A two-locus model of speciation.

Speciation is considered as the evolution of partial or complete cross-incompatibility between the carriers of genes (at a locus called "object locus") that distinguish the prospective species populations. The mating relations at the object locus are modified by the alleles at a second mating modifier locus. Based on a widely applicable concept of fitness and mating preference, it is shown that heterozygote disadvantage in fitness at the object locus is necessary for speciation, which corroborates Wallace's hypothesis. It is pointed out that the difference between sympatric and parapatric speciation essentially lies in the mechanisms stabilizing the polymorphism required at the object locus as a prerequisite for speciation. In the presence of recombination between the object and mating modifier locus speciation may be prevented by forces maintaining gametic phase imbalance between these loci such as can result from unidirectional gene flow between parapatric populations.     

On the evolution of genetic incompatibility systems. IV. Modification of response to an existing antigen polymorphism under partial selfing

A 2-locus model of the evolution of self-incompatibility in a population practicing partial selfing is presented. An allele is introduced at a modifier locus which influences the strength of the rejection reaction expressed by the style in response to antigens recognized in pollen. Two causes of inbreeding depression are investigated. First, offspring viability depends solely on the source (self or non-self) of the fertilizing pollen. Second, offspring viability declines with the expression of recessive deleterious alleles, segregating at a third (disease) locus, which exhibit an imperfect association with antigen alleles. Evolutionary changes occurring at the disease locus are not considered in this study. The condition under which a modifier allele that intensifies the incompatibility reaction increases when rare depends upon the number of antigens, the frequency of recessive deleterious alleles at the disease locus, and the level of association between the antigen locus and the disease locus. It is the improvement of viability among offspring derived by outcrossing, rather than the prevention of self-fertilization, that may represent the primary evolutionary function of genetic incompatibility systems.     

On the Evolution of Genetic Incompatibility Systems. VI. a Three-Locus Modifier Model for the Origin of Gametophytic Self-Incompatibility

Recent genetic analyses have demonstrated that self-incompatibility in flowering plants derives from the coordinated expression of a system of loci. To address the selective mechanisms through which a genetic system of this kind evolves, I present a three-locus model for the origin of gametophytic self-incompatibility. Conventional models assume that a single locus encodes all physiological effects associated with self-incompatibility and that the viability of offspring depends only on whether they were derived by selfing or outcrossing. My model explicitly represents the genetic determination of offspring viability by a locus subject to symmetrically overdominant selection. Initially, the level of expression of the proto-S locus is insufficient to induce self-incompatibility. Weak gametophytic self-incompatibility arises upon the introduction of a rare allele at an unlinked modifier locus which enhances the expression of the proto-S locus. While conventional models predict that the origin of self-incompatibility requires at least two- to threefold levels of inbreeding depression, I find that the comparatively low levels of inbreeding depression generated by a single overdominant locus can ensure the invasion of an enhancer of self-incompatibility under sufficiently high rates of receipt of self-pollen. Associations among components of the incompatibility system promote the origin of self-incompatibility. Enhancement of heterozygosity at the initially neutral proto-S locus improves offspring viability through associative overdominance. Further, the modifier that enhances the expression of self-incompatibility develops a direct association with heterozygosity at the overdominant viability locus. These results suggest that the evolutionary processes by which incompatibility systems originate may differ significantly from those associated with their breakdown. The genetic mechanism explored here may apply to the evolution of other systems that restrict reproduction, including maternal-fetal incompatibility in mammals.     

Population genetics of modifiers of meiotic drive. I. The solution of a special case and some general implications

This is a study of the formal population genetics of a two locus model where the alleles at one locus are subject to meiotic drive and zygotic selection and the only effect of the other locus is the modification of drive intensity. A complete analytic solution is obtained for a biologically reasonable special case. It is then argued, partially with the aid of computer analysis, that with moderate relaxation of assumptions of the special case, the conclusions derived from that case still hold. These conclusions are that if there is linkage a stable two locus polymorphism can result. There is permanent linkage disequilibrium with the loosing allele at the drive locus in coupling with the suppressor allele at the modifier locus, and the driven allele coupled with the modifier allele which enhances drive. It is suggested that this result explains how the SD system in Drosophila maintains its integrity in natural populations.     

