Viscum album in Japan: Chromosomal translocations,maintenance of heterozygosity and the evolution of dioecy

The genusViscum is very suitable for study of structural rearrangements in chromosomes, having very large chromosomes, low basic number and very little polyploidy. An extensive survey of the dioecious speciesV. album (n=10) in Japan has revealed the widespread occurrence of several different chromosomal translocation complexes. Male plants are always heterozygous for large sex-associated translocation complexes, having 6II ⊙8 (six bivalents and a ring-of-eight) or 5II ⊙10 or rarely 4II ⊙12 at meiosis. Female plants are homozygous for these complexes, usually having 10II. There is also a floating ⊙4 which occurs in both male and female plants. Female plants may be heterozygous for another ⊙4 or ⊙6, which do not occur in male plants. Models are presented to account for the relationship between all of the translocations involved. The high levels of translocation heterozygosity are probably important in maintaining heterozygosity in the species for large complexes of adaptive genes. However the sex-associated permanent translocation heterozygosity may have originally been established as a mechanism to stabilize dioecy based on non-allelic unlinked genes for maleness and femaleness.     

Meiotic chromosomal aberrations in wild populations of Podophyllum peltatum

Meiotic chromosomal aberrations observed in wild populations of the plant Podophyllum peltatum include incomplete homologous pairing, non-homologous pairing, and inversion heterozygosity in pachytene; univalents, asymmetrical bivalents, and translocation heterozygosity in metaphase-I; bridge and fragments in anaphase-I; and non-disjunction as detected in anaphase-II. Incomplete homologous pachytene pairing is believed to result in non-homologous pairing and in the formation of metaphase-I univalents. The unequal distribution and precocious division of univalents in anaphase-I leads to non-disjunction. Non-disjunction chromosomes (varying in frequency from 0.0 to 24.6%) appear to be distributed among the genome on the basis of chromosome length. Asymmetrical bivalents and anaphase-I side-arm bridges are believed to be caused by chromatid breakage and fusion rather than inversion heterozygosity. Of the 135 clones examined, 20 were found to be heterozygous for translocations. The possibility of widespread distribution of some translocations is suggested.     

Distribution of heterozygous translocations and aneuploid spermatocyte frequency in domestic sheep

In an analysis of the chromosomes in 1556 metaphase II figures from 26 translocation-carrying rams and 84 figures from 3 normal rams (54, XY), there were no hypermodal cells in the normal rams but 35 such cells in the translocation rams, giving mean aneuploid frequencies of 3.92 percent, 4.29 percent, and 5.29 percent for the rams heterozygous for 1, 2, and 3 translocations, respectively. The translocation chromosome in the heterozygous state was found to segregate evenly during meiosis. Although the effect of heterozygosity was to increase the aneuploid frequency of the ram it is seen that the frequency in all translocation-carrying rams is low and it is thought that these cells are eliminated during spermatogenesis and therefore do not reduce the fertility of the sheep.     

Further evidence of normal fertility and the formation of balanced gametes in sheep with one or more different Robertsonian translocations.

Mating experiments are described for sheep with three different Robertsonian translocations in the single heterozygous t1, t2 and t3, homozygous t1t1 and t3t3 and double heterozygous t1t2 and t1t3 state. The experiments were designed to investigate several previously reported unusual chromosome segregation ratios in sheep, to test the fertility of translocation heterozygous ewes mated to rams of normal karyotype and to test both the fertility and segregation patterns of sheep which were double translocation heterozygotes. The fertility of the translocation heterozygous ewes was normal as assessed from conception to first service, numbers of non-conceiving ewes and lambing percentages. Two types of double translocation heterozygous rams mated to ewes of normal karyotype produced regular chromosome segregation patterns in their progeny and the matings were of normal fertility. Double translocation heterozygous ewes were also fertile. Four sheep were bred with 51 chromosomes. Two of these were triple heterozygotes with three different Robertsonian translocations 51,xy,t1t2t3 and 51,xx,t1t2t3 and two were homozygous for one translocation and heterozygous for the others, namely 51,xx,t1t2t3 and 51,xxt1t3t3. All sheep were phenotypically normal. It is concluded that the t1,t2 and t3 Robertsonian translocations of sheep do not affect reproductive performance significantly.     

