Translocations in DICTYOSTELIUM DISCOIDEUM

Fourteen translocations of independent origin were identified in Dictyostelium discoideum on the basis of segregation anomalies of diploids heterozygous for these chromosome rearrangements, all of which led to the cosegregation of unlinked markers. Many of these translocations were discovered in strains mutagenized with MNNG or in strains carrying mutations affecting DNA repair; however, spontaneous translocations were also obtained. Haploid mitotic recombinants of the rearranged linkage groups were produced from diploids heterozygous for the translocations at frequencies of up to 5% of viable haploid segregants; this is at least a ten-fold higher frequency than that seen with diploids not heterozygous for translocations (approximately 0.1%). These haploid recombinants included both translocated and nontranslocated strains. The T354(II, VII) translocation and possibly the T357(IV, VII) translocation reduce the chromosome number to n = 6; haploids carrying 11 other translocations all have karyotypes with n = 7. Genetic characterization of the T357(IV, VII) translocation showed that the bwnA and whiC loci normally found on linkage group IV were physically linked to the linkage group VII loci couA, phgA, bsgB and cobA.     

Mitotic Crossing over and Nondisjunction in Translocation Heterozygotes of Aspergillus

To analyze mitotic recombination in translocation heterozygotes of A. nidulans two sets of well-marked diploids were constructed, homo- or heterozygous for the reciprocal translocations T1(IL;VIIR) or T2(IL;VIIIR) and heterozygous for selective markers on IL. It was found that from all translocation heterozygotes some of the expected mitotic crossover types could be selected. Such crossovers are monosomic for one translocated segment and trisomic for the other and recovery depends on the relative viabilities of these unbalanced types. The obtained segregants show characteristically reduced growth rates and conidiation dependent on sizes and types of mono- and trisomic segments, and all spontaneously produce normal diploid sectors. Such secondary diploid types either arose in one step of compensating crossing over in the other involved arm, or—more conspicuously—in two steps of nondisjunction via a trisomic intermediate.—In both of the analyzed translocations the segments translocated to IL were extremely long, while those translocated from IL were relatively short. The break in I for T1(I;VII) was located distal to the main selective marker in IL, while that of T2(I;VIII) had been mapped proximal but closely linked to it. Therefore, as expected, the selected primary crossover from the two diploids with T2( I;VIII) in coupling or in repulsion to the selective marker, showed the same chromosomal imbalance and poor growth. These could however be distinguished visually because they spontaneously produced different trisomic intermediates in the next step, in accordance with the different arrangement of the aneuploid segments. On the other hand, from diploids heterozygous for T1( I;VII) mitotic crossovers could only be selected when the selective markers were in coupling with the translocation; these crossovers were relatively well-growing and produced frequent secondary segregants of the expected trisomic, 2n+VII, type. For both translocations it was impossible to recover the reciprocal crossover types (which would be trisomic for the distal segments of I and monosomic for most of groups VII or VIII) presumably because these were too inviable to form conidia.—In addition to the selected segregants of expected types a variety of unexpected ones were isolated. The conditions of selection used favour visual detection of aneuploid types, even if these produce only a few conidial heads and are not at a selective advantage. For T2(I;VIII) these "non-selected" unbalanced segregants were mainly "reciprocal" crossovers of the same phenotype and imbalance as the selected ones. For T1(I;VII) two quite different types were obtained, both possibly originating with loss of the small VII–I translocation chromosome. One was isolated when the selective marker in repulsion to T1(I;VII) was used and, without being homo- or hemizygous for the selective marker, it produced stable sectors homozygous for this marker. The other was obtained from both coupling and repulsion diploids and showed a near-diploid genotype; it produced practically only haploid stable sectors of the type expected from monosomics, 2n–1 for the short translocation chromosome.     