On the evolution of genetic incompatibility systems. V. Origin of sporophytic self-incompatibility in response to overdominance in viability

Conditions for the origin of partial sporophytic self-incompatibility (SSI) are obtained from two quantitative models, which differ with respect to the determination of offspring viability. Offspring viability depends solely on the source (self or nonself) of the fertilizing pollen in the first model, which describes changes only at a primitive S-locus itself. Two loci evolve in the second model: overdominant viability selection maintains an arbitrary number of alleles at one locus, with SSI under the control of a separate locus. In both cases, the origin of SSI requires that the relative change in the numbers of offspring derived by the two reproductive modes compensate for the twofold cost of outcrossing. In the first model studied, the viability of inbred offspring fully determines the relative change in the numbers of inbred and outbred offspring produced. In the second model, the relative change in offspring numbers depends in addition on associations between the S-locus and the viability locus. Because these two-locus associations are comparable in magnitude to the differences between the viabilities of inbred and outbred offspring, SSI can arise under less restrictive conditions than expected from the one-locus model. Greater allelic multiplicity at the viability locus facilitates the origin of SSI by reducing the relative viability of inbred offspring. Tight linkage between the S-locus and the viability locus and high rates of receipt of self-pollen promote the generation and maintenance of associations between the S-locus and the viability locus. In populations in which more than two viability alleles are maintained, the active S-allele can invade even in the absence of linkage with the viability locus. The present study establishes that incompatibility systems can arise in response to identity disequilibrium between a modifier of incompatibility and a locus subject to overdominant viability selection; in particular, compensation for the twofold cost of outcrossing does not require preexisting gametic level disequilibria.     

Supergenes in polymorphic land snails. I. Partula taeniata.

The general colour of the shell in Partula taeniata is controlled by at least two loci. One of these (C) has a series of six alleles which determine the yellow (Y) and neutral brown (N) series of colours. Alleles for darker colours are dominant to those for lighter colours, but dominance is not always complete. The pink (P) colours are determined by a second locus (P) which modified the expression of the lighter alleles of the C locus. Orangeshell colour segregates with yellow but its allelic relationship is unknown. Colour of the lip is controlled by a locus (L) with pink lip dominant to white lip. The colour of the spire is determined by a locus (S) with dark (N4) spire dominant to light spire. An intermediate spire colour shows the same pattern of inheritance and may represent the effect of another allele. Banding of the shell is dominant to absence of bands, with two loci (B1 and B2) determining the type of banding. An allele at B1 produces the frenata pattern; an allele at B2 produces zonata; together they produce lyra. All the loci for which linkage data are available are linked so strongly that the whole array may be considered a supergene. Self-fertilisation takes place primarily during early reproductive life. About 20 per cent of the young of the first mating of an individual are produced by selfing, but over the whole reproductive span the frequency is only about 2-5 per cent. There is inconclusive evidence for heterozygote advantage of banded individuals.     

Studies of esterase 6 in Drosophila melanogaster. I. The genetics of a posttranslational modification

A locus has been found, an allele of which causes a modification of some allozymes of the enzyme esterase 6 in Drosophila melanogaster. There are two alleles of this locus, one of which is dominant to the other and results in increased electrophoretic mobility of affected allozymes. The locus responsible has been mapped to 3-56.7 on the standard genetic map (Est-6 is at 3-36.8). Of 13 other enzyme systems analyzed, only leucine aminopeptidase is affected by the modifier locus. Neuraminidase incubations of homogenates altered the electrophoretic mobility of esterase 6 allozymes, but the mobility differences found are not large enough to conclude that esterase 6 is sialylated.This work was supported by NIH Grant No. GM23706 and PHS Grant SO7RR7031 to Rollin C. Richmond and by NIH Genetics Training Grant No. 82 to Indiana University.     