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.     

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.     

CHROMOSOME RACES AND STRUCTURAL HETEROZYGOSITY IN CALYCADENIA CILIOSA GREENE (ASTERACEAE)

A biosystematic study of Calycadenia ciliosa resulted in the recognition of five homoploid (n = 6) chromosome races. The cytogenetic evidence indicates that each of these races is differentiated from its nearest relative by a single reciprocal chromosome translocation, although at one point in the evolutionary history a pericentric inversion may have been a concurrent cytological event. The data also show that the chromosome phylogeny includes two instances of redundant translocation. Mean pollen stainabilities of interracial hybrids range from 16–80%. In a survey of four natural populations 30–60% of the individuals sampled were found to be structurally heterozygous for reciprocal chromosome translocations. Pericentric inversion heterozygosity was also detected in one population. Another population contained morphologically indistinguishable individuals separated by as little as 120 m that were differentiated by a minimum of four chromosome translocations. These observations were compared with similar instances in other species in an effort to determine their significance in the process of plant evolution.     

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.     

Recombination in the X chromosome of Drosophila melanogaster females heterozygous for two autosomal translocations

X chromosome recombination was measured in females carrying two 2; 3-translocations. Total X chromosome recombination values varied according to the amount of structural heterozygosity between the two translocations. The results support the hypothesis that the observed effects of autosomal translocation homozygosity on recombination in the X chromosome are due to homozygosity for position effects of the translocation breakpoints and are not due to chromosome discontinuity.     

Chromosomal Sites Necessary for Normal Levels of Meiotic Recombination in DROSOPHILA MELANOGASTER. I. Evidence for and Mapping of the Sites

Meiotic exchange was measured in females heterozygous for a normal sequence X chromosome and for each of eleven T(1;4)s and each of sixteen T(1;Y)s. The results indicate that the X chromosome can be divided into five intervals, such that heterozygosity for a breakpoint in one interval strongly suppresses exchange within that interval, but has little or no effect on exchange in other intervals. The boundaries between these intervals are identified and mapped to regions 3C4-6/7, 7A-7E, 11A and proximal to 18C on the standard salivary map; each boundary is located at (or within a small region containing) a major constriction (i.e., a block of intercalary heterochromatin).--Exchange was examined in females heterozygous for translocations broken within the constriction at 11A. The results imply that a boundary occupies only a subregion of the entire constriction and is subdivisible by translocation breakpoints. Several other properties of boundaries have been elucidated. Finally, the relationship of these data to a simple model of meiotic pairing proposed by I. Sandler (1956) and to the role of intercalary heterochromatin in the meiotic process is discussed.     

Chromosomal polymorphism in the house mouse (Mus domesticus) of Greece and Yugoslavia

