Evidence for Dobzhansky-Muller incompatibilites contributing to the sterility of hybrids between Mimulus guttatus and M. nasutus.

Both chromosomal rearrangements and negative interactions among loci (Dobzhansky-Muller incompatibilities) have been advanced as the genetic mechanism underlying the sterility of interspecific hybrids. These alternatives invoke very different evolutionary histories during speciation and also predict different patterns of sterility in artificial hybrids. Chromosomal rearrangements require drift, inbreeding, or other special conditions for initial fixation and, because heterozygosity per se generates any problems with gamete formation, F1 hybrids will be most infertile. In contrast, Dobzhansky-Muller incompatibilities may arise as byproducts of adaptive evolution and often affect the segregating F2 generation most severely. To distinguish the effects of these two mechanisms early in divergence, we investigated the quantitative genetics of hybrid sterility in a line cross between two members of the Mimulus guttatus species complex (M. guttatus and M. nasutus). Hybrids showed partial male and female sterility, and the patterns of infertility were not consistent with the action of chromosomal rearrangements alone. F2 and F1 hybrids exhibited equal decreases in pollen viability (> 40%) relative to the highly fertile parental lines. A large excess of completely pollen-sterile F2 genotypes also pointed to the segregation of Dobzhansky-Muller incompatibility factors affecting male fertility. Female fertility showed a pattern similarly consistent with epistatic interactions: F2 hybrids produced far fewer seeds per flower than F1 hybrids (88.0 +/- 2.8 vs. 162.9 +/- 8.5 SE, respectively) and either parental line, and many F2 genotypes were completely female sterile. Dobzhansky-Muller interactions also resulted in the breakdown of several nonreproductive characters and appear to contribute to correlations between male and female fertility in the F2 generation. These results parallel and contrast with the genetics of postzygotic isolation in model animal systems and are a first step toward understanding the process of speciation in this well-studied group of flowering plants.

Pollen discounting and the evolution of selfing in Arenaria uniflora (caryophyllaceae).

Although most models of mating system evolution assign a central role to the male transmission advantage of selfing genotypes, empirical data on the male fitness consequences of increased self-pollination are still uncommon. Here, I use measures of pollen import and export by focal plants in genotyped arrays to investigate the effects of floral morphology and pollination environment on self and outcross male function. Plants from an autogamous population of Arenaria uniflora (Caryophyllaceae) exhibit complete pollen discounting relative to closely related outcrossers, as do morphologically intermediate F1 hybrids between the two populations. However, the low cumulative male fitness of hybrids probably results from reduced pollen number or competitive ability, rather than a nonlinear relationship with floral morphology. When surrounded by selfers, plants from the outcrosser population self-fertilize at nearly the same rate as selfers (>80%), but have much lower self male fitness due to reduced fruit set. Because outcross siring success is also extremely low (<8%) in this treatment, these mate-limited outcrossers are at male fitness disadvantage to both pseudocleistogamous selfers and nonlimited outcrossers. The relative male fitness of plants with different mating systems appears dependent on the ecological context, as well as on morphological trade-offs.

Toxin compartmentation and delivery in the Cnidaria: the nematocyst's tubule as a multiheaded poisonous arrow.

With the aid of dialysis and ion exchange chromatography, a new polypeptide toxin was purified from the tentacles of the Mediterranean jellyfish Rhopilema nomadica. The amino acid sequence of the N-terminal segment of the new toxin revealed that it is a phospholipase A2 (PhA2) toxin closely resembling those previously isolated from reptile and hymenopterous venoms. The occurrence of a PhA2 toxin in the jellyfish tentacles may explain both their local (dermanecrotic) and systemic (cardiac-respiratory) effects upon human envenomation. We used an antibody raised against the above toxin as a probe to explore, for the first time, the site of toxin allocation in cnidarian nematocysts and its morphological route of delivery. Our immunocytochemical approach revealed that the toxin is stored on the outer ("cytoplasmic") surface of the inverted tubule folded in the capsule of the resting nematocyst. During discharge the toxin is translocated to the internal surface surrounding the lumen of the everting tubule, and its delivery via extended spirally arrayed barbs is apparently propelled by the high hydrostatic pressure of the capsule. This is a unique example where subcellular translocation and transfer of a polypetide is driven by mechanical forces.

Review of the literature: severe hyperphosphatemia.

A patient with a markedly elevated serum phosphorus level (23.9 mg/dL) is described, followed by a brief review of severe hyperphosphatemia. Elevated serum phosphorus levels may be artifactual or true. True hyperphosphatemia is usefully subdivided according to (a) whether phosphorus is added to the extracellular fluid from a variety of exogenous or endogenous sources, or (b) whether the urinary excretion of phosphorus is reduced from either decreased glomerular filtration or increased tubular reabsorption. Severe hyperphosphatemia, defined herein as levels of 14 mg/dL or higher, is almost invariably multifactorial--usually resulting from addition of phosphorus to the extracellular fluid together with decreased phosphorus excretion. The hyperphosphatemia of the patient described herein appeared to result from a combination of dietary phosphorus supplementation, acute renal failure, acute pancreatitis, and ischemic bowel disease, complicated by lactic acidosis.

Combined computed tomographic and radionuclide imaging in the long-term follow-up of children with primary intra-axial intracranial neoplasms.

Radionuclide and computed tomographic (CT) scanning was performed for the long-term follow-up of 63 patients who had been treated for primary intracranial central nervous system tumors. This group included 23 children with supratentorial lesions and 40 with infratentorial lesions. The results of imaging were correlated with clinical assessment and the results of cytologic evaluation of the cerebrospinal fluid and, when available, surgical or autopsy findings. The sensitivity, specificity, and positive predictive value of both CT and radionuclide scanning were evaluated for each type of tumor. These two modalities play a complementary role in the long-term follow-up of children with primary intra-axial neoplasms.

Mineralization of elastin extracted from human aortic tissues.

Elastin was isolated from human aortic tissues by extraction with formic acid. The aortas came from individuals of both sexes with ages ranging from 18 to 67 years. The elastin was treated to remove pre-existing nuclei for hydroxyapatite and mineralized in vitro by incubation in a solution metastable to hydroxyapatite. The results showed an increased rate of mineralization in vitro as the age of the individual from whom the aorta was obtained increased. The lag period in mineral formation seen with elastin samples obtained from individuals below the age 40, disappeared with older elastin. Amino-acid analysis of the elastin samples confirmed earlier results by indicating an increase in the more polar amino acids in the older elastin samples. Exposure of the extracted elastin samples to several proteolytic enzymes including elastase confirmed the identity of the material as elastin.