Study of callus tissues from different parts ofNigella sativa (Ranunculaceae)

Summary Callus tissues were produced from leaf, stem and seed ofNigella sativa in modified Murashige and Skoog's medium. Depending on the origin of explants, the time of callus initiation and also the properties of the callus tissue were found to be different, especially regarding chromosome instability.The authors are grateful to Prof. A.K. Sharma for his advice and suggestions during the course of work.     

Induction of chromosome aberrations and chlorophyll mutations in plants by methylisocyanate (MIC) gas

Summary Seeds ofSolanum surattense Burm. f. collected from areas of Bhopal (India) affected by methylisocyanate gas showed chromosome aberrations in root cells, and growth retardation and chlorophyll mutation of seedlings, the frequencies of which varied from one locality to another.     

The role of disparity-sensitive cortical neurons in signalling the direction of self-motion

Movement of an observer through the environment generates motion on the retina. This optic flow provides information about the direction of self-motion, but only if it contains differential motion of elements at different depths. If the observer tracks a stationary object while moving in a direction different from his line of sight, the images of objects in the foreground and in the background move in opposite directions. We have found neurons in the cerebral cortex of monkeys that prefer one direction of motion when the disparity of a stimulus corresponds to foreground motion and prefer the opposite direction when the disparity corresponds to background motion. We propose that these neurons contribute a signal about the direction of self-motion.     

Use of an 125I-labelled DNA ligand to probe DNA structure

It no longer seems likely that DNA molecules in situ have a uniform conformation, represented by the classical B-form helix. For example, recent structural studies have shown that in certain conditions DNA can have a left-handed (so-called Z-form) helix, and have revealed extensive sequence-dependent variations of B-DNA helical parameters. Such sequence-dependent variations in DNA structure can be investigated in solution with reagents that bind to DNA in a conformation-dependent manner, and cut one or both strands of the double-helix at the site of binding, as, for example, has been shown for the endonuclease DNase I3. We describe here a simple way to endow a DNA-binding ligand with the ability to cleave DNA--labelling with 125I. The radiochemical damage associated with 125I decay induces a double-stranded DNA break. Using this technique we have shown that a sequence of four consecutive A X T base pairs is a necessary, but not sufficient, condition for strong binding to DNA of the bis-benzamide Hoechst 33258--presumably the other important factor is the conformation of the double-helix at the site of the (A/T)4 sequence. We suggest 125I-Hoechst 33258 may be a useful new probe of DNA structure.     

Eutrophication causes speciation reversal in whitefish adaptive radiations

Species diversity can be lost through two different but potentially interacting extinction processes: demographic decline and speciation reversal through introgressive hybridization. To investigate the relative contribution of these processes, we analysed historical and contemporary data of replicate whitefish radiations from 17 pre-alpine European lakes and reconstructed changes in genetic species differentiation through time using historical samples. Here we provide evidence that species diversity evolved in response to ecological opportunity, and that eutrophication, by diminishing this opportunity, has driven extinctions through speciation reversal and demographic decline. Across the radiations, the magnitude of eutrophication explains the pattern of species loss and levels of genetic and functional distinctiveness among remaining species. We argue that extinction by speciation reversal may be more widespread than currently appreciated. Preventing such extinctions will require that conservation efforts not only target existing species but identify and protect the ecological and evolutionary processes that generate and maintain species.