Chromosomal arrangement of leghemoglobin genes in soybean.

A cluster of four different leghemoglobin (Lb) genes was isolated from AluI-HaeIII and EcoRI genomic libraries of soybean in a set of overlapping clones which together include 45 kilobases (kb) of contiguous DNA. These four genes, including a pseudogene, are present in the same orientation and are arranged in the order: 5'-Lba-Lbc1-Lb psi-Lbc3-3'. The intergenic regions average 2.5 kb. In addition to this main Lb locus, there are other Lb genes which do not appear to be contiguous to this locus. A sequence probably common to the 3' region of Lb loci was found flanking the Lbc3 gene. The 3' flanking region of the main Lb locus also contains a sequence that appears to be expressed more abundantly in root tissue. Another sequence which is primarily expressed in root and leaf is found 5' to two Lb loci. Overall, the main leghemoglobin locus is similar in structure to the mammalian globin gene loci.     

Mitochondrial Genetics VII. Allelism and Mapping Studies of Ribosomal Mutants Resistant to Chloramphenicol, Erythromycin and Spiramycin in S. CEREVISIAE

We have isolated 15 spontaneous mutants resistant to one or several antibiotics like chloramphenicol, erythromycin and spiramycin. We have shown by several criteria that all of them result from mutations localized in the mitochondrial DNA. The mutations have been mapped by allelism tests and by two- and three-factor crosses involving various configurations of resistant and sensitive alleles associated in cis or in trans with the mitochondrial locus omega which governs the polarity of genetic recombination. A general mapping procedure based on results of heterosexual (omega(+)x omega(-)) crosses and applicable to mutations localized in the polar segment is described and shown to be more resolving than that based on results of homosexual crosses. Mutations fall into three loci which are all linked and map in the following order: omega-R(I)-R(II)-R(III). The first locus is very tightly linked with omega while the second is less linked to the first. Mutations of similar resistance phenotype can belong to different loci and different phenotypes to the same locus. Mutations confer antibiotic resistance on isolated mitochondrial ribosomes and delineate a ribosomal segment of the mitochondrial DNA. Homo- and hetero-sexual crosses between mutants of the ribosomal segment and those belonging to the genetically unlinked ATPase locus, O(I), have been performed in various allele configurations. The polarity of recombination between R(I), R(II), R(III) and O(I) decreases as a function of the distance of the R locus from the omega locus rather than as a function of the distance of the R locus from the O(I) locus.     

卤代烷烃脱卤酶基因在拟南芥中反式失活系统的建立

以细菌Xanthobacter autotrophicus卤代烷烃脱卤酶基因为遗传负选择标记,建立了该基因在拟南芥中反式失活的实验系统,在卤代[烷烃脱卤酶转基因的拟南芥中,有1株表现为转基因失活,离体核run-off转录实验表明为基因转录后沉默（这里特指沉默位点）,用这一转基因沉墨植株与同源转基因高效表达植株（这里特指同源转基因位点）杂交,结果96％的F1代植表现为同源基因反式失活,将F1代植株自交,使部分沉默位点与反式失活的同源转基因位点分离,结果200株子代中有42株表现DhlA活性,158株无DhlA活性,即 dhlA沉默植株与表达植株之比为3．76：1,表明沉默位点是以孟德尔显性因子方式使同源转基因位点反式失活的。     

Satiation in cutaneous saltation.

The extent of cutaneous saltation (the illusory displacement of a tap presented to one skin locus by another tap occurring close in time at another locus) was modified by a "preconditioning" stimulus presented prior to and at a site distant from the saltatory test pattern. The 10-sec vibratory preconditioning (PC) stimulus appears to be analogous to inspection figures that "satiate" the perceptual field in experiments on figural aftereffects, producing changes in the perceived size, position, or shape of subsequent stimuli. The direction of displacement of the saltatory phantom was always away from the locus of the prior PC stimulus, consistent with results observed in studies of visual and kinesthetic aftereffects. Th- amount of repulsion and the rate at which the saltatory phantom returned to its initial position depend on the intensity, locus, and number of PC stimuli. As with figural aftereffects, these results resist explanation by peripheral mechanisms such as adaptation.     

