Somatic sex determination
nation occurs in developing B lymphocytes (q.v.). V (D) J recombination (q.v.) results in the joining of any one of many variable Ig gene segments to one of a few constant segments. The arrangement that re- sults is different in the cells that produce the anti- body from all other somatic cells and germ cells. Compare with somatic crossing over. somatic sex determination the genetic and devel- opmental process that specifies sexual identity and sex-specific development of the somatic cells of an organism. Compare with germ line sex determination. See sex determination. somatoclonal variation the appearance of new traits in plants that regenerate from a callus in tissue culture. Some of the variations represent single nu- cleotide changes; others involve chromosomal trans- locations, losses, or duplications. Much of the varia- tion occurs during tissue culture, rather than as a result of unmasking the variation present in the par- ent plant. See gametoclonal variation. somatocrinin growth hormone releasing hormone. See human growth hormone. somatomammotropin See human growth hormone.
somatostatin a polypeptide hormone that stimu- lates the release of growth hormone by the pituitary and of insulin and glucagon by the pancreas. The gene for this 14 amino acid peptide was chemically synthesized, spliced into a plasmid, and cloned in E. coli. The transformed bacteria secreted somatostatin, and this led to the first commercial production of a synthetic human protein. See Appendix C, 1977, Itakura et al.; human growth hormone. somatotropin See human growth hormone. sonicate subject (a biological sample) to ultrasonic vibration so as to fragment the cells, macromole- cules, and membranes. A biological sample that has been subjected to such treatment. Sonic hedgehog (Shh) the vertebrate homolog of the Drosophila gene hedgehog (q.v.). In humans Shh has been mapped to 7q36. Shh encodes a signal pro- tein that controls the patterning of the ventral neural tube, the anterior-posterior limb axis, and the ven- tral somites. Sordaria fimicola an ascomycete fungus often used in studies of gene conversion (q.v.). sorting See protein sorting. sorting signals segments several amino acids long in proteins that target them to their final destina- tions. For example, the nuclear-targeting signal is four to eight amino acids in length, and it contains several positively charged residues and usually one or more prolines. The targets can occur at a variety of places in different nuclear proteins. The peroxisomal-target- ing signal is usually located near the carboxy termi- nus of the protein, and it consists of three amino acids (serine, lysine, and leucine). There are also tar- geting signals that cause specific proteins to be re- tained in the ER or Golgi apparatus or to be targeted to lysosomes. In animals like Caenorhabditis or Dro- sophila, about 5% of the proteins contain sorting sig- nals that direct them to mitochondria. However, in plants, like Arabidopsis, nearly 25% of the nuclear genes direct encoded proteins to either chloroplasts or mitochondria. See protein sorting. SOS boxes the operator sequences in E. coli DNA that are recognized by a repressor called the LexA protein.
This protein represses several loci involved in DNA repair functions. See regulon, SOS response. SOS response a cellular response to extensive DNA damage in which certain genes, called SOS genes, are sequentially activated in order to repair the damaged DNA. In E. coli about 20 such genes have been identified, including lexA, whose product normally represses the SOS genes. The remainder include genes such as uvrA, uvrB, recA, sulA, and umuC. Among the functions assigned to these genes are recombinational repair, nucleotide excision re- pair, inhibition of cell division, and error-prone re- pair. Normally, SOS genes are repressed by the LexA protein, which binds to operator sequences, called SOS boxes (q.v.), upstream of each of these genes. When DNA is damaged, single-stranded re- gions become exposed, and these interact with the RecA protein (q.v.) to form a complex (RecA*), which acquires protease activity and facilitates the cleavage of the LexA repressor (q.v.). The cleaved LexA protein is unable to bind DNA, thus allowing the SOS genes to be de-repressed.
