possible activity may have decreased on encysted stages in the large intestinal lining. In any event, after treatment of some horses with moxidectin, the life cycle of small strongyles is shorter now than at the onset of usage of this compound.     

Fatal necrotic enteritis associated with Clostridium perfringens in wild crows (Corvus macrorhynchos)

Sporadic outbreaks of fatal enteritis occurred among free-living wild crows (‘large billed’ or ‘wok’ crow; Corvus macrorhynchos) in an open-air park in Japan in 2002. Eight crows were found dead during February, followed by two more in September, and five of the eight were examined histopathologically. At necropsy, all cases showed a markedly dilated small intestine, especially the jejunum and ileum, with large amounts of gas, and dark red to greenish–brown soft content. The necrotic intestinal wall was markedly thickened with multifocal haemorrhages. All cases had multifocal white foci in the liver, and four cases showed marked splenomegaly. Histologically, there was severe necrotic enteritis characterized by extensive mucosal necrosis and multifocal haemorrhages, as well as inflammatory cell infiltrations. A prominent pseudo-membrane formation was noted in the affected intestine. Severe adhesive peritonitis was also observed in three cases. Gram-positive bacilli were present in large numbers in the lumen, and in and around necrotic lesions in the affected intestine. The bacilli were positive for Clostridium perfringens enterotoxin type A by immunohistochemistry, and were also positive for C. perfringens type A using the immunofluorescence method. C. perfringens was isolated by anaerobic culture from the intestinal contents. The present enteritis was thought to be induced by proliferated C. perfringens in the intestine, and to be the cause of death.     

Introgression of distant mitochondria into the genome of <Emphasis Type="Italic">Gyrodactylus salaris</Emphasis>: Nuclear and mitochondrial markers are necessary to identify parasite strains

Novel combinations of mitochondrial DNA (CO1) and internal transcribed spacers of nuclear ribosomal DNA (ITS) were detected among Gyrodactylus parasites on brown trout (Salmo trutta L.), rainbow trout (Oncorhynchus mykiss (Walbaum)), and Ohrid trout (Salmo letnica (Karaman)) from salmonid farms in Poland and Macedonia. Some clones differed from standard ITS only by ≤ 4 nucleotides, but they belonged to a mtDNA clade that differed from the Northern European lineages of G. salaris by d _MCL = 0.266 ± 0.108 (maximum composite likelihood distance). The divergence of d _MCL = 0.013 ± 0.005 within the alien mtDNA clade suggested introgression from an unknown maternal ancestor into the G. salaris Malmberg genome 137 to 57 kyr ago (or, less probably, repeated introgression). A comparable modern hybrid was detected based on permanently heterozygous ITS (28 bp/1264 = 2.2%) in a clone that is widespread throughout Finnish rainbow trout farms. This was a F₁ hybrid of maternal G. pomeraniae Kuusela, Ziętara et Lumme (on roach, Rutilus rutilus (L.)) and G. lavareti Malmberg (on whitefish, Coregonus lavaretus (L.)). The mtDNA of the parental species differed by d _MCL = 0.290 ± 0.130. The observations emphasize that both nuclear and maternally-inherited DNA markers are necessary to characterize sexually-produced lineages or clones of Gyrodactylus. The hybridization events demonstrated predict incongruence of mitochondrial vs. nuclear gene trees, i.e., reticulate evolution in species trees.