Genetic Analysis of the Reciprocal Translocation T2(I;VIII) of Aspergillus Using the Technique of Mitotic Mapping in Homozygous Translocation Diploids

A UV-induced sulphite-requiring mutant (sD50) consistently shows mitotic linkage to groups I and VIII in haploids from heterozygous mapping diploids. This linkage was found to be due to a reciprocal translocation T2(I;VIII) which could not be separated from the sulphite requirement in about 100 tested progeny from heterozygous crosses, and both may well have been induced by the same mutational event. T2(I;VIII) is the first case of a reciprocal translocation in Aspergillus which showed meiotic linkages between markers of different linkage groups, and, in addition, involved chromosome arms containing markers suitable for complete mapping by the technique of mitotic recombination in homozygous translocation diploids.-Using various selective markers, haploid segregants and diploid crossovers of all possible types were isolated from homozygous translocation diploids. (1) Haploid segregants showed new linkage relationships in T/T diploids: all available markers of VIII now segregated as a group with the majority of the markers of I, except for the markers of the left tip of I. These formed a separate linkage group and are presumably translocated to VIII. (2) Diploid mitotic crossovers confirmed this information and showed that the orientation of the translocated segments was unchanged. These findings conclusively demonstrate that T2(I;VIII) is a reciprocal translocation due to an exchange of the left tip of group I with the long right arm of group VIII.-Since the position of the break on VIIIR was found to be at sD50 this marker could be used to map the break on IL by meiotic recombination in heterozygous crosses. In addition, such crosses showed reduced recombination around the breaks, so that it was possible to sequence markers which normally are barely linked.     

Detection and Identification of Translocations by Increased Specific Nondisjunction in ASPERGILLUS NIDULANS

A meiotic technique for visual detection of translocations has been applied to ten mitotically identified interchanges, and three new translocations were discovered using this method. Testcrosses between "standard" strains and potential translocation strains-e.g. strains with newly induced mutants or descendants from translocation crosses-are inspected for the frequency of abnormal-looking colonies. In all heterozygous translocation crosses "abnormals" are increased at least tenfold compared to the average control level of 0.15%. Most of these are disomics, and can be recognized by their characteristic phenotypes. Each translocation produces a few specific types, since nondisjunction is increased mainly in the linkage groups involved in the translocation (50-100-fold over control values). Therefore, translocations were not only detected but often tentatively assigned to linkage groups from the analysis of the disomic progeny in crosses. In addition, this technique allows reciprocal and nonreciprocal translocations to be distinguished, since only the latter produce one-third phenotypically abnormal duplication progeny. While results are clearcut in most cases, occasionally problems are encountered, e.g. when morphological mutants segregate in crosses, or when other genetic factors which increase or reduce the frequency of nondisjunction are present in certain strains.     

Translocations of Chromosome End-Segments and Facultative Heterochromatin Promote Meiotic Ring Formation in Evening Primroses

Due to reciprocal chromosomal translocations, many species of Oenothera (evening primrose) form permanent multichromosomal meiotic rings. However, regular bivalent pairing is also observed. Chiasmata are restricted to chromosomal ends, which makes homologous recombination virtually undetectable. Genetic diversity is achieved by changing linkage relations of chromosomes in rings and bivalents via hybridization and reciprocal translocations. Although the structural prerequisite for this system is enigmatic, whole-arm translocations are widely assumed to be the mechanistic driving force. We demonstrate that this prerequisite is genome compartmentation into two epigenetically defined chromatin fractions. The first one facultatively condenses in cycling cells into chromocenters negative both for histone H3 dimethylated at lysine 4 and for C-banding, and forms huge condensed middle chromosome regions on prophase chromosomes. Remarkably, it decondenses in differentiating cells. The second fraction is euchromatin confined to distal chromosome segments, positive for histone H3 lysine 4 dimethylation and for histone H3 lysine 27 trimethylation. The end-segments are deprived of canonical telomeres but capped with constitutive heterochromatin. This genomic organization promotes translocation breakpoints between the two chromatin fractions, thus facilitating exchanges of end-segments. We challenge the whole-arm translocation hypothesis by demonstrating why reciprocal translocations of chromosomal end-segments should strongly promote meiotic rings and evolution toward permanent translocation heterozygosity. Reshuffled end-segments, each possessing a major crossover hot spot, can furthermore explain meiotic compatibility between genomes with different translocation histories.     

