Johannes H. Van WYK

Johannes H. Van WYK
Stellenbosch University | SUN

About

118
Publications
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2,153
Citations
Citations since 2016
20 Research Items
894 Citations
2016201720182019202020212022050100150
2016201720182019202020212022050100150
2016201720182019202020212022050100150
2016201720182019202020212022050100150

Publications

Publications (118)
Article
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The emergence of widespread morphological malformations in the reproductive system of wildlife is generating increasing concerns. This concern is because the observed malformities may be linked to pollution by pesticides and other chemicals. The amphibian declines, for example, have been linked to pesticide pollution among other factors. Using an e...
Article
Full-text available
Thyroid hormones play critical roles in body growth and development as well as reproduction. They also influence the activities of a wider variety of tissues and biological functions, such as osmoregulation, metabolism, and especially metamorphosis in organisms, such as frogs. These complex activities of thyroid hormones are prone to disruption by...
Preprint
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Vol.:(0123456789)1 3Archives of Environmental Contamination and Toxicology https://doi.org/10.1007/s00244-020-00756-5Lethal andTeratogenic Impacts ofImazapyr, Diquat Dibromide, andGlufosinate Ammonium Herbicide Formulations Using Frog Embryo Teratogenesis Assay‑Xenopus (FETAX)OluwaseunOlusegunBabalola1,2 · JohannesChristoffTruter1,3· Johan...
Article
Ample evidence around the world exists suggesting a link between exposure to glyphosate, toxicity and perturbed physiological functions in non-target organisms. Although glyphosate formulations are widely used for weed and alien plant management, their ecotoxicological information remain scanty. Using the 96-hour Frog Embryo Teratogenesis Assay-Xen...
Article
Biological control of cyanobacteria is a viable means of controlling nuisance bloom occurrences; however the majority of studies done are against Microcystis sp., with a commonly lytic effect caused. Filamentous cyanobacteria such as Oscillatoria are not as extensively studied in this area of biological control and are often part of Microcystis dom...
Article
Full-text available
Biological control of cyanobacteria is a well-researched area with a central focus on laboratory-scale studies. Numerous reports have been made on algicidal isolates, with bacteria as a major component of the antagonists. The research in this review draws a brief summary of what is currently known in the area of freshwater cyanobacteria being inhib...
Article
Full-text available
Demographic differences in resource use are key components of population and species ecology across the animal kingdom. White sharks (Carcharodon carcharias) are migratory, apex predators, which have undergone significant population declines across their range. Understanding their ecology is key to ensuring that management strategies are effective....
Article
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The present histological study of adult female common smoothhound shark Mustelus mustelus demonstrates that the species can store sperm in their oviducal glands. In addition, single paternity of a litter of 22 pups is reported, contributing to further knowledge regarding the genetic and behavioural mating strategy of M. mustelus.
Article
Full-text available
Results from this study of the white shark Carcharodon carcharias include measurements obtained using a novel photographic method that reveal significant differences between the sexes in the relationship between tooth cuspidity and shark total length, and a novel ontogenetic change in male tooth shape. Males exhibit broader upper first teeth and in...
Article
Full-text available
Thyroid function and nutritional indicators were measured in obese, steatitis-affected Mozambique tilapia Oreochromis mossambicus from Loskop Reservoir (LR), South Africa. Plasma thyroid hormones (especially T3) and thyroid follicle histomorphology revealed high levels of activity in every aspect of the thyroid cascade measured in fish from LR comp...
Article
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Polychlorinated biphenyls are a broad family of persistent environmental pollutants of enormous environmental significance, because despite being prohibited, they are still found in alarming concentrations in the environment, enough to trigger environmental biomonitoring programs. Associated to this need to continue identifying and quantifying them...