Polymorphism in the structure of the yeast mitochondrial tRNA synthesis locus.

Yeast mitochondrial DNA contains a genetic locus, called the tRNA synthesis locus, which codes for information necessary for mitochondrial tRNA biosynthesis. A 9S RNA molecule coded by this locus is thought to be the trans-acting element required for the removal of 5' extensions from tRNA precursors. The DNA coding for this RNA maps to a region of mitochondrial DNA known to contain strain specific restriction site polymorphisms. Comparison of the tRNA synthesis locus in two such strains by sequence analysis demonstrates that the restriction enzyme polymorphisms are due to the deletion/insertion of a 50 base pair GC-rich element in the 5' flanking sequence of the 9S RNA coding region. There are also several differences between the 9S RNA coding region of these two strains which do not interfere with the tRNA synthesis function.     

Supergenes in polymorphic land snails. II. Partula suturalis.

The colour and banding of the shell of Partula suturalis are controlled by a single locus (M) with a series of at least six alleles. MX, giving apex as a homozygote, is dominant to MF1, giving frenata, which is dominant to the other alleles. MF2 is similar to MF1 except in its relation with MA. MF2MA produces bisecta and provides a striking example of a heterozygote that is qualitatively different from both homozygotes for the alleles producing it. MA gives atra as a homozygote and is dominant to MC and MS. MC, giving cestata as a homozygote, is recessive to all except MS. MS, giving strigata, is the universal recessive. It is suggested that the locus may be complex. The direction of coiling of the shell is determined by the H locus with HS (sinistrality) dominant to HD (dextrality). The expression of coiling is delayed by one generation, the maternal genotype determining the phenotype of the offspring. M and H are not linked. Self-fertilisation occurs infrequently and non-randomly.     

Rearrangements of the transposable mating-type cassettes of fission yeast.

The fission yeast, Schizosaccharomyces pombe, switches mating type every few cell divisions. Switching is controlled by the genes of the mating-type locus, which consists of three components, mat1, mat2-P and mat3-M, each separated by approximately 15 kb. Copy transposition of P (Plus) or M (Minus) information from mat2-P or mat3-M into the expression locus mat1 mediates cell type switching. The mating-type locus undergoes events at high frequency (10(-2)-10(-6)) which stabilize one or other mating type. These events are shown to be rearrangements which result in either deletion or insertion of DNA between cassettes.     

人Xp21．1－p21．3上3．5MbYAC重叠群构建及物理图谱分析

用Ａｌｕ－ＰＣＲ指纹图谱法分析了人Ｘｐ２１．１－ｐ２１．３上一系列的酵母人工染色体（ｙｅａｓｔａｒｔｉｆｉｃｉａｌｃｈｒｏｍｏｓｏｍｅ,ＹＡＣ）克隆,发现其中的两个ＹＡＣ克隆构成包含ＤＸＳ１６６位点的重叠群,而且这一重叠群与以前构建的包含ＤＭＤ基因全序列的ＹＡＣ重叠群相连接,ＹＡＣ克隆末端探针交叉杂交证实了这一重叠,使这一ＹＡＣ重叠群至少延伸至ＤＸＳ１６６位点,形成一个跨度为３．５Ｍｂ的ＹＡＣ重叠群。基于这些重叠的ＹＡＣ克隆绘制了这一区域的大尺度限制酶切图谱,并在这一图谱上定位了ＤＸＳ１６６位点,从而确定了ＤＸＳ１６６位点与ＤＭＤ基因的物理关系。这一工作为ＤＭＤ基因的５＇远端调控作用研究及该区域未知基因的克隆奠定了基础。     

Mapping the Treacher Collins syndrome locus to 5q31.3----q33.3.