Most Uv-Induced Reciprocal Translocations in SORDARIA MACROSPORA Occur in or near Centromere Regions

In fungi, translocations can be identified and classified by the patterns of ascospore abortion in asci from crosses of rearrangement x normal sequence. Previous studies of UV-induced rearrangements in Sordaria macrospora revealed that a major class (called type III) appeared to be reciprocal translocations that were anomalous in producing an unexpected class of asci with four aborted ascospores in bbbbaaaa linear sequence (b = black; a = abortive). The present study shows that the anomalous type III rearrangements are, in fact, reciprocal translocations having both breakpoints within or adjacent to centromeres and that bbbbaaaa asci result from 3:1 disjunction from the translocation quadrivalent.-Electron microscopic observations of synaptonemal complexes enable centromeres to be visualized. Lengths of synaptonemal complexes lateral elements in translocation quadrivalents accurately reflect chromosome arm lengths, enabling breakpoints to be located reliably in centromere regions. All genetic data are consistent with the behavior expected of translocations with breakpoints at centromeres.-Two-thirds of the UV-induced reciprocal translocations are of this type. Certain centromere regions are involved preferentially. Among 73 type-III translocations, there were but 13 of the 21 possible chromosome combinations and 20 of the 42 possible combinations of chromosome arms.     

Meiotic pairing and segregation of translocation quadrivalents in yeast

Meiotic pairing and segregation were studied in three different heterozygous reciprocal translocation strains of the baker’s yeast, Saccharomyces cerevisiae. Pachytene translocation quadrivalents were identified by a combination of immunofluorescence and fluorescence in situ hybridization and the karyotypes of meiotic products were determined by pulsed-field gel electrophoresis. The translocations differed with respect to the relative sizes of the chromosomes involved and the positions of translocation breakpoints, and produced translocation quadrivalents of widely different shapes. This allowed us to study the influence of the morphology of quadrivalents on their segregation behaviour. In all cases alternate predominated over adjacent segregation. 3:1 disjunction of chromosomes was more frequent when translocation breakpoints were close to the centromeres. If a translocation breakpoint was distant from the centromere, the occurrence of an intervening chiasma influenced the pattern of segregation. In general, quadrivalent formation and segregation resembled the behaviour of translocation heterozygotes in most higher eukaryotes. We therefore conclude that, although chromosome condensation does not occur in yeast metaphase, centromere orientation and chromosome disjunction are governed in a way similar to that of higher eukaryotes. Received: 6 February 1998; in revised form: 19 May 1998 / Accepted: 23 May 1998     

Analysis of synaptonemal complexes in two fertile heterozygous boars, both carriers of a reciprocal translocation involving an acrocentric chromosome

An electron microscopic study of synaptonemal complexes in two heterozygous fertile boars, one a carrier of a 4;14 reciprocal translocation and the second a carrier of this translocation associated with a 3;7 reciprocal translocation, is reported. The results showed heterologous pairing in almost all quadrivalents, as well as a lack of XY-quadrivalent association. This seemed to be a common feature of translocations in pigs, even if at least one acrocentric chromosome is involved, and may represent a significant meiotic mechanism that prevents spermatocyte loss, while the production of genetically unbalanced gametes leads to loss of progeny through abortion.     

An algorithm for analyzing linkages affected by heterozygous translocations: QuadMap

Heterozygous chromosome rearrangements such as reciprocal translocations are most accurately displayed as two-dimensional linkage maps. Standard linkage mapping software packages, such as MapMaker, generate only one-dimensional maps and so reciprocal translocations appear as clusters of markers, even though they originate from two nonhomologous chromosomes. To more accurately map these regions, researchers have developed statistical methods that use the variance in map distance to distinguish among the four segments (two translocation, two interstitial) of the translocation. In this study, we describe modifications to one of these protocols, that proposed by Livingstone et al. (2000). We also introduce QuadMap, a new software application for dissecting heterozygous translocation-affected linkage maps.     