Article
Full-text available
Knowledge regarding the potential impacts of crude oil on endocrine signalling in freshwater aquatic vertebrates is limited. In this study, the expression of selected genes as biomarkers for altered endocrine signalling were studied in African clawed frog, Xenopus laevis tadpoles, and juvenile Mozambique tilapia, Oreochromis mossambicus exposed to...
Article
Water treatment is an important ecosystem service in natural systems and wetlands are well-known to have increased functionality in this regard. The rehabilitation of productive wetlands plays an important role in improving this functionality in degraded wetlands, but it is not well-known to what extent these activities improve acid mine drainage i...
Article
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Since the industrial revolution, the impact of effluents produced by human activities on ecosystems has been a major international environmental concern. This study was aimed at observing the changes in water and sediment qualities at a watershed level of two different river systems facing the same land use practices, but impacted to different degr...
Article
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Endocrine-disrupting contaminant (EDC) loads in rivers, and the associated risk to wildlife, may be linked to different anthropogenic stressors occupying river catchments. The aims of this study were to evaluate seasonal and spatial variation in steroid estrogen loads, and (anti)estrogenic and (anti)androgenic activity in a river catchment (upper O...
Article
Full-text available
Top-predators may be extremely vulnerable to environmental contaminants, such as organochlorines (OCs) and polycyclic aromatic hydrocarbons (PAHs), mostly because of their position in the trophic web. In this study, the use of skin biopsy is proposed as a sensitive non-lethal technique for the toxicological assessment of white shark (Carcharod...
Article
Various waterborne anthropogenic contaminants disrupt the endocrine systems of wildlife and humans, targeting reproductive pathways, among others. Very little is known, however, regarding the occurrence of endocrine disruptive activity in South African freshwater ecosystems, and coastal ecosystems have not been studied in this regard. In a first at...
Article
Full-text available
Although it is known that environmental chemicals can affect the oestrogenic system, far less attention has been paid to chemicals interacting with the androgen receptor (AR). Pesticides, particularly fungicides, have been shown to competitively bind or affect expression of the AR in an inhibiting manner. Few studies have addressed anti-androgenic...
Article
This paper addresses and illustrates, both analytically as well as by means of simulation, the equivalence of a cyclically rotated complete complementary coded (CRCCC) code division multiple access orthogonal frequency division multiplexed (CDMA-OFDM) BPSK/QPSK system and a narrowband uncoded BPSK/QPSK reference system. The equivalence can be attri...
Conference Paper
Full-text available
White sharks are protected in several countries under national legislation through the Convention of Migratory Species (CMS) and the Convention of the International Trade in Endangered Wild Flora and Fauna (CITES), considered as vulnerable by the International Union for Conservation of Nature (IUCN). The conservation status of this top predator cal...
Article
Full-text available
Ectotherms are known to function optimally within a specific range of body temperatures, bordered by two setpoint values (i.e. preferred temperature range; Tp). Preferred body temperature stands central in understanding behavioural thermoregulation in a thermally dynamic temperate environment. In this study we determined Tp for the group-living cor...
Article
The Oreochromis mossambicus (Peters) population inhabiting Lake Loskop, South Africa, is characterized by a high incidence of obesity and pansteatitis. We investigated potential links between the impaired health of Lake Loskop O. mossambicus and the endocrine system by assessing the expression of selected genes associated with the thyroid and adren...
Chapter
Despite wetland importance as habitat for early live stages of amphibian species and the legislation designed to protect wetlands, they are currently at risk from a number of sources that include coal mining. It has been estimated that half of the world's total wetlands have been lost already. Within Southern Africa, the trend is similar, with an e...
Article
Acid mine drainage (AMD) is a global environmental concern due to detrimental impacts on river ecosystems. Little is however known regarding the biological impacts of neutralized AMD on aquatic vertebrates despite excessive discharge into watercourses. The aim of this investigation was to evaluate the endocrine modulatory potential of neutralized A...
