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ser serine. See amino acid. SER smooth endoplasmic reticulum. serial analysis of gene expression (SAGE) a method that allows the quantitative and simultane- ous analysis of a large number of transcripts. Short diagnostic sequence tags can be isolated from a tis- sue (e.g., pancreas), concatenated, and cloned. Se- quencing of many (perhaps 1,000) tags reveals a gene expression pattern characteristic of the target tissue’s function. See proteome, transcriptome. serial homology the resemblance between differ- ent members of a single, linearly arranged series of structures within an organism (the vertebrae are an example). serial symbiosis theory the theory that eukaryo- tic cells evolved from bacterial ancestors by a series of symbiotic associations. In its most modern form, it suggests that the mitochondria and microtubule organizing systems of present-day eukaryotes evolved from bacteria and spirochaetes that lived as symbi- onts in a line of single-celled eukaryotes that were the ancestors of both fungi and animals. A subline of these protoctists subsequently entered an endo- symbiosis with cyanobacteria. These evolved into chloroplasts, and the algae and plant lineages devel- oped from this group.

See Appendix C, 1978, Schwartz and Dayhoff; 1981, Margulis; 1986, Shih et al.; cryptomonads, cyanelles, endosymbiont theory, Pelomyxa, ribosomes of organelles, Rickettsia prowa- zeki, symbiogenesis. sericins a group of proteins found in silk (q.v.). sericulture the culture of Bombyx mori (q.v.) for the purpose of silk production. serine See amino acid. serine proteases a family of homologous enzymes which require the amino acid serine in their active site and appear to use the same mechanism for catal- ysis. Members include enzymes involved in digestion (trypsin, chymotrypsin, elastase), blood coagulation (thrombin), clot dissolution (plasmin), complement fixation (Cl protease), pain sensing (kallikrein), and fertilization (acrosomal enzymes). serology the study of the nature, production, and interactions of antibodies and antigens. serotonin a cyclic organic compound (5-hydroxy- tryptamine) that causes certain smooth muscles to contract rapidly and increases capillary permeability. The symptoms of anaphylaxis are due in large part to serotonin released from platelets that accumulate in the capillary bed of the lung. Serotonin also plays an important role in the metabolism of the central nervous system.

serotype an antigenic property of a cell (bacterial cell, red blood cell, etc.) identified by serological methods. serotype transformation See antigenic conversion. serum the fluid remaining after the coagulation of blood. seta See chaeta. sevenless a gene in Drosophila melanogaster that controls the development of R7, the seventh photo- receptor cell within an ommatidium (q.v.). In the absence of the wild-type allele of sevenless, R7 devel- ops as a cone cell. It appears that the protein en- coded by this cell fate gene is a membrane-bound receptor that transmits positional information that controls the type of differention the R7 cell under- goes.

See Appendix C, 1986, Tomlinson and Ready; developmental control genes, ommatidium, selector genes. severe acute respiratory syndrome (SARS) See coronavirus. Sewall Wright effect the concept advanced by S.

Wright in 1955 that alleles may be fixed or lost, es- pecially from small populations, because of random sampling errors and without regard to their adaptive values. sex 1. in its broadest sense, sex is any process that recombines in a single organism genes derived from more than a single source. In prokaryotes, sex may involve genetic recombination between two autopoie- tic cells, or between an autopoietic cell (like E. coli) and a nonautopoietic episome (like phage lambda). Eukaryotic sex always involves two autopoietic organ- isms and leads to the alternating generation of haploid and diploid cells.

