COMPARATIVE SPERMATOLOGY OF FOUR SYMPATRIC SPECIES OF SIPHONARIA (PULMONATA: BASOMMATOPHORA)

The fine structure of the modified sperm and spermatogenesisof four sympatric species of Siphonaria is described. The morphologyof the sperm of all species is very similar. The head, whichis about 6 µm long, is composed of a nucleus with fibrouschromatin capped by an acrosome (about 1 µm long) comprisedof an acrosomal pedestal and apical vesicle. The midpiece hasa mitochondrial derivative which surrounds a single glycogenhelix, posterior to which is a glycogen piece. Although differencesbetween each species exist, the value of sperm morphology forpurposes of taxonomy in this genus is questioned. Comparisonwith other basommatophorans however suggests that sperm morphologymay be of value at a higher taxo-nomic level. The morphologicalchanges that occur during spermatogenesis are similar to thosedescribed for other molluscs with modified sperm, except thatduring early spermiogenesis the Golgi body and smooth endoplasmicreticulum become highly developed. This proliferation of theSER and Golgi occurs at the same time as elongation of the spermatid.Throughout spermatogenesis, the germ cells are closely associatedwith a somatic cell which, because of structural similaritieswith the somatic cell of mammalian seminiferous epithelium,has been termed a Sertoli cell. After the spermatids have beenreleased from the Sertoli cells of the testis, maturation continuesin the hermaphrodite duct where the acrosome reaches its finalsize and glycogen accumulates in the glycogen compartment ofthe mid-piece. (Received 25 April 1990; accepted 1 September 1990)     

Some effects of photoperiod on larviposition and fresh weight-gain in Myzus persicae

ABSTRACT. The larviposition of adult apterous Myzus persicae previously entrained to LD 18:6 showed a marked rhythm both under LD 12:12 (on plants reared in LD 18:6) and under LL (on plants reared in LL). No rhythm was detectable in larviposition by adults reared in LL. Larviposition peaked towards the end of the photophase in LD 12:12. Fresh-weight gain also showed a circadian rhythm with the greatest weight increases during the photophase. Re-entrainment from LD 12:12 to DL 12:12 was not complete after 10 days. It is concluded that the changes in the light cycle did not affect the aphids through the plant.     

COMPARISON OF THE STRUCTURE OF THE PALLIAL TENTACLES OF SEVEN SPECIES OF SOUTH AFRICAN PATELLID LIMPET

The ultrastructure of the pallial tentacles of seven speciesof patellid limpet is described. The tip of most of the tentaclesexamined bears a crown of long cilia, whereas the shaft of thetentacles has small tufts (5–10 µm diameter) ofshorter cilia. Sections through the ciliated tufts show themto be composed of several cells, each bearing cilia. The ciliacontain 5–7 central microtubules and therefore do nothave the conventional 9 + 2 arrangement of microtubules. Nerveprocesses run from the base of each ciliated cell to a nervebundle in the centre of the tentacle, suggesting a sensory function.Estimates of densities of ciliated tufts suggest that the territoriallimpets (Patella cochlear and P. longicosta) have the greaternumber of tufts. Electron dense plate-like structures are foundin the centre of the pallial tentacles of Patella cochlear,P. longicosta, P. granularis, P. barbara and Helcion pruinosus.Each plate is about 0.2 µm wide and is surrounded by adouble membrane. It is suggested that these may play a rolein scattering or reflecting light and thus form part of thedermal light sensing ability of these animals. (Received 26 January 1987;     

The distribution and movement of apterous Myzus persicae on rapidly growing turnip plants

Abstract. 1. The spatial distribution of both adult apterous and nymphal M.persicae was related to the "age" of the leaf.
2. This distribution was bimodal and was maintained even on rapidly "ageing" leaves by frequent emigration.
3. The spatial distribution was also found to change between regions of a leaf, the densest populations occurring at the base of all leaves, but with the populations most evenly distributed on the young and old leaves.
4. The spatial distribution also changed during the nymphal period, the youngest nymphs being found predominantly on the lamina, the fourth instar and adults on the largest available veins.
5. On rapidly growing plants, individual M.persicae may move to a new feeding site as frequently as every 24–48 h.
6. Some of the causes and consequences of this frequent dispersal are discussed.     

VARIATIONS IN INFESTATIONS OF SIPHONARIA CAPENSIS AND S. CONCINNA(PULMONATA) BY LARVAL TREMATODES IN SOUTHERN AFRICA

The prevalence of digenean trematode parasites in Siphonariacapensis (Pulmonata) was examined in populations from 24 sites,and in S. concinna from 10 sites along the coast of southernAfrica from 1989 to 1991. Sporocysts containing cercariae werefound in the digestive gland of both species. To date, two typesof cercaria have been isolated from S. capensis and one typefrom S. concinna, these types possibly representing three distinctspecies of trematode. In approximately 30% of parasitized S.capensis and 10% of parasitized S. concinna the digestive glandwas completely destroyed. In both species of Siphonaria therewas considerable temporal and spatial variation in parasiteprevalence. At some locations up to 40% of S. capensis wereinfected whereas at others, no parasites were found. Prevalenceof trema-todes in S. concinna rarely exceeded 12%. At severalsites the prevalence of infestation within the limpets increasedwith increasing host size. (Received 8 June 1992; accepted 6 July 1992)     

Differential Effects of the Substituted Pyridazinone Herbicide Sandoz 9785 on Lipid Composition and Biosynthesis in Photosynthetic and Non-Photosynthetic Marine Microalgae: II. FATTY ACID COMPOSITION

The effect of the substituted pyridazinone, 4-chloro-5-(dimethylamino)-2-phenyl-3(2H)-pyridazinone(Sandoz 9785), on fatty acid synthesis in two photosyntheticspecies (Chroomonas salina and Nannochloropsis oculata) andone non-photosynthetic species (Crypthecodiniun cohnii) of marinemicroalgae were examined. Effects were more obvious in C. salinathan in C. cohnii or N. oculata. In C. cohnii the relative distributionamongst polyunsaturated fatty acids (PUFA) of radioactivityincorporated from ¹⁴C-acetate was not influenced by the herbicideto any great extent and no major changes in the fatty acid compositionof lipid fractions were observed. In C. salina, Sandoz 9785reduced the proportions of radioactivity recovered in 20: 5(n–3) and 22: 6 (n–3) of the phospholipid fraction.The distribution pattern of radioactivity in the fatty acidsof monogalactosyldiacylglycerol (MGDG) in this species was notgreatly affected by the herbicide whereas its presence significantlyreduced the proportions recovered in 18: 4 and 20: 1 in digalactosyldiacylglycerol(DGDG). The level of 20: 5 (n–3) in DGDG of C. salinawas increased from 4.0 to 19.8% by growing the algae in thepresence of Sandoz 9785. The only notable effect of the herbicideon the synthesis of PUFA in N. oculata was a reduction from18.3% to 11.3% of the proportion of radioactivity recoveredin 20: 5 in phospholipids. The herbicide had no effect on thedistribution of radioactivity in PUFA of galactolipids or onthe fatty acid composition of lipid fractions. The results arediscussed in relation to the potential role of galactolipidsand phospholipids as substrates for desaturations involved inthe formation of long chain PUFA in marine microalgae. Key words: Microalgae, herbicide, fatty acids