Conference Paper
The bit error rate of a multiple-input multiple-output (MIMO) multi-carrier code division multiple access (MC-CDMA) communication system has been estimated through Monte Carlo simulation. Cyclically rotated complete complementary codes (CRCCCs) were employed in a code reuse scheme along with orthogonal space-time block codes (OSTBCs) and orthogonal...
Conference Paper
Full-text available
The detection of cellular communications signals was investigated, and a prototype detector was implemented. Global System for Mobile Communications (GSM) signals were detected in recorded data and the correct frequency channel was determined. Inter-channel interference (ICI) was observed and methods to reduce it are discussed. Detection was achiev...
Conference Paper
In the last few decades various studies have shown that top predators fishes are sensitive to the toxicological effects of certain compounds, including organochlorine (OCs), in particular the large class od Endocrine Disrupting Chemicals (EDCs), and Polyciclic Aromatic Hydrocarbons (PAHs). Top-predators may be extremely vulnerable to contaminants m...
Article
The present study reports the effect of high molecular weight bacterial fructan (levan) and glucan (reuteran) on growth and carbohydrate partitioning in transgenic sugarcane plants. These biopolymers are products of bacterial glycosyltransferases, enzymes that catalyze the polymerization of glucose or fructose residues from sucrose. Constructs, tar...
Article
In this paper we present a novel multiple access scheme based on a combination of multi-carrier OFDMA techniques and a multi-dimensional spread spectrum modem that makes use of rotated mutually orthogonal complete complementary codes. A multi-dimensional code-division multiple access (MD-CDMA) modem with recent designs of perfectly orthogonal compl...
Article
In lizards, the epidermal glands of the femoral and precloacal regions are involved in the production of semiochemicals. In addition to its femoral glands, the giant girdled lizard, or sungazer, Cordylus giganteus, which is endemic to South Africa, has generation glands as an additional potential source of semiochemicals. These epidermal glands are...
Article
In a high-altitude population of the Drakensberg crag lizard, Pseudocordylus melanotus, most adult males, unlike adult females, are brightly coloured, and they are larger in body size and have relatively larger heads than females. Three basic colour phases can be distinguished among the brightly coloured adult males. No apparent differences in repr...
Article
Previous research has shown that accurate evaluation of environmental water samples for estrogenic activity requires a panel of in vitro and in vivo bioassays, which are based on different molecular and cellular action mechanisms. In the current study, a test battery containing four assays was used to analyze water from the Eerste River, South Afri...
Article
We investigated inter- and intraspecific variation in epidermal gland characteristics in cordylid lizards. Our particular interest was whether a causal relationship exists between the presence of generation glands and lifestyle, i.e. in rock-dwelling or ground-dwelling lizards. We established that both femoral and generation glands are always prese...
Article
Lingually mediated detection of prey chemicals is widespread in one major clade of lizards, Scleroglossa, but rare in the other, Iguania. It is absent in all ambush-foraging families tested and present in all actively foraging families. In Iguania, prey chemical discrimination is known only in the herbivorous Iguanidae; in Scleroglossa, it was here...
Article
Exposure of aquatic organisms to 17beta-estradiol (E(2)) induces a variety of estrogen-responsive genes, including vitellogenin (vtg)-the precursor protein of egg yolk in oviparous animals and to date the single most used gene product in screening for estrogenic endocrine disruption. Transcription regulation of vtg by E(2) is dependent on binding o...
Article
The yolk-precursor lipoprotein, vitellogenin (VTG) has been widely recognized as a biomarker for the detection of estrogenic activity in water-borne chemical pollutants. We characterized the expression status of this important constituent of reproduction in the Mozambique tilapia (Oreochromis mossambicus), a tilapiine freshwater fish species indige...
Article
a b s t r a c t The yolk-precursor lipoprotein, vitellogenin (VTG) has been widely recognized as a biomarker for the detection of estrogenic activity in water-borne chemical pollutants. We characterized the expression sta-tus of this important constituent of reproduction in the Mozambique tilapia (Oreochromis mossambicus), a tilapiine freshwater fi...