Meiosis results in the formation of haploid gametes that unite in the process of fertiliza- tion to restore the diploid condition. Prokaryotic sex probably evolved more than 3 billion years ago, in the Archean era, while meiotic sex evolved among the protoctists late in the Proterozoic era, about 1 billion

Sex cell

years ago. 2. a classification of organisms or parts of organisms according to the kind of gamete pro- duced; larger, nutrient-rich gametes are female; smaller, nutrient-poor gametes are male. Meiosis in some organisms produces morphologically indistin- guishable isogametes, in which case the sexes are ar- bitrarily designated “plus” and “minus.” An individ- ual that produces both male and female gametes is monoecious (plant) or hermaphroditic (animal). sex cell gamete (q.v.). sex chromatin a condensed mass of chromatin representing an inactivated X chromosome. Each X chromosome in excess of one forms a sex chromatin body in the mammalian nucleus. See Barr body, late- replicating X chromosome. sex chromosomes the homologous chromosomes that are dissimilar in the heterogametic sex. See X chromosome, W, Z chromosomes, Y chromosomes. sex comb a row of bristles arranged like the teeth of a comb on the forelegs of male Drosophila. To- ward the end of the courtship ritual (q.v.) the male drums these tibial bristles on the dorsal surface of the female’s abdomen. Sex combs play a crucial role in determining acceptance or rejection by the female during courtship.

sex-conditioned character a phenotype that is conditioned by the sex of the individual. For exam- ple, a sex-conditioned, autosomal gene may behave as a dominant in males and as a recessive in females. Furthermore, in the homozygous female the condi- tion may be expressed to a minor degree. Human baldness is an example of a sex-conditioned charac- ter. Also called a sex-influenced character. sex determination the genetic or environmental process by which sexual identity is established in an organism, beginning with the initial commitment by embryonic cells to a particular sexual fate and end- ing with sex-specific terminal differentiation. The sex of most dioecious species is established by geno- typic sex determination (q.v.). In mammals sex is specified by the nature of the sperm that fertilizes the egg. Y-bearing sperm produce male zygotes; X- bearing sperm, female zygotes.

The Y chromosome gene, SRY (q.v.) specifies testis formation in the em- bryonic gonad, and feminization occurs when it is absent. in Drosophila and C. elegans sex determina- tion is initiated by an assessment of the X:A ratio (q.v.). This ratio is communicated to master switch gene(s), which in turn influence downstream genes that effect sex-specific differentiation. In hymenop- teran insects sex is determined by haplodiploidy (q.v.).

Some dioecious species (e.g., many reptiles) employ environmental sex determination (q.v.). See Appendix C, 1902, McClung; 1925, Bridges; 1958, Page et al.; androgen receptor gene, genic balance, germ line sex determination, Klinefelter syndrome, so- matic sex determination, Turner syndrome. sexduction the process whereby a fragment of ge- netic material from one bacterium is carried with the sex factor F to a second bacterium. sex factor See fertility factor. sex gene pool theory of speciation a theory of speciation (q.v.) applicable to all higher sexual or- ganisms and based on evidence for the rapid evolu- tion of sexual traits. This theory partitions the spe- cies gene pool (q.v.) into (1) those genes that primarily affect mating and reproduction and (2) those that are essential for other aspects of the life of the organism (its development, metabolism, via- bility, etc.). According to this theory, genes in pools 1 and 2 are selected differentially, with sex-related genes changing preferentially during the early stages of speciation.

See Appendix C, 2000, Singh and Ku- lathinal; mate choice, sexual selection. sex hormone any hormone produced by or influ- encing the activity of gonads: e.g., gonadotropins, es- trogens, androgens. Sex hormones are responsible for development of certain secondary sexual charac- teristics (e.g., growth of facial hair and muscular de- velopment in men). sex index in Drosophila, the ratio of X chromo- somes to autosome sets (A) (e.g., a male has a sex index of 0.5, a female is 1.0, a metamale 0.33, and a metafemale 1.5). sex-influenced character sex-conditioned charac- ter (q.v.). sex-limited character a phenotype expressed in only one sex, although it may be due to a sex-linked or autosomal gene. Examples: the recessive, female


sterile genes of Drosophila; the genes influencing milk and egg production in farm animals. sex linkage a special case of linkage occurring when a gene that produces a certain phenotypic trait (often unrelated to primary or secondary sexual characters) is located on the X chromosome. The re- sult of this situation is that in certain crosses the phenotypic trait in question may be observed only in individuals of the heterogametic sex, differences between reciprocal crosses (q.v.) may also be ob- served, and the trait will be observed much less fre- quently among members of the homogametic sex.

