Analysis of the functional significance of linkage group conservation in Drosophila

Linkage groups, as defined by chromosome arms in Drosophila melanogaster, appear to have remained largely intact within the genus Drosophila and, possibly, within the higher Diptera per se. We hypothesized that linkage group conservation might have a functional basis (possibly related to interphase chromosome arrangement). To test this hypothesis, a series of autosomal 2–3 translocations were synthesized, creating many new linkage groups. A total of 167 2–3 translocations were recovered, cytologically analyzed to determine their polytene chromosome breakpoints, and tested for homozygous viability and fertility. The breakpoints associated with homozygous viable translocations were randomly distributed throughout the genome, indicating that the linear continuity of the linkage groups could be disrupted quite extensively. Inter se complementation crosses between homozygous lethal translocations having similar breakpoints further confirmed this result, documenting that, at least with respect to homozygous viability, the linear integrity of the autosomal linkage groups was not of major functional significance. Fertility analysis of the homozygous translocations also indicated that sterility could not be a single major factor. Having concluded that linkage group conservation is not based on important functional interactions between specific linked chromosomal segments, or due principally to the sterility of new linkages, the problem of linkage group conservation remains unsolved. Several possible selective factors are discussed, principally segregational load and inbreeding depression, which may contribute to the elimination of new linkage rearrangements.     

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.     

A reciprocal translocation radically reshapes sex‐linked inheritance in the common frog

X and Y chromosomes can diverge when rearrangements block recombination between them. Here we present the first genomic view of a reciprocal translocation that causes two physically unconnected pairs of chromosomes to be coinherited as sex chromosomes. In a population of the common frog (Rana temporaria), both pairs of X and Y chromosomes show extensive sequence differentiation, but not degeneration of the Y chromosomes. A new method based on gene trees shows both chromosomes are sex‐linked. Furthermore, the gene trees from the two Y chromosomes have identical topologies, showing they have been coinherited since the reciprocal translocation occurred. Reciprocal translocations can thus reshape sex linkage on a much greater scale compared with inversions, the type of rearrangement that is much better known in sex chromosome evolution, and they can greatly amplify the power of sexually antagonistic selection to drive genomic rearrangement. Two more populations show evidence of other rearrangements, suggesting that this species has unprecedented structural polymorphism in its sex chromosomes.     

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.     

Estimation of the proportion of genetically unbalanced spermatozoa in the semen of boars carrying chromosomal rearrangements using FISH on sperm nuclei

Many chromosomal rearrangements are detected each year in France on young boars candidates for reproduction. The possible use of these animals requires a good knowledge of the potential effect of the rearrangements on the prolificacy of their mates. This effect can be estimated by an accurate determination of the rate of unbalanced spermatozoa in the semen of boars which carry the rearrangements. Indeed, these spermatozoa exhibiting normal fertilizing ability are responsible for an early embryonic mortality, and then, for a decrease of the litter sizes. The "spermFISH" technique, i.e. fluorescent in situ hybridization on decondensed sperm heads, has been used on several occasions in Man, in this perspective. In livestock species, this method was formerly used mainly for semen sexing purposes. We used it, for the first time, to estimate the rates of imbalance in the semen of four boars carrying chromosomal rearrangements: two reciprocal translocations, rcp(3;15)(q27;q13) and rcp(12;14)(q13;q21), as well as two independent cases of trisomy 18 mosaicism. The rates of unbalanced gametes were relatively high for the two reciprocal translocations (47.83% and 24.33%, respectively). These values differed from the apparent effects of the rearrangements estimated using a limited number of litters: a decrease in prolificacy of 23% (estimation obtained using the results of 6 litters) and 39% (57 litters), respectively for the 3/15 and 12/14 translocations. The imbalance rates were much lower for the trisomy mosaics (0.58% and 1.13%), suggesting a very moderate effect of this special kind of chromosomal rearrangement.     

Reciprocal Translocations and Translocation Disomics of Aspergillus and Their Use for Genetic Mapping