A total of 88 wild mice from the Dalmatian coast of Yugoslavia (35 animals), and Peloponnesus (30 animals) and Thebes (23 animals) on mainland Greece were karyotyped. In all but five animals Robertsonian translocations were found. Mice from the Dalmatian region were homozygous for translocations Rb(5.15), Rb(6.12), Rb(8.17), Rb(9.13), and Rb(10.14); they were homo-or heterozygous for the translocation Rb(1.11). Some of them lacked the Rb(1.11) translocation altogether so that the diploid numbers in the Yugoslavian mice were 2n=28, 29, 30, or 40. The mice from the vicinity of Olympia in northwestern Peloponnesus were homozygous for eight Robertsonian translocations: Rb(1.3), Rb(2.5), Rb(4.6), Rb(8.12), Rb(9.16), Rb(10.14), Rb(11.17), and Rb(13.15). Their diploid chromosome number was therefore 2n=24. Mice from the vicinity of Patras in northwest Peloponnesus carried all except the first three of these eight translocations; their chromosome number was 2n=30. Finally, the mice from Thebes were homozygous for translocations Rb(2.15), Rb(4.14), Rb(5.12), and Rb(10.13). They were homo- or heterozygous for Rb(6.9), Rb(8.17), and Rb(1.11); some mice lacked the Tb(1.11) translocation altogether. The translocations Rb(6.9)40Tu and Rb(10.13)42Tu represent new arm combinations not found previously in any wild mouse population. the remaining translocations have previously been found in different Mediterranean countries, in Scotland and in southern Germany. The findings suggest that each translocation arose only once and that different translocations have come together in different populations to generate a unique karyotype characterizing this population.     

Illegitimate recombination leading to allelic loss and unbalanced translocation in p53-mutated human lymphoblastoid cells.

Allelic loss and translocation are critical mutational events in human tumorigenesis. Allelic loss, which is usually identified as loss of heterozygosity (LOH), is frequently observed at tumor suppressor loci in various kinds of human tumors. It is generally thought to result from deletion or mitotic recombination between homologous chromosomes. In this report, we demonstrate that illegitimate (nonhomologous) recombination strongly contributes to the generation of allelic loss in p53-mutated cells. Spontaneous and X-ray-induced LOH mutations at the heterozygous thymidine kinase (tk) gene, which is located on the long arm of chromosome 17, from normal (TK6) and p53-mutated (WTK-1) human lymphoblastoid cells were cytogenetically analyzed by chromosome 17 painting. We observed unbalanced translocations in 53% of LOH mutants spontaneously arising from WTK-1 cells but none spontaneously arising from TK6 cells. We postulate that illegitimate recombination was occurring between nonhomologous chromosomes after DNA replication, leading to allelic loss and unbalanced translocations in p53-mutated WTK-1 cells. X-ray irradiation, which induces DNA double-strand breaks (DSBs), enhanced the generation of unbalanced translocation more efficiently in WTK-1 than in TK6 cells. This observation implicates the wild-type p53 protein in the regulation of homologous recombination and recombinational DNA repair of DSBs and suggests a possible mechanism by which loss of p53 function may cause genomic instability.     

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.     

Kinetics of oogenesis in mice heterozygous for Robertsonian translocations

The total number of oocytes at different postmating time intervals (18-40 days) was determined in mice homozygous and heterozygous for different Robertsonian (Rb) translocations, of both laboratory and feral origin. The number of oocytes was lower in heterozygous than in homozygous mice throughout the period studied. Independently of the genetic background (i.e. laboratory or feral), structural heterozygosity had a progressive detrimental effect on oocyte numbers: open, or chain diakinetic configurations had a greater detrimental effect than close, or ring, configurations. The genetic background, however, affected the ovarian constitution in terms of the total number of germ cells, which are more numerous in laboratory than in feral mice. The kinetics of oogenesis seems to be faster in feral than in laboratory mice. At the light of the data here presented, and of those already available from the literature on male and female gametogenesis in conditions of structural heterozygosity, it appears that factors other than unsaturation of pairing sites or interference with pachytene X-chromosome inactivation have to be considered. In the wild, the reduced oocyte numbers in Rb heterozygous female can contribute to the retention of isolated populations in contact zones.     

Manipulating animal social interactions to enhance translocation impact

Species’ ecological interactions, evolutionary trajectories, and survival strategies are intertwined with their social relationships. Conservation translocations can disrupt social systems, interrupting the mechanisms driving population and ecosystem trends. We outline a research framework to provide targeted tools for translocation practitioners, where appropriate, while advancing the theoretical understanding of social resiliency.     

