protoplast fusion a mechanism for achieving ge- netic transformation by joining two protoplasts or joining a protoplast with any of the components of another cell. Protostomia one of the two major subdivisions of the Bilateria. It contains the annelids, molluscs, and several smaller phyla. The protostome egg under- goes spiral cleavage (q.v.), and each of the cells pro- duced is determined to serve as the progenitor of a specific type of tissue.
The blastopore (q.v.) becomes the adult mouth, and the anus forms anew at the end of the gastrula sac. Compare with Deuteros- tomia. See Appendix A. prototroph 1. an organism that is able to subsist on a carbon source and inorganic compounds. For most bacteria, the carbon source could be a sugar; green plants use carbon dioxide. 2. a microbial strain that is capable of growing on a defined minimal me- dium; wild-type strains are usually regarded as pro- totrophs. Protozoa a kingdom erected in Cavalier-Smith’s classification to contain the majority of unicellular heterotrophic eukaryotes. Protozoa contain 80 S ri- bosomes, they lack chloroplasts, and their unduli- podia lack mastigonemes (q.v.). See Chromista. provirus 1. a virus that is integrated into a host cell chromosome and is transmitted from one cell gener- ation to another without causing lysis of the host. 2. more specifically, a duplex DNA sequence in an eukaryotic chromosome (corresponding to the ge- nome of an RNA retrovirus) that is transmitted from one cell generation to another without causing lysis of the host. Such proviruses are often associated with transformation of cells to the cancerous state.
See mouse mammary tumor virus. proximal toward or nearer to the place of attach- ment (of an organ or appendage). In the case of a chromosome, the part closest to the centromere. Prunus the genus that includes P. amygdalus, the almond; P. armeniaca, the apricot; P. avium, the cherry; P. domestica, the plum; P. persica, the peach. Przewalski horse (pronounced she-val-ski) a horse that once roamed the vast grasslands of central Asia, but now is found only in zoological parks. See Equus przewalskii. pseudoalleles genes that behave as alleles in the cis-trans test (q.v.) but can be separated by crossing over.
See Appendix A, 1949, Green and Green. pseudoautosomal genes See human pseudoau- tosomal region. Pseudocoelomata a subdivision of the Protosto- mia containing animals having a body cavity that is not lined with peritoneum. The space is formed by dispersion of mesenchyme. See Appendix A. pseudocopulation the mode of pollination in cer- tain orchids in which structures of the flower closely resemble a female insect, and the male insects at- tempting copulation serve to transfer pollen from one flower to another. pseudodiploid a condition in which the chromo- some number of a cell is the diploid number charac- teristic of the organism but, as a consequence of chromosomal rearrangements, the karyotype is ab- normal and linkage relationships may be disrupted.
pseudodominance the phenotypic expression of a recessive allele on one chromosome as a conse- quence of deletion of the dominant allele from the homolog. pseudoextinction disappearance of a taxon by vir- tue of its being evolved by anagenesis into another taxon. pseudogamy the parthenogenetic development of an ovum following stimulation (but not fertiliza- tion) by a male gamete or gametophyte; synony- mous with gynogenesis. pseudogene a gene bearing close resemblance to a known gene at a different locus, but rendered non- functional by additions or deletions in its structure that prevent normal transcription and/or translation. Pseudogenes are usually flanked by direct repeats of 10 to 20 nucleotides; such direct repeats are consid- ered to be a hallmark of DNA insertion.
Two classes of pseudogenes exist: (1) Traditional pseudogenes (as exemplified in the globin gene families) appear to have originated by gene duplication and been subse- quently silenced by point mutations, small inser- tions, and deletions; they are usually adjacent to functional copies and show evidence of being under some form of selective constraint for several millions of years after their formation. (2) Processed pseudo- genes lack introns, possess a remnant of a poly-A tail, are often flanked by short direct repeats, and are usually unassociated with functional copies; all of which suggests their formation by the integration into germ-line DNA of a reverse-transcribed pro- cessed RNA. Processed pseudogenes are rare in yeast and Drosophila, but common in mammals. For ex- ample, in humans there are 20 pseudogenes that are believed to have arisen from actin and beta tubulin
mRNAs. See Appendix C, 1977, Jacq et al.; hemoglo- bin genes, leprosy bacterium, orphons, processed gene. pseudohermaphroditism a condition in which an individual has gonads of one sex and secondary sex- ual characters of the other sex or of both sexes. Pseudohermaphrodites are designated as male or fe- male with reference to their sex chromosome consti- tution or the type of gonadal tissue present. Pseudomonas a genus of Gram-negative, motile bacteria that grow as free living organisms in soil, river water, marshes, and coastal marine, habitats and as pathogens of plants and animals. Geneticists often study strains of P. aeruginosa which are resis- tant to antibiotics and disinfectants and are responsi- ble for many infections in humans. This species is the predominant cause of mortality in patients with cystic fibrosis (q.v.). The bacterium is characterized by a single polar flagellum. Its genome contains 6.3 mbp of DNA and about 5,570 ORFs have been identified. Lysogeny (q.v.) is common in P. aerugi- nosa. The 6.2 mbp genome of P.
putida has also been sequenced and found to contain 5,420 ORFs. P. put- ida is a species with diverse metabolic and transport systems, which colonizes soil and water habitats, as well as the roots of crop plants. It has unusual abili- ties in breaking down aromatic and other toxic com- pounds, and it can tolerate heavy metals. See Appen- dix A, Bacteria, Proteobacteria; bacteriocins, bioremediation. pseudotumor an aggregation of blackened cells in Drosophila larvae, pupae, and adults of certain geno- types. Such “tumors” result from encapsulation dur- ing the larval stage of certain tissues by hemocytes and subsequent melanization of these masses. pseudouridine See rare bases. pseudovirion a synthetic virus consisting of the protein coat from one virus and the DNA from a foreign source.