Two new techniques are described for genetic mapping of reciprocal translocations in A. nidulans, which can be used to locate centromeres and meiotically unlinked markers. They both make use of unbalanced disomics from heterozygous translocation crosses. These are mainly hyperhaploids of two classes: either typical-looking n + 1 with a normal chromosome in addition to a haploid set containing the translocation, or translocation disomics. When large chromosome segments are involved, such disomics, as well as stable aneuploids and duplication types, show characteristic phenotypes and can be classified visually. The first method maps translocation breaks qualitatively, since translocated markers can be identified when translocation disomics are analyzed for heterozygous markers. The second method measures meiotic linkage of any marker to the translocation breaks when allele ratios in the balanced haploid sectors of either or both classes of disomics are determined: linked markers show reciprocal deviations from 1:1—In addition, it can be shown that frequencies of nondisjunction and recovery of specific translocation disomics both depend on the relative position of the break within a chromosome arm. Such information can provide a rough estimate of the positions of breaks for a new translocation.—Using these techniques, as well as mitotic mapping in homo- and heterozygous translocation diploids, four reciprocal translocations were mapped. From these results, information on the sequence and orientation of most of the "meiotic fragments" of the current maps (groups III, VI, VII and VIII) was obtained, and the position of the centromeres of groups VI and VII were identified. Translocation disomics are also used to map meiotically unlinked single genes, e.g. oliA of group VII, to specify chromosome segments.     

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.     

A reciprocal translocation between ’Garfi’ almond and ’Nemared’ peach

A map with 51 markers (46 RFLPs and five isozymes) was constructed using an interspecific F₂ population between ’Garfi’ almond (Prunus amygdalus Batsch.) and ’Nemared’ peach [Prunus persica (L.) Batsch.]. This map was developed by selecting markers covering most of the distance of the eight linkage groups from previously constructed Prunus maps. The markers studied in this population mapped to seven linkage groups instead of the eight expected in Prunus. Markers belonging to groups 6 and 8 in previous maps formed a single group in the ’Garfi’×’Nemared’ F₂ and several marker pairs placed in different groups in other maps exhibited tight linkages. The study of pollen fertility and chromosome behavior during meiosis in the F₁ generation allowed us to confirm the hypothesis that a reciprocal translocation exists between ’Garfi’ and ’Nemared’. Based on independent evidence of linkage between markers and pollen fertility data in the F₂ population, we concluded that the breakpoint of the reciprocal translocation was placed between markers AC50 and AG26A in group 6 and between markers AG112A and FG230A in group 8. Received: 28 June 2000 / Accepted: 17 October 2000     

The karyotype of Nothoscordum arenarium Herter (Gilliesioideae, Alliaceae): A populational and cytomolecular analysis

The genus Nothoscordum Kunth comprises approximately 20 species native to South America. Karyologically, the genus is remarkable for its large chromosomes and Robertsonian translocations. Variation in chromosome number has been recorded in a few polyploid species and it is unknown among diploids. This study presents the chromosome number and morphology of 53 individuals of seven populations of N. arenarium Herter (2n = 10). In addition, karyotype analyses after C-banding, staining with CMA and DAPI, and in situ hybridization with 5S and 45S rDNA probes were performed in six individuals from one population. All individuals exhibited 2n = 10 (6M + 4A), except for one tetraploid (2n = 20, 12M + 8A) and one triploid (2n = 15, 9M + 6A) plant. C-banding revealed the presence of CMA(+) /DAPI (-) heterochromatin in the short arm and in the proximal region of the long arm of all acrocentric chromosomes. The 45S rDNA sites co-localized with the CMA (+) regions of the acrocentrics short arms, while the 5S rDNA probe only hybridized with the subterminal region of a pair of metacentric chromosomes. A change in the pattern of CMA bands and rDNA sites was observed in only one individual bearing a reciprocal translocation involving the long arm of a metacentric and the long arm of an acrocentric chromosome. These data suggest that, despite isolated cases of polyploidy and translocation, the karyotype of N. arenarium is very stable and the karyotypic instability described for other species may be associated with their polyploid condition.     

Cytogenetic investigations onOenothera wolfii (Onagraceae)

Oenothera wolfii, endemic in coastal Northern California and Southern Oregon, has been analysed cytogenetically. It is a permanent structural heterozygote composed of two nearly identical genomic complexes which are closely related to the neighboring maritime ecotype of the homozygousOe. elata subsp.hookeri. — Oe wolfii is believed to have evolved recently from its homozygous ancestor by the accumulation of reciprocal translocations and the acquisition of balanced lethals. Forms such asOe. wolfii represent an important connecting link in our understanding of the evolution of the complexheterozygous species ofOenothera. Dedicated to Prof. Dr.Josef Straub for His 75th birthday.     

Pollen germination and tube growth in rye (Secade cereale L.) homozygous and heterozygous for reciprocal translocations

Summary To contribute to the knowledge of the role of reciprocal translocations in rye, a component of fertility was estimated by comparing germination and pollen tube growth in homozygous and heterozygous plants for reciprocal translocations. The results obtained indicate that there are no differences in germination and pollen tube growth rate when homozygous and heterozygous plants as a whole are compared. However, there are significant differences in pollen tube growth between plants carrying different translocations. This suggests that the chromosome constitution of a plant is relevant to these fitness-estimating parameters together with its particular genetic background.     