Article
Full-text available
Pedioplanis burchelli is a small oviparous lacertid lizard that inhabits rocky montane areas in the highveld grassland biome of South Africa. We studied its reproductive cycle by monthly monitoring of gravimetric, morphometric, and histological changes of the reproductive system of males and females throughout 1999. Reproduction took place in sprin...
Article
The hypothesis that low skin reflectance (melanism) provides an advantage for thermoregulation under cold conditions has received mixed support in ectothermic vertebrates. We selected a model system, three allopatric closely related species of cordylid lizards that differ in skin reflectance, to test this hypothesis. Cordylus niger and Cordylus oel...
Article
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During the mid 1990s a potentially serious, chronic syndrome was reported in well-managed beef and dairy herds from unrelated parts of South Africa. Farmers reported that it manifested as various combinations of decreased production, decreased weaning masses, apparent immune breakdown in previously immunocompetent animals, increased reproductive di...
Article
Oreochromis mossambicus is targeted as a sentinel species for assay development to detect disruption of estrogen-dependent pathways in Southern Africa. Aromatase--an enzyme encoded by either of two cytochrome P450 19 (cyp19) genes in O. mossambicus is the only enzyme able to catalyze the aromatization of androgens to estrogens. As part of backgroun...
Article
We investigated possible differences in the consumption of termites (Microhodotermes viator) by individuals in different sized groups of Cordylus cataphractus during different times of the year. Scats, collected once a month from small (2-3 individuals), medium (4-10 individuals) and large C. cataphractus groups (more than 10 individuals), from Jan...
Article
Histological changes during the ovarian cycle of the oviparous gekkonid lizard, Hemidactylus mabouia are described. The ovaries each contained a single germinal epithelium (bed) with numerous oogonia and primordial follicles. Similar to other squamates, the ovarian stroma was characterized by follicles in different stages of development, and may co...
Article
Full-text available
The giant girdled lizard or sungazer, Cordylus giganteus, is endemic to South Africa. It has been suggested that in this species, as in other lizard species, epidermal glands in the femoral, pre-cloacal regions, and cloacal glands are the main sources of semiochemicals and that these secretions could play an important role at different levels of th...
Article
Whether melanism plays a significant role in thermoregulation has been a persistent question in studies of thermal biology of ectotherms. This review provides a synthesis of the thermal melanism hypothesis which states that dark individuals (i.e. lower skin reflectance) are at an advantage under conditions of low temperature since they heat up fast...
Article
Full-text available
Preferred body temperatures (T sel) of ectotherms are important for ecological and evolutionary studies. In lizards, the measurement of T sel is controversial for several reasons, generally related to hypotheses addressing how T sel may evolve in the wild. Although seldom explicitly tested, evolutionary hypotheses of adaptation to local climate req...
Article
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With the introduction of VDSL2 and the implemen-tation of Fibre-to-the-Curb (FTTC), the theoretical limit of DSL technology is pushed even further than currently known ADSL technology. Current research is investigating the possibility of symmetric DSL at 1 Gbps (GDSL). This paper addresses the design and implementation of a com-plete complementary...
Article
Asymmetric Digital Subscriber Line (ADSL) is becoming a relatively matured technology as many telecommunication service providers offer this technology to customers. The actual throughput of ADSL is highly dependent on the number of simultaneous users within a cable grouping. In order to guarantee throughput to any user at any time, one must be abl...
Article
Full-text available
The distribution of calcium ion (Ca-ion) was demonstrated in the mantle epithelial cells by means of two-step chemical precipitation of Ca-ion in the pulmonate snail, Euhadra hickonis. K-oxalate/K-antimonate chemical replacement was done using a simultaneous computerized microwave-stimulated fixation method. These precipitates in the mantle epithel...
