Base-compositional biases and the bat problem. I. DNA-hybridization melting curves based on AT- and GC-enriched tracers

We explored the interordinal relationships of mammals using DNA-DNA hybridization, with particular reference to the much-debated problem of whether the megabats and microbats are more closely related to each other than the megabats are to primates. To try to improve resolution when taxa are distantly related and the melting points of hybrids are low and difficult to distinguish, we increased the GC content of DNA by a fractionation method that used the same melting-point apparatus used in the hybridization studies. When we used GC-rich DNA as the tracer to make hybrids, the melting point of the self-hybrid shifted to a higher temperature as expected, but the behaviour of heterologous hybrids varied with the taxa being compared. When the melting point of the heterologous hybrid also shifted to a higher temperature so that the two compared taxa maintained the same or proportional distance, we called this 'following behaviour', because the heterologous hybrid made with GC-rich tracer 'followed' the GC-rich self-hybrid to higher temperatures. We also commonly saw anomalous behaviour, where the melting point of the heterologous hybrid shifted to a lower temperature when compared with an AT-rich hybrid. In these anomalous cases, the distance measured between the taxa increased markedly as a result of GC-enrichment, indicating that an underestimate of distance may have resulted from AT bias in DNA. This inference was supported by the finding that it was rare to observe a decrease in measured distance between taxa using GC-rich DNA, but very common to find an increase as would be expected from the generally higher AT contents of eutherian DNAs. Moreover, the most extreme cases, where distances changed most using GC-rich DNA, were usually those involving comparisons between taxa known to have the most extreme AT-biases among mammals, such as the megabats and rhinolophoid (including megadermatid) microbats. Our results show consistent underestimates of measured differences between eutherian taxa with extreme AT-biases

Genomic Evolution: Flying DNA

The extremely high AT content of bat DNA complicates the reconstruction of bat phylogeny from DNA sequence data, but may help throw light on genomic evolution

Quantitative comparison of the limits on visual spatial resolution set by the ganglion cell layer in twelve species of reef teleosts

A diverse range of retinal specializations are examined in twelve species of reef teleosts and estimates of the spatial resolution of neurons within the ganglion cell layer calculated using Matthiessen's ratio. Upper limits of between 4 and 27 cycles per degree were found to facilitate acute vision into frontal and eccentric space, utilizing temporal and nasal area centralis, respectively. Upper limits of between 3 and 20 cycles per degree were found in horizontal areas of acute vision across the retinal meridian. These areas are thought to be used for panoramic vision and may, in one species, indicate the relative importance of this region in comparison to the temporal area centralis. Comparisons are made between ganglion cell acuity and other morphological and behavioural acuities calculated in previous studies

Cone photoreceptors lacking oil droplets in the retina of the echidna, Tachyglossus aculeatus (Monotremata)

The echidna, Tachyglossus aculeatus, a monotreme mammal, is thought to possess an all-rod retina (O'Day, 1952). This study provides anatomical evidence for the presence of cone-like photoreceptors in the retina of the echidna. The cones, which constitute 10-15% of the photoreceptors, have all of the ultrastructural characteristics previously shown in the cones of placental mammals, and, like the cones of other animals (Blanks & Johnson, 1984), they bind peanut agglutinin. Unlike the cones of another monotreme, the platypus, the cones of the echidna retina do not possess oil droplets. Twin cones, pairs of cones in which there is no obvious difference in the size, shape, or ultrastructural features of the members of a pair, are common. The density of cones varies from 9000 cells/mm2in the superior periphery to 22,000 cells/mm2in the central retina. Nearest-neighbor analysis suggests that the cone mosaic in the echidna retina results from the presence of single and twin cones in a relatively regular array

Testable corollaries, a conceptual error, and neural correlates of Grush's synthesis

As fundamental researchers in the neuroethology of efference copy, we were stimulated by Grush's bold and original synthesis. In the following critique, we draw attention to ways in which it might be tested in the future, we point out an avoidable conceptual error concerning emulation that Grush seems to share with other workers in the field, and we raise questions about the neural correlates of Grush's schemata that might be probed by neurophysiologists

Some observations on the visual optics of kingfishers (Aves, Coraciformes, Alcedinidae)

We studied visual optics using ophthalmoscopy in six species of coraciform birds, five species from the family Alcedinidae (kingfishers) and one from the family Meropidae (bee-eaters). All six species had large angular separations between the two foveae of one eye (angle α); angle α was greater than 40° in all cases, the largest separation so far reported for any group of vertebrates. In all kingfishers, but not in the bee-eater, the plane containing the projections of both foveae in one eye (angle θ{symbol}) was rotated from the horizontal plane so that the projection of the monocular fovea was lower than the projection of the binocular fovea. Retinal ganglion cell isodensity maps were obtained for the sacred kingfisher (Halcyon sancta) and laughing kookaburra (Dacelo gigas). These maps were constructed in the usual way for the peripheral regions of the retina. For the high-density, multi-layered, central region of the retinal ganglion cell layer, we used a combined retinal wholemount-cross-sectional technique. It was discovered that the 'horizontal streak', formed by the elliptically shaped contours of retinal ganglion cell isodensity, deviated inferiorly at its nasal extent from the line of the foveae. This deviation had the same sign, and slightly greater magnitude, as the rotation (angle θ{symbol}) observed ophthalmoscopically when the eyes were in their primary position. Our new observations provide new insights into the functional significance of the bifoveate visual organization. In particular, the relationship between angle α and angle θ{symbol} suggests one strategy which could be used by kingfishers to maximize their visual capabilities when hunting from a perch above the substrate

Retinal topography in reef teleosts. I. Some species with well-developed areae but poorly-developed streaks

The retinal ganglion cell layer of five species of teleosts has been studied from Nissl-stained whole-mounts and the distribution of neuronal elements determined quantitatively. Isodensity contour maps of neurons in the ganglion cell layer revealed areas of high density (areae centrales) predominantly in the temporal retina, but other areae were also found in the nasal and dorso-nasal retina. Neuronal densities within the ganglion cell layer at the areae centrales ranged from 0.4 x 10(4) to 4.7 x 10(4) cells/mm2. Species that were found to lack a horizontal streak of high ganglion cell density appear to be those whose behaviour suggests they possess an interrupted view of the sand-water horizon and are 'enclosed' species. Concentric density contours around an area centralis seem to be associated with enclosed environments. The relationship between retinal topography and niche is also discussed