Genes residing on the Y chromosome will influence only the heterogametic sex. See Appendix C, 1820, Nasse; 1910, 1911, Morgan; sex chromosomes. sex pilus See F pilus. sex ratio the relative proportion of males and fe- males of a specified age distribution in a population. sex ratio organisms spiroplasmas (q.v.) responsi- ble for male-specific lethality in certain Drosophila species. SROs are transmitted in the ooplasm, and they kill male embryos.

The Y chromosome is not associated with SRO-induced male lethality, and therefore only embryos with two X chromosomes can withstand the infection. Sex ratio spiroplasmas are referred to as sex ratio spirochaetes in the earlier literature. Spiroplasma poulsonii, a sex ratio spiro- plasma of Drosophila willistoni, has been successfully cultivated on a complex, cell-free medium. Its ge- nome contains 2.04 × 106 bp of DNA. sex reversal the change from functioning as one sex to functioning as the other sex. The change may be a normal occurrence (see consecutive sexuality) or it may be experimentally or environmentally in- duced. sexual differentiation the process by which sex- ual determination is phenotypically expressed through the proper development of sexual organs and char- acteristics. sexual isolation ethological isolation (q.v.). sexually dimorphic trait a morphological trait seen only in one sex of a species. Examples would be the antlers of elk, the manes of lions, the brilliant tail plumage of peacocks, and the sex combs of fruit flies. All these characters are found in males, not fe- males.

See mate choice. sexually antagonistic genes genes that generate phenotypes beneficial to one sex but deleterious to the other. For example, a gene that produces a col- orful pattern on the sides of a fish might be useful for males that participate in mating displays to at- tract females. It would be deleterious to females if it made them more vunerable to predation. The accu- mulation of such genes on the Y chromosome would ensure that they are expressed only in males, pro- vided mechanisms evolve that prevent recombina- tion between the X and Y chromosomes in males. sexual reproduction reproduction involving fu- sion of haploid gamete nuclei, which result from meiosis. See parasite theory of sex. sexual selection selection at mating; the concept first proposed by Charles Darwin that in certain spe- cies there occurs a struggle between males for mates and that characteristics enhancing the success of those bearing them would have value and be perpet- uated irrespective of their general value in the strug- gle for existence. In the current literature, sexual se- lection is generally subdivided into intrasexual and epigamic selection. Intrasexual selection is what Dar- win had in mind and involves the power to conquer other males in battle.

In epigamic selection, the fe- male is the active selective agent, making a choice from among a field of genetically variable males of her own species. The results of epigamic selection may be seen in the elaborate sexual displays of cer- tain male birds and insects. Sexual selection for par- ticular types of males may be the first stage in speci- ation. Rapid speciation driven by sexual selection of this type has been shown to occur in the drosophi- lids of the Hawaiian Islands and the cichlid fishes of Lake Victoria, Africa. See Appendix C, 1871, Dar- win; 1981, Lande; 1985, Carson; Hawaiian Drosoph- ilidae, mate choice, prezygotic isolation mechanism, sex gene pool theory of speciation. sexuparous producing offspring by sexual repro- duction.

The term is used to describe the sexual phase in a species that alternates sexual reproduc- tion with parthenogenesis. This phenomenon occurs in aphids, for example. Sgo the symbol for a gene and the protein it en- codes that ensures the cohesion of centromeres dur- ing the first meiotic division. The protein binds to centromeric cohesins and prevents the separase en- zyme from cleaving them. Sgo is the acronym for Sugoshin—”guardian spirit” in Japanese. Sgo is de- graded during telophase I. Therefore the centro- meric cohesin molecules can be cleaved by separases during the second meiotic division, and sister chro- matids can then separate. See cohesin, meiosis, sep- arase. SGSI a gene in yeast that is similar in base se- quence to the human gene that causes Werner syn- drome (q.v.). In both species, mutations at this locus

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