ELECTRON MICROSCOPE STUDY OF A DIPLOGRAPTUS SPECIES

Specimens of Diplograptus sp. from the late Middle Ordovician Lebanon Limestone in central Tennessee have been isolated from the matrix and examined with transmission electron microscopes at 60 and 650 kv. The fine structure of the fusellar layer in the metasicula and in the thecae is a mesh formed from fibers. The cortical layer has two sublayers, one of which is a mesh that is closely similar to the mesh of the fusellar layer and the other is formed from two sets of long fibers that are oriented at moderate to high angles to each other. The prosicula outer wall is a loosely-woven, open mesh formed of fibers that are 0.080–0.165 microns in diameter. The spiral thread appears to be a band that is also formed of interwoven fibers. The longitudinal rod is a bundle of long fibers. Prosicula microstructures of this Diplograptus are significantly different from those in an Orthograptus of the O. quadrimucronatus (Hall) group.     

Evolution rampant: house mice on Madeira

House mice are extra-ordinary animals – extra -ordinary in the literal sense of that word. They are pests – but also a valued laboratory animal. They are generalized rodents – and successful in habitats from tundra to tropics and from sea-level to high altitudes. They have differentiated into a perplexity of taxa, yet differ little in their general morphology. They were long scorned by ecologists as recently arrived commensals, but are increasingly illuminating evolutionary processes as new techniques are applied to their study. Local forms, once valued only by taxonomists, are proving ever more interesting as their genetics are probed. In 1992, Mathias & Mira described the apparently unexciting characteristics of mice living on the two main islands of the Madeira group, 600 km west of continental Portugal. Then in 2000, Britton-Davidian et al. discovered that there were at least six chromosomal (Robertsonian) races on the main island. In the past decade, studies of molecular and mitochondrial genomes have shown an array of variables and posed questions about the origins and subsequent evolution of these island mice. In this issue of Molecular Ecology , Förster et al. report on the mtDNA haplotypes found on the island and in mainland Portugal, discuss the probable source of the island colonizers, and consider data which might give information about the timing of the colonizing event(s).     

Gene flow and hybridization following introduction of Mus domesticus into an established population

Seventy-seven house mice ( Mus domesticus ) from Eday, Orkney were released into a long-established population on the Isle of May (56^o 12'N) in 1982. Introduced allozymes, mt and Y-chromosome DNA, and Robertsonian chromosomes spread rapidly, reaching approximate stability c . 3 years later at frequencies different to those in both parental populations. The hybrid population was morphometrically intermediate between the two parents. This is a preliminary summary only; full details will be published elsewhere.     

The Effect of Ethylene on the Growth and Development of Wild-type and Mutant Arabidopsis thaliana (L.) Heynh

Some developmental responses of wild-type and mutant strainsof Arabidopsis thaliana to ethylene have been investigated (shootand root elongation, isodiametric cell expansion, plumular hookangle, ethylene biosynthesis, peroxidase activity). One suchmutant (eti 5) shows little or no response to ethylene at concentrationsup to 10000 µl 1^–1 in any test; the other mutantsshow reduced responses. The ability of germinated seedlingsto emerge through sand is directly proportional to their sensitivityto ethylene. It is suggested that this finding supports thehypothesis that increased ethylene production in response tomechanical impedance is involved, via its effects on stem thickeningand hook closure, in the emergence process. Arabidopsis thaliana (L.) Heynh., ethylene insensitive mutants, seedling emergence     

Physiological variation in the dog-whelk Nucella lapillus, L. either side of a cline in allozyme and karyotype frequencies

Genetic constitution in the intertidal gastropod Nucella lapillus influences variation in shell shape and growth rate which in turn are correlated with such habitat variables as wave action and temperature. We have investigated the response to hyperosmotic stress of samples from a cline in karyotype and allozyme frequencies and shell shape. Animals with a shell shape associated with environments where temperature and desiccation stress are important respond less to hyperosmotic stress than animals living in a high wave energy environment. With regard to the interaction between shell shape, physiology and habitat, animals with elongate shells associated with protected shores are shown to exhibit a reduced response to hyperosmotic stress compared to animals with a more spherical shell shape; this is discussed in relation to the production of an adaptive phenotype.