Association of Isozymes with a Reciprocal Translocation in Cultivated Rye (SECALE CEREALE L.)

In rye (Secale cereale L. cv. "Ailés") the progeny of a cross between a structural heterozygote for a reciprocal translocation (involving the 1R chromosome) and a homozygote for the standard chromosome arrangement were analyzed for the electrophoretic patterns of eight different leaf isozymes and also for their meiotic configuration at metaphase I.——The Got-3 and Mdh-2b loci are linked to each other and also to the reciprocal translocation. The Mdh-2b locus is located in the interstitial segment of the 3Rq chromosome arm, with an estimated distance of 8 cM to the breakpoint. Therefore, the reciprocal translocation involves the 1R and 3R chromosomes.——Also, the Mdh-1 and 6-Pgd-2 loci are linked (16 ± 3 cM) and have been located on the 2Rq arm. Finally, the Per-3 and Per-4 loci are located on the 2Rp chromosome arm at an estimated distance of 26 ± 4 cM.     

Meiotic nuclear reorganization: switching the position of centromeres and telomeres in the fission yeast Schizosaccharomyces pombe.

In fission yeast meiotic prophase, telomeres are clustered near the spindle pole body (SPB; a centrosome-equivalent structure in fungi) and take the leading position in chromosome movement, while centromeres are separated from the SPB. This telomere position contrasts with mitotic nuclear organization, in which centromeres remain clustered near the SPB and lead chromosome movement. Thus, nuclear reorganization switching the position of centromeres and telomeres must take place upon entering meiosis. In this report, we analyze the nuclear location of centromeres and telomeres in genetically well-characterized meiotic mutant strains. An intermediate structure for telomere-centromere switching was observed in haploid cells induced to undergo meiosis by synthetic mating pheromone; fluorescence in situ hybridization revealed that in these cells, both telomeres and centromeres were clustered near the SPB. Further analyses in a series of mutants showed that telomere-centromere switching takes place in two steps; first, association of telomeres with the SPB and, second, dissociation of centromeres from the SPB. The first step can take place in the haploid state in response to mating pheromone, but the second step does not take place in haploid cells and probably depends on conjugation-related events. In addition, a linear minichromosome was also co-localized with authentic telomeres instead of centromeres, suggesting that telomere clustering plays a role in organizing chromosomes within a meiotic prophase nucleus.     

Strukturheterozygotie beiVicia faba

Summary The karyotypes and the meiotic behaviour of two spontaneous reciprocal translocations and one pericentric inversion are described. One of these translocations was characterized by chiasma-formation in the interstitial-segments, the other was not. The types of orientation and distribution of the chromosomes from the chain-configurations in meta-anaphase I are specified, compared mutually and with those of translocation rings and chains in maize and the similarities and differences are pointed out. In the range of the heterozygous pericentric inversion there was no pairing in meiotic prophase and therefore no chiasma formation and sterility. It will be tried to cross especially one of the translocations with the inversion-type in order to rebuild the karyotype ofVicia faba. In this case it would be possible to distinguish between all five pairs of small chromosomes according to their morphological structure, a possibility very important in localizing chromosome aberrations on the chromosomes ofVicia faba.

