DISPARITY: MORPHOLOGICAL PATTERN AND DEVELOPMENTAL CONTEXT

Abstract:  The distribution of organic forms is clumpy at any scale from populations to the highest taxonomic categories, and whether considered within clades or within ecosystems. The fossil record provides little support for expectations that the morphological gaps between species or groups of species have increased through time as it might if the gaps were created by extinction of a more homogeneous distribution of morphologies. As the quantitative assessments of morphology have replaced counts of higher taxa as a metric of morphological disparity, numerous studies have demonstrated the rapid construction of morphospace early in evolutionary radiations, and have emphasized the difference between taxonomic measures of morphological diversity and quantitative assessments of disparity. Other studies have evaluated changing patterns of disparity across mass extinctions, ecomorphological patterns and the patterns of convergence within ecological communities, while the development of theoretical morphology has greatly aided efforts to understand why some forms do not occur. A parallel, and until recently, largely separate research effort in evolutionary developmental biology has established that the developmental toolkit underlying the remarkable breadth of metazoan form is largely identical among Bilateria, and many components are shared among all metazoa. Underlying this concern with disparity is a question about temporal variation in the production of morphological innovations, a debate over the relative significance of the generation of new morphologies vs. differential probabilities of their successful introduction, and the relative importance of constraint, convergence and contingency in the evolution of form.     

The Effect of Dietary Sterol on the Activity of Fatty Acid Desaturases Isolated from Tetrahymena setosa

Tetrahymena setosa has a nutritional requirement for micro amounts of sterol, a requirement which is also satisfied by relatively large amounts of either intact phospholipids or a mixture of unsaturated fatty acids normally found in these ciliates. Three microsomal fatty acyl-CoA desaturases have been isolated from T. setosa and partially characterized. These enzymes which can account for the formation of the majority of the ciliate's unsaturated fatty acids, include: a Δ9, a Δ12 and a Δ6 desaturase which catalyze the transformation of stearoyl-CoA to oleic acid, of oleoyl-CoA to linoleic acid and of linoleoyl-CoA to ?-linolenic acid, respectively. The stearoyl CoA desaturase required NAD (or NADP), ATP and free CoA; the Δ6 and Δ12 desaturases required NADP, but not ATP or CoA. Cellular levels of the three desaturases were highest in mid-logarithmic phase cells and lowest in stationary phase cells. In order to determine if there was a relationship between the sterol requirement and the ability of the organism to desaturate, T. setosa was grown in a synthetic medium supplemented with either cholesterol or a phospholipid which permits growth in the absence of cholesterol, or with both phospholipid and cholesterol. Cells grown with phospholipid alone had only half as much stearoyl-CoA and oleoyl-CoA desaturase activity as cells of identical culture age grown either on cholesterol alone or on cholesterol plus phospholipid.     

Expression of H-Y Antigen in the Guppy (Lebistes reticulatus)

Expression of a mammalian cross-reactive H-Y antigen on the surface of cells derived from the male guppy ( Lebistes reticulatus ) is demonstrated. This finding further establishes the evolutionary conservation of H-Y antigen among lower vertebrates and provides a basis for speculation on the possible evolutionary association between H-Y antigen and sex determination.     

The origin of metazoan development: a palaeobiological perspective

The rapid diversification of early Metazoa remains one of the most puzzling events in the fossil record. Several models have been proposed to explain a critical aspect of this event: the origin of Metazoan development. These include the origin of the eukaryotic cell, environmental triggers, key innovations or selection among cell lineages. Here, the first three hypotheses are evaulated within a phylogenetic framework using fossil, molecular and developmental evidence. Many elements of metazoan development are widely distributed among unicellular eukaryotes, yet only 3 of the 23 multicellular eukaryotic lineages evolved complex development. Molecular evidence indicates the lineage leading to the eukaryotic cell is nearly as old as the eubacterial and archaebacterial lineages, although the symbiotic events established that the eukaryotic cell probably occurred about 1.5 billion years ago. Yet Metazoa did not appear until 1000 to 600 million years ago (Myr), suggesting the origin of metazoan development must be linked to either an environmental trigger, perhaps an increase in atmospheric oxygen, or key innovations such as the development of collagen. Yet the first model fails to explain the unique appearance of complex development in Metazoa, while the latter fails to explain the simultaneous diversification of several ‘protist’ groups along with the Metazoa. A more complete model of the origin of metazoan development combines environmental triggering of a series of innovations, with successive innovations generating radiations of metazoan clades as lineages breached functional thresholds. The elaboration of new cell classes and the appearance of such developmental innovations as cell sheets may have been of particular importance. Evolutionary biologists often implicitly assume that evolution is a uniformitarian, time-homogeneous process without strong temporal asymmetries in evolutionary mechanisms, rate or context. Yet evolutionary patterns do exhibit such asymmetries, raising the possibility that such innovations as metazoan development impose non-uniformities of evolutionary process.     