Treacher Collins syndrome is an autosomal dominant disorder of abnormal craniofacial development. Linkage analysis was performed in Treacher Collins families with restriction fragment length or microsatellite polymorphisms associated with eight loci previously mapped to 5q31----qter. Positive lod scores were obtained for four loci, D5S119, D5S207, D5S209, and D5S210, which map to 5q31.3----q33.3. The Treacher Collins syndrome locus was linked closest to locus D5S210, which is associated with microsatellite polymorphisms, with a maximum lod score of 8.65 at theta = 0.02. The Treacher Collins syndrome locus was excluded from locus ADRB2R, which maps to 5q31----q32, and loci D5S22, D5S61, and D5S43, which map to 5q34----qter. There was no evidence for genetic heterogeneity among eight families with variable expression of the condition.     

Induction of recessive lethal and specific locus mutations in the zebrafish with ethyl nitrosourea.

Recessive lethal mutations and mutations at the gol-1 locus were induced in the zebrafish by exposure of mature sperm to the alkylating agent ethyl nitrosourea (ENU). Embryonic lethal phenotypes were recognized among the parthenogenetic progeny of mutagenized animals or among the progeny of daughters of mutagenized animals. Novel specific locus mutations were identified by the failure of mutagenized chromosomes to complement pre-existing mutant alleles at the gol-1 locus. Each mutagenized individual harboured approximately 10 embryonic lethal mutations in its germ line and about 1 in 500 mutagenized animals harboured a new mutation at the gol-1 locus. Three lines of evidence indicate that the majority of mutations that were recovered following treatment of mature sperm with ENU were probably point mutations. First, the soma and germ lines of mutagenized animals were mosaic, as expected following simple alkylation of sperm DNA. Second, mutations induced by ENU at the gol-1 locus affected pigmentation but not viability, unlike the majority of mutations induced at this locus with gamma-irradiation. Third, the ratio of specific locus:recessive lethal mutations induced by ENU was approximately 50-fold lower than the ratio observed following mutagenesis with gamma-rays. Comparison of the incidence with which embryonic recessive lethal mutations were induced with the incidence with which specific locus mutations arose indicates that there are greater than 5000 genes essential to the development and viability of the zebrafish embryo.     

Lymphocyte alloantigens of the horse. III. ELY-2.1: a lymphocyte alloantigen not coded for by the MHC

A new polymorphic locus of the horse which has several unusual properties is described. The suggested name for the locus is ELY-2 . The gene product of one allele at this locus, designated ELY-2.1 , has been identified with antisera raised as a result of pregnancy. Antibody to ELY-2.1 was first detected on day 55 after conception in the serum of a mare in first pregnancy. This early onset of antibody is similar to that seen for antibody to ELA antigens, and suggests that the source of the antigenic stimulus may be the tissue of the equine endometrial cups.
The antisera identifying ELY-2.1 are cytotoxic and kill all peripheral blood lymphocytes from horses carrying the antigen. ELY-2.1 is a cell surface molecule expressed on lymphocytes, erythrocytes, and platelets. Other cell types have not been investigated. The overall phenotypic frequency of ELY-2.1 in several horse breeds was 16 %. The ELY-2.1 antigen is controlled by an autosomal, dominant gene which is not coded by the ELA region (the major histocompatibility complex of the horse), nor is it identical to the ELY-1 locus, which codes for another cell surface alloantigen of equine lymphocytes. Stimulator cells carrying ELY-2.1 did not induce proliferation of ELY-2.1 negative responder cells in mixed cultures of horse lymphocytes. Attempts to raise alloantisera to other alleles of the ELY-2 locus through immunization with lymphocytes were unsuccessful. It is possible that the alternate allele(s) does not code for a gene product which is expressed. The function and biochemical nature of the ELY-2.1 molecule are unknown.