See phenotypic mixing, reassortment virus. pseudo-wild type the wild phenotype of a mu- tant, produced by a second (suppressor) mutation. psilophytes early vascular plants that were transi- tional between algae and true plants. They had branches but no leaves. P site See translation. psoralens photosensitive cross-linking reagents that act on specific base-paired regions of nucleic acids. See trimethylpsoralen. P strain the paternally contributing strain of Dro- sophila in a P-M hybrid dysgenesis cross. P strains differ genetically from M strains in that they contain multiple P factors in their genomes. See hybrid dys- genesis, M strain, P elements. 32P suicide inactivation of phages due to the decay of radiophosphorus molecules incorporated into their DNA. psychosis a generic term covering any behavioral disorder of a far-reaching and prolonged nature. PTC abbreviation for phenylthiocarbamide or plas- ma thromboplastin component (both of which see).
PTK protein tyrosine kinase. See Src. pteridine See Drosophila eye pigments. pteridophytes the ferns, horsetails, club mosses, and other vascular spore-bearing plants. pteroylglutamic acid folic acid (q.v.). pterygote an insect belonging to a division that in- cludes all winged species. Some pterygotes fleas) are wingless, but they are believed to have been derived from winged ancestors. See aptery- gotes, Appendix A. PTTH prothoracicotropic hormone (q.v.). Pu abbreviation for any purine (e.g., adenine or guanine). See R3. Compare with Py. puff See chromosomal puff.
pufferfish See Takifugu rubripes and Tetraodon ni- groviridis. pulse-chase experiment an experimental tech- nique in which cells are given a very brief exposure (the pulse) to a radioactively labeled precursor of some macromolecule, and then the metabolic fate of the label is followed during subsequent incubation in a medium containing only the nonlabeled precur- sor (the chase). pulsed-field gradient gel electrophoresis a tech- nique for separating DNA molecules by subjecting them to alternately pulsed, perpendicularly oriented electrical fields. The technique has allowed separa- tion of the yeast genome into a series of molecules that ranged in weight between 40 and 1800 kilo- bases and represent intact chromosomes. See Appen- dix C, 1984, Schwartz and Cantor. pulvillus the last segment of the foot in an insect. It has a pad with a claw on either side.
punctuated equilibrium a term describing a pat- tern seen in the fossil record of relatively brief epi- sodes of speciation followed by long periods of spe- cies stability. Although this pattern conflicts with the pattern of gradualism (q.v.), no special develop- mental, genetic, or ecological mechanisms are re- quired to explain it. Both the gradual and the punc- tuated shifting equilibrium pattern can be simulated from mathematical equations that include only terms for random mutation, natural selection, and population size. See Appendix C, 1972, Eldredge and Gould; 1985, Newman et al. Punnett square the checkerboard method com- monly used to determine the types of zygotes pro- duced by a fusion of gametes from the parents.
The results allow the computation of genotypic and phe- notypic ratios. This matrix was first shown in a text- book by R. C. Punnett titled Mendelism and pub- lished in 1911. pupal period the developmental period between pupation and eclosion. puparium formation the formation of a pupal case by the tanning of the skin molted from the last-in- star larval insect. pupation that stage in the metamorphosis of the insect signaled by the eversion of the imaginal discs. purebred derived from a line subjected to inbreed- ing (q.v.). pure culture a culture that contains only one spe- cies of microorganism. See Appendix C, 1881, Koch. pure line a strain of an organism that is homozy- gous because of continued inbreeding. purine See bases of nucleic acids.
puromycin an antibiotic that, because of its struc- tural resemblance to the terminal aminoacylated adenosine group of aminoacyl tRNA, becomes in- corporated into the growing polypeptide chain and causes the release of incompleted polypeptide chains (which are terminated with a puromycin residue) from the ribosome. P value probability value. A decimal fraction showing the number of times an event will occur in a given number of trials. See probability of an event. Py abbreviation for any pyrimidine (e.g., thymine, cytosine, uracil). See Y. Compare with Pu.
pycnosis the contraction of the nucleus into a compact, strongly staining mass, taking place as the cell dies. pycocin See bacteriocin. pyloric stenosis the constriction of the valve be- tween the stomach and intestine, a congenital disor- der of high heritability. pyrenoid a small, round protein granule sur- rounded by a starch sheath found embedded in the chloroplasts of certain algae and liverworts. pyrethrins diterpene insecticides found in plant tissues. These compounds were first extracted from pyrethrum (chrysanthemum) flowers. pyridoxal phosphate the coenzyme of both amino acid decarboxylating enzymes and transami- nating enzymes.
pyrimidine See bases of nucleic acids. pyrimidine dimer the compound formed by UV irradiation of DNA whereby two thymine residues, or two cytosine residues, or one thymine and one cytosine residue occupying adjacent positions in the polynucleotide strand become covalently joined. See thymine dimer. pyronin Y a basic dye often used in cytochemistry. In 2 M magnesium chloride at pH 5.7, pyronin Y stains only undegraded RNA. See methyl green.
pyrrole molecules ring-shaped compounds con-taining one nitrogen and four carbon atoms that are components of porphyrin (q.v.) molecules.