We report a high-quality draft of the genome sequence of the grey, short-tailed opossum (Monodelphis domestica). As the first metatherian ('marsupial') species to be sequenced, the opossum provides a unique perspective on the organization and evolution of mammalian genomes. Distinctive features of the opossum chromosomes provide support for recent theories about genome evolution and function, including a strong influence of biased gene conversion on nucleotide sequence composition, and a relationship between chromosomal characteristics and X chromosome inactivation. Comparison of opossum and eutherian genomes also reveals a sharp difference in evolutionary innovation between protein-coding and non-coding functional elements. True innovation in protein-coding genes seems to be relatively rare, with lineage-specific differences being largely due to diversification and rapid turnover in gene families involved in environmental interactions. In contrast, about 20% of eutherian conserved non-coding elements (CNEs) are recent inventions that postdate the divergence of Eutheria and Metatheria. A substantial proportion of these eutherian-specific CNEs arose from sequence inserted by transposable elements, pointing to transposons as a major creative force in the evolution of mammalian gene regulation.

From the assumptiox: that the microscopic behavior of the liquid in an unsaturated porous medium is controlled by the phYSlcallaws of surface tension and viscous flow, differential equations governing the macrosc:opic flow it; su~h a .medium ar: deduced. ~o sp~cial pore-shape assumptions are required, but one topolOgIcal approXImatlOn IS needed; I.e., that neither Isolated drops nor isolated bubbles occur. Several nonessential simplifying assumptions are used; i.e., that the macroscopic properties of the medium the character of the liquid, and the pressure of the gas are independent of position, time, and direction.' The macroscopic equations are obtained in a fully reduced form, permitting comparison between two mediaor between two flow systems-that differ only by scaling factors. A no:vel feature of this calculation is its prediction that the liquid-transmission and liquid-capacity proper~es of :m unsaturated me?ium will exhi~it hysteresis in their dependences upon tile liquid-gas pressure dlfferentlai, p. The properties of the medium depend upon the pressure history but are invariant to monotonic timescale distortions of that history. Such time-invariant functionals have been termed by the authors "hysteresis functions," symbolized by the subscript, H, e.g. FH(P). Although methods for measuring an~ ~escribin~ the char:,-cteristics of sl.'ecific "h~steresis functions" have not yet been developed, the general vabdlty of thiS analysIs can be studIed expenmentally bv testing predictions that are contained in the reduced variables. .

A numerical model is explored which simulates frost heave in saturated, granular, air-free, solute-free soil. It is based on equations developed from fundamental thermomechanical considerations and previous laboratory investigations. Although adequate data are lacking for strict experimental verification of the model, we note that simulations produce an overall course of events together with significant specific features which are familiar from laboratory experience. Simulated heave histories show proper sensitivities in the shapes and orders of magnitude of output responses and in the relations between crucial factors such as heave rate, freezing rate, and overburden. ed mathematical models, attempting to reduce uncertainty by including superior representations of relevant physics and thermodynamics. The overall offerings in this domain and many of the underlying concepts are reviewed by O'Neill [1983]. For further discussion of some of the basic concepts and equations adduced below, the reader is referred to this review and to the textbook treatment by Miller [1980]. Among responses to the challenge is what has been called 281 282 O'NEILL AND MILLER: FROST HEAVE MODEL the rigid ice model of frost heave. This model is comprised of a set of macroscopic equations which result from an analysis of the microscopic features of the formation of ice from water within the pores of a soil [Miller, 1978]. At present, the physical formulation is limited to solute-free, air-free soils of negligible compressibility. A simplified set of equations applicable only to very simple quasi steady states has been solved by Miller and Koslow [1980]. A strategy for obtaining numerical solutions of the full set of equations for simple boundary conditions has also been developed [O'Neill and Miller, 1982]. In this paper we expand on the physical basis of that formulation and its mathematical expression and implementation. Improved tactics have produced a more flexible model so that its behavior can be better explored. We report examples of the results of such explorations below. PHASE EQUILIBRIAConcepts of phase equilibrium applicable at a microscopic level are most familiar in the context of water and air in (ice-free) porous media. It is useful to paraphrase these to establish a rationale for the treatment of ice-water equlibria in air-free soil. The adjective microscopic, as used in this paper, refers to scales that are large relative to the dimensions of a single molecule or the length of chemical bonds but small compared to the scale of observation. Relevant scales are those comparable to the dimension of grains, interstices between them, and even "long-range" surface adsorption force fields. A well-known example of the last is the virtual force field that arises if there is a diffuse electrical double layer at a grain surface. In this discussion, no particular model of surface adsorption will be invoked. Instead, we merely use a more general proposition that for whatever reasons, liquid water very close to a grain surface is attract...

After infection with the digenetic trematode Echinostoma paraensei, hemolymph of the snail Biomphalaria glabrata contains lectins comprised of 65-kDa subunits that precipitate polypeptides secreted by E. paraensei intramolluscan larvae. Comparable activity is lacking in hemolymph of uninfected snails. Three different cDNAs with sequence similarities to peptides derived from the 65-kDa lectins were obtained and unexpectedly found to encode fibrinogen-related proteins (FREPs). These FREPs also contained regions with sequence similarity to Ig superfamily members. B. glabrata has at least five FREP genes, three of which are expressed at increased levels after infection. Elucidation of components of the defense system of B. glabrata is relevant because this snail is an intermediate host for Schistosoma mansoni, the most widely distributed causative agent of human schistosomiasis. These results are novel in suggesting a role for invertebrate FREPs in recognition of parasite-derived molecules and also provide a model for investigating the diversity of molecules functioning in nonself-recognition in an invertebrate.

No abstract