A Jurassic mammal from South America

The Jurassic period is an important stage in early mammalian evolution, as it saw the first diversification of this group, leading to the stem lineages of monotremes and modern therian mammals. However, the fossil record of Jurassic mammals is extremely poor, particularly in the southern continents. Jurassic mammals from Gondwanaland are so far only known from Tanzania and Madagascar, and from trackway evidence from Argentina. Here we report a Jurassic mammal represented by a dentary, which is the first, to our knowledge, from South America. The tiny fossil from the Middle to Late Jurassic of Patagonia is a representative of the recently termed Australosphenida, a group of mammals from Gondwanaland that evolved tribosphenic molars convergently to the Northern Hemisphere Tribosphenida, and probably gave rise to the monotremes. Together with other mammalian evidence from the Southern Hemisphere, the discovery of this new mammal indicates that the Australosphenida had diversified and were widespread in Gondwanaland well before the end of the Jurassic, and that mammalian faunas from the Southern Hemisphere already showed a marked distinction from their northern counterparts by the Middle to Late Jurassic.     

Comparative assessment of large-scale data sets of protein-protein interactions

Comprehensive protein protein interaction maps promise to reveal many aspects of the complex regulatory network underlying cellular function. Recently, large-scale approaches have predicted many new protein interactions in yeast. To measure their accuracy and potential as well as to identify biases, strengths and weaknesses, we compare the methods with each other and with a reference set of previously reported protein interactions.     

Spontaneous epigenetic variation in the Arabidopsis thaliana methylome

Heritable epigenetic polymorphisms, such as differential cytosine methylation, can underlie phenotypic variation. Moreover, wild strains of the plant Arabidopsis thaliana differ in many epialleles, and these can influence the expression of nearby genes. However, to understand their role in evolution, it is imperative to ascertain the emergence rate and stability of epialleles, including those that are not due to structural variation. We have compared genome-wide DNA methylation among 10 A. thaliana lines, derived 30 generations ago from a common ancestor. Epimutations at individual positions were easily detected, and close to 30,000 cytosines in each strain were differentially methylated. In contrast, larger regions of contiguous methylation were much more stable, and the frequency of changes was in the same low range as that of DNA mutations. Like individual positions, the same regions were often affected by differential methylation in independent lines, with evidence for recurrent cycles of forward and reverse mutations. Transposable elements and short interfering RNAs have been causally linked to DNA methylation. In agreement, differentially methylated sites were farther from transposable elements and showed less association with short interfering RNA expression than invariant positions. The biased distribution and frequent reversion of epimutations have important implications for the potential contribution of sequence-independent epialleles to plant evolution.     

Saturation models: A brief survey and critique

The authors review and compare the papers in this issue on saturation and logistic growth with special emphasis on the structure of the underlying probability models. Influence diagrams are used to illustrate the dependence and independence assumptions and the way in which uncertainty is reflected in key model assumptions and parameters characterizing the models.     

Induction of hair growth by implantation of cultured dermal papilla cells

Mammalian hairs are formed by differentiation and keratinization of cells produced in the epidermal matrix (Figs 3, 4). Using the rodent vibrissa follicle as a model, transplantation studies have shown that the dermal papilla, a discrete population of specialized fibroblasts, is of prime importance in the growth of hair. Papillae induce hair growth when implanted into follicles and can interact with skin epidermis to form new hair follicles. When grown in culture, papilla cells display singular morphological and behavioural characteristics compared with connective tissue cells from other skin sources. We report here that serially cultured adult papilla cells can induce the growth of hair when implanted into follicles which otherwise would not grow hairs. This finding presents an opportunity to characterize properties distinguishing the papilla cell population from other skin fibroblasts, and, more specifically, those which control hair growth. The eventual application of this work to human hair replacement techniques can also be envisaged.     

Distribution of thyrotropin releasing hormone (TRH), alpha-melanocyte-stimulating hormone (alpha-MSH) and somatostatin in the skin of the green frog (Rana esculenta)]

High concentrations of Thyrotrophin-Releasing Hormone (TRH) have been found in the dorsal skin of the Frog Rana esculenta, lower levels being measured in the ventral skin. alpha-MSH and somatostatin were undetectable in these tissues. Nor was TRH detected in the blood of these animals. The concentration of somatostatin in the pancreas was similar to that of the hypothalamus and twice or one hundred times higher than in the intestine or stomach respectively.