INBREEDING DEPRESSION IN TWO MIMULUS TAXA MEASURED BY MULTIGENERATIONAL CHANGES IN THE INBREEDING COEFFICIENT

In mixed-mating plant populations, one can estimate the relative fitness of selfed progeny w by measuring the inbreeding coefficient F and selfing rate s of adults of one generation, together with F of adults in the following generation (after selection). In the first application of this multigenerational method, we estimated F and s for adults over three consecutive generations in adjacent populations of two annual Mimulus taxa: the outbreeding M. guttatus and the inbreeding M. platycalyx. This gave estimates of w for the last two generations. Although average multilocus selfing rates were high in both taxa (0.63 in M. guttatus; 0.84 in M. platycalyx), the relative fitness of selfed progeny averaged only 0.19 in M. guttatus and 0.32 in M. platycalyx. An alternative estimator for w that incorporates biparental inbreeding gave even lower estimates of w. These values are significantly below the 0.5 threshold thought to favor selfing, and show that partially selfing populations can harbor substantial genetic load. In accordance with the purging hypothesis, the more highly selfing M. platycalyx showed marginally lower inbreeding depression than M. guttatus in both years (P = 0.08). Inbreeding depression and selfing rates also varied among years in concert among taxa. Several sources of bias are discussed, but computer simulations indicate it is unlikely that w is biased downwards by linkage of marker loci to load loci.     

REPRODUCTIVE ASSURANCE MECHANISMS IN THREE TAXA OF THE MIMULUS GUTTATUS COMPLEX (SCROPHULARIACEAE)

The evolution of inbreeding in plants has often been attributed to selection for the ability to set seed in the absence of mates or pollinators. Mechanisms of reproductive assurance in five populations of mixed mating Mimulus guttatus, three populations of inbreeding M. platycalyx, and two populations of inbreeding M. nasutus were examined in a pollinator-free greenhouse. Reproductive assurance was manifested in all populations by autofertility, vegetative reproduction, or both. The inbreeding taxa had significantly greater levels of autofertility and less vegetative reproduction. Three modes of autofertility were identified: 1) due to corolla abscission only, occurring in three M. guttatus populations; 2) due to both corolla abscission and direct anther-stigma contact by curling of the lower stigmatic lobe into the anthers, occurring in two M. guttatus populations; and 3) direct stigma-anther contact by stigma curling alone prior to corolla abscission, found in each M. platycalyx and M. nasutus population. Stigma-anther distance and its interaction with stigma curling contributed to differences in autofertility among populations. Significant levels of intrapopulation quantitative genetic variation were found for seven of ten traits examined; average levels were similar between inbreeding and mixed mating populations. Genetic variation within populations for autofertility per se was not detected, but significant levels controlling stigma-anther distance were found in two M. guttatus populations. These results show that evolution of inbreeding by natural selection for reproductive assurance is possible in Mimulus, and illustrate the complex changes in floral dynamics and morphology it may involve.     

Tripeptides as selective inhibitors of src-SH2 phosphoprotein interactions

Summary This paper describes the synthesis of phosphorylated peptides of the general structural Ac-Tyr(PO₃H₂)-Glu-Xaa_NH₂, where Xaa represents a hydrophobic -amino acid of d-configuration. These peptides displayed activities in the micromolar range in inhibiting src-SH2 domain/epidermal growth factor receptor interactions.     

Overexpression of the influenza virus polymerase can titrate out inhibition by the murine Mx1 protein.

The murine Mx1 protein is an interferon-inducible protein which confers selective resistance to influenza virus infection both in vitro and in vivo. The precise mechanism by which the murine Mx1 specifically inhibits replication of influenza virus is not known. Previously, sensitive replication systems for influenza virus ribonucleoprotein, in which a synthetic influenza virus-like ribonucleoprotein is replicated and transcribed by influenza virus proteins provided in trans, have been developed. With these systems, the antiviral activity of the murine Mx1 protein was examined. It was found that continued expression of influenza polymerase polypeptides via vaccinia virus vectors can titrate out the inhibitory action of the murine Mx1 protein. This titration of inhibitory activity also occurs when the viral PB2 protein alone is overexpressed, suggesting that an antiviral target for the murine Mx1 polypeptide is the viral PB2 protein.     

Inactivation of maize phosphoenolpyruvate carboxylase by urea

Phosphoenolpyruvate carboxylase purified from leaves of maize (Zea mays, L.) is sensitive to the presence of urea. Exposure to 2.5 m urea for 30 min completely inactivates the enzyme, whereas for a concentration of 1.5 m urea, about 1 h is required. Malate appears to have no effect on inactivation by urea of phosphoenolpyruvate carboxylase. However, the presence of 20 mm phosphoenolpyruvate or 20 mm glucose-6-phosphate prevents significant inactivation by 1.5 m urea for at least 1 h. The inactivation by urea is reversible by dilution. The inhibition by urea and the protective effects of phosphoenolpyruvate and glucose-6-phosphate are associated with changes in aggregation state.     

Selective digestion of mouse metaphase chromosomes

Metaphase chromosomes prepared from colcemid-treated mouse L929 cells by non-ionic detergent lysis exhibit distinct heterochromatic centromere regions and associated kinetochores when viewed by whole mount electron microscopy. Deoxyribonuclease I treatment of these chromosomes results in the preferential digestion of the chromosomal arms leaving the centromeric heterochromatin and kinetochores apparently intact. Enrichment in centromere material after DNase I digestion was quantitated by examining the increase in 10,000xg pellets of the 1.691 g/cc satellite DNA relative to main band DNA. This satellite species has been localized at the centromeres of mouse chromosomes by in situ hybridization. From our analysis it was determined that DNase I digestion results in a five to six-fold increase in centromeric material. In contrast to the effect of DNase I, micrococcal nuclease was found to be less selective in its action. Digestion with this enzyme solubilized both chromosome arms and centromeres leaving only a small amount of chromatin and intact kinetochores.