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Mit 15 Abbildungen     

Uncoupling of sexual reproduction from homologous recombination in homozygous Oenothera species

Salient features of the first meiotic division are independent segregation of chromosomes and homologous recombination (HR). In non-sexually reproducing, homozygous species studied to date HR is absent. In this study, we constructed the first linkage maps of homozygous, bivalent-forming Oenothera species and provide evidence that HR was exclusively confined to the chromosome ends of all linkage groups in our population. Co-segregation of complementary DNA-based markers with the major group of AFLP markers indicates that HR has only a minor role in generating genetic diversity of this taxon despite its efficient adaptation capability. Uneven chromosome condensation during meiosis in Oenothera may account for restriction of HR. The use of plants with ancient chromosomal arm arrangement demonstrates that limitation of HR occurred before and independent from species hybridizations and reciprocal translocations of chromosome arms-a phenomenon, which is widespread in the genus. We propose that consecutive loss of HR favored the evolution of reciprocal translocations, beneficial superlinkage groups and ultimately permanent translocation heterozygosity.     

Techniques for Manipulating Chromosomal Rearrangements and Their Application to DROSOPHILA MELANOGASTER. II. Translocations

Translocations have long been valued for their segregational properties. This paper extends the utility of translocations by considering recombinational derivatives of pairs of simple reciprocal translocations. Three major derivative structures are noted. One of these derivatives is suitable for use in half-tetrad experiments. A second should find use in recombining markers with translocation breakpoints. The third is an insertional-tandem duplication: it has a section of one chromosome inserted into a heterologue with a section of the latter chromosome tandemly repeated about the breaks of the insert. All of these structures are contained in "constellations" of chromosomes that regularly segregate aneuploid-1 products (informationally equivalent to nonrecombinant adjacent-1 segregants) for one of the parental translocations but do not segregate euploid products. This is in contrast to the parental T₁/T₂ constellations which segregate euploid products but not aneuploid-1 products. Methods are described for selecting translocation recombinants on the basis of this dichotomy. Several examples of translocation recombinants have been recovered with these techniques, and the recombination frequencies seem to be consistent with those observed for crossovers between inversion breakpoints. Recombinant chromosomes tend to disjoin, but it is observed that the tendency may vary according to the region involved in the recombination, and it is suggested that this difference reflects a difference in chiasmata terminalization times. Special consideration is given to insertional-tandem duplications. Large insertional-tandem duplications are useful in cytogenetic screens. Small insertional-tandem duplications are useful in gene dosage studies and other experiments that require an insert from one chromosome to another. Large duplications can be deleted to form small duplications. To generate a small insert for a specified region, it is only necessary to have one translocation with a breakpoint flanking the region of interest. The second translocation can have a breakpoint quite far from the region: an insertional-tandem duplication containing the region that has one closely flanking breakpoint can be deleted to create a smaller duplication that has two closely flanking breakpoints.     

Analysis of translocations observed in three different populations. I. Reciprocal translocations

A sample of 437 reciprocal translocations was classified into three groups according to their method of ascertainment (Group I = couples with repeated abortions; Group II = karyotypically unbalanced carriers; Group III = balanced translocation heterozygotes). Statistical analysis showed that the distributions of chromosome breaks observed in the three groups could not be accounted for by chromosome arm length alone. In couples with repeated abortions, an excess of breaks in 7p, 17p, and 22q was found, whereas in the balanced translocation heterozygotes an excess of breaks was found only in 11q. An excess of breaks was found in arms 9p, 14p, 18p, 18q, 21q, and 22q in karyotypically unbalanced probands. A significant decrease of breaks in the medial chromosome regions was accompanied by a concomitant increase in the terminal regions in all groups. The three groups demonstrated different distributions of chromosome arm involvement in the observed translocations. Balanced translocation heterozygotes had the highest frequency of large (greater than the length of 4p) translocated segments and an excess in the frequency of large-large translocations, whereas karyotypically unbalanced probands had the highest frequency of small (shorter than 21q) translocations and an excess in the frequency of small-small translocations. For each type of chromosomal imbalance observed, the balanced translocation heterozygotes demonstrated the greatest potential imbalance and the karyotypically unbalanced probands the least.     

