Molecular evolution of olfactomedin

Olfactomedin is a secreted polymeric glycoprotein of unknown function,originally discovered at the mucociliary surface of the amphibian olfactoryneuroepithelium and subsequently found throughout the mammalian brain. As afirst step toward elucidating the function of olfactomedin, itsphylogenetic history was examined to identify conserved structural motifs.Such conserved motifs may have functional significance and provide targetsfor future mutagenesis studies aimed at establishing the function of thisprotein. Previous studies revealed 33% amino acid sequence identity betweenrat and frog olfactomedins in their carboxyl terminal segments. Furtheranalysis, however, reveals more extensive homologies throughout themolecule. Despite significant sequence divergence, cysteines essential forhomopolymer formation such as the CXC motif near the amino terminus areconserved, as is the characteristic glycosylation pattern, suggesting thatthese posttranslational modifications are essential for function.Furthermore, evolutionary analysis of a region of 53 amino acids of fish,frog, rat, mouse, and human olfactomedins indicates that an ancestralolfactomedin gene arose before the evolution of terrestrial vertebrates andevolved independently in teleost, amphibian, and mammalian lineages.Indeed, a distant olfactomedin homolog was identified in Caenorhabditiselegans. Although the amino acid sequence of this invertebrate protein islonger and highly divergent compared with its vertebrate homologs, theprotein from C. elegans shows remarkable similarities in terms of conservedmotifs and posttranslational modification sites. Six universally conservedmotifs were identified, and five of these are clustered in the carboxylterminal half of the protein. Sequence comparisons indicate that evolutionof the N-terminal half of the molecule involved extensive insertions anddeletions; the C-terminal segment evolved mostly through point mutations,at least during vertebrate evolution. The widespread occurrence ofolfactomedin among vertebrates and invertebrates underscores the notionthat this protein has a function of universal importance. Furthermore,extensive modification of its N-terminal half and the acquisition of aC-terminal SDEL endoplasmic-reticulum- targeting sequence may have enabledolfactomedin to adopt new functions in the mammalian central nervoussystem.     

"Protistan" nomenclature: analysis and refutation of some potential objections

The nomenclature used for higher taxonomic categories of protista arose under the influence of the two-kingdom system, and is widely recognized as being evolutionarily misleading. The occurrence and promulgation of multiple, contradictory taxonomic systems have added to the confusion. Recently, we proposed a solution to this problem which involves identifying the largest taxa that are widely recognized to be monophyletic, and naming them using a commonly recognizable prefix and the suffix "protista". Thus, nomenclatural prejudice is eliminated, and, by keeping the system informal and thus circumventing the Linnaean system, our system remains flexible in accommodating new data yet should retain a great degree of stability. Here we discuss possible criticisms of our proposal, such as whether the introduction of more terms could lead to further taxonomic and nomenclatural instability, and conclude that our proposal remains a most reasonable solution to the problem of protistan nomenclature.     

Occurrence of microbodies in chloromonadophycean algae

Microbody-like organelles occur in the cytoplasm of two chloromonadophycean algae,Vacuolaria virescens Cienkowsky andGonyostomum semen Diesing. Microbodies ofVacuolaria andGonyostomum have a granular matrix which lacks a crystalloid core; they are often present in close association with elements of the endoplasmic reticulum. The occurrence of microbodies in other algae is briefly reviewed.     

Cytotaxonomy and breeding system of the genusBiscutella (Cruciferae)

Chromosome counts were determined for 46 populations ofBiscutella representing 28 taxa. The genus was found to contain diploid taxa with 2n = 12, 16 and 18, tetraploid taxa with 2n = 36 and hexaploid taxa having 2n = 54.B. laevigata L. s. l. consists of diploid and tetraploid populations which are poorly differentiated morphologically. TetraploidB. laevigata s. l. and hexaploidB. variegata Boiss. & Reuter (s. l.) are characterized by chromosomal instability. The variation in chromosome numbers and the occurrence of polyploidy is discussed in relation to the taxonomy of the genus. An investigation of the breeding system showed that most of the annual species were self-compatible and partly inbreeding and most of the perennial species self-incompatible and, therefore, outbreeding, while one annual species,B. cichoriifolia Loisel., showed both systems.