A new model for chemotactic signal transduction in Dictyostelium discoideum

Dictyostelium discoideum amoebae were employed to study the refractoriness and adaptation of the rapid (5sec) accumulation of actin in their Triton-insoluble cytoskeletons following stimulation with specific chemoattractants. Amoebae became refractory within 10sec for this response but no adaptation occurred during this period. Amoebae desensitized for one attractant were not desensitized for another and responses to stimulation with a mixture of attractants were approximately additive. The characteristics of these processes are compared to published studies of adaptation in other chemoattractant-induced responses and a new model for the chemotactic signal transduction pathway is formulated. We conclude that intracellular cGMP accumulation may be on a separate branch of the pathway from the actin response.     

Genetic effects of habitat contraction on Lumholtz's tree-kangaroo (Dendrolagus lumholtzi) in the Australian Wet Tropics

Lumholtz's tree-kangaroo (Dendrolaguslumholtzi) is one of two species oftree-kangaroo resident in the tropicalrainforests of north-eastern Australia. Thespecies is confined to the Wet Tropics region,with its distribution centred on the AthertonTablelands. While D. lumholtzi wasexposed to periodic large-scale climaticfluctuations during the Quaternary that haveeffectively acted as natural fragmentationevents, the species is currently under pressurefrom anthropogenic disturbance and habitatfragmentation. This study aimed to assess thelevel of genetic diversity in D.lumholtzi by examining hypervariablemicrosatellite loci and the control region ofmitochondrial DNA (mtDNA) in 21 individualsfrom a single 20 ha forest fragment, and from afurther 24 animals collected throughout theAtherton Tablelands. Results suggest that D. lumholtzi has relatively low levels ofgenetic diversity which is uniformlydistributed throughout the Atherton Tablelands;a pattern congruent with data from many othervertebrates endemic to the Australian WetTropics. It is suggested that Pleistoceneclimatic fluctuations, which resulted inlarge-scale rainforest contractions, haveimposed an ancient population bottleneck on theancestral D. lumholtzi population. Theapparent over-riding influence of thesenatural, historical effects on the geneticstructure of D. lumholtzi populations,will complicate attempts to assess the geneticimpact of current anthropogenic habitat lossand fragmentation.