Centromere Orientation of Quadrivalents of Heterozygous Translocations and an Autoploid of GOSSYPIUM HIRSUTUM L

Cytological observations of quadrivalents of heterozygous translocations in Gossypium hirsutum L. demonstrate that, in addition to alternate-1 and alternate-2 orientations, a third alternate orientation (alternate-3), which occurs as a three-dimensional, V-type configuration, can be identified.—Two additional types of disjunctions, the centromere orientations of which are rotational modifications of either adjacent or alternate configurations, were also observed in quadrivalents of a translocation heterozygote. These two types are rare, and both appear in the form of the Roman numeral X . The X and the alternate-3 types also occur in quadrivalents of an autoploid of G. hirsutum.—The two X types, along with adjacent-1, adjacent-2, alternate-1 and alternate-2 orientations, represent the six possible types of planar 2 x 2 random orientation of the four centromeres of a quadrivalent. Including the three-dimensional alternate-3 type, there are seven types of orientation.     

Multiple Ty-mediated chromosomal translocations lead to karyotype changes in a wine strain of Saccharomyces cerevisiae

Enological strains of Saccharomyces cerevisiae display a high level of chromosome length polymorphism, but the molecular basis of this phenomenon has not yet been clearly defined. In order to gain further insight into the molecular mechanisms responsible for the karyotypic variability, we examined the chromosomal constitution of a strain known to possess aberrant chromosomes. Our data revealed that the strain carries four rearranged chromosomes resulting from two reciprocal translocations between chromosomes III and I, and chromosomes III and VII. The sizes of the chromosomal fragments exchanged through translocation range from 40 to 150?kb. Characterization of the breakpoints indicated that the translocations involved the RAHS of chromosome III, a transposition hot-spot on the right arm of chromosome I and a region on the left arm of chromosome VII. An analysis of the junctions showed that in all cases Ty elements were present and suggested that the translocations result from recombination between transposable Ty elements. The evidence for multiple translocations mediated by Ty elements in a single strain suggests that spontaneous Ty-driven rearrangement could be quite common and may play a major role in the alteration of karyotypes in natural and industrial yeasts.     

Translocations, the Predominant Cause of Total Sterility in Sons of Mice Treated with Mutagens

Histological and cytological analyses of the testes were carried out in 42 sterile sons of males treated in the spermatozoal or spermatid stage with 250 mg/kg ethyl methanesulfonate (EMS) alone or after prefeeding with butylated hydroxytoluene (BHT); or treated with 200 R X-rays. Of the 42 sterile males, 17 had some mature spermatids, nine were blocked at diakinesis, 15 were blocked in pachytene, and one lacked spermatogenic cells altogether, having Sertoli cells only. Mitotic (spermatogonial) metaphases could therefore be analyzed in 41 of the males and meiotic configurations in 26.-(1) None of the males showed abnormalities in chromosome number, such as monosomy, trisomy, or mosaicism for either of these conditions. Certain classes of chromosome abnormalities that have been found associated with male sterility in other investigations, namely trisomies, XXY's, and X-autosome translocations, are not expected from treatment of 19A + Y cells when F(1) males are studied. (2) A very high percentage of the sterile males carried translocations. Direct meiotic evidence for this was found in 22 of the animals. In addition, 11 of the 16 that were blocked (or virtually blocked) in pachytene, and thus could be analyzed in mitosis only, consistently showed one abnormally short chromosome (or, one short plus one long), which presumably had resulted from unequal exchange (or sizable deficiency). Of the meiotically detected translocation males, 1 carried a T(A;Y), 17 had single autosomal translocations, and 4 had multiple autosomal rearrangements involving three, four, four, and six breaks, respectively. In addition, three males showed failure of X-Y pairing. (3) Translocations that cause sterility, rather than partial sterility, in males appear to be those in which at least one of the breaks occurs close to one end of a chromosome. The mitotic and meiotic evidences for this were found to be correlated. (4) It is proposed that many cases of induced F(1) male sterility may be the result of position effects produced when paracentromeric regions are translocated to euchromatic regions of certain other chromosomes. Since many translocations that produce partial sterility in the female cause complete sterility in the male, the male must be assumed to be more susceptible to disturbances of fertility by the postulated mechanism. (5) There is evidence that EMS, especially in the lower dose range, more often breaks chromosomes near one of their ends than does X-irradiation.     

