Luc De Meester

Inbreeding and outbreeding depression in Daphnia

Egg-to-adult viability of sexual offspring in Daphnia magna is lower for selfed (average: 43.0%) than for outcrossed families (average: 74.7%). This suggests that intraclonal mating is not the rule in Daphnia populations. For a given family, hatching rate of eggs resulting from interpopulation crosses is lower than for intrapopulation crosses. This breakdown in hatching responses may result in the effective gene flow between Daphnia populations being severely reduced, offering an explanation for the apparent paradox of genetic differentiation of Daphnia populations in spite of efficient dispersal.Egg-to-adult viability of sexual offspring in Daphnia magna is lower for selfed (average: 43.0%) than for outcrossed families (average: 74.7%). This suggests that intraclonal mating is not the rule in Daphnia populations. For a given family, hatching rate of eggs resulting from interpopulation crosses is lower than for intrapopulation crosses. This breakdown in hatching responses may result in the effective gene flow between Daphnia populations being severely reduced, offering an explanation for the apparent paradox of genetic differentiation of Daphnia populations in spite of efficient dispersal.

An analysis of the phototactic behaviour of Daphnia magna clones and their sexual descendants

Daphnia magna clones, isolated from different natural populations, were inbred and crossbred. Some aspects of the heredity of the phototactic behaviour, quantified by an index, were analysed. Heritability in the broad sense was determined for several assemblages of clones through calculation of the clonal repeatability. Heritability sensu strictu was determined by offspring-parent regressions. Frequency distributions of a phototactic index of several inbred and crossbred families are given.The results indicate a significant contribution of additive genetic variance to the total phenotypic variance. Comparison of heritability estimates suggests an important genetic interaction component.Some implications of these results on the variability of vertical migration patterns in natural populations are discussed.

Life-histories and habitat selection in Daphnia - Divergent life-histories of D. magna clones differing in phototactic behaviour.

To test the hypothesis of co-adaptation of life histories and daytime vertical distribution (vertical migration behaviour) in Daphnia, life history characteristics were analysed for two positively, three negatively, and four intermediately phototactic Daphnia magna clones. Clones with different phototactic behaviour were found to have divergent life history strategies, with positively phototactic clones being good exploiters under the non-limiting conditions provided in the laboratory, i.e. low density (1 ind./1), high food concentration (6,5–7 105Scenedesmus cells/ml, restored daily) and high temperature (20° C). They realized a high intrinsic rate of increase at a small adult body size through rapid development, at a cost of producing small neonates. Negatively and intermediately phototactic clones had larger adult body sizes, and produced larger neonates that were more starvation-resistant than those of positively phototactic clones. Selection for high intrinsic rate of increase in intermediately phototactic clones was mediated through the production of large clutches.

The genetics of phototaxis in Daphnia magna: Existence of three phenotypes for vertical migration among parthenogenetic females

Using a cloning technique, we revealed the existence of three phototactically distinct types in Daphnia magna, viz. negatives, positives, and “gipsies”. The latter migrate continuously between a low and a high light intensity. The expression of this behaviour is genetically determined, although positives and negatives can reversibly switch into “gipsies”.It is proposed that two opposite genotypes exist, with the “gipsy” behaviour epigenetically induced. All juveniles behave more or less uniformly, irrespective of their origin. The phototactic responses develop rather late during ontogenesis.

An estimation of the heritability of phototaxis in Daphnia magna Straus

The phototactic responses of four clones of Daphnia magna were experimentally analysed. Broad-sense heritability of this behavioural character was estimated through an analysis of variance, and it was very high under the standardised experimental conditions of this study.SummaryThe phototactic responses of four clones of Daphnia magna were experimentally analysed. Broad-sense heritability of this behavioural character was estimated through an analysis of variance, and it was very high under the standardised experimental conditions of this study.

Evidence for sex-related differences in phototactic behaviour of Streptocephalus proboscideus (Crustacea: Anostraca)

The phototactic behaviour of adults of the Sudanese fairy shrimpStreptocephalus probiscideus was studied under laboratory conditions. Males were less negatively phototactic than females. This was also evident when colour filters were used. females only became little less negatively phototactic under yellow light, whereas males showed a strong positively phototactic response. The response to the positioning of a yellow filter was stronger than to the use of a red or blue filter for both sexes. The laboratory findings were compared with casual field observations onStreptocaphalus torvicornis that indicate differential vertical distribution between the sexes and a nocturnal vertical migration. Migratory behaviour with ascent starting at dusk is also predicted forS. proboscideus. This behaviour may reduce common stress factors in desert pools such as photodamage, visual predation pressure, and high surface temperatures.

Laboratory observations on the vertical distribution of a tropical pelagic flatworm ( Mesostoma sp.) in relation to satiation

Mesostoma sp., isolated from different lakes in Brazil, was taken to the laboratory and fed Moina sp. The vertical position of satiated and non-satiated individuals was monitored in 25 cm high columns exposed to natural sunlight. Phototactic responses were studied under standardised experimental conditions.Satiated individuals resided nearer to the bottom than non-satiated Mesostoma. Non-satiated individuals showed an irregular vertical distribution due to swimming behaviour, whereas satiated animals remained stationary.

Contributions to the Cladocera fauna from Papua New Guinea

Twenty-eight taxa of the Cladocera are identified in collections from Papua New Guinea, 17 being new records for New Guinea, bringing the total number of Cladocera taxa reported for this region to 39. Most of the taxa are circumtropical. One species (Sarsilatona papuana) is endemic to Papua New Guinea and northern Australia. The species list includes two species that are normally listed as Holarctic:Alonella nana andAlona rustica. Widespread genera such asDaphnia, Pleuroxus, Disparalona, Acroperus were strikingly absent from the Papua New Guinean material.