Conference Paper
Asymmetric digital subscriber line (ADSL) is a high-speed wireline access technology capable of supporting up to 8 Mbps downstream and 1 Mbps upstream in a bandwidth of 1.1 MHz. ADSL uses discrete multitone (DMT) modulation, allowing it to optimally adapt to changing line conditions, including line length, line topology and interference. Various te...
Article
A rapid whole blood culture (WBC) assay system was developed to monitor the inflammatory potential of water samples collected in the Manguang Municipal area, South Africa. Several systems associated with the Modder River catchment area were investigated in this preliminary study to find out whether there were potential problems in this catchment ar...
Article
Nuptial pads are areas, digits, and forearm characterized by epidermal keratin hooks and dermal breeding glands opening on the epidermis surface in male frogs. The presence of the epidermal hooks and secretory activity of the breeding glands are androgen-dependent and considered functioning during mating. Chemical pollutants released into the aquat...
Conference Paper
The loop plant is the only part of the telephone network that has stayed structurally almost the same for the best part of a century. Due to the increasing demand for broadband services, the time has come for a powerful upgrade of the entire outside loop plant, which includes upgrading the network to optical fibre. Initial provisioning of new servi...
Conference Paper
The capacity for an asymmetric digital subscriber line (ADSL) system is determined in a network environment, including other typical services, such as ISDN, high-bit-rate DSL (HDSL) and E1 leased lines. Additive white Gaussian noise (AWGN), near-end crosstalk (NEXT) and far-end crosstalk (FEXT) are included as noise sources.
Article
A sensitive ex vivo bioassay for detecting environmental estrogens and estrogen mimics was developed using Xenopus liver slice culture. Vitellogenin synthesis by these cultures was used as biomarker for estrogenic activity of environmental water samples. Sensitization of the assay for estrogens and mimics was accomplished by employing tissue from a...
Article
Full-text available
The aim of this study was to investigate the endotoxin-induced interleukin-6 (IL-6) release in whole blood cultures from samples taken at rest, 24 h post-exercise, from a control group of recreationally trained individuals (C), a group of highly trained triathletes (TA) and a group of highly trained professional rugby players (RP). Fifteen RP [mean...
Article
Full-text available
Polyclonal antibodies raised against chicken apoll was characterised for its use in Western blotting and ELISA detection systems of apoll in chicken plasma. The antibody has a high avidity and specificity for apolipoprotein II (apoII). Western blots show that the antibody reacts with a single band at 15 kDa. The antibody was used for setting up bot...
Article
Despite recent interest in refuge use, refuge characteristics and their relationships to refuge-associated antipredatory defences have been relatively neglected. These topics were studied experimentally in the Cape girdled lizard, Cordylus cordylus. Lizards used crevices in novel situations under uncertain risk and when confronted by a human simula...
Article
A rapid whole blood culture (WBC) assay system was developed to monitor the inflammatory potential of water samples collected in the Western Cape, South Africa. Water contaminated with inflammatory substances induced the pro-inflammatory hormone interleukin 6 (IL-6). All water samples collected from the Eerste River, Stellenbosch, induced IL-6 secr...
Article
We observed lizards in artificial crevices to examine behaviors that make them harder for predators to dislodge from crevices or burrows. We used four focal species and made additional observations of 13 cordyliform species. There were pronounced differences in defensive behaviors within crevices among species. Within crevices, representatives of s...
Article
Cordylus giganteus is endemic to the Highveld grasslands of South Africa. Owing to limited distribution and extensive habitat destruction, it may be regarded as an endangered species. C. giganteus individuals fed during eight months of the year. The mass of food in the stomachs and remains in the hind guts indicated that a high degree of foraging s...
Article
Full-text available
Two lizard foraging modes, ambush and active foraging, are usually quantified by the variables MPM (movements per minute) and PTM (proportion of time spent moving), but both variables may be affected by behaviors other than foraging. We introduce PAM, the proportion of attacks on prey discovered while lizards are moving (in relation to total attack...