When the DNA has been repaired, RecA becomes inactivated, LexA is no longer cleaved and accumulates in the cell, and the SOS genes are shut down. In addition to accu- rate, error-free repair, the SOS response also induces DNA repair that leads to mutagenesis, i.e., error- prone or mutagenic repair, in which the DNA tem- plate is read with reduced fidelity. Thus, in the pres- ence of extensive DNA damage the cells survive, al- beit at the cost of introducing some errors in their DNA. The acronym, SOS, is derived from “Save Our Souls,” the Morse code signal given by ships in danger and conveys that this is an emergency re- sponse in cells that are in danger of dying. See Ap- pendix C, 1967, Witkins. South African clawed frog See Xenopus.
Southern blotting a technique, developed by E. M. Southern, for transferring electrophoretically re- solved DNA segments from an agarose gel to a nitro- cellulose filter paper sheet via capillary action. Sub- sequently, the DNA segment of interest is probed with a radioactive, complementary nucleic acid, and its position is determined by autoradiography. A similar technique, referred to as northern blotting, is used to identify RNAs. For example, an electropher- ogram containing a multitude of different mRNAs could be probed with a radioactive cloned gene. In cases where proteins have been separated electro- phoretically, a specific protein on an electrophero- gram can be identified by the western blotting proce- dure. In this case, the probe is a radioactively labeled antibody raised against the protein in question. See Appendix C, 1975, Southern; 1977, Alwine et al.; probe. soybean See Glycine max.
spaced training referring to experiments on mem- ory during which repeated training sessions are given with short rest intervals between the sessions. The term massed training refers to repeated training ses- sions with no such rest intervals. Comparisons of the results from both types of experiments have shown that the memory generated immediately after train- ing is short lived and disruptable. During a rest pe- riod such short-term memory (STM) is consolidated into a longer-lasting, more stable, long-term memory (LTM). For the consolidation of STM into LTM, the syntheses of the protein products of specific genes are required. See CREBs. spacer DNA untranscribed segments of eukaryotic and some viral genomes flanking functional genetic regions (cistrons). Spacer segments usually contain repetitive DNA. The function of spacer DNA is not presently known, but it may be important for synap- sis. See transcribed spacer. special creation a nonscientific philosophy assert- ing that each species has originated through a sepa- rate act of divine creation by processes that are not now in operation in the natural world. specialized 1. an organism having a narrow range of tolerance for one or more ecological conditions. 2. a species having a relatively low potential for further evolutionary change; the opposite of generalized. specialized transduction See transduction. speciation 1. the splitting of an ancestral species into daughter species that coexist in time; horizontal evolution or speciation; cladogenesis. 2. the gradual transformation of one species into another without an increase in species number at any time within the lineage; vertical evolution or speciation; phyletic evolution or speciation.
See Appendix C, 1954, Mayr; 1975, King and Wilson; 1985, Carson; 2000, Singh and Kulathinal; alloparapatric speciation, allopatric speciation, cichlid fishes, evolution, founder effect, Hawaiian Drosophilidae, parapatric speciation, peripa- tric speciation, punctuated equilibrium, selector genes, sex gene pool theory of speciation, sexual selection, silversword alliance, sympatric speciation. species 1. biological (genetic) species: reproduc- tively isolated systems of breeding populations. 2. pa- leospecies (successional species): distinctly different appearing assemblages of organisms as a consequence of species transformation (q.v.). 3. taxonomic (mor- phological; phenetic) species: phenotypically distinc- tive groups of coexisting organisms. 4. microspecies (agamospecies): asexually reproducing organisms (mainly bacteria) sharing a common morphology and physi- ology (biochemistry). 5. biosystematic species (eco- species) populations that are isolated by ecological factors rather than ethological isolation (q.v.). species group superspecies (q.v.). species selection a form of group selection (q.v.) in which certain species (produced by cladogenesis) continue the cladogenic process and others become extinct.
species transformation the transformation of a species (A) into another (species B) during the pas- sage of time. Species transformation does not in- crease the number of species, since species A and B do not coexist in time. See anagenesis, speciation, vertical evolution. specific activity the ratio of radioactive to non- radioactive atoms or molecules of the same kind. Sometimes given as the number of atoms of radio- isotope per million atoms of stable element. Also ex- pressed in curies per mole. specific immune suppression an immune re- sponse in which the initial exposure to a particular antigen results in the loss of the ability of the organ- ism to respond to subsequent exposures of that anti- gen, but not to different antigens. See immunological tolerance. specific ionization the number of ion pairs per unit length of path of the ionizing radiation in a given medium (per micron of tissue, for example). specificity selective reactivity between substances: e.g., between an enzyme and its substrate, between a hormone and its cell-surface receptor, or between an antigen and its corresponding antibody.
