Faculty of Natural and Agricultural Sciences
School of Biological Sciences
Department of Zoology and Entomology
Selected Highlights from Research Findings
Viral diseases constitute a serious problem in the profitable production of grapevines. In South Africa, grapevine leafroll disease caused by the virus GLRaV-3 is of major importance in commercial vineyards. Plants infested with the disease show reduced productivity through delayed ripening of grapes and a decrease in sugar content. Several mealybug and soft scale species have been identified as vectors of the virus worldwide. As part of a larger research programme by Winetech, its transmission biology in South Africa is being investigated. The study is being undertaken in collaboration with the ARC-Plant Protection Research Institute and the ARC-Infruitec/Nietvoorbij, with support from THRIP. The vine mealybug Planococcus ficus, the most abundant scale insect in South African vineyards, has been identified as the most efficient vector. A transmission assay, which is required as support technology for the development of virus-resistant grapevines, has been developed based on the transmission characteristics of P. ficus. The study has also developed a method to detect GLRaV-3 in individual mealybugs, including first-instar nymphs. This result now makes it possible to determine directly the percentage of mealybugs or the percentage of different developmental stages in a population carrying the virus. In addition, a species-specific polymerase chain reaction (PCR) technique has been developed for the identification of P. ficus. Differentiating P. ficus from other mealybug species occurring on grapevines has so far relied on morphological characteristics. The use of a species-specific PCR allows for rapid and reliable identification of P. ficus. The method is therefore not only of importance for the current study but also allows other researchers to identify P. ficus without having to be trained in morphological character analysis.
Contact person: Dr K Krüger.
At Marion Island (46°54'S, 37°45'E) two clearly defined growth periods were demonstrated, when the population decreased (1986-1997) and a subsequent stabilization (1997-2004). Concomitantly age at sexual maturity declined and pregnancy rates increased as the population declined, indicating a compensatory response. A relative increase in food availability, concomitant with the population decline, promoted earlier sexual maturity, higher weaning weights of pups, and more rapid growth of juveniles when population abundance was lower. These are important discoveries because changes in the growth and behaviour of apex predator populations are generally believed to reflect large-scale environmental changes and as such suggests that there has been a major alteration in the southern Indian Ocean biome where southern elephant seals from Marion Island spend most (about 75%) of their lives to procure their food. Foraging largely upstream from the island – usually at a mean depth of about 500 m – they are capable of travelling to foraging areas 3 000 km distant over the Antarctic continental shelf as well as to the Agulhas Plateau to the north, within sight of Cape Town. Continued collaboration with the Australian Antarctic Division, Alfred Wegner Institute for Polar and Marine Research, and the Hubbs-Sea World Research Institute resulted in satellite tracking of the elephant seals from Marion Island for periods up to 10 months and to depths of almost 2000 m! The project continues with major financial and logistical support from the Departments of Science and Technology (DST) and Environmental Affairs and Tourism (DEAT) within SANAP, and the National Research Foundation focus area programme.
Contact person: Prof MN Bester.
The nectars of hummingbird- and sunbird-pollinated plants differ in both sugar type and concentration. It has been commonly assumed that these differences can be attributed to selection pressure from birds, largely determined by their sugar preferences and digestive physiology. We examined the sugar preferences of whitebellied sunbirds Nectarinia talatala and broadtailed hummingbirds Selasphorus platycercus across a range of sucrose and equicaloric hexose solutions. Sunbirds and hummingbirds showed similar patterns in sugar preferences. Sunbirds preferred hexoses only when offered very dilute diets and showed slight but insignificant preference for sucrose when offered more concentrated diets. Hummingbirds showed slight (insignificant) hexose preference only on dilute diets at low ambient temperature. In contrast to previous studies, hummingbirds showed no significant sucrose preference, and the difference in results may have a methodological basis. Our findings do not support an ornithocentric explanation for nectar composition. Plant physiology and opportunist nectar feeders may also be influencing nectar sugars.
Contact person: Prof SW Nicolson.
African mole-rats show a range of social structure from strictly solitary to truly social representative species. Recent work by the mole-rat research group has further shown that patterns emerge with regard to the method of ovulation and degree of penile ornamentation exhibited by the male. Mole-rats that are strictly solitary tend to exhibit induced ovulation caused by the vaginal stimulation resulting from the male penis that is heavily ornamented with epidermal spines. However, the truly social aseasonally breeding species that occupy arid environments have females that exhibit spontaneous ovulation and the males lack ornamentation to the penis. All African mole-rats show reduced visual abilities and as a consequence their reliance on and response to light appears to be poor. The solitary species of mole-rats appear to be responsive to light and are able to entrain their locomotory activity rhythms to different photoperiods much more readily than the social species, which have more sloppy rhythms and a slower response period to entrainment. This pattern is further highlighted when the amount of c-fos expression (a protein expressed in the neuron in response to light stimulation) in the suprachiasmatic nucleus (the site of the biological clock) is measured in response to a light pulse presented at different times during a 24 hour period of darkness. Solitary species exhibit patterns of c-fos expression similar to that of normally sighted rodents, whereas the response in the social species is poor. The African mole-rats are proving to be ideal models to investigate trends in reproductive and physiological phenomena in relation to increasing sociality.
Contact person: Prof NC Bennett.
Whales tend to do things slowly - they are long-lived and relatively slow breeders and only reach maturity after about a decade. Research attempting to establish their population dynamics need to be conducted patiently. The notoriously variable marine environment also cause fluctuation around most measurements, which adds to the time needed to detect a change with a meaningful level of statistical significance. In the case of large whales, a decade or more of observations may be needed to get a rate of increase that is significantly different from zero. The Whale Unit has been particularly fortunate in being able to maintain continuity of survey effort for southern right whales over more than three decades, building on work first started when Dr Best was employed by the old Sea Fisheries Research Institute of the Department of Environmental Affairs. Aerial surveys carried out since 1969, and more especially by helicopter since 1979, have established that right whales on the southern coast of South Africa have been increasing at 7% per year for the last three decades. Photographs taken on the helicopter surveys enable the tracking of individual whales, so that details of 793 adult females, that have produced 2,298 calves, have been catalogued. These data allow a number of population parameters to be measured: the normal interval between calves is three years, for instance, and the average age at which a female first calves is about eight years. Survival rates for adult females are high — around 98,6% — and for juvenile females in their first year of life about 91,3%. Because this is an expanding population, however, survival may be unusually high. One of the most interesting possibilities would be to be able to continue these surveys until the population starts to approach some carrying capacity, and to see which parameters start to change and how. Estimates of historical carrying capacity have been made, based largely on historical catch levels, from which it is believed that southern right whales are now somewhere between 10 and 20% of their original numbers. But, besides the uncertainty surrounding such estimates, there is no guarantee that the environment in the 21st century is equivalent to that in the late 18th century, when right whales were unexploited. Right whale sightings in many old whaling grounds, such as off Namibia, Mozambique and southern Angola, are still infrequent, and whether the South African whales will extend their range into those waters, or whether they were separate populations that were even more depleted, is still unknown. For the moment it is perhaps sufficient to dwell on the suggestion that there are now more right whales in South African waters than are predicted to have been for about 150 years.
Contact person: Dr PB Best.
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