Linkage group-chromosome correlation in Podospora anserina

Summary Two reciprocal translocations have been studied in an attempt to establish a one-to-one assignment of the seven linkage groups of P. anserina to specific chromosomes. Genetical data demonstrated clearly the relationships of the two translocations with three linkage groups. Cytological observations at meiosis in crosses heterozygous for the chromosomal interchanges showed the characteristic cross-like figures and confirmed the fact that the two translocations had one chromosome in common. The correspondence of the three largest chromosomes, including the one bearing the nucleolar organizer, with three well known linkage groups has thus been established. Evidence is also given that loci exhibiting a percentage of second division segregation as high as 98% are not at the end of the chromosomes.     

A COSII genetic map of the pepper genome provides a detailed picture of synteny with tomato and new insights into recent chromosome evolution in the genus Capsicum

We report herein the development of a pepper genetic linkage map which comprises 299 orthologous markers between the pepper and tomato genomes (including 263 conserved ortholog set II or COSII markers). The expected position of additional 288 COSII markers was inferred in the pepper map via pepper–tomato synteny, bringing the total orthologous markers in the pepper genome to 587. While pepper maps have been previously reported, this is the first complete map in the sense that all markers could be placed in 12 linkage groups corresponding to the 12 chromosomes. The map presented herein is relevant to the genomes of cultivated C. annuum and wild C. annuum (as well as related Capsicum species) which differ by a reciprocal chromosome translocation. This map is also unique in that it is largely based on COSII markers, which permits the inference of a detailed syntenic relationship between the pepper and tomato genomes—shedding new light on chromosome evolution in the Solanaceae. Since divergence from their last common ancestor is approximately 20 million years ago, the two genomes have become differentiated by a minimum number of 19 inversions and 6 chromosome translocations, as well as numerous putative single gene transpositions. Nevertheless, the two genomes share 35 conserved syntenic segments (CSSs) within which gene/marker order is well preserved. The high resolution COSII synteny map described herein provides a platform for cross-reference of genetic and genomic information (including the tomato genome sequence) between pepper and tomato and therefore will facilitate both applied and basic research in pepper. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

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.     

Chromosome Rearrangements That Involve the Nucleolus Organizer Region in Neurospora

In ~3% of Neurospora crassa rearrangements, part of a chromosome arm becomes attached to the nucleolus organizer region (NOR) at one end of chromosome 2 (linkage group V). Investigations with one inversion and nine translocations of this type are reported here. They appear genetically to be nonreciprocal and terminal. When a rearrangement is heterozygous, about one-third of viable progeny are segmental aneuploids with the translocated segment present in two copies, one in normal position and one associated with the NOR. Duplications from many of the rearrangements are highly unstable, breaking down by loss of the NOR-attached segment to restore normal chromosome sequence. When most of the rearrangements are homozygous, attenuated strands can be seen extending through the unstained nucleolus at pachytene, joining the translocated distal segment to the remainder of chromosome 2. Although the rearrangements appear genetically to be nonreciprocal, molecular evidence shows that at least several of them are physically reciprocal, with a block of rDNA repeats translocated away from the NOR. Evidence that NOR-associated breakpoints are nonterminal is also provided by intercrosses between pairs of translocations that transfer different-length segments of the same donor-chromosome arm to the NOR.     

Induction and transmission of wheat-Haynaldia villosa chromosomal translocations

In order to develop more wheat-Haynaldia villosa translocations involving different chromosomes and chromosome segments of H. villosa, T. durum-H, villosa amphiploid was irradiated with ^60Co γ-rays at doses of 800, 1,200, and 1,600 rad. Pollen collected from the spikes 1, 2, and 3 days after irradiation were transferred to emasculated spikes of the common wheat cv. ‘Chinese Spring＇. Genomic in situ hybridization was used to identify wheat-H, villosa chromosome translocations in the M1 generation. Transmission of the identified translocation chromosomes was analyzed in the BC1, BC2, and BC3 generations. The results indicated that all three irradiation doses were highly efficient for inducing wheat-alien translocations without affecting the viability of the M1 seeds. Within the range of 800-1,600 rad, both the efficiency of translocation induction and the frequency of interstitial chromosome breakage-fusion increased as the irradiation dosage increased. A higher translocation induction frequency was observed using pollen collected from the spikes 1 day after irradiation over that of 2 or 3 days after irradiation. More than 70% of the translocations detected in the M1 generation were transmitted to the BC1 through the female gametes. All translocations recovered in the BC1 generation were recovered in the following BC2, and BC3 generations. The transmission ability of different translocation types in different genetic backgrounds showed an order of ‘whole-arm translocation 〉 small alien segment translocation 〉 large alien segment translocation＇, through either male or female gametes, In general, the transmission ability through the female gametes was higher than that through the male gametes. By this approach, 14 translocation lines that involved different H. villosa chromosomes have been identified in the BC3 using EST-STS markers, and eight of them were homozygous.     