specificity factors proteins that temporarily asso- ciate with the core component of RNA polymerase and determine to which promoters the enzyme will bind (e.g., the sigma factor, q.v.). See antispecificity factor. specimen screen the support for sections to be viewed under the electron microscope consisting of a disc made of copper or gold mesh. spectrin a protein that is a major component of the plasma membranes of animal cells. It is com- posed of two different polypeptide chains, alpha and beta, which form heterodimers. Each polypeptide contains tandemly repeated sequences that can fold upon themselves and so give the spectrin filament great flexibility. In the cell membrane, spectrin fil- aments form a pentagonal network in which their ends attach to junctions made of actin and other proteins. Spectrin has been identified as one of the molecular components of the spectrosome (q.v.) and the fusome (q.v.) in Drosophila. See peripheral pro- tein. spectrophotometer an optical system used in biol- ogy to compare the intensity of a beam of light of specified wave length before and after it passes through a light-absorbing medium. See microspec- trophotometer. spectrosome a prominent spectrin-rich, spherical mass found in the cytoplasm of germ line stem cells (q.v.) and cytoblasts (q.v.) in the Drosophila ovary. The spectrosome is rich in cytoskeletal proteins such as actin (q.v.), α- and β-spectrin, the adducin-like Hts protein, and ankyrin (q.v.).
This organelle is though to be a precursor of the fusome (q.v.) and to anchor the mitotic spindle during germ line stem cell and cystoblast divisions. hts and α-spectrin muta- tions eliminate spectrosome and fusome formation and result in aberrant mitotic spindle orientation during germ line stem cell and cystoblast/cystocyte divisions. See adducin, ankyrin, cystocyte divisions, hu-li tai shao (hts), spectrin. speech-language disorder 1 an extremely rare condition, showing autosomal dominant inheritance, that affects a British family (the KE family) and causes a severe language disorder. The afflicted indi- viduals are unable to learn certain rules of grammar and tense, and they cannot enunciate certain verbal patterns. The gene involved is FOXP2, located at 7q31, and it spans approximately 600 kb of DNA. Two functional copies of FOXP2 are required for the acquisition of normal spoken language. The gene contains over 20 exons, and it spans approximately 600 kb of DNA. There are at least four mRNAs transcribed from FOXP2 by alternative splicing (q.v.), and these transcripts are plentiful in the fetal brain. The most common splice form encodes a pro- tein 715 amino acids long. A segment of this protein contains a DNA-binding site, suggesting that it func- tions as a regulator of transcription.
Homologs of FOXP2 have been identified in the chimpanzee, the gorilla, and the orangutan. The FOXP2 proteins of the apes are all identical to each other, but the hu- man protein has different amino acids at two sites. Perhaps these changes gave new properties to the protein which influenced neural systems in ways that eventually led to the acquisition of speech. See Appendix C, 2001, Lai et al. spelt Triticum spelta (N = 21), the oldest of the cultivated hexaploid wheats, grown since the latter days of Roman Empire. See wheat. Spemann-Mangold organizer named after Hans Spemann and Hilde Mangold who published the de- tails of their tissue implantation experiments in 1924. They showed that tissue from the dorsal blas- topore lip of the amphibian gastrula can induce a secondary body axis in another embryo. The second- ary brain and spinal cord did not arise from the transplanted cells, but from the presumptive ventral epidermis of the host. They concluded that the im- planted material contained diffusible “organizing factors” that determined the future differentiation of the adjacent host tissues. The Spemann-Mangold or- ganizer is now known to play a vital role during de- velopment in all members of the Chordata (q.v.). See blastoporal lip, chordamesoderm, gastrulation, goosecoid, Triton, xenograft. S period See cell cycle. sperm a single male gamete or spermatozoon. Sperm can also refer to multiple male gametes or spermatozoa. spermateleosis spermiogenesis (q.v.). spermatheca the organ in a female or a hermaph- rodite which receives and stores the spermatozoa donated by the mate. spermatid one of four haploid cells formed during meiosis in the male. Spermatids without further di- vision transform into spermatozoa, a process known as spermiogenesis (q.v.). spermatocyte a diploid cell that undergoes meio- sis and forms four spermatids. A primary spermato- cyte undergoes the first of the two meiotic divisions and gives rise to two secondary spermatocytes. Each of these divides to produce two haploid spermatids.
