transvection a type of position effect (q.v.) in Dro- sophila where the ability of one allele of a gene to influence the activity of another on an opposite ho- molog requires that these two loci be synapsed. For example, a1/a1 and a2/a2 individuals both show mu- tant phenotypes, whereas the a1/a2 hybrids appear to be wild types. However, if a rearrangement is in- troduced that prevents the pairing of a1 and a2, the mutant phenotype is now expressed. For this reason, the phenomenon has also been called synapsis-depen- dent allelic complementation. The model that explains transvection assumes that each gene consists of a regulatory element (RE) and a nearby transcription unit (TU). In a1 the regulatory element is functional, while the transcription unit is defective. In a2 the reverse is true. In the hybrid (diagrammed here), the normal activity of the gene is restored because the a1 regulatory element can enhance the activity of the a2 transcription unit, provided that the synapsis of the homologs keeps RE close to TU. See Appendix C, 1945, Lewis; cis-trans configurations, somatic pairing.
trend an apparently directional evolutionary change in a character within a lineage, a chronocline. For example, many mammalian lineages show a ten- dency to increase in size over part of their evolution- ary history. See orthogenesis. Treponema pallidum the bacterium that causes the sexually transmitted disease syphilis. It is an obli- gate parasite of humans. Successful chemotherapy of syphilis began with Salvarsan (q.v.). After 30 years, it was replaced by penicillin. The complete genome sequence of T. pallidum has been recently completed and shown to contain 1.14 × 106 nucleotide base pairs within which are 1,041 ORFs.
See Appendix A, Prokaryotae, Bacteria, Spirochaetae; Appendix C, 1909, Ehrlich et al.. TRF2 the abbreviation for telomeric repeat-bind- ing factors 2 (q.v.). triallelic referring to a polyploid in which three different alleles exist at a given locus. In a tetraploid, A1A2A3A3 and A1A2A2A3 would be examples.
Triassic the most ancient of the Mesozoic periods, during which the first dinosaurs and mammals arose. Gymnosperms and ferns were the dominant plants. At the end of the Triassic, a mass extinction wiped out about 25% of all animal families. Pangaea began to break apart.
See continental drift, geologic time divisions. Tribolium a genus of flour beetles containing T. castaneum and T. confusum, genetically well-known species. At present about 125 mutations are known for castaneum and 75 for confusum. See Appendix A, Animalia, Insecta, Coleoptera; Hox genes. tricarboxylic acid cycle a synonym for citric acid cycle (q.v.). trichloroacetic acid a compound commonly used to precipitate proteins during biochemical extrac- tions.
trichocyst the minute protrusible, spindle-shaped organelles in the ectoplasm of many ciliates. trichogen cell a large cell that secretes the long, tapering hair of the insect bristle. A smaller tormo- gen cell forms the circular chitinous socket around the base of the bristle. trichogyne a receptive hypha extending from the ascogenous mycelium of a fungus such as Neuro- spora. Trichoplax adhaerans a marine species which represents the simplest animal phylum, the Placo- zoa. In culture these animals are a few millimeters in diameter and about 10 µm thick.
There are four cell types: (a) the flat covering cells of the dorsal epi- thelium, each with a single cilium, (b) ventral cylin- drical, ciliated epithelial cells, (c) ventral, non-cili- ated gland cells that secrete digestive enzymes, and (d) cells containing contractile fibers in their cyto- plasm which reside in the space between the dorsal and ventral epithelia. The ventral ciliated cells are adhesive and capable of resorbing materials that have been digested from the substrate.
Trichoplax has the smallest genome of all animals surveyed to date, 40 mb, and its haploid chromosome number is 6. A Pax gene has been isolated and shown to be expressed in the margin of Trichoplax. The gene product may specify the differentiation of the fiber cells that are regarded as protomesodermal cells. See Pax genes. triethylenethiophosphoramide See Thio-tepa.