Article
Male and female Cordylus cordylus can discriminate between tiles labeled by their own pheromones and tiles labeled by individuals of the same sex, as shown by elevated tongue-flick rates in which the tongue contacts only air above the tiles labeled by other individuals and tongue-flicks in which the tongue contacts the tiles themselves. Potential p...
Article
Pseudocordylus capensis, a melanistic lizard, is regarded as a basal species in the phylogeny of the family Cordylidae and is endemic to the Cape Fold Mountains. Data for this study were obtained from measurements and dissections of museum specimens (N = 68). Sexual maturity in both sexes is attained at around 80-90 mm snout-vent length (SVL). Body...
Article
MOUTON, P. LE F.N., and J. H. VAN WYK. 1997. Adaptive radiation in cordyliform lizards: An overview. Afr. J. Herpetol. 46(2): 78–88.Ecophysiological and behavioral character evolution in the Cordyliformes is discussed. Arguments are based on Lang's generic-level phylogenetic tree for the Cordyliformes, but with Platysaurus taken as the most basal g...
Article
Full-text available
The first quantitative data on foraging mode in the cordyliform lizards reveal different foraging behaviours between and within families. All species of cordylids studied (four Cordylus, two Pseudocordylus, and one Platysaurus) are ambush foragers. However, the species of Cordylus and Pseudocordylus microlepidotus are the most extreme ambushers. Th...
Article
The cordylid lizard Cordylus cordylus, a member of a family in which pheromonal communication was previously unreported, detects conspecific pheromones and responds differently to male and female pheromones. In an arena containing a clean ceramic tile or one marked by a male or female conspecific, males tongue flicked male-labeled and female-labele...
Article
We examined the reproductive cycles of two species of the only oviparous genus Platysaurus in the predominantly viviparous family Cordylidae, endemic to Southern Africa. The reproductive cycles of Platysaurus capensis and P. minor were determined using data obtained from field caught and museum specimens. Males of both species exhibit testicular re...
Article
We examined the reproductive cycles of two species of the only oviparous genus Platysaurus in the predominantly viviparous family Cordylidae, endemic to Southern Africa. The reproductive cycles of Platysaurus capensis and P. minor were determined using data obtained from field caught and museum specimens. Males of both species exhibit testicular re...
Article
Cordylus giganteus is a large, terrestrial, viviparous lizard endemic to the Highveld grasslands of South Africa. It has a limited distribution and is regarded by conservationists as threatened. The testicular cycle and associated seasonal variation in the sexual accessory structures, together with plasma testosterone levels, were studies over a pe...
Article
A new crag lizard of the genus Pseudocordylus is described from the Western Cape Province, South Africa. It is an archaic melanistic form, occurring in sympatry with other archaic melanistic lizard and mollusc taxa within the mistbelt of the Hottentotsholland Mountains, part of the Cape Folded Mountains. With a maximum snout-vent length of less tha...
Article
The extent of sexual selection in two girdled lizard species was evaluated by measuring sexual dimorphism in those characters normally affected by sexual selection. Neither Cordylus niger, a cool-adapted species, nor C. cordylus, a warm-adapted species, displayed any notable sexual differences in asymptotic body sizes, but both species displayed sl...
Article
MOUTON, P. LE F.N., and J.H. VAN WYK. 1994. Taxonomic status of geographical isolates in the Cordylus minor complex (Reptilia: Cordylidae): A description of three new species. J. Herpetol. Assoc. Afr. 43: 6–18.Three new species of girdled lizard in the genus Cordylus are described from the Karoo and Namaqualand in South Africa. All three species cl...
Article
Strike-induced chemosensory searching (SICS) was not detected experimentally in the cordylid lizard,Cordylus cordylus. Both components of SICS, a post-strike elevation in tongue-flick rate (PETF) and searching movements for attacked and released prey, were absent. The findings are consistent with previous data showing that PETF and/or SICS are lack...