spermatogenesis the developmental process that results in the formation of mature sperm in an or- ganism. Spermatogenesis involves a series of events, including mitotic divisions in spermatogonia (q.v.), meiotic divisions in spermatocytes, and morphologi- cal changes in spermatids that lead to the formation of mature spermatozoa. Many of these events re- quire interactions between the germ line (q.v.) and the surrounding soma (q.v.). See spermatocyte, sper- matid, spermatozoon, spermiogenesis. spermatogonia mitotically active cells in the go- nads of male animals that are the progenitors of pri- mary spermatocytes. Spermatophyta in older taxonomies the division of the plant kingdom containing the contemporary dominant flora. Spermatophytes are characterized by the production of pollen tubes and seeds. All an- giosperms and gymnosperms are included in the Spermatophyta. See Appendix A. spermatozoon a single male gamete or sperm (plural, spermatozoa). When the word is used as an advective the spelling is spermatozoan. sperm bank a depository where samples of human semen are stored in liquid nitrogen at −196°C; when needed, perhaps years later, a sample can be thawed and used in artificial insemination. spermiogenesis the series of morphological and chemical changes that transform the spermatids re- sulting from the meiotic divisions of a spermatocyte into functional spermatozoa. In most animals, excess cytoplasm is expelled from the spermatid, and the acrosome (q.v.) and the flagellum (q.v.) are formed.
An interesting exception is found in the Nematoda which have amoeboid sperm. sperm polymorphism the production of normal and aberrant sperm during spermatogenesis. The normal sperm are called eupyrene, those containing subnormal numbers of chromosomes are oligopyrene, and those lacking a nucleus altogether are apyrene. Apyrene and oligopyrene sperm are formed by cer- tain snails (Viviparus malleatus is an example) and moths (Bombyx mori), but the function of these ab- normal gametes is unknown. sperm sharing a phenomenon occurring in Brazil- ian freshwater snails of the genus Bioaphalaria in which a simultaneous hermaphrodite (acting me- chanically as a male) transfers sperm to its partner that was collected when it functioned as a female in a previous mating.
Sperm sharing may occur both within and between species. The term sperm com- merce refers to the transfer of a sperm donor’s own sperm along with exogenous sperm from a previous mating. See hermaphrodite. Sphaerocarpus donellii a species of liverwort used in the classic mutagenesis experiments which showed that the wave length of UV specifically ab- sorbed by DNA, not protein, was the most effective in producing mutations. See Appendix A, Plantae, Bryophyta, Hapaticae; Appendix C, 1939, Knapp et al.; ultraviolet radiation. S phase See cell cycle. sphenophytes horse tails, a group of plants that originated during the Devonian and is represented today by the rush Equisetum. In Carboniferous for- ests, sphenophytes grew to heights of 15 meters. spheroplast a protoplast (q.v.) to which some cell wall remnants are attached. For example, a rod- shaped bacterium treated with lysozyme becomes spherical because the enzyme removes peptidogly- can components that give rigidity to the cell wall. sphingomyelin a molecule belonging to a family of compounds that occur in the myelin sheath of nerves. All sphingomyelins contain sphingosine, phosphorylcholine, and a fatty acid.
sphingosine an amino dialcohol component of the sphingolipids, which are abundant in the brain.
spike an inflorescence, such as the catkin of the pussy willow, in which the flowers arise directly from a central axis, the rachis. See raceme.