Karyotyping of Neurospora crassa using synaptonemal complex spreads of translocation quadrivalents.

The purposes of the present research are (i) to establish the karyotype of Neursopora crassa using visualization of kinetochores in the synaptonemal complex (SC) spreads, (ii) to assign each chromosome to a linkage group, and (iii) to examine chromosome pairing and recombination nodules in quadrivalents. Two strains containing reciprocal translocations were used: T(I;II)4637, which involves linkage groups I and II, and alcoy, which contains 3 independent translocations involving I and II, IV and V, and III and VI. Visualization of kinetochores in the spreads requires the use of freshly prepared fixatives. Kinetochore locations and arm ratios were documented in all 7 N. crassa chromosomes. This new information, based on kinetochore position, arm ratios, chromosome length, and quadrivalent analyses, enabled unequivocal confirmation of chromosome assignments to genetic linkage groups. Chromosome pairing in a translocation quadrivalent starts at the 4 terminal regions, and proceeds right up to the translocation break point. Recombination nodules are found in all 4 arms of quadrivalents. The ability to identify a specific chromosome to a genetic linkage group together with the ability to visualize recombination nodules and their locations will allow future cytological analysis of recombination events.     

Breakpoint distribution in male-linked translocations in Anopheles stephensi Liston

A series of translocations involving the male chromosome and chromosome 3 was analyzed in Anopheles stephensi. Using three genetic markers in 3R, namely sp, dp, and Bl, the recombination distance between the breakpoint and each of the three markers was assessed. On the basis of control recombination it was possible to assign the breakpoint to the chromosome relative to the three markers. It was shown that the majority of breakpoints were located in the vicinity of dp-Bl and translocations were identified that showed complete linkage with each of the markers. The results are compared with the published data on radiation-induced breakage and used to interpret the difficulties that have been experienced in producing a genetic sexing system in this species.     

Unusual heterostyly: style dimorphism and self-incompatibility are not tightly associated in Lithodora and Glandora (Boraginaceae)

Background and Aims

Heterostyly is a floral polymorphism characterized by the reciprocal position of stamens and stigmas in different flower morphs in a population. This reciprocal herkogamy is usually associated with an incompatibility system that prevents selfing and intra-morph fertilization, termed a heteromorphic incompatibility system. In different evolutionary models explaining heterostyly, it has been alternately argued that heteromorphic incompatibility either preceded or followed the evolution of reciprocal herkogamy. In some models, reciprocal herkogamy and incompatibility have been hypothesized to be linked together during the evolution of the heterostylous system.

Methods

We examine the incompatibility systems in species with different stylar polymorphisms from the genera Lithodora and Glandora (Boraginaceae). We then test whether evolution towards reciprocal herkogamy is associated with the acquisition of incompatibility. To this end, a phylogeny of these genera and related species is reconstructed and the morphological and reproductive changes that occurred during the course of evolution are assessed.

Key Results

Both self-compatibility and self-incompatibility are found within the studied genera, along with different degrees of intra-morph compatibility. We report for the first time extensive variability among members of the genus Glandora and related species in terms of the presence or absence of intraspecies polymorphism and heteromorphic incompatibility. Overall, our results do not support a tight link between floral polymorphism and incompatibility systems.

Conclusions

The independent evolution of stylar polymorphism and incompatibility appears to have occurred in this group of plants. This refutes the canonical view that there is strong linkage between these reproductive traits.     

A strategy for generation and balancing of autosome: Y chromosome translocations

We describe a method for generation and maintenance of translocations that move large autosomal segments onto the Y chromosome. Using this strategy we produced (2;Y) translocations that relocate between 1.5 and 4.8 Mb of the 2^nd chromosome.. All translocations were easily balanced over a male-specific lethal 1 (msl-1) mutant chromosome. Both halves of the translocation carry visible markers, as well as P-element ends that enable molecular confirmation. Halves of these translocations can be separated to produce offspring with duplications and with lethal second chromosome deficiencies . Such large deficiencies are otherwise tedious to generate and maintain.