In Vitro Studies on the Biosynthesis of the Surface Glycoprotein of Trypanosoma congolense1,2

Tritiated leucine, glucosamine, mannose, and galactose were incorporated into the variant specific surface glycoprotein (VSG) of Trypanosoma congolense in vitro. The uptake of the precursors is shown by SDS-polyacrylamide electrophoresis and fluorography, by assay of the radioactivity in immunoprecipitates obtained with specific antisera, and by the isolation of the labeled antigens by affinity chromatography on concanavalin A-sepharose and isoelectric focusing. The in vitro labeled VSG exhibits the same degree of microheterogeneity as that observed in the VSG isolated from trypanosomes grown in animals. Analysis of the incorporated sugars after hydrolysis of the glycoprotein showed that glucosamine and mannose were utilized in biosynthesis of the carbohydrate moiety directly whereas galactose was converted possibly to other intermediates before being incorporated into the antigen. Tunicamycin completely prevented the incorporation of the radiolabeled sugars into the surface glycoprotein. The unglycosylated VSG with a molecular weight of 47 kDa had completely lost its size heterogeneity.     

The Pathway of Assimilate Flow from Source to Sink in Urtica dioica L., Studied with14C under Ambient Atmospheric Conditions

The contribution of individual vascular bundles of the stemto the flow of assimilates from a selected source leaf to thesink regions was investigated inUrtica dioica L., a plant witha decussate leaf arrangement. Two homologous sets of eight vascularstrands were recognized, arranged in mirror symmetry in thestem internodes. In each set, three of the bundles were identifiedas traces of one leaf merging into the vascular system of thestem one node below the origin of the leaf. The main bundleof a stem-half bifurcates at each end of the internode intotwo subdominant bundles, which combine in the next but one nodeto form the dominant bundle again. Each set of vascular strandsalso contains two minor bundles which pass more or less withoutinterruption through the whole stem. The uppermost mature source leaf (leaf number 5 as counted fromthe tip) was exposed to¹⁴CO₂in a closed gas circuit. The concentrationof the carbon-labelled CO₂was maintained at the ambient CO₂levelto maintain the natural source strength of the leaf. By theend of the usual nocturnal dark phase, carbon from the sourceleaf had been imported predominantly by sink leaves of the sameorthostichy. Lesser, but significant amounts of radiocarbonwere also incorporated into the sink leaves of the adjacenttwo orthostichies via the marginal leaf traces. In spite ofthe junction of the vascular strands in the nodes and an interfascicularconnection of the stem bundles, randomization of the photosynthatesfrom individual leaves was minimal in the vascular system ofthe stem in the upward direction, and also low in the flux tothe roots. Substantial amounts of radioactivity were also foundin the lately-formed xylem elements of the vascular strandsand their interfascicular connections, indicating active secondarygrowth. Assimilate distribution; source–sink connections; Urtica dioica ; vascular architecture     

Development and localization of microsatellite markers for the sibling species Chironomus riparius and Chironomus piger (Diptera: Chironomidae)

Five variable microsatellite loci are reported for the nonbiting midge species Chironomus riparius and Chironomus piger. All loci show considerable intraspecific variation and species‐specific alleles, which allow to discriminate among the two closely related species and their interspecific hybrids, and to estimate genetic diversity within and between populations. Additionally, the loci were localized on C. riparius polytene chromosomes to verify their single copy status and investigate possible chromosomal linkage. The described markers are used in different studies with regard to population and ecological genetics and evolutionary ecotoxicology of Chironomus.