Identifizierung der genetischen Geschlechtschromosomen bei der monogenen Schmeißfliege Chrysomya rufifacies (Calliphoridae,Diptera)

Previous investigations have shown the sex determination in the monogenic blowfly Chrysomya rufifacies to be controlled by a cytologically not discernible homogamety-heterogamety mechanism in the female. Female-producing (thelygenic) females are assumed to be heterozygous for a dominant female sex realizer (F′) with sex-predetermining properties, while male-producing (arrhenogenic) females as well as males are supposed to be homozygous for the recessive allele (f). In order to identify the genetic sex chromosomes of C. rufifacies among its five pairs of long euchromatic chromosomes (nos.1–5) plus one pair of small heterochromatic ones (no. 6), all chromosomes were marked by reciprocal translocations induced by X-ray treatment of adult males. The inheritance of thirteen different heterozygous translocations has been analyzed. All of the translocations (eleven) between two of the four longer chromosomes did not show sex-linked inheritance, thus demonstrating the autosomal character of the chromosomes nos 1, 2, 3 and 4. The same is true for the translocation T6 (2/6). Therefore the small heterochromatic chromosome no. 6, corresponding to the morphologically differentiated sex chromosomes within the amphogenic calliphorid species, remains without sex determining function in the monogenic fly. This could be confirmed by the analysis of monosomic (monosomy-6) and trisomic (trisomy-6) individuals, which resulted from meiotic non-disjunction in T6/+ translocation heterozygotes. Contrary to these translocations, the heterozygous 5/2 translocation (T14) exhibited sex-linked inheritance: There was but a very low frequency (0,76 per cent) of recombinants resulting from crossing-over between F′/f and the translocation breakage point in thelygenic F₁ T14/+ females. The sex-linked inheritance of T14 was confirmed by the progeny of a thelygenic F₁ T14/+ female crossed to a homozygous T14/T14 translocation male. Among the offspring of that F₁ T14/+ female, which had received the translocation from its father, all of the F₂ T14/+ females were thelygenic compared to their arrhenogenic T14/T14 sisters. These results prove that the chromosomes of pair no. 5 genetically act as X′X-XX sex chromosomes in C. rufifacies.     

Determining the order of genes,centromeres, and rearrangement breakpoints inNeurospora by tests of duplication coverage

Nontandem segmental duplications provide a useful alternative to conventional recombination mapping for determining gene order in a haploid organism such asNeurospora. When an insertional or terminal rearrangement is crossed by Normal sequence, a class of progeny is produced that have a precisely delimited chromosome segment duplicated. In such Duplication progeny, a recessive gene in the Normal-sequence donor chromosome may or may not be masked (“covered”) by its dominant wild-type allele in the translocation-sequence recipient chromosome. Coverage depends upon whether the gene in question is left or right of the rearrangement breakpoint. The recessive gene will be heterozygous and covered (not expressed) if its locus is within the duplicated segment, but it will be haploid and expressed if the locus is outside the segment. Not only genes but also centromeres can be mapped by means of duplications, because genes included in. the same viable duplication must reside in the same chromosome arm. - Numerous sequences in the current genetic maps ofN. crassa have been determined using duplications. Gene order in the albino region and in the centromere region of linkage group I provide examples. Over 50 insertional or terminal rearrangements are available from which nontandem duplications of defined content can be obtained at will; collectively these cover about 75% of the genome. - Intercrosses between partially overlapping chromosome rearrangements also produce Duplication progeny containing two copies of regions between the breakpoints. The 180 mapped reciprocal translocations and inversions include numerous overlapping combinations that can be used for duplication mapping.