Heterozygosity-based assortative mating in blue tits (Cyanistes caeruleus): implications for the evolution of mate choice

The general hypothesis of mate choice based on non-additive genetic traits suggests that individuals would gain important benefits by choosing genetically dissimilar mates (compatible mate hypothesis) and/or more heterozygous mates (heterozygous mate hypothesis). In this study, we test these hypotheses in a socially monogamous bird, the blue tit (Cyanistes caeruleus). We found no evidence for a relatedness-based mating pattern, but heterozygosity was positively correlated between social mates, suggesting that blue tits may base their mating preferences on partner''s heterozygosity. We found evidence that the observed heterozygosity-based assortative mating could be maintained by both direct and indirect benefits. Heterozygosity reflected individual quality in both sexes: egg production and quality increased with female heterozygosity while more heterozygous males showed higher feeding rates during the brood-rearing period. Further, estimated offspring heterozygosity correlated with both paternal and maternal heterozygosity, suggesting that mating with heterozygous individuals can increase offspring genetic quality. Finally, plumage crown coloration was associated with male heterozygosity, and this could explain unanimous mate preferences for highly heterozygous and more ornamented individuals. Overall, this study suggests that non-additive genetic traits may play an important role in the evolution of mating preferences and offers empirical support to the resolution of the lek paradox from the perspective of the heterozygous mate hypothesis.     

Direct segregation analysis of reciprocal translocations: a study of 283 sperm karyotypes from four carriers.

Using the technique of in vitro human-hamster fertilization, sperm of four men heterozygous for 4 reciprocal translocations--t(4;17),t(5;13),t(6;7), and t(9;18)--was studied. Frequencies of numerical abnormalities unrelated to the translocations range from 8.3% to 13.3%, and the incidence of imbalances ranges from 23.0% to 66.0%. Results are pooled with data from the nine other reciprocal translocations reported elsewhere, and the combined data demonstrate that male meiotic segregation is not random: whatever the type of translocation may be, the distribution of imbalances in sperm is constant, with approximately 72.0% adjacent 1, 18.5% adjacent 2, and 9.5% 3:1 segregations. The same prevalence of adjacent 1 segregations as that reported at term for translocations of paternal origin is observed. There is a strong postzygotic elimination process; for a given translocation it affects selectively the maximum-imbalance zygotes so that imbalanced segregations observed at term are always predetermined.     

Analysis of meiotic segregation in a man heterozygous for two reciprocal translocations using the hamster in vitro penetration system.

Sperm chromosomal complements of a man heterozygous for two reciprocal translocations and exhibiting the karyotype 46,XY,t(5;11) (p13;q23.2),t(7;14)(q11;q24.1) were analyzed following in vitro fusion with golden hamster zona-free eggs (the hamster in vitro penetration [HIP] system). Products of alternate, adjacent 1, and 3:1 segregation at meiosis I of both translocation quadrivalents were recovered, and the analysis of their output, which was dissimilar between the two translocations, permitted prediction of probable sites of chiasma formation in the chromosomes involved in the translocation. These data, which comprise the first reported analysis of the products of two translocations in a single individual (hence, in a common genetic background), emphasize the uniqueness in genetic behavior of individual translocations; they further demonstrate the usefulness of the HIP system to carry out such studies.     

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.     

Meiotic confirmation of the identification of chromosomes 9 and 13 in T(9; 19)163H,T(5; 13)264 Ca,and T(1; 13)70H stocks in Mus musculus

Quinacrine fluorescent banding patterns of chromosomes 9 and 13 are very similar in mitotic preparations of Mus musculus. Meiotic studies were carried out in male and female mice heterozygous for two translocations involving these chromosomes to determine whether the translocations have a common chromosome. The results indicate that chromosome 9 is involved in the T163H translocation but not in either the T70H or T264Ca translocations. The T70H and T264Ca translocations, but not the T163H, have chromosome 13 in common. These results support the interpretations based on mitotic studies.